Armstrong, NeilNeil A. Armstrong was an American astronaut. He was the first person to set foot on the moon.Neil A. Armstrong was an American astronaut. He was the first person to set foot on the moon. Image credit: NASABorn in 1930, Neil A. Armstrong, a United States astronaut, was the first person to set foot on the moon. On July 20, 1969, Armstrong and Buzz Aldrin landed the Apollo 11 lunar module Eagle on the moon. Armstrong left the module and explored the lunar surface. Upon taking his first step onto the moon, he said: "That's one small step for a man, one giant leap for mankind." But the word a was lost in radio transmission.

Armstrong was born on Aug. 5, 1930, on his grandparents' farm in Auglaize County, Ohio. He moved with his family to several Ohio communities before they settled in Wapakoneta when Neil was 13 years old. Armstrong developed an interest in flying at an early age. His love of airplanes grew when he went for his first plane ride in a Ford Tri-Motor, a "Tin Goose," at the age of 6. From then on, he was fascinated by aviation.

In 1947, Armstrong entered Purdue University. He began studies in aeronautical engineering. But in 1949, the United States Navy called him to active duty. Armstrong became a Navy pilot and was sent to Korea in 1950, near the start of the Korean War. In Korea, he flew 78 combat missions in Navy Panther jets.

In 1952, Armstrong returned to Purdue. He earned a bachelor's degree in aeronautical engineering there in 1955.

Armstrong was a civilian test pilot assigned to test the X-15 rocket airplane before becoming an astronaut in 1962. He made his first space flight in 1966 on Gemini 8 with David R. Scott. The two men performed the first successful docking of two vehicles in space -- the Gemini 8 and an uninhabited Agena rocket.

Armstrong resigned from the United States astronaut program in 1970. Also in 1970, he earned a master's degree in aerospace engineering at the University of Southern California. From 1971 to 1979, Armstrong was a professor of aerospace engineering at the University of Cincinnati. In 1986, he was named vice chairman of a presidential commission investigating the breakup of the space shuttle Challenger. From 1982 to 1992, Armstrong served as chairman of the board of Computing Technologies for Aviation, a company that develops software for flight scheduling.

Aurora An aurora is a natural display of light in the sky that can be seen with the unaided eye only at night. An auroral display in the Northern Hemisphere is called the aurora borealis,

or the northern lights. A similar phenomenon in the Southern Hemisphere is called the aurora australis. Auroras are the most visible effect of the sun's activity on the earth's atmosphere.

Most auroras occur in far northern and southern regions. They appear chiefly as arcs, clouds, and streaks. Some move, brighten, or flicker suddenly. The most common color in an aurora is green. But displays that occur extremely high in the sky may be red or purple. Most auroras occur about 60 to 620 miles (97 to 1,000 kilometers) above the earth. Some extend lengthwise across the sky for thousands of miles or kilometers.

Auroral displays are associated with the solar wind, a continuous flow of electrically charged particles from the sun. When these particles reach the earth's magnetic field, some get trapped. Many of these particles travel toward the earth's magnetic poles. When the charged particles strike atoms and molecules in the atmosphere, energy is released. Some of this energy appears in the form of auroras.

Auroras occur most frequently during the most intense phase of the 11-year sunspot cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in number. Violent eruptions on the sun's surface, known as solar flares, are associated with sunspots. Electrons and protons released by solar flares add to the number of solar particles that interact with the earth's atmosphere. This increased interaction produces extremely bright auroras. It also results in sharp variations in the earth's magnetic field called magnetic storms. During these storms, auroras may shift from the polar regions toward the equator.

A bar magnet has a magnetic field like that of the sun. Field lines, which represent the field, exit the north pole and enter the south pole. Image credit: World Book diagram by Precision Graphics

Comet A comet (KOM iht) is an icy body that releases gas or dust. Most of the comets that can be seen from Earth travel around the sun in long, oval orbits. A comet consists of a solid nucleus (core) surrounded by a cloudy atmosphere called the coma and one or two tails. Most comets are too small or too faint to be seen without a telescope. Some comets, however, become visible to the unaided eye for several weeks as they pass close to the sun. We can see comets because the gas and dust in their comas and tails reflect sunlight. Also, the gases release energy absorbed from the sun, causing them to glow.

Astronomers classify comets according to how long they take to orbit the sun. Short-period comets need less than 200 years to complete one orbit, while long-period comets take 200 years or longer.

Astronomers believe that comets are leftover debris from a collection of gas, ice, rocks, and dust that formed the outer planets about 4.6 billion years ago. Some scientists believe that comets originally brought to Earth some of the water and the carbon-based molecules that make up living things.

Parts of a comet

The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty snowball. The ice consists mainly of frozen water but may include other frozen substances, such as ammonia, carbon dioxide, carbon monoxide, and methane. Scientists believe the nucleus of some comets may be fragile because several comets have split apart for no apparent reason.

As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on the surface of the nucleus, spewing gas and dust particles into space. This gas and dust forms the comet's coma. Radiation from the sun pushes dust particles away from the coma. These particles form a tail called the dust tail. At the same time, the solar wind -- that is, the flow of high-speed electrically charged particles from the sun-converts some of the comet's gases into ions (charged particles). These ions also stream away from the

Halley's Comet becomes visible to the unaided eye about every 76 years as it nears the sun. Image credit: Lick Observatory

coma, forming an ion tail. Because comet tails are pushed by solar radiation and the solar wind, they always point away from the sun.

Most comets are thought to have a nucleus that measures about 10 miles (16 kilometers) or less across. Some comas can reach diameters of nearly 1 million miles (1.6 million kilometers). Some tails extend to distances of 100 million miles (160 million kilometers).

The life of a comet

Scientists think that short-period comets come from a band of objects called the Kuiper belt, which lies beyond the orbit of Pluto. The gravitational pull of the outer planets can nudge objects out of the Kuiper belt and into the inner solar system, where they become active comets. Long-period comets come from the Oort cloud, a nearly spherical collection of icy bodies about 1,000 times farther away from the sun than Pluto's orbit. Gravitational interactions with passing stars can cause icy bodies in the Oort cloud to enter the inner solar system and become active comets.

Comets lose ice and dust each time they return to the inner solar system, leaving behind trails of dusty debris. When Earth passes through one of these trails, the debris become meteors that burn up in the atmosphere. Eventually, some comets lose all their ices. They break up and dissipate into clouds of dust or turn into fragile, inactive objects similar to asteroids.

The long, oval-shaped orbits of comets can cross the almost circular orbits of the planets. As a result, comets sometimes collide with planets and their satellites. Many of the impact craters in the solar system were caused by collisions with comets.

Studying comets

Scientists learned much about comets by studying Halley's Comet as it passed near Earth in 1986. Five spacecraft flew past the comet and gathered information about its appearance and chemical composition. Several probes flew close enough to study the

Comets that pass near the sun come from two groups of comets near the outer edge of the solar system, according to astronomers. The disk-shaped Kuiper belt contributes comets that orbit the sun in fewer than 200 years. The Kuiper belt lies beyond Pluto's orbit, which extends to about 4.6 billion miles (7.4 billion kilometers) from the sun. The Oort cloud provides comets that take longer to complete their orbits. The outer edge of the Oort cloud may be 1,000 times farther than the orbit of Pluto. Image credit: World Book diagram by Terry Hadler, Bernard Thornton Artists

nucleus, which is normally concealed by the comet's coma. The spacecraft found a roughly potato-shaped nucleus measuring about 9 miles (15 kilometers) long. The nucleus contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar to Halley's Comet.

Scientists unexpectedly found the nucleus of Halley's Comet to be extremely dark black. They now believe that the surface of the comet, and perhaps most other comets, is covered with a black crust of dust and rock that covers most of the ice. These comets release gas only when holes in this crust rotate toward the sun, exposing the interior ice to the warming sunlight.

Another comet nucleus that has been seen by spacecraft cameras is that of Comet Borrelly. During a flyby in 2001, the Deep Space 1 spacecraft observed a nucleus about half the size of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight.

In 1994, astronomers observed a comet named Shoemaker-Levy 9, which had split into more than two dozen pieces, crashing into the planet Jupiter. One of the most active comets seen in more than 400 years was Comet Hale-Bopp, which came within 122 million miles (197 million kilometers) of Earth in 1997. This was not an especially close approach for a comet. However, Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas. The nucleus was estimated to be about 18 to 25 miles (30 to 40 kilometers) across.

In 2004, the U.S. spacecraft Stardust passed near the nucleus of Comet Wild 2 and gathered samples from the comet's coma. Stardust was scheduled to return the samples to Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014. Rosetta carried a small probe designed to land on the comet's nucleus.

The space probe Giotto passed near Halley's Comet on March 14, 1986. Giotto returned dramatic close-up images of the comet, including this one. Image credit: European Space Agency

Europa 

Europa, (yu ROH puh), is a large moon of Jupiter. Its surface is made of ice, which may have an ocean of water beneath it. Such an ocean could provide a home for living things. The surface layer of ice or ice and water is 50 to 100 miles (80 to 160 kilometers) deep. The satellite has an extremely thin atmosphere. Electrically charged particles from Jupiter's radiation belts continuously bombard Europa.

Europa is one of the smoothest bodies in the solar system. Its surface features include shallow cracks, valleys, ridges, pits, blisters, and icy flows. None of them extend more than a few hundred yards or meters upward or downward. In some places, huge sections of the surface have split apart and separated. The surface of Europa has few impact craters (pits caused by collisions with asteroids or comets). The splitting and shifting of the surface and disruptions from below have destroyed most of the old craters.

Europa's interior is hotter than its surface. This internal heat comes from the gravitational forces of Jupiter and Jupiter's other large satellites, which pull Europa's interior in different directions. As a result, the interior flexes, producing heat in a process known as tidal heating. The core of Europa may be rich in iron, but most of the satellite is made of rock.

Europa's diameter is 1,940 miles (3,122 kilometers), slightly smaller than Earth's moon. Europa takes 3.55 days to orbit Jupiter at a distance of 416,900 miles (670,900 kilometers). The Italian astronomer Galileo discovered Europa in 1610. Much of what is known about it comes from data gathered by a space probe, also named Galileo, that orbited Jupiter from 1995 to 2003.

The surface of Europa, a moon of Jupiter, consists mostly of huge blocks of ice that have cracked and shifted about, suggesting that there may be an ocean of liquid water underneath. Image credit: NASA

Global Warming 

Global warming is an increase in the average temperature of Earth's surface. Since the late 1800's, the global average temperature has increased about 0.7 to 1.4 degrees F (0.4 to 0.8 degrees C). Many experts estimate that the average temperature will rise an additional 2.5 to 10.4 degrees F (1.4 to 5.8 degrees C) by 2100. That rate of increase would be much larger than most past rates of increase.

Scientists worry that human societies and natural ecosystems might not adapt to rapid climate changes. An ecosystem consists of the living organisms and physical environment in a particular area. Global warming could cause much harm, so countries throughout the world drafted an agreement called the Kyoto Protocol to help limit it.

Causes of global warming

Climatologists (scientists who study climate) have analyzed the global warming that has occurred since the late 1800's. A majority of climatologists have concluded that human activities are responsible for most of the warming. Human activities contribute to global warming by enhancing Earth's natural greenhouse effect. The greenhouse effect warms Earth's surface through a complex process involving sunlight, gases, and particles in the atmosphere. Gases that trap heat in the atmosphere are known as greenhouse gases.

The main human activities that contribute to global warming are the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in automobiles, in factories, and in electric power plants that provide energy for houses and office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees and other plants remove CO2 from the air during photosynthesis, the process they use to produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate at which the gas is removed from the atmosphere or by the decomposition of dead vegetation.

A small number of scientists argue that the increase in greenhouse gases has not made a measurable difference in the temperature. They say that natural processes could have caused global warming. Those processes include increases in the energy emitted (given off) by the sun. But the vast majority of climatologists believe that increases in the sun's energy have contributed only slightly to recent warming.

The impact of global warming

Continued global warming could have many damaging effects. It might harm plants and animals that live in the sea. It could also force animals and plants on land to move to new habitats. Weather patterns could change, causing flooding, drought, and an increase in damaging storms. Global warming could melt enough polar ice to raise the sea level. In certain parts of the world, human disease could spread, and crop yields could decline.

Harm to ocean life

Through global warming, the surface waters of the oceans could become warmer, increasing the stress on ocean ecosystems, such as coral reefs. High water temperatures can cause a damaging process called coral bleaching. When corals bleach, they expel the algae that give them their color and nourishment. The corals turn white and, unless the water temperature cools, they die. Added warmth also helps spread diseases that affect sea creatures.

Changes of habitat

Widespread shifts might occur in the natural habitats of animals and plants. Many species would have difficulty surviving in the regions they now inhabit. For example, many flowering plants will not bloom without a sufficient period of winter cold. And human occupation has altered the landscape in ways that would make new habitats hard to reach or unavailable altogether.

Weather damage

Extreme weather conditions might become more frequent and therefore more damaging. Changes in rainfall patterns could increase both flooding and drought in some areas. More hurricanes and other tropical storms might occur, and they could become more powerful.

Rising sea level

Continued global warming might, over centuries, melt large amounts of ice from a vast sheet that covers most of West Antarctica. As a result, the sea level would rise throughout the world. Many coastal areas would experience flooding, erosion, a loss of wetlands, and

Thousands of icebergs float off the coast of the Antarctic Peninsula after 1,250 square miles (3,240 square kilometers) of the Larsen B ice shelf disintegrated in 2002. The area of the ice was larger than the state of Rhode Island or the nation of Luxembourg. Antarctic ice shelves have been shrinking since the early 1970's because of climate warming in the region. Image credit: NASA/Earth Observatory

an entry of seawater into freshwater areas. High sea levels would submerge some coastal cities, small island nations, and other inhabited regions.

Threats to human health

Tropical diseases, such as malaria and dengue, might spread to larger regions. Longer-lasting and more intense heat waves could cause more deaths and illnesses. Floods and droughts could increase hunger and malnutrition.

Changes in crop yields

Canada and parts of Russia might benefit from an increase in crop yields. But any increases in yields could be more than offset by decreases caused by drought and higher temperatures -- particularly if the amount of warming were more than a few degrees Celsius. Yields in the tropics might fall disastrously because temperatures there are already almost as high as many crop plants can tolerate.

Limited global warming

Climatologists are studying ways to limit global warming. Two key methods would be (1) limiting CO2 emissions and (2) carbon sequestration -- either preventing carbon dioxide from entering the atmosphere or removing CO2 already there.

Limiting CO2 emissions

Two effective techniques for limiting CO2 emissions would be (1) to replace fossil fuels with energy sources that do not emit CO2, and (2) to use fossil fuels more efficiently.

Alternative energy sources that do not emit CO2 include the wind, sunlight, nuclear energy, and underground steam. Devices known as wind turbines can convert wind energy to electric energy. Solar cells can convert sunlight to electric energy, and various devices can convert solar energy to useful heat. Geothermal power plants convert energy in underground steam to electric energy.

Alternative sources of energy are more expensive to use than fossil fuels. However, increased research into their use would almost certainly reduce their cost.

Carbon sequestration could take two forms: (1) underground or underwater storage and (2) storage in living plants.

Underground or underwater storage would involve injecting industrial emissions of CO2 into underground geologic formations or the ocean. Suitable underground formations include natural reservoirs of oil and gas from which most of the oil or gas has been removed. Pumping CO2 into a reservoir would have the added benefit of making it easier

to remove the remaining oil or gas. The value of that product could offset the cost of sequestration. Deep deposits of salt or coal could also be suitable.

The oceans could store much CO2. However, scientists have not yet determined the environmental impacts of using the ocean for carbon sequestration.

Storage in living plants

Green plants absorb CO2 from the atmosphere as they grow. They combine carbon from CO2 with hydrogen to make simple sugars, which they store in their tissues. After plants die, their bodies decay and release CO2. Ecosystems with abundant plant life, such as forests and even cropland, could tie up much carbon. However, future generations of people would have to keep the ecosystems intact. Otherwise, the sequestered carbon would re-enter the atmosphere as CO2.

Agreement on global warming

Delegates from more than 160 countries met in Kyoto, Japan, in 1997 to draft the agreement that became known as the Kyoto Protocol. That agreement calls for decreases in the emissions of greenhouse gases.

Emissions targets

Thirty-eight industrialized nations would have to restrict their emissions of CO2 and five other greenhouse gases. The restrictions would occur from 2008 through 2012. Different countries would have different emissions targets. As a whole, the 38 countries would restrict their emissions to a yearly average of about 95 percent of their 1990 emissions. The agreement does not place restrictions on developing countries. But it encourages the industrialized nations to cooperate in helping developing countries limit emissions voluntarily.

Industrialized nations could also buy or sell emission reduction units. Suppose an industrialized nation cut its emissions more than was required by the agreement. That country could sell other industrialized nations emission reduction units allowing those nations to emit the amount equal to the excess it had cut.

Several other programs could also help an industrialized nation earn credit toward its target. For example, the nation might help a developing country reduce emissions by replacing fossil fuels in some applications.

Approving the agreement

The protocol would take effect as a treaty if (1) at least 55 countries ratified (formally approved) it, and (2) the industrialized countries ratifying the protocol had CO2

emissions in 1990 that equaled at least 55 percent of the emissions of all 38 industrialized countries in 1990.

In 2001, the United States rejected the Kyoto Protocol. President George W. Bush said that the agreement could harm the U.S. economy. But he declared that the United States would work with other countries to limit global warming. Other countries, most notably the members of the European Union, agreed to continue with the agreement without United States participation.

By 2004, more than 100 countries, including nearly all the countries classified as industrialized under the protocol, had ratified the agreement. However, the agreement required ratification by Russia or the United States to go into effect. Russia ratified the protocol in November 2004. The treaty was to come into force in February 2005.

Analyzing global warming

Scientists use information from several sources to analyze global warming that occurred before people began to use thermometers. Those sources include tree rings, cores (cylindrical samples) of ice drilled from Antarctica and Greenland, and cores drilled out of sediments in oceans. Information from these sources indicates that the temperature increase of the 1900's was probably the largest in the last 1,000 years.

Computers help climatologists analyze past climate changes and predict future changes. First, a scientist programs a computer with a set of mathematical equations known as a climate model. The equations describe how various factors, such as the amount of CO2 in the atmosphere, affect the temperature of Earth's surface. Next, the scientist enters data representing the values of those factors at a certain time. He or she then runs the program, and the computer describes how the temperature would vary. A computer's representation of changing climatic conditions is known as a climate simulation.

In 2001, the Intergovernmental Panel on Climate Change (IPCC), a group sponsored by the United Nations (UN), published results of climate simulations in a report on global warming. Climatologists used three simulations to determine whether natural variations in climate produced the warming of the past 100 years. The first simulation took into account both natural processes and human activities that affect the climate. The second simulation took into account only the natural processes, and the third only the human activities.

The climatologists then compared the temperatures predicted by the three simulations with the actual temperatures recorded by thermometers. Only the first simulation, which took into account both natural processes and human activities, produced results that corresponded closely to the recorded temperatures.

The IPCC also published results of simulations that predicted temperatures until 2100. The different simulations took into account the same natural processes but different

patterns of human activity. For example, scenarios differed in the amounts of CO2 that would enter the atmosphere due to human activities.

The simulations showed that there can be no "quick fix" to the problem of global warming. Even if all emissions of greenhouse gases were to cease immediately, the temperature would continue to increase after 2100 because of the greenhouse gases already in the atmosphere.

Hurricane A hurricane is a powerful, swirling storm that begins over a warm sea. Hurricanes form in waters near the equator, and then they move toward the poles.

The winds of a hurricane swirl around a calm central zone called the eye surrounded by a band of tall, dark clouds called the eyewall. The eye is usually 10 to 40 miles (16 to 64 kilometers) in diameter and is free of rain and large clouds. In the eyewall, large changes in pressure create the hurricane's strongest winds. These winds can reach nearly 200 miles (320 kilometers) per hour. Damaging winds may extend 250 miles (400 kilometers) from the eye.

Hurricanes are referred to by different labels, depending on where they occur. They are called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons if they occur in the Northwest Pacific Ocean, west of an imaginary line called the International Date Line. Near Australia and in the Indian Ocean, they are referred to as tropical cyclones.

Hurricanes are most common during the summer and early fall. In the Atlantic and the Northeast Pacific, for example, August and September are the peak hurricane months. Typhoons occur throughout the year in the Northwest Pacific but are most frequent in summer. In the North Indian Ocean, tropical cyclones strike in May and November. In the South Indian Ocean, the South Pacific Ocean, and off the coast of Australia, the hurricane season runs from December to March. Approximately 85 hurricanes, typhoons, and tropical cyclones occur in a year throughout the world. In the rest of this article, the term hurricane refers to all such storms.

Hurricane conditions

Hurricane winds swirl about the eye, a calm area in the center of the storm. The main mass of clouds shown in this photograph measures almost 250 miles (400 kilometers) across. The hurricane, named Andrew, struck the Bahamas, Florida, and Louisiana in 1992, killing 65 people and causing billions of dollars in damage. Image credit: NASA

Hurricanes require a special set of conditions, including ample heat and moisture, that exist primarily over warm tropical oceans. For a hurricane to form, there must be a warm layer of water at the top of the sea with a surface temperature greater than 80 degrees F (26.5 degrees C).

Warm seawater evaporates and is absorbed by the surrounding air. The warmer the ocean, the more water evaporates. The warm, moist air rises, lowering the atmospheric pressure of the air beneath. In any area of low atmospheric pressure, the column of air that extends from the surface of the water -- or land -- to the top of the atmosphere is relatively less dense and therefore weighs relatively less.

Air tends to move from areas of high pressure to areas of low pressure, creating wind. In the Northern Hemisphere, the earth's rotation causes the wind to swirl into a low-pressure area in a counterclockwise direction. In the Southern Hemisphere, the winds rotate clockwise around a low. This effect of the rotating earth on wind flow is called the Coriolis effect. The Coriolis effect increases in intensity farther from the equator. To produce a hurricane, a low-pressure area must be more than 5 degrees of latitude north or south of the equator. Hurricanes seldom occur closer to the equator.

For a hurricane to develop, there must be little wind shear -- that is, little difference in speed and direction between winds at upper and lower elevations. Uniform winds enable the warm inner core of the storm to stay intact. The storm would break up if the winds at higher elevations increased markedly in speed, changed direction, or both. The wind shear would disrupt the budding hurricane by tipping it over or by blowing the top of the storm in one direction while the bottom moved in another direction.

The life of a hurricane

Meteorologists (scientists who study weather) divide the life of a hurricane into four stages: (1) tropical disturbance, (2) tropical depression, (3) tropical storm, and (4) hurricane.

Tropical disturbance is an area where rain clouds are building. The clouds form when moist air rises and becomes cooler. Cool air cannot hold as much water vapor as warm air can, and the excess water changes into tiny droplets of water that form clouds. The clouds in a tropical disturbance may rise to great heights, forming the towering thunderclouds that meteorologists call cumulonimbus clouds.

Cumulonimbus clouds usually produce heavy rains that end after an hour or two, and the weather clears rapidly. If conditions are right for a hurricane, however, there is so much heat energy and moisture in the atmosphere that new cumulonimbus clouds continually form from rising moist air.

Tropical depression is a low-pressure area surrounded by winds that have begun to blow in a circular pattern. A meteorologist considers a depression to exist when there is low

pressure over a large enough area to be plotted on a weather map. On a map of surface pressure, such a depression appears as one or two circular isobars (lines of equal pressure) over a tropical ocean. The low pressure near the ocean surface draws in warm, moist air, which feeds more thunderstorms.

The winds swirl slowly around the low-pressure area at first. As the pressure becomes even lower, more warm, moist air is drawn in, and the winds blow faster.

Tropical storm

When the winds exceed 38 miles (61 kilometers) per hour, a tropical storm has developed. Viewed from above, the storm clouds now have a well-defined circular shape. The seas have become so rough that ships must steer clear of the area. The strong winds near the surface of the ocean draw more and more heat and water vapor from the sea. The increased warmth and moisture in the air feed the storm.

A tropical storm has a column of warm air near its center. The warmer this column becomes, the more the pressure at the surface falls. The falling pressure, in turn, draws more air into the storm. As more air is pulled into the storm, the winds blow harder.

Each tropical storm receives a name. The names help meteorologists and disaster planners avoid confusion and quickly convey information about the behavior of a storm. The World Meteorological Organization (WMO), an agency of the United Nations, issues four alphabetical lists of names, one for the North Atlantic Ocean and the Caribbean Sea, and one each for the Eastern, Central, and Northwestern Pacific. The lists include both men's and women's names that are popular in countries affected by the storms.

Except in the Northwestern and Central Pacific, the first storm of the year gets a name beginning with A -- such as Tropical Storm Alberto. If the storm intensifies into a hurricane, it becomes Hurricane Alberto. The second storm gets a name beginning with B, and so on through the alphabet. The lists do not use all the letters of the alphabet, however, since there are few names beginning with such letters as Q or U. For example, no Atlantic or Caribbean storms receive names beginning with Q, U, X, Y, or Z.

Because storms in the Northwestern Pacific occur throughout the year, the names run through the entire alphabet instead of starting over each year. The first typhoon of the year might be Typhoon Nona, for example. The Central Pacific usually has fewer than five named storms each year.

The system of naming storms has changed since 1950. Before that year, there was no formal system. Storms commonly received women's names and names of saints of both genders. From 1950 to 1952, storms were given names from the United States military alphabet -- Able, Baker, Charlie, and so on. The WMO began to use only the names of women in 1953. In 1979, the WMO began to use men's names as well.

Hurricane

A storm achieves hurricane status when its winds exceed 74 miles (119 kilometers) per hour. By the time a storm reaches hurricane intensity, it usually has a well-developed eye at its center. Surface pressure drops to its lowest in the eye.

In the eyewall, warm air spirals upward, creating the hurricane's strongest winds. The speed of the winds in the eyewall is related to the diameter of the eye. Just as ice skaters spin faster when they pull their arms in, a hurricane's winds blow faster if its eye is small. If the eye widens, the winds decrease.

Heavy rains fall from the eyewall and bands of dense clouds that swirl around the eyewall. These bands, called rainbands, can produce more than 2 inches (5 centimeters) of rain per hour. The hurricane draws large amounts of heat and moisture from the sea.

The path of a hurricane

Hurricanes last an average of 3 to 14 days. A long-lived storm may wander 3,000 to 4,000 miles (4,800 to 6,400 kilometers), typically moving over the sea at speeds of 10 to 20 miles (16 to 32 kilometers) per hour.

Hurricanes in the Northern Hemisphere usually begin by traveling from east to west. As the storms approach the coast of North America or Asia, however, they shift to a more northerly direction. Most hurricanes turn gradually northwest, north, and finally northeast. In the Southern Hemisphere, the storms may travel westward at first and then turn southwest, south, and finally southeast. The path of an individual hurricane is irregular and often difficult to predict.

All hurricanes eventually move toward higher latitudes where there is colder air, less moisture, and greater wind shears. These conditions cause the storm to weaken and die out. The end comes quickly if a hurricane moves over land, because it no longer receives heat energy and moisture from warm tropical water. Heavy rains may continue, however, even after the winds have diminished.

Hurricane damage

Hurricane damage results from wind and water. Hurricane winds can uproot trees and tear the roofs off houses. The fierce winds also create danger from flying debris. Heavy rains may cause flooding and mudslides.

Hurricane winds on the ocean surface swirl counterclockwise around a calm eye in the Northern Hemisphere. Image credit: World Book illustrations by Bruce Kerr

The most dangerous effect of a hurricane, however, is a rapid rise in sea level called a storm surge. A storm surge is produced when winds drive ocean waters ashore. Storm surges are dangerous because many coastal areas are densely populated and lie only a few feet or meters above sea level. A 1970 cyclone in East Pakistan (now Bangladesh) produced a surge that killed about 266,000 people. A hurricane in Galveston, Texas, in 1900 produced a surge that killed about 6,000 people, the worst natural disaster in United States history.

Hurricane watchers rate the intensity of storms on a scale called the Saffir-Simpson scale, developed by American engineer Herbert S. Saffir and meteorologist Robert H. Simpson. The scale designates five levels of hurricanes, ranging from Category 1, described as weak, to Category 5, which can be devastating. Category 5 hurricanes have included Hurricane Camille, which hit the United States in 1969; Hurricane Gilbert, which raked the West Indies and Mexico in 1988; and Hurricane Andrew, which struck the Bahamas, Florida, and Louisiana in 1992.

Forecasting hurricanes

Meteorologists use weather balloons, satellites, and radar to watch for areas of rapidly falling pressure that may become hurricanes. Specially equipped airplanes called hurricane hunters investigate budding storms.

If conditions are right for a hurricane, the National Weather Service issues a hurricane watch. A hurricane watch advises an area that there is a good possibility of a hurricane within 36 hours. If a hurricane watch is issued for your location, check the radio or television often for official bulletins. A hurricane warning means that an area is in danger of being struck by a hurricane in 24 hours or less. Keep your radio tuned to a news station after a hurricane warning. If local authorities recommend evacuation, move quickly to a safe area or a designated hurricane shelter.

Moon 

Moon is Earth's only natural satellite and the only astronomical body other than Earth ever visited by human beings. The moon is the brightest object in the night sky but gives off no light of its own. Instead, it reflects light from the sun. Like Earth and the rest of the solar system, the moon is about 4.6 billion years old.

The moon is much smaller than Earth. The moon's average radius (distance from its center to its surface) is 1,079.6 miles (1,737.4 kilometers), about 27 percent of the radius of Earth.

The moon is also much less massive than Earth. The moon has a mass (amount of matter) of 8.10 x 1019 tons (7.35 x 1019 metric tons). Its mass in metric tons would be written out as 735 followed by 17 zeroes. Earth is about 81 times that massive. The moon's density (mass divided by volume) is about 3.34 grams per cubic centimeter, roughly 60 percent of Earth's density.

Because the moon has less mass than Earth, the force due to gravity at the lunar surface is only about 1/6 of that on Earth. Thus, a person standing on the moon would feel as if his or her weight had decreased by 5/6. And if that person dropped a rock, the rock would fall to the surface much more slowly than the same rock would fall to Earth.

Despite the moon's relatively weak gravitational force, the moon is close enough to Earth to produce tides in Earth's waters. The average distance from the center of Earth to the center of the moon is 238,897 miles (384,467 kilometers). That distance is growing -- but extremely slowly. The moon is moving away from Earth at a speed of about 1 1/2 inches (3.8 centimeters) per year.

The temperature at the lunar equator ranges from extremely low to extremely high -- from about -280 degrees F (-173 degrees C) at night to +260 degrees F (+127 degrees C)

The moon's surface shows striking contrasts of light and dark. The light areas are rugged highlands. The dark zones were partly flooded by lava when volcanoes erupted billions of years ago. The lava froze to form smooth rock. Image credit: Lunar and Planetary Institute

The distance to the moon is measured to an accuracy of 5 centimeters by a laser beam sent from Earth. The beam bounces off a laser reflector placed on the moon by astronauts, and returns to Earth. Image credit: World Book diagram by Bensen Studios

in the daytime. In some deep craters near the moon's poles, the temperature is always near -400 degrees F (-240 degrees C).

The moon has no life of any kind. Compared with Earth, it has changed little over billions of years. On the moon, the sky is black -- even during the day -- and the stars are always visible.

A person on Earth looking at the moon with the unaided eye can see light and dark areas on the lunar surface. The light areas are rugged, cratered highlands known as terrae (TEHR ee). The word terrae is Latin for lands. The highlands are the original crust of the moon, shattered and fragmented by the impact of meteoroids, asteroids, and comets. Many craters in the terrae exceed 25 miles (40 kilometers) in diameter. The largest is the South Pole-Aitken Basin, which is 1,550 miles (2,500 kilometers) in diameter.

The dark areas on the moon are known as maria (MAHR ee uh). The word maria is Latin for seas; its singular is mare (MAHR ee). The term comes from the smoothness of the dark areas and their resemblance to bodies of water. The maria are cratered landscapes that were partly flooded by lava when volcanoes erupted. The lava then froze, forming rock. Since that time, meteoroid impacts have created craters in the maria.

The moon has no substantial atmosphere, but small amounts of certain gases are present above the lunar surface. People sometimes refer to those gases as the lunar atmosphere. This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density) zone of particles surrounding an airless body. Mercury and some asteroids also have an exosphere.

In 1959, scientists began to explore the moon with robot spacecraft. In that year, the Soviet Union sent a spacecraft called Luna 3 around the side of the moon that faces away from Earth. Luna 3 took the first photographs of that side of the moon. The word luna is Latin for moon.

On July 20, 1969, the U.S. Apollo 11 lunar module landed on the moon in the first of six Apollo landings. Astronaut Neil A. Armstrong became the first human being to set foot on the moon.

In the 1990's, two U.S. robot space probes, Clementine and Lunar Prospector, detected evidence of frozen water at both of the moon's poles. The ice came from comets that hit the moon over the last 2 billion to 3 billion years. The ice apparently has lasted in areas that are always in the shadows of

The first people on the moon were U.S. astronauts Neil A. Armstrong, who took this picture, and Buzz Aldrin, who is pictured next to a seismograph. A television camera and a United States flag are in the background. Their lunar module, Eagle, stands at the right. Image credit: NASA

crater rims. Because the ice is in the shade, where the temperature is about -400 degrees F (-240 degrees C), it has not melted and evaporated.

This article discusses Moon (The movements of the moon) (Origin and evolution of the moon) (The exosphere of the moon) (Surface features of the moon) (The interior of the moon) (History of moon study).

The movements of the moon

The moon moves in a variety of ways. For example, it rotates on its axis, an imaginary line that connects its poles. The moon also orbits Earth. Different amounts of the moon's lighted side become visible in phases because of the moon's orbit around Earth. During events called eclipses, the moon is positioned in line with Earth and the sun. A slight motion called libration enables us to see about 59 percent of the moon's surface at different times.

Rotation and orbit

The moon rotates on its axis once every 29 1/2 days. That is the period from one sunrise to the next, as seen from the lunar surface, and so it is known as a lunar day. By contrast, Earth takes only 24 hours for one rotation.

The moon's axis of rotation, like that of Earth, is tilted. Astronomers measure axial tilt relative to a line perpendicular to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. The tilt of Earth's axis is about 23.5 degrees from the perpendicular and accounts for the seasons on Earth. But the tilt of the moon's axis is only about 1.5 degrees, so the moon has no seasons.

Another result of the smallness of the moon's tilt is that certain large peaks near the poles are always in sunlight. In addition, the floors of some craters -- particularly near the south pole -- are always in shadow.

The moon completes one orbit of Earth with respect to the stars about every 27 1/3 days, a period known as a sidereal month. But the moon revolves around Earth once with respect to the sun in about 29 1/2 days, a period known as a synodic month. A sidereal month is slightly shorter than a synodic month because, as the moon revolves around Earth, Earth is revolving around the sun. The moon needs some extra time to "catch up" with Earth. If the moon started on its orbit from a spot between Earth and the sun, it would return to almost the same place in about 29 1/2 days.

A synodic month equals a lunar day. As a result, the moon shows the same hemisphere -- the near side -- to Earth at all times. The other hemisphere -- the far side -- is always turned away from Earth.

People sometimes mistakenly use the term dark side to refer to the far side. The moon does have a dark side -- it is the hemisphere that is turned away from the sun. The location of the dark side changes constantly, moving with the terminator, the dividing line between sunlight and dark.

The lunar orbit, like the orbit of Earth, is shaped like a slightly flattened circle. The distance between the center of Earth and the moon's center varies throughout each orbit. At perigee (PEHR uh jee), when the moon is closest to Earth, that distance is 225,740 miles (363,300 kilometers). At apogee (AP uh jee), the farthest position, the distance is 251,970 miles (405,500 kilometers). The moon's orbit is elliptical (oval-shaped).

Phases

As the moon orbits Earth, an observer on Earth can see the moon appear to change shape. It seems to change from a crescent to a circle and back again. The shape looks different from one day to the next because the observer sees different parts of the moon's sunlit surface as the moon orbits Earth. The different appearances are known as the phases of the moon. The moon goes through a complete cycle of phases in a synodic month.

The moon has four phases: (1) new moon, (2) first quarter, (3) full moon, and (4) last quarter. When the moon is between the sun and Earth, its sunlit side is turned away from Earth. Astronomers call this darkened phase a new moon.

The next night after a new moon, a thin crescent of light appears along the moon's eastern edge. The remaining portion of the moon that faces Earth is faintly visible because of earthshine, sunlight reflected from Earth to the moon. Each night, an observer on Earth can see more of the sunlit side as the terminator, the line between sunlight and dark, moves westward. After about seven days, the observer can see half a full moon, commonly called a half moon. This phase is known as the first quarter because it occurs one quarter of the way through the synodic month. About seven days later, the moon is on the side of Earth opposite the sun. The entire sunlit side of the moon is now visible. This phase is called a full moon.

About seven days after a full moon, the observer again sees a half moon. This phase is the last quarter, or third quarter. After another seven days, the moon is between Earth and the sun, and another new moon occurs.

As the moon changes from new moon to full moon, and more and more of it becomes visible, it is said to be waxing. As it changes from full moon to new moon, and less and less of it can be seen, it is waning. When the moon appears smaller than a half moon, it is called crescent. When it looks larger than a half moon, but is not yet a full moon, it is called gibbous (GIHB uhs).

Like the sun, the moon rises in the east and sets in the west. As the moon progresses through its phases, it rises and sets at different times. In the new moon phase, it rises with

the sun and travels close to the sun across the sky. Each successive day, the moon rises an average of about 50 minutes later.

Eclipses occur when Earth, the sun, and the moon are in a straight line, or nearly so. A lunar eclipse occurs when Earth gets directly -- or almost directly -- between the sun and the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a full moon. A solar eclipse occurs when the moon gets directly -- or almost directly -- between the sun and Earth, and the moon's shadow falls on Earth. A solar eclipse can occur only during a new moon.

During one part of each lunar orbit, Earth is between the sun and the moon; and, during another part of the orbit, the moon is between the sun and Earth. But in most cases, the astronomical bodies are not aligned directly enough to cause an eclipse. Instead, Earth casts its shadow into space above or below the moon, or the moon casts its shadow into space above or below Earth. The shadows extend into space in that way because the moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun.

Libration

People on Earth can sometimes see a small part of the far side of the moon. That part is visible because of lunar libration, a slight rotation of the moon as viewed from Earth. There are three kinds of libration: (1) libration in longitude, (2) diurnal (daily) libration, and (3) libration in latitude. Over time, viewers can see more than 50 percent of the moon's surface. Because of libration, about 59 percent of the lunar surface is visible from Earth.

Libration in longitude occurs because the moon's orbit is elliptical. As the moon orbits Earth, its speed varies according to a law discovered in the 1600's by the German astronomer Johannes Kepler. When the moon is relatively close to Earth, the moon travels more rapidly than its average speed. When the moon is relatively far from Earth, the moon travels more slowly than average. But the moon always rotates about its own axis at the same rate. So when the moon is traveling more rapidly than average, its rotation is too slow to keep all of the near side facing Earth. And when the moon is traveling more slowly than average, its rotation is too rapid to keep all of the near side facing Earth.

Diurnal libration is caused by a daily change in the position of an observer on Earth relative to the moon. Consider an observer who is at Earth's equator when the moon is full. As Earth rotates Diurnal libration enables an observer

on Earth to see around one edge of the moon, then the other, during a single night. The libration occurs because Earth's rotation changes the observer's viewpoint by a distance equal to the diameter of Earth. Image credit: World Book illustration

from west to east, the observer first sees the moon when it rises at the eastern horizon and last sees it when it sets at the western horizon. During this time, the observer's viewpoint moves about 7,900 miles (12,700 kilometers) -- the diameter of Earth -- relative to the moon. As a result, the moon appears to rotate slightly to the west.

While the moon is rising in the east and climbing to its highest point in the sky, the observer can see around the western edge of the near side. As the moon descends to the western horizon, the observer can see around the eastern edge of the near side.

Libration in latitude occurs because the moon's axis of rotation is tilted about 6 1/2 degrees relative to a line perpendicular to the moon's orbit around Earth. Thus, during each lunar orbit, the moon's north pole tilts first toward Earth, then away from Earth. When the lunar north pole is tilted toward Earth, people on Earth can see farther than normal along the top of the moon. When that pole is tilted away from Earth, people on Earth can see farther than normal along the bottom of the moon.

Origin and evolution of the moon

Scientists believe that the moon formed as a result of a collision known as the Giant Impact or the "Big Whack." According to this idea, Earth collided with a planet-sized object 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off Earth's surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon.

The rapid joining together of the small bodies released much energy as heat. Consequently, the moon melted, creating an "ocean" of magma (melted rock).

The magma ocean slowly cooled and solidified. As it cooled, dense, iron-rich materials sank deep into the moon. Those materials also cooled and solidified, forming the mantle, the layer of rock beneath the crust.

As the crust formed, asteroids bombarded it heavily, shattering and churning it. The largest impacts may have stripped off the entire crust. Some collisions were so powerful that they almost split the moon into pieces. One such collision created the South Pole-Aitken Basin, one of the largest known impact craters in the solar system.

About 4 billion to 3 billion years ago, melting occurred in the mantle, probably caused by radioactive elements deep in the moon's interior. The resulting magma erupted as dark, iron-rich lava, partly flooding the heavily cratered surface. The lava cooled and solidified into rocks known as basalts (buh SAWLTS).

A basalt rock that astronauts brought to Earth from the moon formed from lava that erupted from a lunar volcano. Escaping gases created the holes before the lava solidified into rock. Image credit: Lunar and Planetary Institute

Small eruptions may have continued until as recently as 1 billion years ago. Since that time, only an occasional impact by an asteroid or comet has modified the surface. Because the moon has no atmosphere to burn up meteoroids, the bombardment continues to this day. However, it has become much less intense.

Impacts of large objects can create craters. Impacts of micrometeoroids (tiny meteoroids) grind the surface rocks into a fine, dusty powder known as the regolith (REHG uh lihth). Regolith overlies all the bedrock on the moon. Because regolith forms as a result of exposure to space, the longer a rock is exposed, the thicker the regolith that forms on it.

The exosphere of the moon

The lunar exosphere -- that is, the materials surrounding the moon that make up the lunar "atmosphere" -- consists mainly of gases that arrive as the solar wind. The solar wind is a continuous flow of gases from the sun -- mostly hydrogen and helium, along with some neon and argon.

The remainder of the gases in the exosphere form on the moon. A continual "rain" of micrometeoroids heats lunar rocks, melting and vaporizing their surface. The most common atoms in the vapor are atoms of sodium and potassium. Those elements are present in tiny amounts -- only a few hundred atoms of each per cubic centimeter of exosphere. In addition to vapors produced by impacts, the moon also releases some gases from its interior.

Most gases of the exosphere concentrate about halfway between the equator and the poles, and they are most plentiful just before sunrise. The solar wind continuously sweeps vapor into space, but the vapor is continuously replaced.

During the night, the pressure of gases at the lunar surface is about 3.9 x 10-14 pound per square inch (2.7 x 10-10 pascal). That is a stronger vacuum than laboratories on Earth can usually achieve. The exosphere is so tenuous -- that is, so low in density -- that the rocket exhaust released during each Apollo landing temporarily doubled the total mass of the entire exosphere.

The surface of the moon is covered with bowl-shaped holes called craters, shallow depressions called basins, and broad, flat plains known as maria. A powdery dust called the regolith overlies much of the surface of the moon.

Craters

The vast majority of the moon's craters are formed by the impact of meteoroids, asteroids, and comets. Craters on the moon are named for famous scientists. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer who realized in the 1500's that the planets move about the sun. Archimedes Crater is named for the Greek mathematician Archimedes, who made many mathematical discoveries in the 200's B.C.

The shape of craters varies with their size. Small craters with diameters of less than 6 miles (10 kilometers) have relatively simple bowl shapes. Slightly larger craters cannot maintain a bowl shape because the crater wall is too steep. Material falls inward from the wall to the floor. As a result, the walls become scalloped and the floor becomes flat.

Still larger craters have terraced walls and central peaks. Terraces inside the rim descend like stairsteps to the floor. The same process that creates wall scalloping is responsible for terraces. The central peaks almost certainly form as did the central peaks of impact craters on Earth. Studies of the peaks on Earth show that they result from a deformation of the ground. The impact compresses the ground, which then rebounds, creating the peaks. Material in the central peaks of lunar craters may come from depths as great as 12 miles (19 kilometers).

Surrounding the craters is rough, mountainous material -- crushed and broken rocks that were ripped out of the crater cavity by shock pressure. This material, called the crater ejecta blanket, can extend about 60 miles (100 kilometers) from the crater.

Farther out are patches of debris and, in many cases, irregular secondary craters, also known as secondaries. Those craters come in a range of shapes and sizes, and they are often clustered in groups or aligned in rows. Secondaries form when material thrown out of the primary (original) crater strikes the surface. This material consists of large blocks, clumps of loosely joined rocks, and fine sprays of ground-up rock. The material may travel thousands of miles or kilometers.

Crater rays are light, wispy deposits of powder that can extend thousands of miles or kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes the powder into the upper surface layer. Thus, craters that still have visible rays must be among the youngest craters on the moon.

Craters larger than about 120 miles (200 kilometers) across tend to have central mountains. Some of them also have inner rings of peaks, in addition to the central peak.

Euler Crater has central peaks and slumped walls. The peaks almost certainly formed quickly after the impact that produced the crater compressed the ground. The ground rebounded upward, forming the peaks. The crater walls are slumped because the original walls were too steep to withstand the force of gravity. Material fell inward, away from the walls. This crater, in Mare Imbrium (Sea of Rains), is about 17 1/2 miles (28 kilometers) across. Image credit: Lunar and Planetary Institute

The appearance of a ring signals the next major transition in crater shape -- from crater to basin.

Basins are craters that are 190 miles (300 kilometers) or more across. The smaller basins have only a single inner ring of peaks, but the larger ones typically have multiple rings. The rings are concentric -- that is, they all have the same center, like the rings of a dartboard. The spectacular, multiple-ringed basin called the Eastern Sea (Mare Orientale) is almost 600 miles (1,000 kilometers) across. Other basins can be more than 1,200 miles (2,000 kilometers) in diameter -- as large as the entire western United States.

Basins occur equally on the near side and far side. Most basins have little or no fill of basalt, particularly those on the far side. The difference in filling may be related to variations in the thickness of the crust. The far side has a thicker crust, so it is more difficult for molten rock to reach the surface there.

In the highlands, the overlying ejecta blankets of the basins make up most of the upper few miles or kilometers of material. Much of this material is a large, thick layer of shattered and crushed rock known as breccia (BREHCH ee uh). Scientists can learn about the original crust by studying tiny fragments of breccia.

Maria, the dark areas on the surface of the moon, make up about 16 percent of the surface area. Some maria are named in Latin for weather terms -- for example, Mare Imbrium (Sea of Rains) and Mare Nubium (Sea of Clouds). Others are named for states of mind, as in Mare Serenitatus (Sea of Serenity) and Mare Tranquillitatis (Sea of Tranquility).

Landforms on the maria tend to be smaller than those of the highlands. The small size of mare features relates to the scale of the processes that formed them -- volcanic eruptions and crustal deformation, rather than large impacts. The chief landforms on the maria include wrinkle ridges and rilles and other volcanic features.

Wrinkle ridges are blisterlike humps that wind across the surface of almost all maria. The ridges are actually broad folds in the rocks, created by compression. Many wrinkle ridges are roughly circular, aligned with small peaks that stick up through the maria and outlining interior rings. Circular ridge systems also outline buried features, such as rims of craters that existed before the maria formed.

Rilles are snakelike depressions that wind across many areas of the maria. Scientists formerly thought the rilles might be ancient riverbeds.

A lunar rover is parked near the edge of Hadley Rille, a long channel probably formed by lava 4 billion to 3 billion years ago. The slopes in the background are part of a formation called the Swann Hills. This photo was taken during the Apollo 15 mission in 1971. Astronaut David R. Scott is reaching under a seat to get a camera. Image credit: NASA

However, they now suspect that the rilles are channels formed by running lava. One piece of evidence favoring this view is the dryness of rock samples brought to Earth by Apollo astronauts; the samples have almost no water in their molecular structure. In addition, detailed photographs show that the rilles are shaped somewhat like channels created by flowing lava on Earth.

Volcanic features

Scattered throughout the maria are a variety of other features formed by volcanic eruptions. Within Mare Imbrium, scarps (lines of cliffs) wind their way across the surface. The scarps are lava flow fronts, places where lava solidified, enabling lava that was still molten to pile up behind them. The presence of the scarps is one piece of evidence indicating that the maria consist of solidified basaltic lava.

Small hills and domes with pits on top are probably little volcanoes. Both dome-shaped and cone-shaped volcanoes cluster together in many places, as on Earth. One of the largest concentrations of cones on the moon is the Marius Hills complex in Oceanus Procellarum (Ocean of Storms). Within this complex are numerous wrinkle ridges and rilles, and more than 50 volcanoes.

Large areas of maria and terrae are covered by dark material known as dark mantle deposits. Evidence collected by the Apollo missions confirmed that dark mantling is volcanic ash.

Much smaller dark mantles are associated with small craters that lie on the fractured floors of large craters. Those mantles may be cinder cones -- low, broad, cone-shaped hills formed by explosive volcanic eruptions.

The interior of the moon

The moon, like Earth, has three interior zones -- crust, mantle, and core. However, the composition, structure, and origin of the zones on the moon are much different from those on Earth.

Most of what scientists know about the interior of Earth and the moon has been learned by studying seismic events -- earthquakes and moonquakes, respectively. The data on moonquakes come from scientific equipment set up by Apollo astronauts from 1969 to 1972.

Crust

The average thickness of the lunar crust is about 43 miles (70 kilometers), compared with about 6 miles (10 kilometers) for Earth's crust. The outermost part of the moon's crust is broken, fractured, and jumbled as a result of the large impacts it has endured. This shattered zone gives way to intact material below a depth of about 6 miles. The bottom of

the crust is defined by an abrupt increase in rock density at a depth of about 37 miles (60 kilometers) on the near side and about 50 miles (80 kilometers) on the far side.

Mantle

The mantle of the moon consists of dense rocks that are rich in iron and magnesium. The mantle formed during the period of global melting. Low-density minerals floated to the outer layers of the moon, while dense minerals sank deeper into it.

Later, the mantle partly melted due to a build-up of heat in the deep interior. The source of the heat was probably the decay (breakup) of uranium and other radioactive elements. This melting produced basaltic magmas -- bodies of molten rock. The magmas later made their way to the surface and erupted as the mare lavas and ashes. Although mare volcanism occurred for more than 1 billion years -- from at least 4 billion years to fewer than 3 billion years ago -- much less than 1 percent of the volume of the mantle ever remelted.

Core

Data gathered by Lunar Prospector confirmed that the moon has a core and enabled scientists to estimate its size. The core has a radius of only about 250 miles (400 kilometers). By contrast, the radius of Earth's core is about 2,200 miles (3,500 kilometers).

The lunar core has less than 1 percent of the mass of the moon. Scientists suspect that the core consists mostly of iron, and it may also contain large amounts of sulfur and other elements.

Earth's core is made mostly of molten iron and nickel. This rapidly rotating molten core is responsible for Earth's magnetic field. A magnetic field is an influence that a magnetic object creates in the region around it. If the core of a planet or a satellite is molten, motion within the core caused by the rotation of the planet or satellite makes the core magnetic. But the small, partly molten core of the moon cannot generate a global magnetic field. However, small regions on the lunar surface are magnetic. Scientists are not sure how these regions acquired magnetism. Perhaps the moon once had a larger, more molten core.

There is evidence that the lunar interior formerly contained gas, and that some gas may still be there. Basalt from the moon contains holes called vesicles that are created during a volcanic eruption. On Earth, gas that is dissolved in magma comes out of solution during an eruption, much as carbon dioxide comes out of a carbonated beverage when you shake the drink container. The presence of vesicles in lunar basalt indicates that the deep interior contained gases, probably carbon monoxide or gaseous sulfur. The existence of volcanic ash is further evidence of interior gas; on Earth, volcanic eruptions are largely driven by gas.

History of moon study

Ancient ideas

Some ancient peoples believed that the moon was a rotating bowl of fire. Others thought it was a mirror that reflected Earth's lands and seas. But philosophers in ancient Greece understood that the moon is a sphere in orbit around Earth. They also knew that moonlight is reflected sunlight.

Some Greek philosophers believed that the moon was a world much like Earth. In about A.D. 100, Plutarch even suggested that people lived on the moon. The Greeks also apparently believed that the dark areas of the moon were seas, while the bright regions were land.

In about A.D. 150, Ptolemy, a Greek astronomer who lived in Alexandria, Egypt, said that the moon was Earth's nearest neighbor in space. He thought that both the moon and the sun orbited Earth. Ptolemy's views survived for more than 1,300 years. But by the early 1500's, the Polish astronomer Nicolaus Copernicus had developed the correct view -- Earth and the other planets revolve about the sun, and the moon orbits Earth.

Early observations with telescopes

The Italian astronomer and physicist Galileo wrote the first scientific description of the moon based on observations with a telescope. In 1609, Galileo described a rough, mountainous surface. This description was quite different from what was commonly believed -- that the moon was smooth. Galileo noted that the light regions were rough and hilly and the dark regions were smoother plains.

The presence of high mountains on the moon fascinated Galileo. His detailed description of a large crater in the central highlands -- probably Albategnius -- began 350 years of controversy and debate about the origin of the "holes" on the moon.

Other astronomers of the 1600's mapped and cataloged every surface feature they could see. Increasingly powerful telescopes led to more detailed records. In 1645, the Dutch engineer and astronomer Michael Florent van Langren, also known as Langrenus, published a map that gave names to the surface features of the moon, mostly its craters. A map drawn by the Bohemian-born Italian astronomer Anton M. S. de Rheita in 1645 correctly depicted the bright ray systems of the craters Tycho and Copernicus. Another effort, by the Polish astronomer Johannes Hevelius in 1647, included the moon's libration zones.

By 1651, two Jesuit scholars from Italy, the astronomer Giovanni Battista Riccioli and the mathematician and physicist Francesco M. Grimaldi, had completed a map of the moon. That map established the naming system for lunar features that is still in use.

Determining the origin of craters

Until the late 1800's, most astronomers thought that volcanism formed the craters of the moon. However, in the 1870's, the English astronomer Richard A. Proctor proposed correctly that the craters result from the collision of solid objects with the moon. But at first, few scientists accepted Proctor's proposal. Most astronomers thought that the moon's craters must be volcanic in origin because no one had yet described a crater on Earth as an impact crater, but scientists had found dozens of obviously volcanic craters.

In 1892, the American geologist Grove Karl Gilbert argued that most lunar craters were impact craters. He based his arguments on the large size of some of the craters. Those included the basins, which he was the first to recognize as huge craters. Gilbert also noted that lunar craters have only the most general resemblance to calderas (large volcanic craters) on Earth. Both lunar craters and calderas are large circular pits, but their structural details do not resemble each other in any way.

In addition, Gilbert created small craters experimentally. He studied what happened when he dropped clay balls and shot bullets into clay and sand targets.

Gilbert was the first to recognize that the circular Mare Imbrium was the site of a gigantic impact. By examining photographs, Gilbert also determined which nearby craters formed before and after that event. For example, a crater that is partially covered by ejecta from the Imbrium impact formed before the impact. A crater within the mare formed after the impact.

Describing lunar evolution

Gilbert suggested that scientists could determine the relative age of surface features by studying the ejecta of the Imbrium impact. That suggestion was the key to unraveling the history of the moon. Gilbert recognized that the moon is a complex body that was built up by innumerable impacts over a long period.

In his book The Face of the Moon (1949), the American astronomer and physicist Ralph B. Baldwin further described lunar evolution. He noted the similarity in form between craters on the moon and bomb craters created during World War II (1939-1945) and concluded that lunar craters form by impact.

Baldwin did not say that every lunar feature originated with an impact. He stated correctly that the maria are solidified flows of basalt lava, similar to flood lava plateaus on Earth. Finally, independently of Gilbert, he concluded that all circular maria are actually huge impact craters that later filled with lava.

In the 1950's, the American chemist Harold C. Urey offered a contrasting view of lunar history. Urey said that, because the moon appears to be cold and rigid, it has always been so. He then stated -- correctly -- that craters are of impact origin. However, he concluded

falsely that the maria are blankets of debris scattered by the impacts that created the basins. And he was mistaken in concluding that the moon never melted to any significant extent. Urey had won the 1934 Nobel Prize in chemistry and had an outstanding scientific reputation, so many people took his views seriously. Urey strongly favored making the moon a focus of scientific study. Although some of his ideas were mistaken, his support of moon study was a major factor in making the moon an early goal of the U.S. space program.

In 1961, the U.S. geologist Eugene M. Shoemaker founded the Branch of Astrogeology of the U.S. Geological Survey (USGS). Astrogeology is the study of celestial objects other than Earth. Shoemaker showed that the moon's surface could be studied from a geological perspective by recognizing a sequence of relative ages of rock units near the crater Copernicus on the near side. Shoemaker also studied the Meteor Crater in Arizona and documented the impact origin of this feature. In preparation for the Apollo missions to the moon, the USGS began to map the geology of the moon using telescopes and pictures. This work gave scientists their basic understanding of lunar evolution.

Apollo missions

Beginning in 1959, the Soviet Union and the United States sent a series of robot spacecraft to examine the moon in detail. Their ultimate goal was to land people safely on the moon. The United States finally reached that goal in 1969 with the landing of the Apollo 11 lunar module. The United States conducted six more Apollo missions, including five landings. The last of those was Apollo 17, in December 1972.

The Apollo missions revolutionized the understanding of the moon. Much of the knowledge gained about the moon also applies to Earth and the other inner planets -- Mercury, Venus, and Mars. Scientists learned, for example, that impact is a fundamental geological process operating on the planets and their satellites.

After the Apollo missions, the Soviets sent four Luna robot craft to the moon. The last, Luna 24, returned samples of lunar soil to Earth in August 1976.

Recent exploration

No more spacecraft went to the moon until January 1994, when the United States sent the orbiter Clementine. From February to May of that year, Clementine's four cameras took more than 2 million pictures of the moon. A laser device measured the height and depth of mountains, craters, and other features. Radar signals that Clementine

The Clementine orbiter used radar signals to find evidence of a large deposit of frozen water on the moon. The orbiter sent radar signals to various target points on the lunar surface. The targets reflected some of the signals to Earth, where they were received by large antennas and analyzed. Image credit: Lunar and Planetary Institute

bounced off the moon provided evidence of a large deposit of frozen water. The ice appeared to be inside craters at the south pole.

The U.S. probe Lunar Prospector orbited the moon from January 1998 to July 1999. The craft mapped the concentrations of chemical elements in the moon, surveyed the moon's magnetic fields, and found strong evidence of ice at both poles. Small particles of ice are apparently part of the regolith at the poles.

The SMART-1 spacecraft, launched by the European Space Agency in 2003, went into orbit around the moon in 2004. The craft's instruments were designed to investigate the moon's origin and conduct a detailed survey of the chemical elements on the lunar surface.

Planets A planet is a large, round heavenly body that orbits a star and shines with light reflected from the star. Eight planets orbit the sun in our solar system. In order of increasing distance from the sun, they are: (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus, and (8) Neptune. Many nearly planet-sized objects, called dwarf planets, also orbit the sun. Dwarf planets include Pluto and Ceres. Since 1992, astronomers have discovered many planets orbiting other stars. Traditionally, the term planet has had no formal definition in astronomy. Millions of objects orbit the sun—the most basic characteristic of a planet. But scholars have struggled to devise a simple classification system that distinguishes the smallest worlds from the largest comets, asteroids, and other bodies.

The International Astronomical Union (IAU), the recognized authority in naming heavenly bodies, divides

The sun blazes with energy. On its surface, magnetic forces create loops and streams of gas that extend tens of thousands of miles or kilometers into space. This image was made by photographing ultraviolet radiation given off by atoms of iron gas that are hotter than 9 million degrees F (5 million degrees C). Image credit: NASA/Transition Region & Coronal Explorer

objects that orbit the sun into three major classes: (1) planets, (2) dwarf planets, and (3) small solar system bodies. A planet orbits the sun and no other body. It has so much mass (amount of matter) that its own gravitational pull compacts it into a round shape. In addition, a planet has a strong enough gravitational pull to sweep the region of its orbit relatively free of other objects. A dwarf planet also orbits the sun and is large enough to be round. However, it does not have a strong enough gravitational pull to clear the region of its orbit. Small solar system bodies, including most asteroids and comets, have too little mass for gravity to round their irregular shapes. Many planets, dwarf planets, and other bodies have smaller objects orbiting them called satellites or moons.

The planets in our solar system can be divided into two groups. The innermost four planets—Mercury, Venus, Earth, and Mars—are

small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets have from 38 to 95 percent of Earth's diameter and from 5.5 to 82 percent of Earth's mass.

The outer four planets—Jupiter, Saturn, Uranus, and Neptune—are called gas giants or Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other materials also occur, including traces of ammonia and methane in their atmospheres. They range from 3.9 times to 11.2 times Earth's diameter and from 15 times to 318 times Earth's mass. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of infrared radiation, which is felt as heat, instead of visible light. Scientists think the source of some of the energy is probably the slow compression of the planets by their own gravity.

From its discovery in 1930, Pluto was generally considered a planet. However, its small size and irregular orbit led many astronomers to question whether Pluto should be grouped with worlds such as Earth and Jupiter. Pluto more closely resembles other icy objects found in a region of the outer solar system called the Kuiper belt. In the early 2000’s, astronomers found several such Kuiper belt objects (KBO’s) comparable in size to Pluto. The IAU created the “dwarf planet” classification to describe Pluto and other nearly planet-sized objects.

Observing the planets

People have known the inner six planets of our solar system for thousands of years because they are visible from Earth without a telescope. The outermost three planets—Uranus and Neptune—were discovered by astronomers, beginning in the 1780's. These planets can be seen from Earth with a telescope.

To the unaided eye, the planets look much like the background stars in the night sky. However, the planets move slightly from night to night in relation to the stars. The name

planet comes from a Greek word meaning to wander. The planets and the moon follow the same apparent path through the sky. This path, known as the zodiac, is about 16° wide. At its center is the ecliptic, the apparent path of the sun. If you see a bright object near the ecliptic at night or near sunrise or sunset, it is most likely a planet. You can even see the brightest planets in the daytime, if you know where to look.

Planets and stars also differ in the steadiness of their light when viewed from Earth's surface. Planets shine with a steady light, but stars seem to twinkle.

The twinkling is due to the moving layers of air that surround Earth. Stars are so far away that they are mere points of light in the sky, even when viewed through a telescope. The atmosphere bends the starlight passing through it. As small regions of the atmosphere move about, the points of light seem to dance and change in brightness.

Planets, which are much closer, look like tiny disks through a telescope. The atmosphere scatters light from different points on a planet's disk. However, enough light always arrives from a sufficient number of points to provide a steady appearance.

Orbits

Viewed from Earth's surface, the planets of the solar system and the stars appear to move around Earth. They rise in the east and set in the west each night. Most of the time, the planets move westward across the sky slightly more slowly than the stars do. As a result, the planets seem to drift eastward relative to the background stars. This motion is called prograde. For a while each year, however, the planets seem to reverse their direction. This backward motion is called retrograde.

In ancient times, most scientists thought that the moon, sun, planets, and stars actually moved around Earth. One puzzle that ancient scientists struggled to explain was the annual retrograde motion of the planets. In about A.D. 150, the Greek astronomer Ptolemy developed a theory that the planets orbited in small circles, which in turn orbited Earth in larger circles. Ptolemy thought that retrograde motion was caused by a planet moving on its small circle in an opposite direction from the motion of the small circle around the big circle.

In 1543, the Polish astronomer Nicolaus Copernicus showed that the sun is the center of the orbits of the planets. Our term solar system is based on Copernicus's discovery. Copernicus realized that retrograde motion occurs because Earth moves faster in its orbit than the planets that are farther from the sun. The planets that are closer to the sun move faster in their orbits than Earth travels in its orbit. Retrograde motion occurs whenever Earth passes an outer planet traveling around the sun or an inner planet passes Earth.

In the 1600's, the German astronomer Johannes Kepler used observations of Mars by the Danish astronomer Tycho Brahe to figure out three laws of planetary motion. Although

Kepler developed his laws for the planets of our solar system, astronomers have since realized that Kepler's laws are valid for all heavenly bodies that orbit other bodies.

Kepler's first law says that planets move in elliptical (oval-shaped) orbits around their parent star—in our solar system, the sun. An ellipse is a closed curve formed around two fixed points called foci. The ellipse is formed by the path of a point moving so that the sum of its distances from the two foci remains the same. The orbital paths of the planets form such curves, with the parent star at one focus of the ellipse. Before Kepler, scientists had assumed that the planets moved in circular orbits.

Kepler's second law says that an imaginary line joining the parent star to its planet sweeps across equal areas of space in equal amounts of time. When a planet is close to its star, it moves relatively rapidly in its orbit. The line therefore sweeps out a short, fat, trianglelike figure. When the planet is farther from its star, it moves relatively slowly. In this case, the line sweeps out a long, thin figure that resembles a triangle. But the two figures have equal areas.

Kepler's third law says that a planet's period (the time it takes to complete an orbit around its star) depends on its average distance from the star. The law says that the square of the planet's period—that is, the period multiplied by itself—is proportional to the cube of the planet's average distance from its star—the distance multiplied by itself twice—for all planets in a solar system.

The English scientist, astronomer, and mathematician Isaac Newton presented his theory of gravity and explained why Kepler's laws work in a treatise published in 1687. Newton showed how his expanded version of Kepler's third law could be used to find the mass of the sun or of any other object around which things orbit. Using Newton's explanation, astronomers can determine the mass of a planet by studying the period of its moon or moons and their distance from the planet.

Rotation

Planets rotate at different rates. One day is defined as how long it takes Earth to rotate once. Jupiter and Saturn spin much faster, in only about 10 hours. Venus rotates much slower, in about 243 Earth days.

Most planets rotate in the same direction in which they revolve around the sun, with their axis of rotation standing upright from their orbital path. A law of physics holds that such rotation does not change by itself. So astronomers think that the solar system formed out of a cloud of gas and dust that was already spinning.

Uranus is tipped on its side, however, so that its axes lies nearly level with its paths around the sun. Venus is tipped all the way over. Its axis is almost completely upright, but the planet rotates in the direction opposite from the direction of its revolution around

the sun. Most astronomers think that some other objects in the solar system must have collided with Uranus, Pluto, and Venus and tipped them.

The planets of our solar system

Astronomers measure distances within the solar system in astronomical units (AU). One astronomical unit is the average distance between Earth and the sun, which is about 93 million miles (150 million kilometers). The inner planets have orbits whose diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Because of their different distances from the sun, the temperature, surface features, and other conditions on the planets vary widely.

Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close to the sun that temperatures on its surface can climb as high as 800 degrees F (430 degrees C). But some regions near the planet's poles may be always in shadow, and astronomers speculate that water or ice may remain there. No spacecraft has visited Mercury since the 1970's, when Mariner 10 photographed about half the planet's surface at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by Mercury three times before going into orbit around the planet in 2011.

Venus is known as Earth's twin because it resembles Earth in size and mass, though it has no moon. Venus has a dense atmosphere that consists primarily of carbon dioxide. The pressure of the atmosphere on Venus's surface is 90 times that of Earth's atmosphere. Venus's thick atmosphere traps energy from the sun, raising the surface temperature on Venus to about 870 degrees F (465 degrees C), hot enough to melt lead. This trapping of heat is known as the greenhouse effect. Scientists have warned that a similar process on Earth is causing permanent global warming. Several spacecraft have orbited or landed on Venus. In the 1990's, the Magellan spacecraft used radar -- radio waves bounced off the planet -- to map Venus in detail.

Earth, our home planet, has an atmosphere that is mostly nitrogen with some oxygen. Earth has oceans of liquid water and continents that rise above sea level. Many measuring devices on the surface and in space monitor conditions on our planet. In 1998, the National Aeronautics and Space Administration (NASA) launched the first of a series of satellites called the Earth

The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. Image credit: NASA

Thick clouds of sulfuric acid cover Venus. Image credit: NASA

Earth, our home planet, has oceans of liquid water, and continents that rise above sea level. Image credit: NASA/Goddard Space Flight Center

Observing System (EOS). The EOS satellites will carry remote-sensing instruments to measure climate changes and other conditions on Earth's surface.

Mars is known as the red planet because of its reddish-brown appearance, caused by rusty dust on the Martian surface. Mars is a cold, dry world with a thin atmosphere. The atmospheric pressure (pressure exerted by the weight of the gases in the atmosphere) on the Martian surface is less than 1 percent the atmospheric pressure on Earth. This low surface pressure has enabled most of the water that Mars may once have had to escape into space.

The surface of Mars has giant volcanoes, a huge system of canyons, and stream beds that look as if water flowed through them in the past. Mars has two tiny moons, Phobos and Deimos. Many spacecraft have landed on or orbited Mars.

Jupiter, the largest planet in our solar system, has more mass than the other planets combined. Like the other Jovian planets, it has gaseous outer layers and may have a rocky core. A huge storm system called the Great Red Spot in Jupiter's atmosphere is larger than Earth and

has raged for hundreds of years.

Jupiter's four largest moons -- Io, Europa, Ganymede, and Callisto -- are larger than Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter's equator are three thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a probe into Jupiter's atmosphere. Galileo orbited Jupiter from 1995 to 2003.

Saturn, another giant planet, has a magnificent set of gleaming rings. Its gaseous atmosphere is not as colorful as Jupiter's, however. One reason Saturn is relatively drab is that its hazy upper atmosphere makes the cloud patterns below difficult to see. Another reason is that Saturn is farther than Jupiter from the sun. Because of the difference in distance, Saturn is colder than Jupiter. Due to the temperature difference, the kinds of chemical reactions that color Jupiter's atmosphere occur too slowly to do the same on Saturn.

Saturn's moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up views of Saturn and its rings and moons.

The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was designed to be dropped into Titan's atmosphere.

The planet Mars has clouds in its atmosphere and a deposit of ice at its north pole. Image credit: NASA/JPL/Malin Space Science Systems

The layers of dense clouds around Jupiter appear in a photograph of the planet taken by the Voyager 1 space probe. Image credit: JPL

Saturn is encircled by seven major rings. Image credit: NASA/JPL/Space Science Institute

Uranus was the first planet discovered with a telescope. German-born English astronomer William Herschel found it in 1781. He at first thought he had discovered a comet. Almost 200 years later, scientists detected 10 narrow rings around Uranus when the planet moved in front of a star and the rings became visible. Voyager 2 studied Uranus and its rings and moons close-up in 1986.

Neptune was first observed in 1846 by German astronomer Johann G. Galle after other astronomers predicted its position by studying how it affected Uranus's orbit. In 1989, Voyager 2 found that Neptune had a storm system called the Great Dark Spot, similar to Jupiter's Great Red Spot. But five years later, in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished. Neptune has four narrow rings, one of which has clumps of matter. Neptune's moon Triton is one of the largest in the solar system and has volcanoes that emit plumes of frozen nitrogen.

The Dwarf Planets

The solar system’s dwarf planets consist primarily of rock and ice and feature little or no atmosphere. They lack the mass to sweep their orbits clear, so they tend to be found among populations of similar, smaller bodies.

Ceres ranks as the largest of millions of asteroids found between the orbits of Mars and Jupiter. Ceres has a rocky composition and resembles a slightly squashed sphere. Its longest diameter measures 596 miles (960 kilometers). The Italian astronomer Giuseppe Piazzi discovered Ceres in 1801. As with Pluto, people once widely considered Ceres a planet.

The outer dwarf planets generally lie beyond the orbit of Neptune. Astronomers have had difficulty studying bodies in this region because they are extremely far from Earth. Dozens of them probably fit the IAU’s definition of a dwarf planet. Most of these bodies belong to the Kuiper belt.

Some astronomers have suggested calling the outer dwarf planets plutonians in honor of Pluto, the first one discovered. A body designated 2003 UB313 ranks as the largest dwarf planet, with a diameter of around 1,500 miles (2,400 kilometers). Quaoar «KWAH oh wahr» , a KBO discovered in 2002, measures roughly half the size of Pluto. Sedna, discovered in 2004, measures about three-fourths the size of Pluto and lies nearly three times as far from the sun. Some scientists think Sedna belongs to population of cometlike objects called the Oort cloud, which lies beyond the Kuiper belt.

Uranus appears in true colors, left, and false colors, right, in images produced by combining numerous pictures taken by the Voyager 2 spacecraft. Image credit: JPL

The blue clouds of Neptune are mostly frozen methane. The other object shown is Neptune's moon Triton. Image credit: NASA/JPL

Pluto is so far from Earth that even powerful telescopes reveal little detail of its surface. The Hubble Space Telescope gathered the light for the pictures of Pluto shown here. Image credit: NASA

2003 UB 313 has a surface that contains methane ice and remains around –406 °F (–243 °C). 2003 UB313 has a small moon about 1⁄8 its diameter. The American astronomers Michael E. Brown, Chadwick A. Trujillo, and David L. Rabinowitz announced the discovery of 2003 UB313 in 2005.

Pluto was long considered the ninth planet. The American astronomer Clyde W. Tombaugh discovered Pluto in 1930. Pluto is slightly smaller than 2003 UB313. Its surface also features methane ice. Pluto’s largest satellite, Charon, measures about half the dwarf planet’s diameter. The New Horizons probe, launched in 2006, was designed to observe Pluto during a flyby in 2015.

Planets in other solar systems

Even with the most advanced telescopes, astronomers cannot see planets orbiting other stars directly. The planets shine only by reflected light and are hidden by the brilliance of their parent stars. The planets and their stars are also much farther away than our sun. The nearest star is 4.2 light-years away, compared to 8 light-minutes for the sun. One light-year is the distance that light travels in one year -- about 5.88 trillion miles (9.46 trillion kilometers). Thus, it takes light 4.2 years to reach Earth from the nearest star beyond the sun and only 8 minutes to reach Earth from the sun.

Scientists know of more than 100 stars other than the sun that have planets. Astronomers cannot see planets around distant stars. However, they can detect the planets from tiny changes in the stars' movement and tiny decreases in the amount of light coming from the stars. The changes in a star's movement are caused by the slight pull of the planet's gravity on its parent star. To find new planets, astronomers use a technique called spectroscopy, which breaks down the light from stars into its component rainbow of colors. The scientists look for places in the rainbow where colors are missing. At these places, dark lines known as spectral lines cross the rainbow. The spectral lines change their location in the rainbow slightly as a star is pulled by the gravity of an orbiting planet toward and away from Earth. These apparent changes in a star's light as the star moves are due to a phenomenon known as the Doppler effect. The changes not only show that a planet is present but also indicate how much mass it has.

The amount of light coming from the star decreases when the planet passes in front of the star. The planet blocks some of the starlight, dimming the star.

The first discoveries

Astronomers announced the discovery of the first planets around a star other than our sun in 1992. The star is a pulsar named PSR B1257+12 in the constellation Virgo. Pulsars are dead stars that have collapsed until they are only about 12 miles (20 kilometers) across. They spin rapidly on their axes, sending out radio waves that arrive on Earth as pulses of radio energy. Some pulsars spin hundreds of times each second. If a pulsar has a planet, the planet pulls the star to and fro slightly as it orbits. These pulls cause slight variations

in the radio pulses. From measurements of these variations, the Polish-born American astronomer Alexander Wolszczan and American Dale A. Frail discovered three planets in orbit around PSR B1257+12. The star emits such strong X rays, however, that no life could survive on its planets.

Astronomers soon began to find planets around stars more like the sun. In 1995, Swiss astronomers Michel Mayor and Didier Queloz found the first planet orbiting a sunlike star, 51 Pegasi, in the constellation Pegasus. American astronomers Geoffrey W. Marcy and R. Paul Butler confirmed the discovery and found planets of their own around other stars. In 1999, astronomers announced the first discovery of a multiple-planet system belonging to a sunlike star. They determined that three planets orbit the star Upsilon Andromedae, which is 44 light-years from Earth in the constellation Andromeda.

Also in 1999, American astronomer Gregory W. Henry first detected a dimming of starlight due to the presence of a planet. The star that Henry observed is known as HD 209458, and it is located in Pegasus. Henry measured the star's brightness at the request of Marcy, Butler, and American astronomer Steven S. Vogt, who had previously used the spectroscopic technique to identify this star as a parent of a planet.

Some stars have a planet orbiting them at a distance at which living things could exist. Most scientists consider liquid water essential for life, so a region that is neither too hot nor too cold for liquid water is known as a habitable zone. Although astronomers have found stars with planets in their habitable zones, all the planets found so far are probably gaseous with no solid surface. But they may have solid moons.

In 2001, Marcy announced the discovery of a solar system containing an extremely unusual object. That object and an ordinary planet orbit the star HD 168443, which is 123 light-years away in the constellation Serpens. The object is so unusual because of its mass. It is at least 17 times as massive as Jupiter.

Astronomers are not yet sure how to classify the object. They had not thought that a planet could be as massive as the object is. Before this discovery, the only known heavenly bodies of such mass were dim objects called brown dwarfs. But brown dwarfs form by means of the same process that forms stars, not planets.

Astronomers also have been surprised to find that other solar systems have huge, gaseous planets in close orbits. In our own solar system, the inner planets are rocky and small, and only the outer planets, except for Pluto, are huge and gassy. But several newly discovered planets have at least as much mass as Jupiter, the largest planet in our solar system. Unlike Jupiter, however, these massive planets race around their stars in only a few weeks. Kepler's third law says that for a planet to complete its orbit so quickly, it must be close to its parent star. Several of these giant planets, therefore, must travel around their stars even closer than our innermost planet, Mercury, orbits our sun. Such close orbits would make their surfaces too hot to support life as we know it.

Some newly discovered planets follow unusual orbits. Most planets travel around their stars on nearly circular paths, like those of the planets in our solar system. But a planet around the star 16 Cygni B follows an extremely elliptical orbit. It travels farther from its star than the planet Mars does from our sun, and then draws closer to the star than Venus does to our sun. If a planet in our solar system traveled in such an extreme oval, its gravity would disrupt the orbits of the other planets and toss them out of their paths.

hroughout the early 2000's, astronomers continued to improve techniques for detecting planets, enabling them to discover an increasing variety of planets around other stars. In 2004, astronomers announced the first discoveries of planets much smaller than Jupiter. The newly discovered planets were about the size of Uranus or Neptune. Despite the planets' huge size, astronomers theorized that some of them might be rocky planets rather than gas giants.

How the planets formed

Astronomers have developed a theory about how our solar system formed that explains why it has small, rocky planets close to the sun and big, gaseous ones farther away. Astronomers believe our solar system formed about 4.6 billion years ago from a giant, rotating cloud of gas and dust called the solar nebula. Gravity pulled together a portion of gas and dust at the center of the nebula that was denser than the rest. The material accumulated into a dense, spinning clump that formed our sun.

The remaining gas and dust flattened into a disk called a protoplanetary disk swirling around the sun. Protoplanetary disks around distant stars were first observed through telescopes in 1983. Rocky particles within the disk collided and stuck together, forming bodies called planetesimals. Planetesimals later combined to form the planets. At the distances of the outer planets, gases froze into ice, creating huge balls of frozen gas that formed the Jovian planets.

Hot gases and electrically charged particles flow from our sun constantly, forming a stream called the solar wind. The solar wind was stronger at first than it is today. The early solar wind drove the light elements -- hydrogen and helium -- away from the inner planets like Earth. But the stronger gravity of the giant outer planets held on to more of the planets' hydrogen and helium, and the solar wind was weaker there. So these outer planets kept most of their light elements and wound up with much more mass than Earth.

Astronomers developed these theories when they thought that rocky planets always orbited close to the parent star and giant planets farther out. But the "rule" was based only on our own solar system. Now that astronomers have learned something about other solar systems, they have devised new theories. Some scientists have suggested that the giant planets in other solar systems may have formed far from their parent stars and later moved in closer.

Rocket

 A rocket is a type of engine that pushes itself forward or upward by producing thrust. Unlike a jet engine, which draws in outside air, a rocket engine uses only the substances carried within it. As a result, a rocket can operate in outer space, where there is almost no air. A rocket can produce more power for its size than any other kind of engine. For example, the main rocket engine of the space shuttle weighs only a fraction as much as a train engine, but it would take 39 train engines to produce the same amount of power. The word rocket can also mean a vehicle or object driven by a rocket engine.

Rockets come in a variety of sizes. Some rockets that shoot fireworks into the sky measure less than 2 feet (60 centimeters) long. Rockets 50 to 100 feet (15 to 30 meters) long serve as long-range missiles that can be used to bomb distant targets during wartime. Larger and more powerful rockets lift spacecraft, artificial satellites, and scientific probes into space. For example, the Saturn 5 rocket that carried astronauts to the moon stood about 363 feet (111 meters) tall.

Rocket engines generate thrust by expelling gas. Most rockets produce thrust by burning a mixture of fuel and an oxidizer, a substance that enables the fuel to burn without drawing in outside air. This kind of rocket is called a chemical rocket because burning fuel is a chemical reaction. The fuel and oxidizer are called the propellants.

A chemical rocket can produce great power, but it burns propellants rapidly. As a result, it needs a large amount of propellants to work for even a short time. The Saturn 5 rocket burned more than 560,000 gallons (2,120,000 liters) of propellants during the first 2 3/4 minutes of flight. Chemical rocket engines become extremely hot as the propellants burn. The temperature in some engines reaches o 6000 degrees F (3300 degrees C), much higher than the temperature at which steel melts.

Jet engines also burn fuel to generate thrust. Unlike rocket engines, however, jet engines work by drawing in oxygen from the surrounding air. For more information on jet engines, see Jet propulsion.

Researchers have also developed rockets that do not burn propellants. Nuclear rockets use heat generated by a nuclear fuel to produce thrust. In an electric rocket, electric energy produces thrust.

Military forces have used rockets in war for hundreds of years. In the 1200's, Chinese soldiers fired rockets against attacking armies. British troops used rockets to attack Fort McHenry in Maryland during the War of 1812 (1812-1815). After watching the battle, the American lawyer Francis Scott Key described "the rocket's red glare" in the song "The Star-Spangled Banner." During World War I (1914-1918), the French used rockets to shoot down enemy observation balloons. Germany attacked London with V-2 rockets during World War II (1939-1945). In the Persian Gulf War of 1991 and the Iraq War, which began in 2003, United States troops launched rocket-powered Patriot missiles to intercept and destroy Iraqi missiles.

Rockets are the only vehicles powerful enough to carry people and equipment into space. Since 1957, rockets have lifted hundreds of artificial satellites into orbit around Earth. These satellites take pictures of Earth's weather, gather information for scientific study, and transmit communications around the world. Rockets also carry scientific instruments far into space to explore and study other planets. Since 1961, rockets have launched spacecraft carrying astronauts and cosmonauts into orbit around Earth. In 1969, rockets carried astronauts to the first landing on the moon. In 1981, rockets lifted the first space shuttle into Earth orbit.

This article discusses Rocket (How rockets work) (How rockets are used) (Kinds of rocket engines) (History).

How rockets work

Rocket engines generate thrust by putting a gas under pressure. The pressure forces the gas out the end of the rocket. The gas escaping the rocket is called exhaust. As it escapes, the exhaust produces thrust according to the laws of motion developed by the English scientist Isaac Newton. Newton's third law of motion states that for every action, there is an equal and opposite reaction. Thus, as the rocket pushes the exhaust backward, the exhaust pushes the rocket forward.

The amount of thrust produced by a rocket depends on the momentum of the exhaust -- that is, its total amount of motion. The exhaust's momentum equals its mass (amount of matter) multiplied by the speed at which it exits the rocket. The more momentum the exhaust has, the more thrust the rocket produces. Engineers can therefore increase a rocket's thrust by increasing the mass of exhaust it produces. Alternately, they can increase the thrust by increasing the speed at which the exhaust leaves the rocket.

Parts of a rocket include the rocket engine and the equipment and cargo the rocket carries. The four major parts of a rocket are (1) the payload, (2) propellants, (3) the chamber, and (4) the nozzle.

The payload of a rocket includes the cargo, passengers, and equipment the rocket carries. The payload may consist of a spacecraft, scientific instruments, or even explosives. The space shuttle's payload, for example, is the shuttle orbiter and the mission astronauts and any satellites, scientific experiments, or supplies the orbiter carries. The payload of a missile may include explosives or other weapons. This kind of payload is called a warhead.

Propellants generally make up most of the weight of a rocket. For example, the fuel and oxidizer used by the space shuttle account for nearly 90 percent of its weight at liftoff. The shuttle needs such a large amount of propellant to overcome Earth's gravity and the resistance of the atmosphere.

The space shuttle and many other chemical rockets use liquid hydrogen as fuel. Hydrogen becomes a liquid only at extremely low temperatures, requiring powerful cooling systems. Kerosene, another liquid fuel, is easier to store because it remains liquid at room temperature.

Many rockets, including the space shuttle, use liquid oxygen, or lox, as their oxidizer. Like hydrogen, oxygen must be cooled to low temperatures to become a liquid. Other commonly used oxidizers include nitrogen tetroxide and hydrogen peroxide. These oxidizers remain liquid at room temperature and do not require cooling.

An electric or nuclear rocket uses a single propellant. These rockets store the propellant as a gas or liquid.

The chamber is the area of the rocket where propellants are put under pressure. Pressurizing the propellants enables the rocket to expel them at high speeds.

In a chemical rocket, the fuel and oxidizer combine and burn in an area called the combustion chamber. As they burn, the propellants expand rapidly, creating intense pressure.

Burning propellants create extreme heat and pressure in the combustion chamber. Temperatures in the chamber become hot enough to melt the steel, nickel, copper, and other materials used in its construction. Combustion chambers need insulation or cooling to survive the heat. The walls of the chamber must also be strong enough to withstand intense pressure. The pressure inside a rocket engine can exceed 3,000 pounds per square inch (200 kilograms per square centimeter), nearly 100 times the pressure in the tires of a car or truck.

In a nuclear rocket, the chamber is the area where nuclear fuel heats the propellant, producing pressure. In an electric rocket, the chamber contains the electric devices used to force the propellant out of the nozzle.

The nozzle is the opening at the end of the chamber that allows the pressurized gases to escape. It converts the high pressure of the gases into thrust by forcing the exhaust through a narrow opening, which accelerates the exhaust to high speeds. The exhaust from the nozzle can travel more than 1 mile (1.6 kilometers) per second. Like the chamber, the nozzle requires cooling or insulation to withstand the heat of the exhaust.

Multistage rockets

Many chemical rockets work by burning propellants in a single combustion chamber. Engineers refer to these rockets as single-stage rockets. Missions that require long-distance travel, such as reaching Earth orbit, generally require multiple-stage or multistage rockets. A multistage rocket uses two or more sets of combustion chambers and propellant tanks. These sets, called stages, may be stacked end to end or attached side by side. When a stage runs out of propellant, the rocket discards it. Discarding the empty stage makes the rocket lighter, allowing the remaining stages to accelerate it more strongly. Engineers have designed and launched rockets with as many as five separate stages. The space shuttle uses two stages.

How rockets are used

People use rockets for high-speed, high-power transportation both within Earth's atmosphere and in space. Rockets are especially valuable for (1) military use, (2) atmospheric research, (3) launching probes and satellites, and (4) space travel.

Military use

Rockets used by the military vary in size from small rockets used on the battlefield to giant guided missiles that can fly across oceans. The bazooka is a small rocket launcher carried by soldiers for use against armored vehicles. A person using a bazooka has as much striking power as a small tank. Armies use larger rockets to fire explosives far behind enemy lines and to shoot down enemy aircraft. Fighter airplanes carry rocket-powered guided missiles to attack other planes and ground targets. Navy ships use guided missiles to attack other ships, land targets, and planes.

Powerful rockets propel a type of long-range guided missile called an intercontinental ballistic missile (ICBM). Such a missile can travel 3,400 miles (5,500 kilometers) or more to bomb an enemy target with nuclear explosives. An ICBM generally employs two or three separate stages to propel it during the early part of its flight. The ICBM coasts the rest of the way to its target.

Atmospheric research

A two-stage rocket carries a propellant and one or more rocket engines in each stage. The first stage launches the rocket. After burning its supply of propellant, the first stage falls away from the rest of the rocket. The second stage then ignites and carries the payload into earth orbit or even farther into space. A balloon and a rocket work in much the same way. Gas flowing from the nozzle creates unequal pressure that lifts the balloon or the rocket off the ground. Image credit: World Book diagram

Scientists use rockets to explore Earth's atmosphere. Sounding rockets, also called meteorological rockets, carry such equipment as barometers, cameras, and thermometers high into the atmosphere. These instruments collect information about the atmosphere and send it by radio to receiving equipment on the ground.

Rockets also provide the power for experimental research airplanes. Engineers use these planes in the development of spacecraft. By studying the flights of such planes as the rocket-powered X-1 and X-15, engineers learned how to control vehicles flying many times as fast as the speed of sound.

Launching probes and satellites

Rockets carry crewless spacecraft called space probes on long voyages to explore the solar system. Probes have explored the sun, the moon, and all the planets in our solar system except Pluto. They carry scientific instruments that gather information about the planets and transmit data back to Earth. Probes have landed on the surface of the moon, Venus, and Mars.

Rockets lift artificial satellites into orbit around Earth. Some orbiting satellites gather information for scientific research. Others relay telephone conversations and radio and television broadcasts across the oceans. Weather satellites track climate patterns and help scientists predict the weather. Navigation satellites, such as those that make up the Global Positioning System (GPS), enable receivers anywhere on Earth to determine their locations with great accuracy. The armed forces use satellites to observe enemy facilities and movements. They also use satellites to communicate, monitor weather, and watch for missile attacks. Not only are satellites launched by rockets, but many satellites use small rocket engines to maintain their proper orbits.

Rockets that launch satellites and probes are called launch vehicles. Most of these rockets have from two to four stages. The stages lift the satellite to its proper altitude and give it enough speed -- about 17,000 miles (27,000 kilometers) per hour -- to stay in orbit. A space probe's speed must reach about 25,000 miles (40,000 kilometers) per hour to escape Earth's gravity and continue on its voyage.

Engineers created the first launch vehicles by altering military rockets or sounding rockets to carry spacecraft. For example, they added stages to some of these rockets to increase their speed. Today, engineers sometimes attach smaller rockets to a launch vehicle. These rockets, called boosters, provide additional thrust to launch heavier spacecraft.

Space travel

Rockets launch spacecraft carrying astronauts that orbit Earth and travel into space. These rockets, like the ones used to launch probes and satellites, are called launch vehicles.

The Saturn 5 rocket, which carried astronauts to the moon, was the most powerful launch vehicle ever built by the United States. Before launch, it weighed more than 6 million pounds (2.7 million kilograms). It could send a spacecraft weighing more than 100,000 pounds (45,000 kilograms) to the moon. The Saturn 5 used 11 rocket engines to propel three stages.

Space shuttles are reusable rockets that can fly into space and return to Earth repeatedly. Engineers have also worked to develop space tugs, smaller rocket-powered vehicles that could tow satellites, boost space probes, and carry astronauts over short distances in orbit. For more information on rockets used in space travel, see Space exploration.

Other uses

People have fired rockets as distress signals from ships and airplanes and from the ground. Rockets also shoot rescue lines to ships in distress. Small rockets called JATO (jet-assisted take-off) units help heavily loaded airplanes take off. Rockets have long been used in fireworks displays. Kinds of rocket engines

The vast majority of rockets are chemical rockets. The two most common types of chemical rockets are solid-propellant rockets and liquid-propellant rockets. Engineers have tested a third type of chemical rocket, called a hybrid rocket, that combines liquid and solid propellants. Electric rockets have propelled space probes and maneuvered orbiting satellites. Researchers have designed experimental nuclear rockets.

Solid-propellant rockets burn a rubbery or plastic-like material called the grain. The grain consists of a fuel and an oxidizer in solid form. It is shaped like a cylinder with one or more channels or ports that run through it. The ports increase the surface area of the grain that the rocket burns. Unlike some liquid propellants, the fuel and oxidizer of a solid-propellant rocket do not burn upon contact with each other. Instead, an electric charge ignites a smaller grain. Hot exhaust gases from this grain ignite the main propellant surface.

The temperature in the combustion chamber of a solid-propellant rocket ranges from 3000 to 6000 degrees F (1600 to 3300 degrees C). In most of these rockets, engineers build the chamber walls from high-strength steel or titanium to withstand the pressure

A solid-propellant rocket burns a solid material called the grain. Engineers design most grains with a hollow core. The propellant burns from the core outward. Unburned propellant shields the engine casing from the heat of combustion. Image credit: World Book diagram by Precision Graphics

and heat of combustion. They also may use composite materials consisting of high-strength fibers embedded in rubber or plastic. Composite chambers made from high-strength graphite fibers in a strong adhesive called epoxy weigh less than steel or titanium chambers, enabling the rocket to accelerate its payload more efficiently. Solid propellants burn at a rate of about 0.6 inch (1.5 centimeters) per second.

Solid propellants can remain effective after long storage and present little danger of combusting or exploding until ignited. Furthermore, they do not need the pumping and injecting equipment required by liquid propellants. On the other hand, rocket controllers cannot easily stop or restart the burning of solid propellant. This can make a solid-propellant rocket difficult to control. One method used to stop the burning of solid propellant involves blasting the entire nozzle section from the rocket. This method, however, prevents restarting.

Rocket designers often choose solid propellants for rockets that must be easy to store, transport, and launch. Military planners prefer solid-propellant rockets for many uses because they can be stored for a long time and fired with little preparation. Solid-propellant rockets power ICBM's, including the American Minuteman 2 and MX and the Russian RT-2. They also propel such smaller missiles as the American Hellfire, Patriot, Sparrow, and Sidewinder, and the French SSBS. Solid-propellant rockets often serve as sounding rockets and as boosters for launch vehicles and cruise missiles. They are also used in fireworks.

Liquid-propellant rockets burn a mixture of fuel and oxidizer in liquid form. These rockets carry the fuel and the oxidizer in separate tanks. A system of pipes and valves feeds the propellants into the combustion chamber. In larger engines, either the fuel or the oxidizer flows around the outside of the chamber before entering it. This flow cools the chamber and preheats the propellant for combustion.

A liquid-propellant rocket feeds the fuel and oxidizer into the combustion chamber using either pumps or high-pressure gas. The most common method uses pumps to force the fuel and oxidizer into the combustion chamber. Burning a small portion of the propellants provides the energy to drive the pumps. In the other method, high-pressure gas forces the fuel and oxidizer into the chamber. The gas may be nitrogen or some other gas stored under high pressure or may come from the burning of a small amount of propellants.

A liquid-propellant rocket carries fuel and an oxidizer in separate tanks. The fuel circulates through the engine's cooling jacket before entering the combustion chamber. This circulation preheats the fuel for combustion and helps cool the rocket. Image credit: World Book diagram by Precision Graphics

Some liquid propellants, called hypergols, ignite when the fuel and the oxidizer mix. But most liquid propellants require an ignition system. An electric spark may ignite the propellant, or the burning of a small amount of solid propellant in the combustion chamber may do so. Liquid propellants continue to burn as long as fuel and oxidizer flow into the combustion chamber.

Engineers use thin, high-strength steel or aluminum to construct most tanks that hold liquid propellants. They may also reinforce tanks with composite materials like those used in solid-propellant rocket chambers. Most combustion chambers in liquid-propellant rockets are made of steel or nickel.

Liquid propellants usually produce greater thrust than do equal amounts of solid propellants burned in the same amount of time. Controllers can easily adjust or stop burning in a liquid-propellant rocket by increasing or decreasing the flow of propellants into the chamber. Liquid propellants, however, are difficult to handle. If the fuel and oxidizer blend without igniting, the resulting mixture often will explode easily. Liquid propellants also require complicated pumping machinery.

Scientists use liquid-propellant rockets for most space launch vehicles. Liquid-propellant rockets serve as the main engines of the space shuttle as well as Europe's Ariane rocket, Russia's Soyuz rocket, and China's Long March rocket.

Hybrid rockets combine some of the advantages of both solid-propellant and liquid-propellant rockets. A hybrid rocket uses a liquid oxidizer, such as liquid oxygen, and a solid-fuel grain made of plastic or rubber. The solid-fuel grain lines the inside of the combustion chamber. A pumping system sprays the oxidizer onto the surface of the grain, which is ignited by a smaller grain or torch.

Hybrid rockets are safer than solid-propellant rockets because the propellants are not premixed and so will not ignite accidentally. Also, unlike solid-propellant rockets, hybrid rockets can vary thrust or even stop combustion by adjusting the flow of oxidizer. Hybrid engines require only half the pumping gear of liquid-propellant rockets, making them simpler to build.

Launch vehicles used by European nations include the European Space Agency's Ariane 5 rocket and Russia's A class and Proton rockets. These vehicles carry space probes and artificial satellites into outer space. The A Class rocket has also carried people into space, and the Proton rocket has carried International Space Station modules. Image credit: World Book illustrations by Oxford Illustrators Limited

A key disadvantage of hybrid rockets is that their fuel burns slowly, limiting the amount of thrust they can produce. A hybrid rocket burns grain at a rate of about 0.04 inch (1 millimeter) per second. For a given amount of propellant, hybrid rockets typically produce more thrust than solid rockets and less than liquid engines. To generate more thrust, engineers must manufacture complex fuel grains with many separate ports through which oxidizer can flow. This exposes more of the grain to the oxidizer.

Researchers have used hybrid rockets to propel targets used in missile testing and to accelerate experimental motorcycles and cars attempting land speed records. Their safety has led designers to attempt to develop hybrid rockets for use in human flight. One such rocket would launch from an airplane to carry people to an altitude of about 60 miles (100 kilometers). Researchers have not yet developed hybrid rockets powerful enough to launch human beings into space. Hybrid rockets can produce enough thrust, however, to boost planetary probes or maneuver satellites in orbit. Hybrid rockets could also power escape mechanisms being developed for new launch vehicles that would carry crews.

The safety of hybrid rockets has led engineers to develop them for use in human flight. The Scaled Composites company of Mojave, California, developed a hybrid rocket called SpaceShipOne that launched from an airplane. On June 21, 2004, SpaceShipOne became the first privately funded craft to carry a person into space. It carried the American test pilot Michael Melvill more than 62 miles (100 kilometers) above Earth's surface during a brief test flight.

Researchers have also used hybrid rockets to propel targets used in missile testing and to accelerate experimental motorcycles and cars attempting land speed records. In addition, they have worked to develop hybrid rockets to boost planetary probes, maneuver satellites in orbit, and power crew escape mechanisms for launch vehicles.

Electric rockets use electric energy to expel ions (electrically charged particles) from the nozzle. Solar panels or a nuclear reactor can provide the energy.

In one design, xenon gas passes through an electrified metal grid. The grid strips electrons from the xenon atoms, turning them into positively charged ions. A positively charged screen repels the ions, focusing them into a beam. The beam then enters a negatively

An ion rocket is a kind of electric rocket. Heating coils in the rocket change a fuel, such as xenon, into a vapor. A hot platinum or tungsten ionization grid changes the flowing vapor into a stream of electrically charged particles called ions. Image credit: World Book diagram by Precision Graphics

charged device called an accelerator. The accelerator speeds up the ions and shoots them out through a nozzle.

The exhaust from such rockets travels extremely fast. However, the stream of xenon ions has a relatively low mass. As a result, an electric rocket cannot produce enough thrust to overcome Earth's gravity. Electric rockets used in space must therefore be launched by chemical rockets. Once in space, however, the low rate of mass flow becomes an advantage. It enables an electric rocket to operate for a long time without running out of propellant. The xenon rocket that powered the U.S. space probe Deep Space 1, launched in 1998, fired for a total of over 670 days using only 160 pounds (72 kilograms) of propellant. In addition, small electric rockets using xenon propellant have provided the thrust to keep communications satellites in position above Earth's surface.

Another type of electric rocket uses electromagnets rather than charged screens to accelerate xenon ions. This type of rocket powers the SMART-1 lunar probe, launched by the European Space Agency in 2003.

Nuclear rockets use the heat energy of a nuclear reactor, a device that releases energy by splitting atoms. Some proposed designs would use hydrogen as propellant. The rocket would store the hydrogen as a liquid. Heat from the reactor would boil the liquid, creating hydrogen gas. The gas would expand rapidly and push out from the nozzle.

The exhaust speed of a nuclear rocket might reach four times that of a chemical rocket. By expelling a large quantity of hydrogen, a nuclear rocket could therefore achieve high thrust. However, a nuclear rocket would require heavy shielding because a nuclear reactor uses radioactive materials. The shielding would weigh so much that the rocket could not be practically used to boost a launch vehicle. More practical applications would use small nuclear engines with low, continuous thrust to decrease flight times to Mars or other planets.

Nuclear rocket developers must also overcome public fears that accidents involving such devices could release harmful radioactive materials. Before nuclear rockets can be launched, engineers must convince the public that such devices are safe.

A nuclear rocket uses the heat from a nuclear reactor to change a liquid fuel into a gas. Most of the fuel flows through the reactor. Some of the fuel, heated by the nozzle of the rocket, flows through the turbine. The turbine drives the fuel pump. Image credit: World Book diagram by Precision Graphics

History

Historians believe the Chinese invented rockets, but they do not know exactly when. Historical accounts describe "arrows of flying fire" -- believed to have been rockets -- used by Chinese armies in A.D. 1232. By 1300, the use of rockets had spread throughout much of Asia and Europe. These first rockets burned a substance called black powder, which consisted of charcoal, saltpeter, and sulfur. For several hundred years, the use of rockets in fireworks displays outranked their military use in importance

During the early 1800's, Colonel William Congreve of the British Army developed rockets that could carry explosives. Many of these rockets weighed about 32 pounds (15 kilograms) and could travel 1 3/4 miles (2.7 kilometers). British troops used Congreve rockets against the United States Army during the War of 1812. Austria, Russia, and several other countries also developed military rockets during the early 1800's.

The English inventor William Hale improved the accuracy of military rockets. He substituted three fins for the long wooden tail that had been used to guide the rocket. United States troops used Hale rockets in the Mexican War (1846-1848). During the American Civil War (1861-1865), both sides used rockets.

Rockets of the early 1900's

The Russian school teacher Konstantin E. Tsiolkovsky first stated the correct theory of rocket power. He described his theory in a scientific paper published in 1903. Tsiolkovsky also first presented the ideas of the multistage rocket and rockets using liquid oxygen and hydrogen propellants. In 1926, the American rocket pioneer Robert H. Goddard conducted the first successful launch of a liquid-propellant rocket. The rocket climbed 41 feet (13 meters) into the air at a speed of about 60 miles (97 kilometers) per hour and landed 184 feet (56 meters) away.

During the 1930's, rocket research advanced in Germany, the Soviet Union, and the United States. Hermann Oberth led a small group of German engineers and scientists that experimented with rockets. Leading Soviet rocket scientists included Fridrikh A. Tsander and Sergei P. Korolev. Goddard remained the most prominent rocket researcher in the United States.

During World War II, German engineers under the direction of Wernher von Braun developed the powerful V-2 guided missile. Germany bombarded London and Antwerp, Belgium, with hundreds of V-2's during the last months of the war. American forces captured many V-2 missiles and sent them to the United States for use in research. After the war, von Braun and about 150 other German scientists moved to the United States to continue their work with rockets. Some other German rocket experts went to the Soviet Union.

High-altitude rockets

For several years after World War II, U.S. scientists benefited greatly by conducting experiments with captured German V-2's. These V-2's became the first rockets used for high-altitude research.

The first high-altitude rockets designed and built in the United States included the WAC Corporal, the Aerobee, and the Viking. The 16-foot (4.9-meter) WAC Corporal reached altitudes of about 45 miles (72 kilometers) during test flights in 1945. Early models of the Aerobee climbed about 70 miles (110 kilometers). In 1949, the U.S. Navy launched the Viking, an improved liquid-propellant rocket based chiefly on the V-2. The Viking measured more than 45 feet (14 meters) long, much longer than the Aerobee. But the first models of the Viking rose only about 50 miles (80 kilometers).

Rockets developed by the U.S. armed forces during the 1950's included the Jupiter and the Pershing. The Jupiter had a range of about 1,600 miles (2,600 kilometers), and the Pershing could travel about 450 miles (720 kilometers).

The vehicles shown here helped the United States and the Soviet Union achieve milestones in the exploration of space. The United States no longer builds these rockets, but Russia continues to use the Soviet A Class design in the Soyuz rocket. • Jupiter C, U.S. Lifted Explorer I, the first U.S. satellite, in 1958. 68 feet (21 meters) • Mercury-Redstone, U.S. Launched Alan Shepard in 1961. 83 feet (25 meters) • A Class (Sputnik), Soviet. Boosted Sputnik 1, the first artificial satellite, in 1957. 98 feet (29 meters) Image credit: WORLD BOOK illustrations by Oxford Illustrators Limited

The U.S. Navy conducted the first successful launch of a Polaris underwater missile in 1960. United States space scientists later used many military rockets developed in the 1950's as the basis for launch vehicles.

Rocket-powered airplanes

On Oct. 14, 1947, Captain Charles E. Yeager of the U.S. Air Force made the first supersonic (faster than sound) flight. He flew a rocket-powered airplane called the X-1.

A rocket engine also powered the X-15, which set an unofficial airplane altitude record of 354,200 feet (107,960 meters) in 1963. In one flight, the X-15 reached a peak speed of 4,520 miles (7,274 kilometers) per hour -- more than six times the speed of sound. A privately owned and developed rocket-powered plane called the EZ-Rocket began piloted test flights in 2001.

The space age began on Oct. 4, 1957, when the Soviet Union launched the first artificial satellite, Sputnik 1, aboard a two-stage rocket. On Jan. 31, 1958, the U.S. Army launched the first American satellite, Explorer 1, into orbit with a Juno I rocket.

On April 12, 1961, a Soviet rocket put a cosmonaut, Major Yuri A. Gagarin, into orbit around Earth for the first time. On May 5, 1961, a Redstone rocket launched Commander Alan B. Shepard, Jr., the first American to travel in space. On April 12, 1981, the United States launched the rocket-powered Columbia, the first space shuttle to orbit Earth. For more information on the history of rockets in space travel, see Space exploration.

Rocket research

In the early 2000's, engineers and scientists worked to develop lightweight rocket engines that used safer propellants. They also searched for more efficient propellants that did not require refrigeration. Engineers began designing and testing smaller rocket engines for use in smaller vehicles, such as tiny satellites that may weigh only a few pounds or kilograms when fully loaded.

The vehicles shown here helped the United States and the Soviet Union achieve milestones in the exploration of space. The United States no longer builds these rockets, but Russia continues to use the Soviet A Class design in the Soyuz rocket. • A Class (Vostok), Soviet. Carried Yuri Gagarin, the first person to orbit the earth, in 1961. 126 feet (38 meters) • Saturn 5, U.S. Launched Neil Armstrong, the first person to set foot on the moon, in 1969. 363 feet (111 meters) Image credit: WORLD BOOK illustrations by Oxford Illustrators Limited

Star A star is a huge, shining ball in space that produces a tremendous amount of light and other forms of energy. The sun is a star, and it supplies Earth with light and heat energy. The stars look like twinkling points of light -- except for the sun. The sun looks like a ball because it is much closer to Earth than any other star.

The sun and most other stars are made of gas and a hot, gaslike substance known as plasma. But some stars, called white dwarfs and neutron stars, consist of tightly packed atoms or subatomic particles. These stars are therefore much more dense than anything on Earth.

Stars come in many sizes. The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers). But astronomers classify the sun as a dwarf because other kinds of stars are much bigger. Some of the stars known as supergiants have a radius about 1,000 times that of the sun. The smallest stars are the neutron stars, some of which have a radius of only about 6 miles (10 kilometers).

About 75 percent of all stars are members of a binary system, a pair of closely spaced stars that orbit each other. The sun is not a member of a binary system. However, its nearest known stellar neighbor, Proxima Centauri, is part of a multiple-star system that also includes Alpha Centauri A and Alpha Centauri B.

The distance from the sun to Proxima Centauri is more than 25 trillion miles (40 trillion kilometers). This distance is so great that light takes 4.2 years to travel between the two stars. Scientists say that Proxima Centauri is 4.2 light-years from the sun. One light-year, the distance that light travels in a vacuum in a year, equals about 5.88 trillion miles (9.46 trillion kilometers).

Stars are grouped in huge structures called galaxies. Telescopes have revealed galaxies throughout the universe at distances of 12 billion to 16 billion light-years. The sun is in a galaxy called the Milky Way that contains more than 100 billion stars. There are more than 100 billion galaxies in the universe, and the average number of stars per galaxy may be 100 billion. Thus, more than 10 billion trillion stars may exist. But if you look at the night sky far from city lights, you can see only about 3,000 of them without using binoculars or a telescope.

Stars, like people, have life cycles -- they are born, pass through several phases, and eventually die. The sun was born about 4.6 billion years ago and will remain much as it is for another 5 billion years. Then it will grow to become a red giant. Late in the sun's

A globular cluster is a tightly grouped swarm of stars held together by gravity. This globular cluster is one of the densest of the 147 known clusters in the Milky Way galaxy. Image credit: NASA

lifetime, it will cast off its outer layers. The remaining core, called a white dwarf, will slowly fade to become a black dwarf.

Other stars will end their lives in different ways. Some will not go through a red giant stage. Instead, they will merely cool to become white dwarfs, then black dwarfs. A small percentage of stars will die in spectacular explosions called supernovae.

This article discusses Star (The stars at night) (Names of stars) (Characteristics of stars) (Fusion in stars) (Evolution of stars).

The stars at night

If you look at the stars on a clear night, you will notice that they seem to twinkle and that they differ greatly in brightness. A much slower movement also takes place in the night sky: If you map the location of several stars for a few hours, you will observe that all the stars revolve slowly about a single point in the sky.

Twinkling of stars is caused by movements in Earth's atmosphere. Starlight enters the atmosphere as straight rays. Twinkling occurs because air movements constantly change the path of the light as it comes through the air. You can see a similar effect if you stand in a swimming pool and look down. Unless the water is almost perfectly still, your feet will appear to move and change their shape. This "twinkling" occurs because the moving water constantly changes the path of the light rays that travel from your feet to your eyes.

Brightness of stars. How bright a star looks when viewed from Earth depends on two factors: (1) the actual brightness of the star -- that is, the amount of light energy the star emits (sends out) -- and (2) the distance from Earth to the star. A nearby star that is actually dim can appear brighter than a distant star that is really extremely brilliant. For example, Alpha Centauri A seems to be slightly brighter than a star known as Rigel. But Alpha Centauri A emits only 1/100,000 as much light energy as Rigel. Alpha Centauri A seems brighter because it is only 1/325 as far from Earth as Rigel is -- 4.4 light-years for Alpha Centauri A, 1,400 light-years for Rigel.

Rising and setting of stars

When viewed from Earth's Northern Hemisphere, stars rotate counterclockwise around a point called the celestial north pole. Viewed from the Southern Hemisphere, stars rotate clockwise about the celestial south pole. During the day, the sun moves across the sky in the same direction, and at the same rate, as the stars. These movements do not result from any actual revolution of the sun and stars. Rather, they occur because of the west-to-east rotation of Earth about its own axis. To an observer standing on the ground, Earth seems motionless, while the sun and stars seem to move in circles. But actually, Earth moves.

Names of stars

Ancient people saw that certain stars are arranged in patterns shaped somewhat like human beings, animals, or common objects. Some of these patterns, called constellations, came to represent figures of mythological characters. For example, the constellation Orion (the Hunter) is named after a hero in Greek mythology.

Today, astronomers use constellations, some of which were described by the ancients, in the scientific names of stars. The International Astronomical Union (IAU), the world authority for assigning names to celestial objects, officially recognizes 88 constellations. These constellations cover the entire sky. In most cases, the brightest star in a given constellation has alpha -- the first letter of the Greek alphabet -- as part of its scientific name. For instance, the scientific name for Vega, the brightest star in the constellation Lyra (the Harp), is Alpha Lyrae. Lyrae is Latin for of Lyra.

The second brightest star in a constellation is usually designated beta, the second letter of the Greek alphabet, the third brightest is gamma, and so on. The assignment of Greek letters to stars continues until all the Greek letters are used. Numerical designations follow.

But the number of known stars has become so large that the IAU uses a different system for newly discovered stars. Most new names consist of an abbreviation followed by a group of symbols. The abbreviation stands for either the type of star or a catalog that lists information about the star. For example, PSR J1302-6350 is a type of star known as a pulsar -- hence the PSR in its name. The symbols indicate the star's location in the sky. The 1302 and the 6350 are coordinates that are similar to the longitude and latitude designations used to indicate locations on Earth's surface. The J indicates that a coordinate system known as J2000 is being used.

Characteristics of stars

A star has five main characteristics: (1) brightness, which astronomers describe in terms of magnitude or luminosity; (2) color; (3) surface temperature; (4) size; and (5) mass (amount of matter). These characteristics are related to one another in a complex way. Color depends on surface temperature, and brightness depends on surface temperature and size. Mass affects the rate at which a star of a given size produces energy and so affects surface temperature. To make these relationships easier to understand, astronomers developed a graph called the Hertzsprung-Russell (H-R) diagram. This graph, a version of which appears in this article, also helps astronomers understand and describe the life cycles of stars.

Magnitude and luminosity

Magnitude is based on a numbering system invented by the Greek astronomer Hipparchus in about 125 B.C. Hipparchus numbered groups of stars according to their brightness as viewed from Earth. He called the brightest stars first magnitude stars, the

next brightest second magnitude stars, and so on to sixth magnitude stars, the faintest visible stars.

Modern astronomers refer to a star's brightness as viewed from Earth as its apparent magnitude. But they have extended Hipparchus's system to describe the actual brightness of stars, for which they use the term absolute magnitude. For technical reasons, they define a star's absolute magnitude as what its apparent magnitude would be if it were 32.6 light-years from Earth.

Astronomers have also extended the system of magnitude numbers to include stars brighter than first magnitude and dimmer than sixth magnitude. A star that is brighter than first magnitude has a magnitude less than 1. For example, the apparent magnitude of Rigel is 0.12. Extremely bright stars have magnitudes less than zero -- that is, their designations are negative numbers. The brightest star in the night sky is Sirius, with an apparent magnitude of -1.46. Rigel has an absolute magnitude of -8.1. According to astronomers' present understanding of stars, no star can have an absolute magnitude much brighter than -8. At the other end of the scale, the dimmest stars detected with telescopes have apparent magnitudes up to 28. In theory, no star could have an absolute magnitude much fainter than 16.

Luminosity is the rate at which a star emits energy. The scientific term for a rate of energy emission is power, and scientists generally measure power in watts. For example, the luminosity of the sun is 400 trillion trillion watts. But astronomers do not usually measure a star's luminosity in watts. Instead, they express luminosities in terms of the luminosity of the sun. They often say, for instance, that the luminosity of Alpha Centauri A is about 1.3 times that of the sun and that Rigel is roughly 150,000 times as luminous as the sun.

Luminosity is related to absolute magnitude in a simple way. A difference of 5 on the absolute magnitude scale corresponds to a factor of 100 on the luminosity scale. Thus, a star with an absolute magnitude of 2 is 100 times as luminous as a star with an absolute magnitude of 7. A star with an absolute magnitude of -3 is 100 times as luminous as a star whose absolute magnitude is 2 and 10,000 times as luminous as a star that has an absolute magnitude of 7.

Color and temperature

If you look carefully at the stars, even without binoculars or a telescope, you will see a range of color from reddish to yellowish to bluish. For example, Betelgeuse looks reddish, Pollux -- like the sun -- is yellowish, and Rigel looks bluish.

A star's color depends on its surface temperature. Astronomers measure star temperatures in a metric unit known as the kelvin. One kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales start at different points. The Kelvin scale starts at -273.15 degrees C. Therefore, a temperature of 0 K equals -273.15 degrees

C, or -459.67 degrees F. A temperature of 0 degrees C (32 degrees F) equals 273.15 K.

Dark red stars have surface temperatures of about 2500 K. The surface temperature of a bright red star is approximately 3500 K; that of the sun and other yellow stars, roughly 5500 K. Blue stars range from about 10,000 to 50,000 K in surface temperature.

Although a star appears to the unaided eye to have a single color, it actually emits a broad spectrum (band) of colors. You can see that starlight consists of many colors by using a prism to separate and spread the colors of the light of the sun, a yellow star. The visible spectrum includes all the colors of the rainbow. These colors range from red, produced by the photons (particles of light) with the least energy; to violet, produced by the most energetic photons.

Visible light is one of six bands of electromagnetic radiation. Ranging from the least energetic to the most energetic, they are: radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays. All six bands are emitted by stars, but most individual stars do not emit all of them. The combined range of all six bands is known as the electromagnetic spectrum.

Astronomers study a star's spectrum by separating it, spreading it out, and displaying it. The display itself is also known as a spectrum. The scientists study thin gaps in the spectrum. When the spectrum is spread out from left to right, the gaps appear as vertical lines. The spectra of stars have dark absorption lines where radiation of specific energies is weak. In a few special cases in the visible spectrum, stars have bright emission lines where radiation of specific energies is especially strong.

An absorption line appears when a chemical element or compound absorbs radiation that has the amount of energy corresponding to the line. For example, the spectrum of the visible light coming from the sun has a group of absorption lines in the green part of the spectrum. Calcium in an outer layer of the sun absorbs light rays that would have produced the corresponding green colors.

Although all stars have absorption lines in the visible band of the electromagnetic spectrum, emission lines are more common in other parts of the spectrum. For instance, nitrogen in the sun's atmosphere emits powerful radiation that produces emission lines in the ultraviolet part of the spectrum.

A spectacular explosion on the star Eta Carinae about 150 years ago produced three huge clouds of gas and dust -- two puffy lobes and a thin disk. Astronomers call Eta Carinae a luminous blue variable star because of its color and because it often becomes very bright -- as it did when the explosion occurred. Image credit: NASA

Size

Astronomers measure the size of stars in terms of the sun's radius. Alpha Centauri A, with a radius of 1.05 solar radii (the plural of radius), is almost exactly the same size as the sun. Rigel is much larger at 78 solar radii, and Antares has a huge size of 776 solar radii.

A star's size and surface temperature determine its luminosity. Suppose two stars had the same temperature, but the first star had twice the radius of the second star. In this case, the first star would be four times as bright as the second star. Scientists say that luminosity is proportional to radius squared -- that is, multiplied by itself. Imagine that you wanted to compare the luminosities of two stars that had the same temperature but different radii. First, you would divide the radius of the larger star by the radius of the smaller star. Then, you would square your answer.

Now, suppose two stars had the same radius but the first star's surface temperature -- measured in kelvins -- was twice that of the second star. In this example, the luminosity of the first star would be 16 times that of the second star. Luminosity is proportional to temperature to the fourth power. Imagine that you wanted to compare the luminosities of stars that had the same radius but different temperatures. First, you would divide the temperature of the warmer star by the temperature of the cooler star. Next, you would square the result. Then, you would square your answer again.

Mass

Astronomers express the mass of a star in terms of the solar mass, the mass of the sun. For example, they give the mass of Alpha Centauri A as 1.08 solar masses; that of Rigel, as 3.50 solar masses. The mass of the sun is 2 Ž 1030 kilograms, which would be written out as 2 followed by 30 zeros.

Stars that have similar masses may not be similar in size -- that is, they may have different densities. Density is the amount of mass per unit of volume. For instance, the average density of the sun is 88 pounds per cubic foot (1,400 kilograms per cubic meter), about 140 percent that of water. Sirius B has almost exactly the same mass as the sun, but it is 90,000 times as dense. As a result, its radius is only about 1/50 of a solar radius.

The Hertzsprung-Russell diagram displays the main characteristics of stars. The diagram is named for astronomers Ejnar Hertzsprung of Denmark and Henry Norris Russell of the United States. Working independently of each other, the two scientists developed the diagram around 1910.

Luminosity classes

Points representing the brightest stars appear toward the top of the H-R diagram; points corresponding to the dimmest stars, toward the bottom. These points appear in groups

that correspond to different kinds of stars. In the 1930's, American astronomers William W. Morgan and Philip C. Keenan invented what came to be known as the MK luminosity classification system for these groups. Astronomers revised and extended this system in 1978. In the MK system, the largest and brightest classes have the lowest classification numbers. The MK classes are: Ia, bright supergiant; Ib, supergiant; II, bright giant; III, giant; IV, subgiant; and V, main sequence or dwarf.

Because temperature also affects the luminosity of a star, stars from different luminosity classes can overlap. For example, Spica, a class V star, has an absolute magnitude of -3.2; but Pollux, a class III star, is dimmer, with an absolute magnitude of 0.7.

Spectral classes

Points representing the stars with the highest surface temperatures appear toward the left edge of the H-R diagram; points representing the coolest stars, toward the right edge. In the MK system, there are eight spectral classes, each corresponding to a certain range of surface temperature. From the hottest stars to the coolest, these classes are: O, B, A, F, G, K, M, and L. Each spectral class, in turn, is made up of 10 spectral types, which are designated by the letter for the spectral class and a numeral. The hottest stars in a spectral class are assigned the numeral 0; the coolest stars, the numeral 9.

A complete MK designation thus includes symbols for luminosity class and spectral type. For example, the complete designation for the sun is G2V. Alpha Centauri A is also a G2V star, and Rigel's designation is B8Ia.

Fusion in stars

A star's tremendous energy comes from a process known as nuclear fusion. This process begins when the temperature of the core of the developing star reaches about 1 million K.

A star develops from a giant, slowly rotating cloud that consists almost entirely of the chemical elements hydrogen and helium. The cloud also contains atoms of other elements as well as microscopic particles of dust.

Due to the force of its own gravity, the cloud begins to collapse inward, thereby becoming smaller. As the cloud shrinks, it rotates more and more rapidly, just as spinning ice skaters turn more rapidly when they pull in their arms. The outermost parts of the cloud form a spinning disk. The inner parts become a roughly spherical clump, which continues to collapse.

The collapsing material becomes warmer, and its pressure increases. But the pressure tends to counteract the gravitational force that is responsible for the collapse. Eventually, therefore, the collapse slows to a gradual contraction. The inner parts of the clump form a protostar, a ball-shaped object that is no longer a cloud, but is not yet a star. Surrounding

the protostar is an irregular sphere of gas and dust that had been the outer parts of the clump.

Combining nuclei

When the temperature and pressure in the protostar's core become high enough, nuclear fusion begins. Nuclear fusion is a joining of two atomic nuclei to produce a larger nucleus.

Nuclei that fuse are actually the cores of atoms. A complete atom has an outer shell of one or more particles called electrons, which carry a negative electric charge. Deep inside the atom is the nucleus, which contains almost all the atom's mass. The simplest nucleus, that of the most common form of hydrogen, consists of a single particle known as a proton. A proton carries a positive electric charge. All other nuclei have one or more protons and one or more neutrons. A neutron carries no net charge, and so a nucleus is electrically positive. But a complete atom has as many electrons as protons. The net electric charge of a complete atom is therefore zero -- the atom is electrically neutral.

However, under the enormous temperatures and pressures near the core of a protostar, atoms lose electrons. The resulting atoms are known as ions, and the mixture of the free electrons and ions is called a plasma.

Atoms in the core of the protostar lose all their electrons, and the resulting bare nuclei approach one another at tremendous speeds. Under ordinary circumstances, objects that carry like charges repel each other. However, if the core temperature and pressure become high enough, the repulsion between nuclei can be overcome and the nuclei can fuse. Scientists commonly refer to fusion as "nuclear burning." But fusion has nothing to do with ordinary burning or combustion.

Converting mass to energy

When two relatively light nuclei fuse, a small amount of their mass turns into energy. Thus, the new nucleus has slightly less mass than the sum of the masses of the original nuclei. The German-born American physicist Albert Einstein discovered the relationship E = mc-squared (E=mc 2) that indicates how much energy is released when fusion occurs. The symbol E represents the energy; m, the mass that is converted; and c-squared (c2), the speed of light squared.

The speed of light is 186,282 miles (299,792 kilometers) per second. This is such a large number that the conversion of a tiny quantity of mass produces a tremendous amount of energy. For example, complete conversion of 1 gram of mass releases 90 trillion joules of energy. This amount of energy is roughly equal to the quantity released in the explosion of 22,000 tons (20,000 metric tons) of TNT. This is much more energy than was released by the atomic bomb that the United States dropped on Hiroshima, Japan, in 1945 during

World War II. The energy of the bomb was equivalent to the explosion of 13,000 tons (12,000 metric tons) of TNT.

Destruction of light nuclei

In the core of a protostar, fusion begins when the temperature reaches about 1 million K. This initial fusion destroys nuclei of certain light elements. These include lithium 7 nuclei, which consist of three protons and four neutrons. In the process involving lithium 7, a hydrogen nucleus combines with a lithium 7 nucleus, which then splits into two parts. Each part consists of a nucleus of helium 4 -- two protons and two neutrons. A helium 4 nucleus is also known as an alpha particle.

Hydrogen fusion

After the light nuclei are destroyed, the protostar continues to contract. Eventually, the core temperature reaches about 10 million K, and hydrogen fusion begins. The protostar is now a star.

In hydrogen fusion, four hydrogen nuclei fuse to form a helium 4 nucleus. There are two general forms of this reaction: (1) the proton-proton (p-p) reaction and (2) the carbon-nitrogen-oxygen (CNO) cycle.

The p-p reaction can occur in several ways, including the following four-step process:

(1) Two protons fuse. In this step, two protons collide, and then one of the protons loses its positive charge by emitting a positron. The proton also emits an electrically neutral particle called a neutrino.

A positron is the antimatter equivalent of an electron. It has the same mass as an electron but differs from the electron in having a positive charge. By emitting the positron, the proton becomes a neutron. The new nucleus therefore consists of a proton and a neutron -- a combination known as a deuteron.

(2) The positron collides with an electron that happens to be nearby. As a result, the two particles annihilate each other, producing two gamma rays.

(3) The deuteron fuses with another proton, producing a helium 3 nucleus, which consists of two protons and one neutron. This step also produces a gamma ray.

(4) The helium 3 nucleus fuses with another helium 3 nucleus. This step produces a helium 4 nucleus, and two protons are released.

The CNO cycle differs from the p-p reaction mainly in that it involves carbon 12 nuclei. These nuclei consist of six protons and six neutrons. During the cycle, they change into

nuclei of nitrogen 15 (7 protons and 8 neutrons) and oxygen 15 (8 protons and 7 neutrons). But they change back to carbon 12 nuclei by the end of the cycle.

Fusion of other elements

Helium nuclei can fuse to form carbon 12 nuclei. However, the core temperature must rise to about 100 million K for this process to occur. This high temperature is necessary because the helium nuclei must overcome a much higher repulsive force than the force between two protons. Each helium nucleus has two protons, so the repulsive force is four times as high as the force between two protons.

The fusion of helium is called the triple-alpha process because it combines three alpha particles to create a carbon 12 nucleus. Helium fusion also produces nuclei of oxygen 16 (8 protons and 8 neutrons) and neon 20 (10 protons and 10 neutrons).

At core temperatures of about 600 million K, carbon 12 can fuse to form sodium 23 (11 protons, 12 neutrons), magnesium 24 (12 protons, 12 neutrons), and more neon 20. However, not all stars can reach these temperatures.

As fusion processes produce heavier and heavier elements, the temperature necessary for further processes increases. At about 1 billion K, oxygen 16 nuclei can fuse, producing silicon 28 (14 protons, 14 neutrons), phosphorus 31 (15 protons, 16 neutrons), and sulfur 32 (16 protons, 16 neutrons).

Fusion can produce energy only as long as the new nuclei have less mass than the sum of the masses of the original nuclei. Energy production continues until nuclei of iron 56 (26 protons, 30 neutrons) begin to combine with other nuclei. When this happens, the new nuclei have slightly more mass than the original nuclei. This process therefore uses energy, rather than producing it.

Evolution of stars

The life cycles of stars follow three general patterns, each associated with a range of initial mass. There are (1) high-mass stars, which have more than 8 solar masses; (2) intermediate-mass stars, with 0.5 to 8 solar masses -- the group that includes the sun; and (3) low-mass stars, with 0.1 to 0.5 solar mass. Objects with less than 0.1 solar mass do not have enough gravitational force to produce the core temperature necessary for hydrogen fusion.

The life cycles of single stars are simpler than those of binary systems, so this section discusses the evolution of single stars first. And because astronomers know much more about the sun than any other star, the discussion begins with the development of intermediate-mass stars.

Intermediate-mass stars

A cloud that eventually develops into an intermediate-mass star takes about 100,000 years to collapse into a protostar. As a protostar, it has a surface temperature of about 4000 K. It may be anywhere from a few times to a few thousand times as luminous as the sun, depending on its mass.

T-Tauri phase

When hydrogen fusion begins, the protostar is still surrounded by an irregular mass of gas and dust. But the energy produced by hydrogen fusion pushes away this material as a protostellar wind. In many cases, the disk that is left over from the collapse channels the wind into two narrow cones or jets. One jet emerges from each side of the disk at a right angle to the plane of the disk. The protostar has become a T-Tauri star, a type of object named after the star T in the constellation Taurus (the Bull). A T-Tauri star is a variable star, one that varies in brightness.

Main-sequence phase

The T-Tauri star contracts for about 10 million years. It stops contracting when its tendency to expand due to the energy produced by fusion in its core balances its tendency to contract due to gravity. By this time, hydrogen fusion in the core is supplying all the star's energy. The star has begun the longest part of its life as a producer of energy from hydrogen fusion, the main-sequence phase. The name of this phase comes from a part of the H-R diagram.

Any star -- whatever its mass -- that gets all its energy from hydrogen fusion in its core is said to be "on the main sequence" or "a main-sequence star." The amount of time a star spends there depends on its mass. The greater a star's mass, the more rapidly the hydrogen in its core is used up, and therefore the shorter is its stay on the main sequence. An intermediate-mass star remains on the main sequence for billions of years.

Red giant phase

When all the hydrogen in the core of an intermediate-mass star has fused into helium, the star changes rapidly. Because the core no longer produces fusion energy, gravity immediately crushes matter down upon it. The resulting compression quickly heats the core and the region around it. The temperature becomes so high that hydrogen fusion begins in a thin shell surrounding the core. This fusion produces even more energy than had been produced by hydrogen fusion in the core. The extra energy pushes against the star's outer layers, and so the star expands enormously.

As the star expands, its outer layers become cooler, so the star becomes redder. And because the star's surface area expands greatly, the star also becomes brighter. The star is now a red giant.

Horizontal branch phase

Eventually, the core temperature reaches 100 million K, high enough to support the triple-alpha process. This process begins so rapidly that its onset is known as helium flash.

As the triple-alpha process continues, the core expands, but its temperature drops. This decrease in temperature causes the temperature of the hydrogen-burning shell to drop. Consequently, the energy output of the shell decreases, and the outer layers of the star contract. The star becomes hotter but smaller and fainter than it had been as a red giant. This change occurs over a period of about 100 million years.

At the end of this period, the star is in its horizontal branch phase, named for the position of the point representing the star on the H-R diagram. The star steadily burns helium and hydrogen, and so its temperature, size, and luminosity do not change significantly. This phase lasts for about 10 million years.

Asymptotic giant phase

When all the helium in the core has fused, the core contracts and therefore becomes hotter. The triple-alpha process begins in a shell surrounding the core, and hydrogen fusion continues in a shell surrounding that. Due to the increased energy produced by the burning in the shells, the star's outer layers expand. The star becomes a giant again, but it is bluer and brighter than it was the first time.

On the H-R diagram, the point representing the star has moved upward and to the right along a line known as the asymptotic (as ihm TOT ihk) giant branch (AGB). The star is therefore called an AGB star.

An AGB star's core is so hot and its gravitational grip on its outermost layers is so weak that those layers blow away in a stellar wind. As each layer blows away, a hotter layer is exposed. Thus, the stellar wind becomes even stronger. Out in space, a succession of new, fast winds slam into old, slow winds that are still moving away from the star. The collisions produce dense shells of gas, some of which cool to form dust.

White dwarf phase

In just a few thousand years, all but the hot core of an AGB star blows away, and fusion ceases in the core. The core illuminates the surrounding shells. Such shells looked like planets through the crude telescopes of astronomers who studied them in the 1800's. As a result, the astronomers called

A planetary nebula with an unusual textured appearance, the cause of which is unknown. This photo was taken by the Hubble Space Telescope. Image credit: NASA

the shells planetary nebulae -- and today's astronomers still do. The word nebulae is Latin for clouds.

After a planetary nebula fades from view, the remaining core is known as a white dwarf star. This kind of star consists mostly of carbon and oxygen. Its initial temperature is about 100,000 K.

Black dwarf phase

Because a white dwarf star has no fuel remaining for fusion, it becomes cooler and cooler. Over billions of years, it cools more and more slowly. Eventually, it becomes a black dwarf -- an object too faint to detect. A black dwarf represents the end of the life cycle of an intermediate-mass star.

High-mass stars, those with more than 8 solar masses, form quickly and have short lives. A high-mass star forms from a protostar in about 10,000 to 100,000 years.

High-mass stars on the main sequence are hot and blue. They are 1,000 to 1 million times as luminous as the sun, and their radii are about 10 times the solar radius. High-mass stars are much less common than intermediate- and low-mass stars. Because they are so bright, however, high-mass stars are visible from great distances, and so many are known.

A high-mass star has a strong stellar wind. A star of 30 solar masses can lose 24 solar masses by stellar wind before its core runs out of hydrogen and it leaves the main sequence.

As a high-mass star leaves the main sequence, hydrogen begins to fuse in a shell outside its core. As a result, its radius increases to about 100 times that of the sun. However, its luminosity decreases slightly. Because the star is now emitting almost the same amount of energy from a much larger surface, the temperature of the surface decreases. The star therefore becomes redder.

As the star evolves, its core heats up to 100 million K, enough to start the triple-alpha process. After about 1 million years, helium fusion ends in the core but begins in a shell outside the core. And, as in an intermediate-mass star, hydrogen fuses in a shell outside that. The high-mass star becomes a bright red supergiant.

When the contracting core becomes sufficiently hot, carbon fuses, producing neon, sodium, and magnesium. This phase lasts only about 10,000 years. A succession of fusion processes then occur in the core. Each successive process involves a different element and takes less time. Whenever a different element begins to fuse in the core, the element that had been fusing there continues to fuse in a shell outside the core. In addition, all the elements that had been fusing in shells continue to do so. Neon fuses to produce oxygen and magnesium, a process that lasts about 12 years. Oxygen then fuses,

producing silicon and sulfur for about 4 years. Finally, silicon fuses to make iron, taking about a week.

Supernovae

At this time, the radius of the iron core is about 1,900 miles (3,000 kilometers). Because further fusion would consume energy, the star is now doomed. It cannot produce any more fusion energy to balance the force of gravity.

When the mass of the iron core reaches 1.4 solar masses, violent events occur. The force of gravity within the core causes the core to collapse. As a result, the core temperature rises to nearly 10 billion K. At this temperature, the iron nuclei break down into lighter nuclei and eventually into individual protons and neutrons. As the collapse continues, protons combine with electrons, producing neutrons and neutrinos. The neutrinos carry away about 99 percent of the energy produced by the crushing of the core.

Now, the core consists of a collapsing ball of neutrons. When the radius of the ball shrinks to about 6 miles (10 kilometers), the ball rebounds like a solid rubber ball that has been squeezed.

All the events from the beginning of the collapse of the core to the rebounding of the neutrons occur in about one second. But more violence is in store. The rebounding of the ball of neutrons sends a spherical shock wave outward through the star. Much of the energy of the wave causes fusion to occur in overlying layers, creating new elements. As the wave reaches the star's surface, it boosts temperatures to 200,000 K. As a result, the star explodes, hurling matter into space at speeds of about 9,000 to 25,000 miles (15,000 to 40,000 kilometers) per second. The brilliant explosion is known as a Type II supernova.

Supernovae enrich the clouds of gas and dust from which new stars eventually form. This enrichment process has been going on since the first supernovae billions of years ago. Supernovae in the first generation of stars enriched the clouds with materials that later went into making newer stars.

Three generations of stars may exist. Astronomers have not found any of what would be the oldest generation, Population III, stars. But they have found members of the other two generations. Population II stars, which would be the second generation, contain relatively small amounts of heavy elements. The more massive ones aged and died quickly, thereby contributing more nuclei of heavy elements to the clouds. For this reason, Population I stars, the third generation, contain the largest amounts of heavy elements. Yet these quantities are tiny compared with the amount of hydrogen and helium in Population I stars. For example, elements other than hydrogen and helium make up from 1 to 2 percent of the mass of the sun, a Population I star.

Neutron stars

After a Type II supernova blast occurs, the stellar core remains behind. If the core has less than about 3 solar masses, it becomes a neutron star. This object consists almost entirely of neutrons. It packs at least 1.4 solar masses into a sphere with a radius of about 6 to 10 miles (10 to 15 kilometers).

Neutron stars have initial temperatures of 10 million K, but they are so small that their visible light is difficult to detect. However, astronomers have detected pulses of radio energy from neutron stars, sometimes at a rate of almost 1,000 pulses per second.

A neutron star actually emits two continuous beams of radio energy. The beams flow away from the star in opposite directions. As the star rotates, the beams sweep around in space like searchlight beams. If one of the beams periodically sweeps over Earth, a radio telescope can detect it as a series of pulses. The telescope detects one pulse for each revolution of the star. A star that is detected in this way is known as a pulsar.

Black holes

If the stellar core remaining after the supernova explosion has about 3 or more solar masses, no known force can support it against its own gravitation. The core collapses to form a black hole, a region of space whose gravitational force is so strong that nothing can escape from it. A black hole is invisible because it traps even light. All its matter is located at a single point in its center. This point, known as a singularity, is much smaller than an atomic nucleus.

Low-mass stars, ranging from 0.1 to 0.5 solar mass, have surface temperatures less than about 4,000 K. Their luminosities are less than 2 percent of the solar luminosity. Low-mass stars use hydrogen fuel so slowly that they may shine as main-sequence stars for 100 billion to 1 trillion years. This life span is longer than the present age of the universe, believed to be 10 billion to 20 billion years. Therefore, no low-mass star has ever died. Nevertheless, astronomers have determined that low-mass stars will never fuse anything but hydrogen. Thus, as these stars die, they will not pass through a red-giant phase. Instead, they will merely cool to become white dwarfs, then black dwarfs.

Binary stars develop from two protostars that form near each other. More than 50 percent of what seem to the unaided eye to be single stars are actually binaries.

One star in a binary system can affect the life cycle of the other if the two stars are sufficiently close together. Between the stars is a location called the Lagrange point, named for the French

Transfer of mass occurs in a binary star system. Matter flows from a sunlike star, in the background in this illustration, to a disk orbiting a white dwarf star, then to the surface of the dwarf. Image credit: Space Telescope Science Institute

mathematician Joseph Louis Lagrange, where the star's gravitational forces are exactly equal. If one of the stars expands so much that its outer layers pass the Lagrange point, the other star will begin to strip away those layers and accumulate them on its surface.

This process, called mass transfer, can take many forms. Mass transfer from a red giant onto a main-sequence companion can add absorption lines of carbon or other elements to the spectrum of the main- sequence star. But if the stars are close together, the material will flow in the opposite direction when the giant star becomes a white dwarf. The matter will spiral in toward the dwarf, forming a hot disk around it. The disk will flare brilliantly in visible and ultraviolet radiation.

If the giant star leaves behind a neutron star or a black hole instead of a white dwarf, an X-ray binary may form. In this case, the matter transferred from the main-sequence star will become extremely hot. When this matter strikes the surface of the neutron star or is pulled into the black hole, it will emit X rays.

In a third case, the red giant becomes a white dwarf, and the main-sequence star becomes a red giant. When enough gas from the giant accumulates on the dwarf's surface, gas nuclei will fuse violently in a flash called a nova. In some cases, so much gas will accumulate that its weight will cause the dwarf to collapse. Almost instantly, the dwarf's carbon will fuse, and the entire dwarf will explode in a Type I supernova. This kind of explosion is so bright that it can outshine an entire galaxy for a few months.

Uranus Uranus, (YUR uh nuhs or yu RAY nuhs), is the seventh planet from the sun. Only Neptune and Pluto are farther away. Uranus is the farthest planet that can be seen without a telescope. Its average distance from the sun is about 1,784,860,000 miles (2,872,460,000 kilometers), a distance that takes light about 2 hours 40 minutes to travel.

Uranus is a giant ball of gas and liquid. Its diameter at the equator is 31,763 miles (51,118 kilometers), over four times that of Earth. The surface of Uranus consists of blue-green

Uranus appears in true colors, left, and false colors, right in images produced by combining numerous pictures taken by the Voyager 2 spacecraft. The false colors emphasize bands of smog around the planet's south pole. The small spots are shadows of dust specks in the camera. Image credit: JPL

clouds made up of tiny crystals of methane. The crystals have frozen out of the planet's atmosphere. Far below the visible clouds are probably thicker cloud layers made up of liquid water and crystals of ammonia ice. Deeper still -- about 4,700 miles (7,500 kilometers) below the visible cloud tops -- may be an ocean of liquid water containing dissolved ammonia. At the very center of the planet may be a rocky core about the size of Earth. Scientists doubt Uranus has any form of life.

Uranus was the first planet discovered since ancient times. British astronomer William Herschel discovered it in 1781. Johann E. Bode, a German astronomer, named it Uranus after a sky god in Greek mythology. Most of our information about Uranus comes from the flight of the United States spacecraft Voyager 2. In 1986, that craft flew within about 50,000 miles (80,000 kilometers) of the planet's cloud tops.

Orbit and rotation

Uranus travels around the sun in an elliptical (oval-shaped) orbit, which it completes in 30,685 Earth days, or just over 84 Earth years. As it orbits the sun, Uranus also rotates on its axis, an imaginary line through its center. The planet's interior (ocean and core) takes 17 hours 14 minutes to spin around once on its axis. However, much of the atmosphere rotates faster than that. The fastest winds on Uranus, measured about two-thirds of the way from the equator to the south pole, blow at about 450 miles per hour (720 kilometers per hour). Thus, this area toward the south pole makes one complete rotation every 14 hours.

Uranus is tilted so far on its side that its axis lies nearly level with its path around the sun. Scientists measure the tilt of a planet relative to a line at a right angle to the orbital plane, an imaginary surface touching all points of the orbit. Most planets' axes tilt less than 30¡. For example, the tilt of Earth's axis is about 23 1/2. But Uranus's axis tilts 98 degrees, so that the axis lies almost in the orbital plane. Many astronomers think that a collision with an Earth-sized planet may have knocked Uranus on its side soon after it was formed.

Uranus has a mass (quantity of matter) 14 1/2 times larger than that of Earth. However, the mass of Uranus is only about 1/20 as large as that of the largest planet, Jupiter.

Uranus has an average density of 1.27 grams per cubic centimeter, or about 1 1/4 times the density of water. Density is the amount of mass in a substance divided by the volume of the substance. The density of Uranus is 1/4 that of Earth, and is similar to that of Jupiter.

The force of gravity at the surface of Uranus is about 90 percent of that at the surface of Earth. Thus, an object that weighs 100 pounds on Earth would weigh about 90 pounds on Uranus.

The atmosphere of Uranus is composed of about 83 percent hydrogen, 15 percent helium, 2 percent methane, and tiny amounts of ethane and other gases. The atmospheric pressure

beneath the methane cloud layer is about 19 pounds per square inch (130 kilopascals), or about 1.3 times the atmospheric pressure at the surface of Earth. Atmospheric pressure is the pressure exerted by the gases of a planet's atmosphere due to their weight.

The visible clouds of Uranus are the same pale blue-green all over the surface of the planet. Images of Uranus taken by Voyager 2 and processed for high contrast by computers show very faint bands within the clouds parallel to the equator. These bands are made up of different concentrations of smog produced as sunlight breaks down methane gas. In addition, there are a few small spots on the planet's surface. These spots probably are violently swirling masses of gas resembling a hurricane.

The temperature of the atmosphere is about -355 degrees F (-215 degrees C). In the interior, the temperature rises rapidly, reaching perhaps 4200 degrees F (2300 degrees C) in the ocean and 12,600 degrees F (7000 degrees C) in the rocky core. Uranus seems to radiate as much heat into space as it gets from the sun. Because Uranus is tilted 98¡ on its axis, its poles receive more sunlight during a Uranian year than does its equator. However, the weather system seems to distribute the extra heat fairly evenly over the planet.

Satellites

Uranus has 21 known satellites. Astronomers discovered the 5 largest satellites between 1787 and 1948. Photographs by Voyager 2 in 1985 and 1986 revealed 10 additional satellites. Astronomers later discovered more satellites by using Earth-based telescopes.

Miranda, the smallest of the five large satellites, has certain surface features that are unlike any other formation in the solar system. These are three oddly shaped regions called ovoids. Each ovoid is 120 to 190 miles (200 to 300 kilometers) across. The outer areas of each ovoid resemble a race track, with parallel ridges and canyons wrapped about the center. But in the center, ridges and canyons crisscross one another randomly.

Magnetic field

Miranda, a satellite of Uranus, has three regions called ovoids whose outer ridges resemble race tracks. Internal geological activity created the ovoids, probably in the past 2 billion years. Image credit: JPL

Uranus has a number of rings around it. Ten of them are dark and narrow, ranging in width from less than 3 miles (5 kilometers) to 60 miles (100 kilometers). They are no more than 33 feet (10 meters) thick. Image credit: NASA

Uranus has a strong magnetic field. The axis of the field (an imaginary line connecting its north and south poles) is tilted 59 degrees from the planet's axis of rotation.

The magnetic field has trapped high-energy, electrically charged particles -- mostly electrons and protons -- in radiation belts around the planet. As these particles travel back and forth between the magnetic poles, they send out radio waves. Voyager 2 detected the waves, but they are so weak that they cannot be detected on Earth.

Contributors: Peter J. Gierasch, Ph.D., Professor of Astronomy, Cornell University. Philip D. Nicholson, Ph.D., Professor of Astronomy, Cornell University.

Artificial Satellites An artificial satellite is a manufactured object that continuously orbits Earth or some other body in space. Most artificial satellites orbit Earth. People use them to study the universe, help forecast the weather, transfer telephone calls over the oceans, assist in the navigation of ships and aircraft, monitor crops and other resources, and support military activities.

Artificial satellites also have orbited the moon, the sun, asteroids, and the planets Venus, Mars, and Jupiter. Such satellites mainly gather information about the bodies they orbit.

Piloted spacecraft in orbit, such as space capsules, space shuttle orbiters, and space stations, are also considered artificial satellites. So, too, are orbiting pieces of "space junk," such as burned-out rocket boosters and empty fuel tanks that have not fallen to Earth. But this article does not deal with these kinds of artificial satellites.

Artificial satellites differ from natural satellites, natural objects that orbit a planet. Earth's moon is a natural satellite.

The Soviet Union launched the first artificial satellite, Sputnik 1, in 1957. Since then, the United States and about 40 other countries have developed, launched, and operated satellites. Today, about 3,000 useful satellites and 6,000 pieces of space junk are orbiting Earth.

Satellite orbits

Satellite orbits have a variety of shapes. Some are circular, while others are highly elliptical (egg-shaped). Orbits also vary in altitude. Some circular orbits, for example, are just above the atmosphere at an altitude of about 155 miles (250 kilometers), while others are more than 20,000 miles (32,200 kilometers) above Earth. The greater the altitude, the longer the orbital period -- the time it takes a satellite to complete one orbit.

A satellite remains in orbit because of a balance between the satellite's velocity (speed at which it would travel in a straight line) and the gravitational force between the satellite and Earth. Were it not for the pull of gravity, a satellite's velocity would send it flying away from Earth in a straight line. But were it not for velocity, gravity would pull a satellite back to Earth.

To help understand the balance between gravity and velocity, consider what happens when a small weight is attached to a string and swung in a circle. If the string were to break, the weight would fly off in a straight line. However, the string acts like gravity, keeping the weight in its orbit. The weight and string can also show the relationship between a satellite's altitude and its orbital period. A long string is like a high altitude. The weight takes a relatively long time to complete one circle. A short string is like a low altitude. The weight has a relatively short orbital period.

Many types of orbits exist, but most artificial satellites orbiting Earth travel in one of four types: (1) high altitude, geosynchronous; (2) medium altitude, (3) sun-synchronous, polar; and (4) low altitude. Most orbits of these four types are circular.

A high altitude, geosynchronous orbit lies above the equator at an altitude of about 22,300 miles (35,900 kilometers). A satellite in this orbit travels around Earth's axis in exactly the same time, and in the same direction, as Earth rotates about its axis. Thus, as seen from Earth, the satellite always appears at the same place in the sky overhead. To boost a satellite into this orbit requires a large, powerful launch vehicle.

A medium altitude orbit has an altitude of about 12,400 miles (20,000 kilometers) and an orbital period of 12 hours. The orbit is outside Earth's atmosphere and is thus very stable. Radio signals sent from a satellite at medium altitude can be received over a large area of Earth's surface. The stability and wide coverage of the orbit make it ideal for navigation satellites.

A sun-synchronous, polar orbit has a fairly low altitude and passes almost directly over the North and South poles. A slow drift of the orbit's position is coordinated with Earth's movement around the sun in such a way that the satellite always crosses the equator at the same local time on Earth. Because the satellite flies over all latitudes, its instruments can gather information on almost the entire surface of Earth. One example of this type of orbit is that of the TERRA Earth Observing System's NOAA-H satellite. This satellite studies how natural cycles and human activities affect Earth's climate. The altitude of its orbit is 438 miles (705 kilometers), and the orbital period is 99 minutes. When the satellite crosses the equator, the local time is always either 10:30 a.m. or 10:30 p.m.

A low altitude orbit is just above Earth's atmosphere, where there is almost no air to cause drag on the spacecraft and reduce its speed. Less energy is required to launch a satellite into this type of orbit than into any other orbit. Satellites that point toward deep space and provide scientific information generally operate in this type of orbit. The Hubble Space Telescope, for example, operates at an altitude of about 380 miles (610 kilometers), with an orbital period of 97 minutes.

Types of artificial satellites

Artificial satellites are classified according to their mission. There are six main types of artificial satellites: (1) scientific research, (2) weather, (3) communications, (4) navigation, (5) Earth observing, and (6) military.

Scientific research satellites gather data for scientific analysis. These satellites are usually designed to perform one of three kinds of missions. (1) Some gather information about the composition and effects of the space near Earth. They may be placed in any of various orbits, depending on the type of measurements they are to make. (2) Other satellites record changes in Earth and its atmosphere. Many of them travel in sun-synchronous, polar orbits. (3) Still others observe planets, stars, and other distant objects. Most of these satellites operate in low altitude orbits. Scientific research satellites also orbit other planets, the moon, and the sun.

Weather satellites help scientists study weather patterns and forecast the weather. Weather satellites observe the atmospheric conditions over large areas.

Some weather satellites travel in a sun-synchronous, polar orbit, from which they make close, detailed observations of weather over the entire Earth. Their instruments measure cloud cover, temperature, air pressure, precipitation, and the chemical composition of the atmosphere. Because these satellites always observe Earth at the same local time of day, scientists can easily compare weather data collected under constant sunlight conditions. The network of weather satellites in these orbits also function as a search and rescue system. They are equipped to detect distress signals from all commercial, and many private, planes and ships.

Other weather satellites are placed in high altitude, geosynchronous orbits. From these orbits, they can always observe weather activity over nearly half the surface of Earth at

A weather satellite called the Geostationary Operational Environmental Satellite observes atmospheric conditions over a large area to help scientists study and forecast the weather. Image credit: NASA

A communications satellite, such as the Tracking and Data Relay Satellite (TDRS) shown here, relays radio, television, and other signals between different points in space and on Earth. Image credit: NASA

the same time. These satellites photograph changing cloud formations. They also produce infrared images, which show the amount of heat coming from Earth and the clouds.

Communications satellites serve as relay stations, receiving radio signals from one location and transmitting them to another. A communications satellite can relay several television programs or many thousands of telephone calls at once. Communications satellites are usually put in a high altitude, geosynchronous orbit over a ground station. A ground station has a large dish antenna for transmitting and receiving radio signals. Sometimes, a group of low orbit communications satellites arranged in a network, called a constellation, work together by relaying information to each other and to users on the ground. Countries and commercial organizations, such as television broadcasters and telephone companies, use these satellites continuously.

Navigation satellites enable operators of aircraft, ships, and land vehicles anywhere on Earth to determine their locations with great accuracy. Hikers and other people on foot can also use the satellites for this purpose. The satellites send out radio signals that are picked up by a computerized receiver carried on a vehicle or held in the hand.

Navigation satellites operate in networks, and signals from a network can reach receivers anywhere on Earth. The receiver calculates its distance from at least three satellites whose signals it has received. It uses this information to determine its location.

Earth observing satellites are used to map and monitor our planet's resources and ever-changing chemical life cycles. They follow sun-synchronous, polar orbits. Under constant, consistent illumination from the sun, they take pictures in

different colors of visible light and non-visible radiation. Computers on Earth combine and analyze the pictures. Scientists use Earth observing satellites to locate mineral deposits, to determine the location and size of freshwater supplies, to identify sources of pollution and study its effects, and to detect the spread of disease in crops and forests.

Military satellites include weather, communications, navigation, and Earth observing satellites used for military purposes. Some military satellites -- often called

A navigation satellite, like this Global Positioning System (GPS) satellite, sends signals that operators of aircraft, ships, and land vehicles and people on foot can use to determine their location. Image credit: NASA

An Earth observing satellite surveys our planet's resources. This satellite, Aqua, helps scientists study ocean evaporation and other aspects of the movement and distribution of Earth's water. Image credit: NASA

"spy satellites" -- can detect the launch of missiles, the course of ships at sea, and the movement of military equipment on the ground.

The life and death of a satellite

Building a satellite

Every satellite carries special instruments that enable it to perform its mission. For example, a satellite that studies the universe has a telescope. A satellite that helps forecast the weather carries cameras to track the movement of clouds.

In addition to such mission-specific instruments, all satellites have basic subsystems, groups of devices that help the instruments work together and keep the satellite operating. For example, a power subsystem generates, stores, and distributes a satellite's electric power. This subsystem may include panels of solar cells that gather energy from the sun. Command and data handling subsystems consist of computers that gather and process data from the instruments and execute commands from Earth.

A satellite's instruments and subsystems are designed, built, and tested individually. Workers install them on the satellite one at a time until the satellite is complete. Then the satellite is tested under conditions like those that the satellite will encounter during launch and while in space. If the satellite passes all tests, it is ready to be launched.

Launching the satellite

Space shuttles carry some satellites into space, but most satellites are launched by rockets that fall into the ocean after their fuel is spent. Many satellites require minor adjustments of their orbit before they begin to perform their function. Built-in rockets called thrusters make these adjustments. Once a satellite is placed into a stable orbit, it can remain there for a long time without further adjustment.

Performing the mission

Most satellites operate are directed from a control center on Earth. Computers and human operators at the control center monitor the satellite's position, send instructions to its computers, and retrieve information that the satellite has gathered. The control center communicates with the satellite by radio. Ground stations within the satellite's range send and receive the radio signals.

A satellite does not usually receive constant direction from its control center. It is like an orbiting robot. It controls its solar panels to keep them pointed toward the sun and keeps its antennas ready to receive commands. Its instruments automatically collect information.

Satellites in a high altitude, geosynchronous orbit are always in contact with Earth. Ground stations can contact satellites in low orbits as often as 12 times a day. During each contact, the satellite transmits information and receives instructions. Each contact must be completed during the time the satellite passes overhead -- about 10 minutes.

If some part of a satellite breaks down, but the satellite remains capable of doing useful work, the satellite owner usually will continue to operate it. In some cases, ground controllers can repair or reprogram the satellite. In rare instances, space shuttle crews have retrieved and repaired satellites in space. If the satellite can no longer perform usefully and cannot be repaired or reprogrammed, operators from the control center will send a signal to shut it off.

Falling from orbit

A satellite remains in orbit until its velocity decreases and gravitational force pulls it down into a relatively dense part of the atmosphere. A satellite slows down due to occasional impact with air molecules in the upper atmosphere and the gentle pressure of the sun's energy. When the gravitational force pulls the satellite down far enough into the atmosphere, the satellite rapidly compresses the air in front of it. This air becomes so hot that most or all of the satellite burns up.

History

In 1955, the United States and the Soviet Union announced plans to launch artificial satellites. On Oct. 4, 1957, the Soviet Union launched Sputnik 1, the first artificial satellite. It circled Earth once every 96 minutes and transmitted radio signals that could be received on Earth. On Nov. 3, 1957, the Soviets launched a second satellite, Sputnik 2. It carried a dog named Laika, the first animal to soar in space. The United States launched its first satellite, Explorer 1, on Jan. 31, 1958, and its second, Vanguard 1, on March 17, 1958.

In August 1960, the United States launched the first communications satellite, Echo I. This satellite reflected radio signals back to Earth. In April 1960, the first weather satellite, Tiros I, sent pictures of clouds to Earth. The U.S. Navy developed the first navigation satellites. The Transit 1B navigation satellite first orbited in April 1960. By 1965, more than 100 satellites were being placed in orbit each year.

Since the 1970's, scientists have created new and more effective satellite instruments and have made use of computers and miniature electronic technology in satellite design and construction. In addition, more nations and some private businesses have begun to purchase and operate satellites. By the early 2000's, more than 40 countries owned satellites, and nearly 3,000 satellites were operating in orbit.

Aviation Aviation is a term that includes all the activities involved in building and flying aircraft, especially airplanes. The first successful airplane flights did not take place until 1903. Yet today, airplanes affect the lives of people almost everywhere. Giant airliners carry passengers and cargo between the world's major cities in a matter of hours. Planes and helicopters rush medicine and other supplies to the farthest islands and deepest jungles. Farmers use airplanes to seed fields, count livestock, and spray crops. Aviation has also changed the way nations make war. Modern warfare depends on the instant striking power of jet fighters and bombers and the rapid supply capabilities of jet transports. Helicopters and other special aircraft are also important in military aviation.

Hundreds of thousands of airplanes are used throughout the world. They range from small planes with room for only a pilot to enormous jumbo jets, which can carry hundreds of passengers. To produce and operate all these airplanes requires the skills of millions of workers in many countries -- from the engineers who design the planes to the mechanics and pilots who service and fly them. Many government agencies also work to make flying safer and more dependable. All these activities together make up the aviation industry. The industry's two major activities are (1) the manufacture of aircraft and aircraft components, such as engines, and (2) the operation of airlines. The manufacture of aircraft, together with the manufacture of spacecraft, missiles, and related electronic equipment, is often called the aerospace industry.

The aviation industry began on Dec. 17, 1903, near Kitty Hawk, North Carolina. That day, Orville and Wilbur Wright -- two brothers who operated a bicycle-manufacturing shop in Dayton, Ohio -- made the world's first successful piloted airplane flights. They had built their airplane after studying the writings of other aviation pioneers and after experimenting with gliders, kites, and wind tunnels.

Within a few years, several small factories in Europe and the United States were producing airplanes. Daredevil fliers bought many of these planes and used them to put on thrilling air shows. The governments of various countries also began to buy airplanes

to build small air forces. The daring feats of the early fliers and the development of military airplanes greatly encouraged the growth of the aviation industry.

By the late 1930's, airplanes had become an important means of transportation. Then, in the 1950's, engineers developed jet airliners -- and air travel grew at an even faster rate. In 1960, the world's airlines carried about 100 million passengers. By the early 2000's, they carried about 1 1/2 billion people annually.

Almost from the beginning of the aviation industry, the governments of most nations have been deeply involved in its activities. Airplanes have such great importance as weapons of war that many countries have encouraged and financed improvements in airplane design for military reasons. Most nations have also supported the development of civil aviation (the operation of nonmilitary aircraft).

Although aviation includes all types of heavier-than-air craft, this article deals chiefly with airplanes. To learn about the two other main types of heavier-than-air craft. The Airplane article traces the history of human efforts to fly and the development of the airplane. It also describes how a plane flies, how pilots navigate, and how planes are built.

This article discusses Aviation (The aviation industry) (Aviation agencies and organizations) (History of the aviation industry) (Careers in aviation).

The aviation industry

The aviation industry can be divided into five branches: (1) aircraft manufacturing, (2) general aviation activities, (3) airline operations, (4) airport operations, and (5) aviation support industries.

Aircraft manufacturing

Aircraft companies produce chiefly airplanes, but many also manufacture gliders, helicopters, and parts for spacecraft. Some parts factories and assembly plants are owned by conglomerates, enormous corporations that control a number of firms in largely unrelated fields. Most of the aircraft used around the world are manufactured in the United States.

The Russian aerospace industry produces aircraft and equipment for use throughout the former Eastern bloc -- that is, the former Soviet Union and its Eastern European allies. Russia also exports military aircraft to many other countries. British Aerospace is the United Kingdom's major manufacturer of aircraft. Europe's other leading aircraft manufacturing countries are France, Germany, Italy, and Spain. Other countries with important aerospace industries include Brazil, Canada, China, India, Israel, Japan, and South Africa. Many other nations have facilities for aircraft repair and maintenance.

Manufacturers produce three main types of airplanes: (1) general aviation planes, (2) commercial transport planes, and (3) military planes. General aviation activities range from business and personal flying to rescue services. Most general aviation planes are small propeller driven airplanes with one or two engines. Many businesses use jets. Commercial transport planes are large airplanes used to carry both passengers and cargo or cargo only. Airlines operate these planes. The smallest commercial transports carry from 20 to 100 passengers, and the largest, called jumbo jets or airbuses, carry several hundred. Most commercial transports are jet planes with two, three, or four engines. Military planes include bombers, fighters, and military transports owned by the governments of various countries and operated by their armed forces.

In some countries, the government wholly or partly owns some or all aircraft companies. All aircraft companies in the United States and some other countries are privately owned. But many depend heavily on government orders for military planes, engines, missiles, or spacecraft. Many U.S. manufacturers -- such as the Boeing Company, General Electric Company, and the Lockheed Martin, Northrop Grumman, and United Technologies corporations -- receive large government contracts.

A modern jet airliner costs millions of dollars to build. A small company cannot afford to build such a plane, and even large companies often have trouble acquiring the necessary funds. Many companies have merged (combined) to cut costs. These mergers have produced some of the world's largest aerospace companies, including Boeing, British Aerospace, and the European Aeronautic Defence and Space Company.

A number of European nations have cooperated in special aircraft-manufacturing projects. For example, the British and French governments formed a partnership called a consortium to share the cost of building a supersonic transport (SST), the Concorde. SST's were designed to carry passengers at speeds faster than that of sound.

General aviation activities include pleasure flying, land surveying, giving flying instructions, inspecting telephone lines, scattering seed, and spraying crops. Another important general aviation activity is using light planes to provide transportation. Most air taxi services, also called commuter airlines, use compact, twin-engine planes to carry passengers -- usually fewer than 20 -- on short flights. They serve small communities and provide connecting flights to large airports. Some air taxi services have planes large enough to carry more than 20 passengers. Some large airlines also provide air taxi service.

Many businesses have their own aircraft that are used to fly officials and salespeople to out-of-town assignments. General aviation planes also carry cargo and passengers in areas of the world that do not have highways or railroads.

In Australia, a specialized aviation service called the Royal Flying Doctor Service supplies medical treatment to people living in remote areas. People who are ill or require medical advice use radio to contact a doctor at the nearest base. The doctor may advise

the patient by radio or may arrange for a light plane to pick up the patient. Air ambulances in other parts of the world provide specially equipped airplanes to fly patients to hospitals.

Airline operations

Almost every country has at least one airline. In some countries, the government owns one or more airlines. For example, Alitalia, Italy's national airline, is mostly state-owned. During the 1990's, many governments encouraged privatization of airlines to curb costs and increase efficiency.

There are two main types of airline service -- scheduled flights and nonscheduled flights. Scheduled flights are made over certain routes according to a timetable. Nonscheduled flights are mainly charter flights for customers who want to hire a plane to fly to a particular place at a particular time.

In the United States, airlines must receive permission from the federal government to use commercial transport planes for scheduled flights. The airlines that the government approves for such flights are called certificated airlines. The term scheduled airlines is often used in the United States for the certificated airlines, though these lines may also make some nonscheduled flights. To receive government certification, an airline's planes and pilots must meet government standards.

Most airlines carry both passengers and cargo. Airliners usually carry a certain amount of freight on passenger flights. Many passenger airlines also operate transport planes that carry only cargo. A few certificated airlines in the United States specialize in carrying cargo and do not make any passenger flights.

Sometimes, airlines have financial problems due to low passenger traffic, debts from purchasing new aircraft, and increasing costs, such as the rising cost of jet fuel. In the 1970's, many airlines cut their airfares and developed various bargain ticket plans to attract passengers. These steps led to huge increases in passenger traffic. High operating costs led many small airlines to merge with larger airlines. By the late 1990's, many airlines had also formed alliances for ticketing and for scheduling certain routes.

In most European countries, the government has combined two or more airlines to form a large national airline. Various European airlines have also formed consortiums to help cut expenses. The members of an airline consortium cooperate in such matters as purchasing aircraft and training pilots.

Airport operations

Airports provide the fuel and the runways, navigation aids, and other ground facilities needed for air travel. Generally, only a few of a country's airports have the facilities to handle large passenger planes. Additional small airfields serve light planes or specialized

aircraft, such as helicopters or seaplanes. Cities or public corporations own most large airports. Most small airports are private airfields owned by organizations or individuals.

Aviation support industries provide a wide variety of supplies and services to airlines, airports, pilots, and passengers. Some companies furnish repair services or fuel for airplanes. Freight forwarders make arrangements for shipping air cargo. Various food services prepare meals to be served on passenger flights. Some insurance brokers specialize in flight insurance, and some lawyers specialize in air law. Private weather bureaus supply pilots with specialized information not provided by government weather services.

Aviation agencies and organizations

Aviation agencies Most countries have government agencies that enforce air safety regulations and handle various economic matters relating to aviation. In the United States, the Federal Aviation Administration (FAA) establishes the rules that all planes must follow when flying in the United States. One of the agency's most important jobs is to operate a network of air route traffic control centers throughout the United States and its territories. Each control center uses radar and radio communications to help planes in its vicinity follow the airways, also called air routes, to which they are assigned. The FAA also issues licenses to pilots. In addition, every newly manufactured airplane must be issued an FAA certificate of airworthiness before it may be flown. This certificate states that the airplane has been inspected and is in good flying condition.

Almost every U.S. state has an agency to regulate and improve aviation within its borders. These agencies handle airport construction, registration of airplanes and pilots, and similar matters. Many local governments also have aviation agencies. These agencies deal mainly with the operation and maintenance of local airports.

In Canada, the federal government regulates civil aviation. The director of civil aviation, under the supervision of the Department of Transport, deals mainly with such matters as registration of aircraft, licensing of pilots, and establishment of air navigation facilities. The Canadian Transport Commission handles the economic regulation of Canadian airlines.

Similar regulatory activities are carried out by national agencies in other countries. Such agencies include the United Kingdom's Civil Aviation Authority (CAA). These agencies are involved in such issues as air-traffic control and registration of airplanes and pilots.

The International Civil Aviation Organization is an agency of the United Nations (UN). Almost every country belongs to the ICAO. The organization sets up common air safety standards among member countries and tries to increase cooperation in other matters concerning international aviation.

Other aviation organizations include various groups that were formed to further their own special interests. These groups include airline operators, airplane manufacturers, and pilots. For example, U.S. and Canadian airline operators belong to the Air Transport Association of America. Operators of international airlines in countries throughout the world belong to the International Air Transport Association.

History of the aviation industry

Beginnings

The successful piloted flights of a powered airplane by Orville and Wilbur Wright in 1903 marked the beginning of the practical aviation industry. After these flights, the Wright brothers tried to sell the design for their plane to the U.S. and various European governments. But they had never made an official public flight, and government leaders were not convinced that their plane could fly.

Meanwhile, a few European inventors had also built airplanes. In the 1890's, the German glider pioneer Otto Lilienthal had manufactured a limited production series of special gliders for experimental use. In 1905, two French fliers, the brothers Charles and Gabriel Voisin, started the world's first airplane-manufacturing company. They began making a few made-to-order planes at a small factory outside Paris. Within a few years, other European fliers also started manufacturing companies. They included Louis Bleriot and the brothers Henri and Maurice Farman in France; and Frederick Handley Page, A. V. Roe, and T. O. M. Sopwith in the United Kingdom.

In 1907, Glenn H. Curtiss, an American flier and airplane designer, started the first airplane company in the United States, in Hammondsport, New York. Curtiss sold his first plane to the newly organized Aeronautic Society of New York for $5,000. This was the first sale of a commercial airplane in the United States.

The Wright brothers had made their first official public flight in 1908 and amazed the world with their airplane's flying ability. That same year, the U.S. Army Signal Corps ordered a specially built Wright plane, for which the government paid $30,000. This was the world's first military plane. In November 1909, a group of wealthy Americans lent the Wright brothers money to start a manufacturing firm, the Wright Company. The company had its factory in Dayton, Ohio, and its headquarters in New York City. In the autumn of 1909, a young automobile mechanic and salesman named Glenn L. Martin began to manufacture airplanes in an abandoned church in Santa Ana, California. Within a few years, his company became a leading U.S. producer of military planes.

The world's first great aviation meeting was held in 1909 near Reims, France. Manufacturers displayed 38 airplanes. Several of the planes on show were offered for sale to the public -- a sign of growing confidence in the airplane.

The first flying regulations

In 1905, a group of French flying enthusiasts established the Federation Aeronautique Internationale (FAI) in Paris. One of the FAI's main duties was to regulate the sport of flying. It also ruled on world speed, altitude, and other flying records. The FAI still has this function. The Aero Club of America was also founded in 1905. It regulated flying in the United States, sponsored exhibitions and races, and issued licenses to U.S. pilots.

In 1908, Kissimmee, Florida, passed the world's first law regulating airplanes. The law required the registration of local aircraft and regulated their speed and altitude when flying over the town. In 1911, Connecticut passed the first state law regulating aviation. The law required anyone who owned or operated an airplane within the state to register the plane and obtain a pilot's license.

World War I (1914-1918)

When World War I began in Europe, even the largest airplane factories turned out only a few planes a year. But the factories quickly increased their production to meet the demands of the warring nations. Airplane builders used newly designed engines to put fighters and bombers into the skies. Such well-known manufacturers as Farman, Handley Page, and Voisin built many of these planes. Other European manufacturers also became famous for their warplanes. They included Morane-Saulnier and Nieuport in France; Fokker and Junkers in Germany; and Bristol, de Havilland, Hawker, Short, and Vickers in the United Kingdom. By the end of the war, designers had created such aircraft as the British Vickers Vimy bomber and the American Curtiss NC-4, both of which flew across the Atlantic Ocean in 1919.

The United States entered the war in 1917 with about 110 military planes. The government immediately set a production goal of 29,000 airplanes a year. But the airplane companies had little or no experience with mass-production methods. The nation's automobile manufacturers, on the other hand, had developed assembly lines before the war and used them to turn out thousands of cars yearly. Various automakers helped set up assembly lines in the airplane factories.

The United States had no designs of its own for bombers or fighters. But American engineers designed a powerful airplane engine called the Liberty. Several U.S. companies began to mass-produce the United Kingdom's de Havilland D.H. 4 bombers and equip them with Liberty engines. The principal producer was the Dayton Wright Aeroplane Company, which was organized in 1917. Wilbur Wright had died of typhoid fever in 1912, and Orville sold his interest in the Wright Company to a group of investors in 1915. Although Orville had no financial interest in the Dayton Wright Company, he allowed the firm to use the Wright name in its title. The companies founded by Curtiss and Martin also became major producers of military planes during the war. Although U.S. factories did not meet their production goal of 29,000 planes a year, they had built almost 15,000 military planes by the end of the war.

In 1916, two airplane companies were established on the West Coast of the United States. They were the Boeing Company, founded in Seattle by William E. Boeing, and the Lockheed Corporation (now Lockheed Martin Corporation), founded in Santa Barbara, California, by the brothers Allan and Malcolm Loughead. The Boeing and Lockheed companies were too small to make many planes during the war. But in time, they became two of the nation's leading aircraft manufacturers.

The first airlines

The Wright brothers and other early fliers occasionally took passengers for short plane rides. In 1910, a Wright airplane flew 70 pounds (32 kilograms) of silk from Dayton to Columbus, Ohio -- perhaps the first air freight shipment in history. The world's first regular airplane passenger service began in the United States in 1914, but it lasted only a few months. A pilot named Tony Jannus used a small seaplane to fly passengers across Tampa Bay, between St. Petersburg and Tampa, Florida. On May 15, 1918, the U.S. government started the world's first permanent airmail service. Army pilots flew the mail between New York City, Philadelphia, and Washington, D.C.

After World War I, thousands of military planes became available for civilian use. In 1919, bombers were used to start nearly 20 small passenger airlines in France, Germany, the United Kingdom, and several other European countries. One of these airlines, founded by Henri and Maurice Farman, began the world's first regular international airline service. The company used old Farman bombers to make weekly passenger flights between Paris and Brussels, Belgium.

By 1924, passenger airlines were operating in 17 European countries as well as in Africa, Australia, and South America. Several of these airlines are still active. They include KLM Royal Dutch Airlines (now part of Air France-KLM) of the Netherlands, Germany's Lufthansa, and Australia's Queensland and Northern Territory Aerial Services (QANTAS). Beginning in the mid-1920's, the governments of many countries started to combine two or more private airlines to form a large national airline. In 1924, the United Kingdom became the first major power to form a national, government-owned airline, Imperial Airways.

Aviation progress

Many small passenger airlines were formed during the early 1920's. But most lasted only a few months because they could not attract enough customers. Most people considered flying a dangerous sport rather than a safe means of transportation.

In the United States, the federal government's main interest in aviation was to improve airmail service. In 1920, airmail routes extended from New York City to San Francisco. Mail planes operated only during the day, however. To help the mail pilots fly their open-cockpit planes at night, the government installed beacon lights at airports along the transcontinental route. Each light could be seen as far as 50 miles (80 kilometers) away.

By 1924, night-flying techniques enabled planes to get mail from New York City to San Francisco in 24 hours.

In 1925, the U.S. Congress passed the Kelly Air Mail Act, which gave private airlines the job of flying the mail. The government then signed contracts with 11 companies formed to carry the mail. Henry Ford, the famous automobile maker, owned one of these airlines. In 1926, Ford's airline became the first airline to carry U.S. mail. Within a few months, all 11 companies were flying mail between major U.S. cities. Some of the airlines also began carrying passengers. In 1926, airlines in the United States carried only about 6,000 passengers. In 1930, they carried more than 400,000.

Several U.S. aircraft companies were also started during the 1920's. In 1920, an engineer named Donald Douglas helped organize an aircraft company in Santa Monica, California. It became the Douglas Company the following year, later part of McDonnell Douglas Corporation, and later still part of the Boeing Company. In 1923, the Consolidated Aircraft Corporation was founded in East Greenwich, Rhode Island. It took over the airplane designs of the Dayton Wright Company. The Pratt and Whitney Company began making aircraft engines in Hartford, Connecticut, in 1925. In 1929, the Curtiss and Wright companies merged to form the Curtiss-Wright Corporation. Grumman Aircraft (now part of Northrop Grumman Corporation) also started business in 1929 on Long Island, New York.

The rapid increase in aviation activity led Congress to pass the Air Commerce Act in 1926. This act was the first federal law to regulate aviation in the United States. It provided for a system of airways and navigation aids across the country. The act also called for rules governing the manufacture of airplanes and the licensing of airplanes and pilots. A Bureau of Air Commerce was set up to carry out these measures.

The industry comes of age

Air transport continued to grow during the early 1930's. By 1935, the United States had four major domestic airlines -- American, Eastern, Transcontinental and Western Air (later called Trans World Airlines), and United. Smaller regional airlines included Braniff, Delta, and Northwest. The country also had a major international airline -- Pan American World Airways (Pan Am) -- which flew to Latin America. Many European governments continued to form large national airlines, such as Air France (now part of Air France-KLM) and Italy's Ala Littoria (now Alitalia).

To meet the growing demand for faster, larger airliners, manufacturers began to produce twin-engine planes, such as the Boeing 247 and the Douglas DC-3. The DC-3 appeared in 1935 and soon became the world's most popular transport plane. A number of companies, including Martin (now Lockheed Martin Corporation) in the United States and Short in the United Kingdom, started to make large, four-engine seaplanes called flying boats. In the 1930's, flying boats made the first passenger flights across oceans. New firms were

also started in the 1930's, such as North American Aviation and United Aircraft (now United Technologies), which took over production of Pratt and Whitney engines.

By the late 1930's, flying was an important means of travel in most of the world. In 1938, the world's airlines carried nearly 3 1/2 million passengers.

The rapid growth of civil aviation created a need for more effective government regulation. In 1938, the U.S. Congress established the Civil Aeronautics Authority to deal with every aspect of civil aviation. The authority included a five-member board, which, in 1940, became the Civil Aeronautics Board. It also included an administrative office, which became the Civil Aeronautics Administration (CAA) in 1940.

World War II (1939-1945)

The peace treaty that ended World War I prohibited the manufacture of military aircraft in Germany. Nevertheless, several German aircraft firms were founded during the 1920's. They included the famous Heinkel and Messerschmitt companies. In the mid-1930's, Heinkel, Messerschmitt, and other German firms, such as Dornier and Junkers, secretly made hundreds of bombers and fighters for the German air force. On Sept. 1, 1939, German dive bombers attacked Poland, and World War II began. One European country after another fell to the Germans. Finally, the United Kingdom was left nearly alone to fight off the German air force. British aircraft companies, such as Avro, de Havilland, Handley Page, Hawker, and Supermarine, quickly increased their production of warplanes.

The United States produced about 2,100 military planes in 1939. Both Germany and Japan had larger air forces. The huge Mitsubishi corporation produced many of Japan's warplanes, including the famous Zero fighter. After the United States entered the war in December 1941, U.S. airplane production increased greatly. More than 40 companies took part in a gigantic effort to supply the United States and its allies with military planes. Many companies enlarged their factories and hired additional workers. Assembly lines began working round the clock. By 1944, production had reached nearly 100,000 transport planes, bombers, and fighters a year.

By the end of the war, U.S. factories had built more than 300,000 aircraft. Germany, Japan, the Soviet Union, and the United Kingdom had also produced many thousands of planes. During the war, aircraft production had become the world's leading manufacturing industry.

A new age of flight

In 1937, British inventor Frank Whittle built the first successful jet engine. The first jet airplanes were developed for military use. Germany flew the first jet aircraft in 1939. By 1942, both the United Kingdom and the United States had developed experimental jet planes for military use.

After World War II, aircraft manufacturers began the development of jet airliners. In 1952, British Overseas Airways Corporation (BOAC), now British Airways, started jet passenger flights with de Havilland Comets. But the flights were stopped after several Comets exploded in the air. Investigators discovered serious flaws in the plane's structure. De Havilland engineers then designed an improved Comet. In 1958, BOAC used the new Comets to begin jet passenger service across the Atlantic Ocean. American companies also built successful jet transports in the late 1950's, and these aircraft quickly dominated international air transportation. In 1959, American Airlines used the first of these -- the Boeing 707 -- to start transcontinental jet service from New York City to Los Angeles.

The beginning of jet airline service created new challenges. Large jetliners carried nearly 200 passengers, and the crash of one of these planes could cause heavy loss of life. In addition, new hazards were created along the world's air routes as airplanes flew faster and in greater numbers than ever before. In 1958, the U.S. government combined the CAA and several other agencies to form the Federal Aviation Agency. The agency was given the job of establishing and enforcing air safety regulations and air traffic procedures in the United States. It was renamed the Federal Aviation Administration in 1967.

By 1970, jet transports had replaced propeller-driven planes on most major airlines. In 1970, Pan Am became the first airline to offer jumbo jet service, using Boeing 747's. France and the United Kingdom began passenger service with their SST, the Concorde, in 1976.

Industry mergers

Beginning in the 1950's, several large aerospace companies were formed by mergers. In 1954, the General Dynamics Corporation took control of Consolidated Vultee (Convair). In 1967, McDonnell Aircraft merged with Douglas Aircraft to form the McDonnell Douglas Corporation, and North American Aviation and Rockwell-Standard merged, forming the North American Rockwell Corporation. In 1973, this firm merged with Rockwell Manufacturing Company to become Rockwell International Corporation.

Internationalization became an important trend in the aviation industry beginning in the 1960's. The term refers to cooperative manufacturing programs in which firms from different nations share research, development, and production costs. The consortium formed by the British and French to build the Concorde SST was an early program of this type. Another successful program has been Airbus. This consortium, which manufactures commercial transport aircraft, has involved most countries in Western Europe.

United States aviation firms moved slowly into internationalization in the 1970's. Manufacturers in Canada, Italy, Japan, and the United Kingdom produced major parts of the McDonnell Douglas DC-10 transport, which began commercial service in 1971. Some U.S. firms have formed partnerships with foreign companies to manufacture

European-designed aircraft in the United States. For example, during the 1980's, McDonnell Douglas produced the British-designed Harrier -- a V/STOL (Vertical/Short Take-Off and Landing plane) -- in partnership with British Aerospace.

Airline safety concerns

Beginning in the 1960's, airliner hijacking, also called air piracy, became a serious problem. In 1970, hijackers throughout the world seized 49 airliners and forced the pilots to fly to destinations off their routes, often to other countries. In the 1980's, terrorist sabotage became a serious risk as several airliners were blown up in flight.

In response to the hijackings, aviation authorities tightened airport security regulations. These regulations include the inspection of aircraft, passengers, and baggage for hidden guns, bombs, or other weapons. Most countries have laws against hijacking and terrorism. But laws differ from country to country. The ICAO develops procedures to help member countries establish consistent methods to prevent and investigate hijackings.

Deregulation of the U.S. airlines

In the late 1970's, the Civil Aeronautics Board began to ease its controls over airline fares and routes in the United States to encourage greater competition and better service. In 1978, Congress passed the Airline Deregulation Act. This law provided for the gradual removal of economic controls of the airline industry. The Civil Aeronautics Board was dissolved in 1984. New airlines soon began to form, and existing ones rapidly expanded their services.

Recent developments

Deregulation in the United States allowed domestic airlines to compete in many international markets. Many U.S. airlines formed alliances with overseas carriers to simplify ticketing. Many U. S. airlines also developed hub and spoke systems. In such a system, many flights connect at a central airport. In manufacturing, several mergers in the 1990's led to the disappearance of historic U.S. airplane builders, such as McDonnell Douglas, which merged into Boeing. International partnerships became increasingly significant, with Airbus capturing one-third of the world market in jet airliner sales in the 1990's.

On Sept. 11, 2001, terrorists hijacked four commercial airplanes, deliberately crashing two into the towers of the World Trade Center in New York City and one into the Pentagon Building outside Washington, D.C. The fourth hijacked plane crashed in Somerset County, Pennsylvania. After the hijackings, U.S. airports and airlines sought new ways to protect against terrorist attacks. Congress passed legislation requiring federal employees to handle all passenger and baggage inspection in U.S. airports by the end of 2002. A newly created agency, the Transportation Security Administration, took over air safety functions from the FAA.

Fears of terrorism and a sluggish world economy contributed to a decline in air travel in the early 2000's. In 2003, British Airways and Air France discontinued all Concorde flights because the flights were no longer profitable.

Careers in aviation

The aviation industry employs many kinds of skilled workers. They include aeronautical engineers, computer specialists, electricians, flight attendants, flight engineers, flying instructors, mechanics, pilots, radar specialists, and radio operators. In the United States, many jobs in the aviation industry require certification from the FAA. For example, air traffic controllers, aviation mechanics, flight engineers, and pilots must have FAA certificates.

Some schools offer courses in preparation for such careers as aviation mechanic, computer specialist, and radio operator. Aeronautical engineering and some other highly skilled professions require a college education. Most pilots obtain their training at flying schools or in military service. Some high schools and colleges also offer courses in flying.

Jobs in general aviation

Many pilots work for air taxi services, business firms, and other organizations that use light planes. In many countries, flying light planes for commercial purposes requires a commercial pilot license. In the United States, the FAA issues these licenses to pilots 18 years old or over who have at least 200 hours of flying experience and who pass the physical, written, and flight examinations.

Jobs with airlines and airports

In most countries, airline pilots and copilots must obtain a special license. They must pass a thorough physical examination, as well as written and flight examinations. In the United States, airline pilots and copilots must have an FAA airline transport pilot license. To obtain this license, they must be at least 23 years old, and have a commercial pilot license and 1,500 hours of flight time.

Some airlines use flight engineers. On long flights, the engineers watch the many instruments in the cockpit that tell how the engines are operating. Most airlines require their flight engineers to have a commercial pilot license. Airlines prefer to hire flight attendants who have some college, business, or nursing training. Skilled mechanics are needed for airliner maintenance.

Jobs in the aircraft industry

Aircraft manufacturers hire electricians, machine tool operators, mechanics, and other skilled workers to make and assemble the many parts of airplanes. The industry also

employs various types of engineers to design aircraft and experienced pilots to test-fly planes.

Jobs with government agencies. Government agencies in many countries hire radar and radio operators to work at air route traffic control centers and airport control towers. They also hire mechanics and pilots to serve as safety agents. Many local aviation agencies also require engineers, mechanics, pilots, and other skilled people. Some large cities hire pilots to serve as flying police officers or to perform rescue services.

Constellation A constellation (KON stuh LAY shuhn) is a group of stars visible within a particular region of the night sky. The word constellation also refers to the region in which a specific group of stars appears. Astronomers have divided the sky into 88 areas, or constellations.

The ancient Greeks, Romans, and people of various other early civilizations observed groups of stars in the northern two-thirds of the sky. They named these groups of stars after animals and mythological characters. For example, the constellation Leo was named for a lion, Pisces for two fish, and Taurus for a bull. The constellations Andromeda, Cassiopeia, Orion, and Perseus are named for heroines and heroes in Greek mythology.

Between the early 1400's and the mid-1700's, European navigators explored the Southern Hemisphere and observed many constellations in the southernmost third of the sky. Mapmakers and explorers named these star groups for scientific instruments and other things as well as for animals. For example, the constellation Telescopium was named for the telescope. Musca was named for the fly, and Tucana for the toucan, a large-billed bird of Central and South America.

Some well-known groups of stars form only part of a constellation. Such smaller groups are called asterisms. For example, the Big Dipper is an asterism that lies in the constellation Ursa Major (Great Bear).

Some constellations can be seen only during certain seasons due to the earth's annual revolution around the sun. The part of the sky visible at night at a particular place gradually changes as the earth moves around the sun. Also, observers at different latitudes see different parts of the sky. An observer at the equator can view all the constellations during the course of a year, but an observer at the North or the South Pole can see only a single hemisphere of constellations.

Extraterrestrial Intelligence Extraterrestrial (EHKS truh tuh REHS tree uhl) intelligence is intelligent life that developed somewhere other than the Earth. No life has been discovered on any planet other than the Earth. However, many scientists have concluded that intelligent life may exist on planets orbiting some of the hundreds of billions of stars in our galaxy, the Milky Way. These scientists base their conclusion on research in such fields as astronomy, biology, planetary science, and paleontology (the study of prehistoric life through fossils). The effort to find evidence that there is extraterrestrial intelligence is often called SETI, which stands for Search for Extraterrestrial Intelligence.

SETI researchers believe that the best way to discover other intelligent life in the galaxy is to look for evidence of technology developed by that life. In the belief that intelligent beings on other worlds would eventually develop radio technology, researchers have used large radio telescopes to search the sky. In 1960, the first SETI experiment unsuccessfully examined two stars at a single radio frequency. After several dozen additional searches, the National Aeronautics and Space Administration (NASA) in 1992 began a two-part project known as the High Resolution Microwave Survey. Researchers searched for weak microwave (short radio wave) signals originating near specific stars that are similar to the sun. They also started to scan the entire sky for strong microwave signals. In 1993, the United States Congress, in a budget-cutting measure, instructed NASA to end the project. SETI research continues in the United States under private support.

In 1998, astronomers began to search for pulses of laser light. The astronomers reasoned that intelligent beings on a planet orbiting a distant star might have developed powerful lasers. The beings might have transmitted brief pulses of laser light into space as a signal to observers on other planets. They would have used pulses so that the observers could distinguish the laser light from the bright, steady light coming from their star. Astronomers on the earth would be able to distinguish powerful pulses that were a few billionths of a second in duration.

Gravitation 

Gravitation is the force of attraction that acts between all objects because of their mass. An object's mass is its amount of matter. Because of gravitation, an object that is near Earth falls toward the surface of the planet. An object that is already on the surface experiences a downward force due to gravitation. We experience this force on our bodies as our weight. Gravitation holds together the hot gases that make up the sun, and it keeps the planets in their orbits around the sun. Another term for gravitation is the force of gravity.

People misunderstood gravitation for centuries. In the 300's B.C., the Greek philosopher and scientist Aristotle taught the incorrect idea that heavy objects fall faster than light objects. People accepted that idea until the early 1600's, when the Italian scientist Galileo corrected it. Galileo said that all objects fall with the same acceleration unless air resistance or some other force acts on them. An object's acceleration is the rate of change of its velocity (speed in a particular direction). Thus, a heavy object and a light object that are dropped from the same height will reach the ground at the same time.

Newton's law of gravitation

Ancient astronomers measured the movements of the moon and planets across the sky. However, no one correctly explained those motions until the late 1600's. At that time, the English scientist Isaac Newton described a connection between the movements of the celestial bodies and the gravitation that attracts objects to Earth.

In 1665, when Newton was 23 years old, a falling apple caused him to question how far the force of gravity reaches. Newton explained his discovery in 1687 in a work called Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy). Using laws of planetary motion discovered by the German astronomer Johannes Kepler, Newton showed how the sun's force of gravity must decrease with the distance from the sun. He then assumed that Earth's gravitation decreases in the same

way with the distance from Earth. Newton knew that Earth's gravitation holds the moon in its orbit around Earth, and he calculated the strength of Earth's gravitation at the distance of the moon. Using his assumption, he calculated what the strength of that gravitation would be at Earth's surface. The calculated result was the same as the strength of the gravitation that would accelerate an apple.

Newton's law of gravitation says that the gravitational force between two objects is directly proportional to their masses. That is, the larger either mass is, the larger is the force between the two objects. The law also says that the gravitational force between two objects is inversely (oppositely) proportional to the distance between the two objects squared (multiplied by itself). For example, if the distance between the two objects doubles, the force between them becomes one-fourth of its original strength. Newton's law is given by the equation F = m1m2 / d 2, where F is the gravitational force between two objects, m1 and m2 are the masses of the objects, and d2 is the distance between them squared.

Until the early 1900's, scientists had observed only one movement that could not be described mathematically using Newton's law -- a tiny variation in the orbit of the planet Mercury around the sun. Mercury's orbit -- like the orbits of the other planets -- is an ellipse, a geometric figure with the shape of a flattened hoop. The sun is not at the exact center of the ellipse. So one point in each orbit is closer to the sun than all other points in that orbit. But the location of the closest point changes slightly each time Mercury revolves around the sun. Astronomers refer to that variation as a precession.

Scientists used Newton's law to calculate the precession. The calculated amount differed slightly from the observed amount.

Einstein's theory of gravitation

In 1915, the German-born physicist Albert Einstein announced his theory of space, time, and gravitation, the general theory of relativity. Einstein's theory completely changed scientists' way of thinking about gravitation. However, it expanded upon Newton's law, rather than contradicting it.

In many cases, Einstein's theory produced results that differed only slightly from results based on Newton's law. For example, when Einstein used his theory to calculate the precession of Mercury's orbit, the result agreed exactly with the observed motion. That agreement was the first confirmation of Einstein's theory.

Einstein based his theory on two assumptions. The first is related to an entity known as space-time, and the second is a rule known as the principle of equivalence.

Space-time

In the complex mathematics of relativity, time and space are not absolutely separate. Instead, physicists refer to space-time, a combination of time and the three dimensions of space -- length, width, and height. Einstein assumed that matter and energy can distort (change the shape of) space-time, curving it; and that gravitation is an effect of the curvature.

The principle of equivalence states that the effects of gravity are equivalent to the effects of acceleration. To understand this principle, suppose you were in a rocket ship so far from any planet, star, or other celestial object that the ship experienced virtually no gravitation. Imagine that the ship was moving forward, but not accelerating -- in other words, that the ship was traveling at a constant speed and in a constant direction. If you held out a ball and released it, the ball would not fall. Instead, it would hover beside you.

But suppose the rocket accelerated by increasing its speed. The ball would appear to fall toward the rear of the ship exactly as if gravity had acted upon it.

Predictions of general relativity

In the years since the calculation of Mercury's precession confirmed Einstein's theory, several observations have verified predictions made with the theory. Some examples include predictions of the bending of light rays and radio waves, the existence of gravity waves and black holes, and the expansion of the universe.

Bending of light rays

Einstein's theory predicts that gravity will bend the path of a light ray as the ray passes near a massive body. The bending will occur because the body will curve space-time. The sun is massive enough to bend rays by an observable amount, and scientists first confirmed this prediction during a total eclipse of the sun in 1919.

Bending and slowing of radio waves

The theory also predicts that the sun will bend radio waves and slow them down. Scientists have measured the sun's bending of radio waves emitted (sent out) by quasars, extremely powerful objects at the centers of some galaxies. The measurements agree well with the prediction.

Researchers measured a delay of radio waves that pass near the sun by sending signals between Earth and the Viking space probes that reached Mars in 1976. Those measurements still represent one of the most precise confirmations of general relativity.

Gravitational waves

General relativity also indicates that massive bodies in orbit around each other will emit waves of energy known as gravitational waves. Since 1974, scientists have confirmed the

existence of gravitational waves indirectly by observing an object known as a binary pulsar. A binary pulsar is a rapidly rotating neutron star that orbits a similar, but unobserved, companion star. A neutron star consists mostly of tightly packed neutrons, particles that ordinarily occur only in the nuclei of atoms.

A pulsar emits two steady beams of radio waves that flow away in opposite directions. As the star rotates on its own axis, the beams sweep around in space like searchlight beams. If one of the radio beams periodically sweeps over Earth, a radio telescope can detect the beam as a series of pulses. By closely observing changes in the pulse rate of a binary pulsar, scientists can determine the pulsar's orbital period -- the time it takes the two stars to completely orbit each other.

Observations of the binary pulsar called PSR 1913 + 16 indicate that its orbital period is decreasing, and astronomers have measured the amount of the decrease. Scientists have also used equations of general relativity to calculate the amount by which the orbital period would decrease if the binary pulsar was radiating away energy as gravitational waves. The calculated amount agrees with the measured amount.

In addition, the pulsar's orbit precesses as the pulsar revolves around the companion star. General relativity predicts the precession rate, and measurements match the prediction with great precision.

Black holes

Einstein's theory predicts the existence of objects called black holes. A black hole is a region of space whose gravitational force is so strong that not even light can escape from it. Researchers have found strong evidence that most very massive stars eventually evolve into black holes, and that most large galaxies have a gigantic black hole at their centers.

Expansion of the universe

In a paper published in 1917, Einstein applied general relativity to cosmology, the study of the universe as a whole. The theory showed that the universe must either expand or contract. In 1917, however, scientists had not yet found any evidence of expansion or contraction. To prevent his theory from disagreeing with the available evidence, Einstein added a term, the cosmological constant, to the theory. That term represented a repulsion (pushing away) of every point in space by the surrounding points, preventing contraction.

But in 1929, the American astronomer Edwin Hubble discovered that distant galaxies are moving away from Earth and that, the more distant a galaxy, the more rapidly it is moving away. Hubble's discovery indicated that the universe is expanding. In response to Hubble's discovery and confirming observations by other astronomers, Einstein abandoned the cosmological constant, calling it his greatest blunder.

The discovery of the expansion of the universe, together with other observations, led to the development of the big bang theory of the origin of the universe. According to that theory, the universe began with a hot, explosive event -- a "big bang." At the beginning of the event, all the matter in the part of the universe we can see was smaller than a marble. Matter then expanded rapidly, and it is still expanding.

Dark energy

Although Einstein called the cosmological constant his greatest blunder, it may turn out to be one of his greatest achievements. Measurements reported in 1998 suggest that the universe is expanding more and more rapidly. Furthermore, the rate of expansion has been increasing as predicted by general relativity with a cosmological constant.

Until the measurements were reported, astronomers generally thought that the rate of expansion was decreasing due to the gravitational attraction of galaxies for one another. The measurements showed that exploding stars known as supernovae in distant galaxies were dimmer than expected and that the galaxies therefore were farther away then expected. But the galaxies could be so far away only if the rate of expansion had begun to increase in the past.

Astronomers have concluded that the increase in the expansion rate is due to an entity that presently opposes gravitation. That entity could be a cosmological constant or something much like it called dark energy. Scientists have not yet developed theories to account for the existence of dark energy, but they know how much of it probably exists. The amount of dark energy in the universe is about twice as much as the amount of matter.

The matter in the universe includes both visible matter and a mysterious substance known as dark matter. Scientists do not know the composition of dark matter. But measurements of the motion of stars and gas clouds in galaxies have led scientists to believe that it exists. Those measurements show that the masses of galaxies are many times larger than the masses of the visible objects in them. These and other observations suggest that the universe has at least 30 times as much dark matter as visible matter.

Gravitation and the age of the universe

Other observations have helped show that the theory of general relativity applies to the whole universe. Cosmologists have calculated the age of the universe using equations of general relativity, the measured rate of expansion of the universe, and estimates of the amounts of dark energy and dark matter. The calculated age, about 14 billion years, agrees well with results determined by two methods that do not involve general relativity: (1) calculations based on the evolution of stars and (2) the radioactive dating of old stars.

Stellar evolution

As a star evolves, its surface temperature and its brightness change in a well-understood way. Astronomers can determine the ages of certain stars by measuring their temperature and brightness, then performing calculations based on their knowledge of stellar evolution. By means of such techniques, astronomers have found stars that may be about 13 billion years old -- but no stars that are clearly older than that.

Radioactive dating of stars is based on the fact that certain chemical elements undergo radioactive decay. In radioactive decay, an isotope (form) of an element turns into an isotope of another element. Radioactive isotopes decay at known rates.

In 2001, scientists working with the European Southern Observatory's Very Large Telescope in Chile applied the radioactive dating technique to an old star in our galaxy, the Milky Way. The researchers studied the isotope uranium 238, whose nucleus contains 92 protons and 146 neutrons. The scientists knew how much uranium the star must have had when it formed, and they measured how much it has now. They then applied their knowledge of decay rates to calculate the age of the star. The most likely age of the star is 12.5 billion years, so the universe is probably older than that. Measurements of the ages of many old stars using another element, thorium, gave similar results.

Christiaan Huygens 

Christiaan Huygens, (HY guhnz), (1629-1695), was a Dutch physicist, astronomer, and mathematician. In 1678, Huygens proposed that light consists of series of waves. He used this theory in investigating the refraction (bending) of light.

Huygens's wave theory competed for many years with the corpuscular theory of the English scientist Isaac Newton. Newton maintained that light is made up of particles. Today, scientists believe that light behaves as both a particle and a wave.

Huygens was born on April 14, 1629, in The Hague, the Netherlands. He studied mathematics and law at the University of Leiden and the College of Orange at Breda. Huygens worked with his brother Constantijn to develop skill in grinding and polishing spherical lenses. With these lenses, they built the most powerful telescopes of their time. Huygens also discovered Saturn's moon Titan and asserted that what astronomers called "Saturn's arms" was a ring. In mathematics, he refined the value of pi . In the 1650's, Huygens invented a clock with a freely suspended pendulum. He died on July 8, 1695.

The European Space Agency honored Huygens's discovery of Titan by naming a space probe after him. The Huygens probe, designed to drop through Titan's atmosphere, was launched aboard the Cassini spacecraft in 1997.

Mars 

Mars is the fourth planet from the sun. The planet is one of Earth's "next-door neighbors" in space. Earth is the third planet from the sun, and Jupiter is the fifth. Like Earth, Jupiter, the sun, and the remainder of the solar system, Mars is about 4.6 billion years old.

Mars is named for the ancient Roman god of war. The Romans copied the Greeks in naming the planet for a war god; the Greeks called the planet Ares (AIR eez). The Romans and Greeks associated the planet with war because its color resembles the color of blood. Viewed from Earth, Mars is a bright reddish-orange. It owes its color to iron-rich minerals in its soil. This color is also similar to the color of rust, which is composed of iron and oxygen.

Scientists have observed Mars through telescopes based on Earth and in space. Space probes have carried telescopes and other instruments to Mars. Early probes were designed to observe the planet as they flew past it. Later, spacecraft orbited Mars and even landed there. But no human being has ever set foot on Mars.

Scientists have found strong evidence that water once flowed on the surface of Mars. The evidence includes channels, valleys, and gullies on the planet's surface. If this interpretation of the evidence is correct, water may still lie in cracks and pores in subsurface rock. A space probe has also discovered vast amounts of ice beneath the surface, most of it near the south pole.

The planet Mars, like Earth, has clouds in its atmosphere and a deposit of ice at its north pole. But unlike Earth, Mars has no liquid water on its surface. The rustlike color of Mars comes from the large amount of iron in the planet's soil. Image credit: NASA/JPL/Malin Space Science Systems

In addition, a group of researchers has claimed to have found evidence that living things once dwelled on Mars. That evidence consists of certain materials in meteorites found on Earth. But the group's interpretation of the evidence has not convinced most scientists.

The Martian surface has many spectacular features, including a canyon system that is much deeper and much longer than the Grand Canyon in the United States. Mars also has mountains that are much higher than Mount Everest, Earth's highest peak.

Above the surface of Mars lies an atmosphere that is about 100 times less dense than the atmosphere of Earth. But the Martian atmosphere is dense enough to support a weather system that includes clouds and winds. Tremendous dust storms sometimes rage over the entire planet.

Mars is much colder than Earth. Temperatures at the Martian surface vary from as low as about -195 degrees F (-125 degrees C) near the poles during the winter to as much as 70 degrees F (20 degrees C) at midday near the equator. The average temperature on Mars is about -80 degrees F (-60 degrees C).

Mars is so different from Earth mostly because Mars is much farther from the sun and much smaller than Earth. The average distance from Mars to the sun is about 141,620,000 miles (227,920,000 kilometers). This distance is roughly 1 1/2 times the distance from Earth to the sun. The average radius (distance from its center to its surface) of Mars is 2,107 miles (3,390 kilometers), about half the radius of Earth.

Characteristics of Mars

Orbit and rotation

Like the other planets in the solar system, Mars travels around the sun in an elliptical (oval) orbit. But the orbit of Mars is slightly more "stretched out" than the orbits of Earth and most of the other planets. The distance from Mars to the sun can be as little as about 128,390,000 miles (206,620,000 kilometers) or as much as about 154,860,000 miles (249,230,000 kilometers). Mars travels around the sun once every 687 Earth days; this is the length of the Martian year.

The distance between Earth and Mars depends on the positions of the two planets in their orbits. It can be as small as about 33,900,000 miles (54,500,000 kilometers) or as large as about 249,000,000 miles (401,300,000 kilometers).

A sunset on Mars creates a glow due to the presence of tiny dust particles in the atmosphere. This photo is a combination of four images taken by Mars Pathfinder, which landed on Mars in 1997. Image credit: NASA/JPL

Like Earth, Mars rotates on its axis from west to east. The Martian solar day is 24 hours 39 minutes 35 seconds long. This is the length of time that Mars takes to turn around once with respect to the sun. The Earth day of 24 hours is also a solar day.

The axis of Mars is not perpendicular to the planet's orbital plane, an imaginary plane that includes all points in the orbit. Rather, the axis is tilted from the perpendicular position. The angle of the tilt, called the planet's obliquity, is 25.19¡ for Mars, compared with 23.45¡ for Earth. The obliquity of Mars, like that of Earth, causes the amount of sunlight falling on certain parts of the planet to vary widely during the year. As a result, Mars, like Earth, has seasons.

Mass and density

Mars has a mass (amount of matter) of 7.08 X 1020 tons (6.42 X 1020 metric tons). The latter number would be written out as 642 followed by 18 zeroes. Earth is about 10 times as massive as Mars. Mars's density (mass divided by volume) is about 3.933 grams per cubic centimeter. This is roughly 70 percent of the density of Earth.

Gravitational force

Because Mars is so much smaller and less dense than Earth, the force due to gravity at the Martian surface is only about 38 percent of that on Earth. Thus, a person standing on Mars would feel as if his or her weight had decreased by 62 percent. And if that person dropped a rock, the rock would fall to the surface more slowly than the same rock would fall to Earth.

Physical features of Mars

Scientists do not yet know much about the interior of Mars. A good method of study would be to place a network of motion sensors called seismometers on the surface. Those instruments would measure tiny movements of the surface, and scientists would use the measurements to learn what lies beneath. Researchers commonly use this technique to study Earth's interior.

Scientists have four main sources of information on the interior of Mars: (1) calculations involving the planet's mass, density, gravity, and rotational properties; (2) knowledge of other planets; (3) analysis of Martian meteorites that fall to Earth; and (4) data gathered by orbiting space probes. They think that Mars probably has three main layers, as Earth has: (1) a crust of rock, (2) a mantle of denser rock beneath the crust, and (3) a core made mostly of iron.

Crust

Scientists suspect that the average thickness of the Martian crust is about 30 miles (50 kilometers). Most of the northern hemisphere lies at a lower elevation than the southern

hemisphere. Thus, the crust may be thinner in the north than in the south.

Much of the crust is probably composed of a volcanic rock called basalt (buh SAWLT). Basalt is also common in the crusts of Earth and the moon. Some Martian crustal rocks, particularly in the northern hemisphere, may be a form of andesite. Andesite is also a volcanic rock found on Earth, but it contains more silica than basalt does. Silica is a compound of silicon and oxygen.

Mantle

The mantle of Mars is probably similar in composition to Earth's mantle. Most of Earth's mantle rock is peridotite (PEHR uh DOH tyt), which is made up chiefly of silicon, oxygen, iron, and magnesium. The most abundant mineral in peridotite is olivine (OL uh veen).

The main source of heat inside Mars must be the same as that inside Earth: radioactive decay, the breakup of the nuclei of atoms of elements such as uranium, potassium, and thorium. Due to radioactive heating, the average temperature of the Martian mantle may be roughly 2700 degrees F (1500 degrees C).

Core

Mars probably has a core composed of iron, nickel, and sulfur. The density of Mars gives some indication of the size of the core. Mars is much less dense than Earth. Therefore, the radius of Mars's core relative to the overall radius of Mars must be smaller than the radius of Earth's core relative to the overall radius of Earth. The radius of the Martian core is probably between 900 and 1,200 miles (1,500 and 2,000 kilometers).

Unlike Earth's core, which is partially molten (melted), the core of Mars probably is solid. Scientists suspect that the core is solid because Mars does not have a significant magnetic field. A magnetic field is an influence that a magnetic object creates in the region around it. Motion within a planet's molten core makes the core a magnetic object. The motion occurs due to the rotation of the planet.

Data from Mars Global Surveyor show that some of the planet's oldest rocks formed in the presence of a strong magnetic field. Thus, in the distant past, Mars may have had a hotter interior and a molten core.

Surface features

The surface of Mars was sampled for signs of life by the Viking 2 lander in 1976. A mechanical sampling arm dug the grooves near the round rock at the lower left. The cylinder at the right covered the sampling device and was ejected after landing. The cylinder is about 12 inches (30 centimeters) long. Image credit: NASA/National Space Science Data Center

Mars has many of the kinds of surface features that are common on Earth. These include plains, canyons, volcanoes, valleys, gullies, and polar ice. But craters occur throughout the surface of Mars, while they are rare on Earth. In addition, fine-grained reddish dust covers almost all the Martian surface.

Plains

Many regions of Mars consist of flat, low-lying plains. Most of these areas are in the northern hemisphere. The lowest of the northern regions are among the flattest, smoothest places in the solar system. They may be so smooth because they were built up from deposits of sediment (tiny particles that settle to the bottom of a liquid). There is ample evidence that water once flowed across the Martian surface. The water would have tended to collect in the lowest spots on the planet and thus would have deposited sediments there.

Canyons

Along the equator lies one of the most striking features on the planet, a system of canyons known as the Valles Marineris. The name is Latin for Valleys of Mariner; a space probe called Mariner 9 discovered the canyons in 1971. The canyons run roughly east-west for about 2,500 miles (4,000 kilometers), which is close to the width of Australia or the distance from Philadelphia to San Diego. Scientists believe that the Valles Marineris formed mostly by rifting, a splitting of the crust due to being stretched.

Individual canyons of the Valles Marineris are as much as 60 miles (100 kilometers) wide. The canyons merge in the central part of the system, in a region that is as much as 370 miles (600 kilometers) wide. The depth of the canyons is enormous, reaching 5 to 6 miles (8 to 10 kilometers) in some places.

Large channels emerge from the eastern end of the canyons, and some parts of the canyons have layered sediments. The channels and sediments indicate that the canyons may once have been partly filled with water.

Volcanoes

Mars has the largest volcanoes in the solar system. The tallest one, Olympus Mons (Latin for Mount Olympus), rises 17 miles (27 kilometers) above the surrounding plains. It is about 370 miles (600 kilometers) in diameter. Three other large volcanoes, called Arsia Mons, Ascraeus Mons, and Pavonis Mons, sit atop a broad uplifted region called Tharsis.

The Valles Marineris system of valleys is about 2,500 miles (4,000 kilometers) long -- roughly one-fifth the distance around the planet Mars. Parts of the system are 6 miles (10 kilometers) deep. Image credit: NASA/National Space Science Data Center

All these volcanoes have slopes that rise gradually, much like the slopes of Hawaiian volcanoes. Both the Martian and Hawaiian volcanoes are shield volcanoes. They formed from eruptions of lavas that can flow for long distances before solidifying.

Mars also has many other types of volcanic landforms. These range from small, steep-sided cones to enormous plains covered in solidified lava. Scientists do not know how recently the last volcano erupted on Mars -- some minor eruptions may still occur.

Craters and impact basins

Many meteoroids have struck Mars over its history, producing impact craters. Impact craters are rare on Earth for two reasons: (1) Those that formed early in the planet's history have eroded away, and (2) Earth developed a dense atmosphere, preventing meteorites that could have formed craters from reaching the planet's surface.

Martian craters are similar to craters on Earth's moon, the planet Mercury, and other objects in the solar systems. The craters have deep, bowl-shaped floors and raised rims. Large craters can also have central peaks that form when the crater floor rebounds upward after an impact.

On Mars, the number of craters varies dramatically from place to place. Much of the surface of the southern hemisphere is extremely old, and so has many craters. Other parts of the surface, especially in the northern hemisphere, are younger and thus have fewer craters.

Some volcanoes have few craters, indicating that they erupted recently. The lava from the volcanoes would have covered any craters that existed at the time of the eruptions. And not enough time has passed since the eruptions for many new craters to form.

Some of the impact craters have unusual-looking deposits of ejecta, material thrown out of the craters at impact. These deposits resemble mudflows that have solidified. This appearance suggests that the impacting bodies may have encountered water or ice beneath the ground.

Mars has a few large impact craters. The largest is Hellas Planitia in the southern hemisphere. Planitia is a Latin word that can mean low plain or basin; Hellas Planitia is also known as the Hellas impact basin. The crater has a diameter of about 1,400 miles (2,300 kilometers). The crater floor is about 5.5 miles (9 kilometers) lower than the surrounding plain.

Channels, valleys, and gullies occur in many regions of Mars, apparently as a result of water

Channels in a Martian crater, in an image taken in 2000 by the Mars Global Surveyor, suggest to scientists that liquid water may have flowed across the surface of Mars in recent times. Image credit: NASA

erosion. The most striking of these features are known as outflow channels. These channels can be as wide as 60 miles (100 kilometers) and as long as 1,200 miles (2,000 kilometers). They appear to have been carved by enormous floods that rushed across the surface. In many cases, the water seems to have escaped suddenly from underground.

Many of the channels do not look like river systems on Earth, with the main river formed from smaller rivers and streams. Rather, those Martian channels arise fully formed from low-lying areas.

Other regions of Mars have much smaller features called valley networks. These networks look more like river systems on Earth. Martian valley networks are up to a few miles or kilometers wide and up to a few hundred miles or kilometers long. The networks are mostly ancient features. They suggest that the Martian climate may once have been warm enough to enable water to exist as a liquid.

The gullies are smaller still. Most of them lie at high latitudes. They may be a result of a leakage of a small amount of ground water to the surface within the past few million years.

Polar deposits

The most interesting features in the polar regions of Mars are thick stacks of finely layered deposits of material. Scientists believe that the layers consist of mixtures of water ice and dust. The deposits extend from the poles to latitudes of about 80 degrees in both hemispheres.

The atmosphere probably deposited the layers over long periods. The layers may provide evidence of seasonal weather activity and long-term changes in the Martian climate. One possible cause of climate changes is variation in the planet's obliquity. This variation alters the amount of sunlight falling on different parts of Mars. The variation in sunlight, in turn, may change the climate. Past climate changes could have affected the rate at which the atmosphere deposited dust and ice into layers.

Lying atop much of the layered deposits in both hemispheres are caps of water ice that remain frozen all year. The layers and overlying caps are several miles or kilometers thick.

In the wintertime, additional seasonal caps form from layers of frost. The seasonal caps are clearly visible through Earth-based telescopes. The frost consists of solid carbon dioxide (CO2) -- also known as "dry ice" -- that has condensed from CO2 gas in the atmosphere. In the deepest part of the winter, the frost extends from the poles to latitudes as low as 45 degrees -- halfway to the equator.

Atmosphere

The atmosphere of Mars contains much less oxygen (O2) than that of Earth. The O2 content of the Martian atmosphere is only 0.13 percent, compared with 21 percent in Earth's atmosphere. Carbon dioxide makes up 95.3 percent of the gas in the atmosphere of Mars. Other gases include nitrogen (N2), 2.7 percent; argon (Ar), 1.6 percent; carbon monoxide (CO), 0.07 percent; and water vapor (H2O), 0.03 percent.

Pressure

At the surface of Mars, the atmospheric pressure is typically only about 0.10 pound per square inch (0.7 kilopascal). This is roughly 0.7 percent of the atmospheric pressure at Earth's surface. When the seasons change on Mars, the atmospheric pressure at the surface there varies by 20 to 30 percent.

Each winter, the condensation of CO2 at the poles removes much gas from the atmosphere. When this happens, the atmospheric pressure due to CO2 gas decreases sharply. The opposite process occurs each summer. In addition, the atmospheric pressure varies as the weather changes during the day, much as on Earth.

Temperature

The atmosphere of Mars is coldest at high altitudes, from about 40 to 78 miles (65 to 125 kilometers) above the surface. At those altitudes, typical temperatures are below -200 degrees F (-130 degrees C). The temperature increases toward the surface, where daytime temperatures of -20 to -40 degrees F (-30 to -40 degrees C) are typical. In the lowest few miles or kilometers of the atmosphere, the temperature varies widely during the day. It can reach -150 degrees F (-100 degrees C) late at night, even near the equator.

Atmospheric temperatures can be warmer than normal when the atmosphere contains much dust. The dust absorbs sunlight and then transfers much of the resulting heat to the atmospheric gases.

Clouds

In the Martian atmosphere, thin clouds made up of particles of frozen CO2 can form at high altitudes. In addition, clouds, haze, and fog composed of particles of water ice are common. Haze and fog are especially frequent in the early morning. At that time, temperatures are the lowest, and water vapor is therefore most likely to condense.

Wind

The Martian atmosphere, like that of Earth, has a general circulation, a wind pattern that occurs over the entire planet. Scientists have studied the global wind patterns of Mars by observing the motions of clouds and changes in the appearance of wind-blown dust and sand on the surface.

Global-scale winds occur on Mars as a result of the same process that produces such winds on Earth. The sun heats the atmosphere more at low latitudes than at high latitudes. At low latitudes, the warm air rises, and cooler air flows in along the surface to take its place. The warm air then travels toward the cooler regions at higher latitudes. At the higher latitudes, the cooler air sinks, then travels toward the equator.

On Mars, the condensation and evaporation of CO2 at the poles influence the general circulation. When winter begins, atmospheric CO2 condenses at the poles, and more CO2 flows toward the poles to take its place. When spring arrives, CO2 frost evaporates, and the resulting gas flows away from the poles.

Surface winds on Mars are mostly gentle, with typical speeds of about 6 miles (10 kilometers) per hour. Scientists have observed wind gusts as high as 55 miles (90 kilometers) per hour. However, the gusts exert much less force than do equally fast winds on Earth. The winds of Mars have less force because of the lower density of the Martian atmosphere.

Dust storms

Some of the most spectacular weather occurs on Mars when dust blows in the wind. Small, swirling winds can lift dust off the surface for brief intervals. These winds create dust devils, tiny storms that look like tornadoes.

Large dust storms begin when wind lifts dust into the atmosphere. The dust then absorbs sunlight, warming the air around it. As the warmed air rises, more winds occur, lifting still more dust. As a result, the storm becomes stronger.

At larger scales, dust storms can blanket areas from more than 200 miles (320 kilometers) to a few thousand miles or kilometers across. The largest storms can cover the entire surface of Mars. Storms of that size are unusual, but they can last for months. The strongest storms can block almost the entire surface from view. Such storms occurred in 1971 and 2001.

Dust storms are most common when Mars is closest to the Sun. More storms occur then because that is when the sun heats the atmosphere the most.

Satellites

Mars has two tiny moons, Phobos and Deimos. The American astronomer Asaph Hall discovered them in 1877 and named them for the sons of Ares. Both satellites are irregularly shaped. The largest diameter of Phobos is about 17 miles (27 kilometers); that of Deimos, about 9 miles (15 kilometers).

The two satellites have many craters that formed when meteoroids struck them. The surface of Phobos also has a complicated pattern of grooves. These may be cracks that developed when an impact created the satellite's largest crater.

Scientists do not know where Phobos and Deimos formed. They may have come into existence in orbit around Mars at the same time the planet formed. Another possibility is that the satellites formed as asteroids near Mars. The gravitational force of Mars then pulled them into orbit around the planet. The color of both satellites is a dark gray that is similar to the color of some kinds of asteroids.

Evolution of Mars

Scientists know generally how Mars evolved after it formed about 4.6 billion years ago. Their knowledge comes from studies of craters and other surface features. Features that formed at various stages of the planet's evolution still exist on different parts of the surface. Researchers have developed an evolutionary scenario that accounts for the sizes, shapes, and locations of those features.

Researchers have ranked the relative ages of surface regions according to the number of impact craters observed. The greater the number of craters in a region, the older the surface there.

However, scientists have not yet determined exactly when the various evolutionary stages occurred. To do that, they would need to know the ages of rocks of surface features representing those stages. They could determine how old such rocks are if they could analyze samples of them in a laboratory. But no space probe has ever brought Martian rocks to Earth.

Scientists have divided the "lifetime" of Mars into three periods. From the earliest to the most recent, the periods are: (1) The Noachian (noh AY kee uhn), (2) the Hesperian, and (3) the Amazonian. Each period is named for a surface region that was created during that period.

The Noachian Period is named for Noachis Terra, a vast highland in the southern hemisphere. During the Noachian Period, a tremendous number of rocky objects of all sizes, ranging from small meteoroids to large asteroids, struck Mars. The impact of those objects created craters of all sizes. The Noachian was also a time of great volcanic activity.

In addition, water erosion probably carved the many small valley networks that mark Mars's surface during the Noachian Period. The presence of those valleys suggests that the climate may have been warmer during the Noachian Period than it is today.

The Hesperian Period

The intense meteoroid and asteroid bombardment of the Noachian Period gradually tapered off, marking the beginning of the Hesperian Period. This period is named for Hesperia Planum, a high plain in the lower latitudes of the southern hemisphere.

During the Hesperian Period, volcanic activity continued. Volcanic eruptions covered over Noachian craters in many parts of Mars. Most of the largest outflow channels on the planet are of Hesperian age.

The Amazonian Period, which is characterized by a low rate of cratering, continues to this day. The period is named for Amazonis Planitia, a low plain that is in the lower latitudes of the northern hemisphere.

Volcanic activity has occurred throughout the Amazonian Period, and some of the largest volcanoes on Mars are of Amazonian age. The youngest geologic materials on Mars, including the ice deposits at the poles, are also Amazonian.

Possibility of life

Mars might once have harbored life, and living things might exist there even today. Mars almost certainly has three ingredients that scientists believe are necessary for life: (1) chemical elements such as carbon, hydrogen, oxygen, and nitrogen that form the building blocks of living things, (2) a source of energy that living organisms can use, and (3) liquid water.

The essential chemical elements likely were present throughout the planet's history. Sunlight could be the energy source, but a second source of energy could be the heat inside Mars. On Earth, internal heat supports life in the deep ocean and in cracks in the crust.

Liquid water apparently carved Mars's large channels, its smaller valleys, and its young gullies. In addition, there are vast quantities of ice within about 3 feet (1 meter) of the surface near the south pole and perhaps near the north pole. Thus, water apparently has existed near the surface over much of the planet's history. And water is probably present beneath the surface today, kept liquid by Mars's internal heat.

In 1996, scientists led by David S. McKay, a geologist at the National Aeronautics and Space Administration's Johnson Space Center in Houston, reported that scientists there had found evidence of microscopic Martian life. They discovered this evidence inside a meteorite that had made its way to Earth. The meteorite had been blasted from the surface of Mars, almost certainly by the impact

A curved, rodlike structure shown in the center of this photo has been referred to as a fossilized Martian creature by some scientists. The structure is about 200 billionths of a meter long and is part of a Martian rock that was found on Earth. Image credit: NASA/Johnson Space Center

of a much larger meteorite. The small meteorite had then journeyed to Earth, attracted by Earth's gravity. The trip may have taken millions of years.

The evidence included complex organic molecules, grains of a mineral called magnetite that can form within some kinds of bacteria, and tiny structures that resemble fossilized microbes. The scientists' conclusions are controversial, however. There is no general scientific agreement that Mars has ever harbored life.

History of Mars study

Observation from Earth

Observing Mars through Earth-based telescopes, early astronomers discovered polar caps that grow and shrink with the seasons. They also found light and dark markings that change their shape and location.

In the late 1800's, the Italian astronomer Giovanni V. Schiaparelli reported that he saw a network of fine dark lines. He called these lines canali, which is Italian for channels. But canali was generally mistranslated as canals. Many other astronomers also reported seeing such features. Among those observers was the American astronomer Percival Lowell, who referred to the features as canals. Lowell speculated that the canals had been built by a Martian civilization.

The canals turned out not to exist. In some cases, the observers had misinterpreted dark, blurry regions that they had actually seen. In other cases, there was no relationship between "canals" and real features.

However, the changing dark and light markings were real. Some scientists thought that the changing patterns might result from the growth and death of vegetation. Much later, other scientists suspected correctly that the cause was the Martian winds. Light and dark materials blow to and fro across the surface.

Observation by spacecraft

Robotic spacecraft began detailed observation of Mars in the 1960's. The United States launched Mariner 4 to Mars in 1964 and Mariners 6 and 7 in 1969. Each flew by Mars about half a year after its launch. The craft took pictures showing that Mars is a barren world, with craters like those on the moon. There was no sign of liquid water or life. The spacecraft observed few of the planet's most interesting features because they happened to fly by only heavily cratered regions.

In 1971, Mariner 9 went into orbit around Mars. This craft mapped about 80 percent of Mars. It made the first discoveries of the planet's canyons and volcanoes. It also found what appear to be dry riverbeds.

The next major mission to Mars was Viking, launched by the United States in 1975. Viking consisted of two orbiters and two landers. Its main goal was to search for life. The orbiters scouted out landing sites for the landers, which touched down in July and September 1976. The landers took the first close-up pictures of the Martian surface, and they sampled the soil. They found no strong evidence for life.

The next two successful probes were Mars Pathfinder, which was a lander, and Mars Global Surveyor, an orbiter. The United States launched both craft in 1996. The main objective of Pathfinder was to demonstrate a new landing system. Inflated air bags cushioned the probe's landing in July 1997. Pathfinder also carried a small roving vehicle called Sojourner. The rover rolled down a ramp to the surface, and then moved from rock to rock. Pathfinder sent spectacular photos back to Earth, and Sojourner analyzed rocks and soil. People throughout the world watched television pictures of Sojourner doing its work.

Mars Global Surveyor carried a group of sophisticated scientific instruments. A laser altimeter used laser beams to determine the elevation of the Martian surface. This instrument produced maps of the entire surface that are accurate to within 1 yard or meter of elevation. An infrared spectrometer determined the composition of some of the minerals on the surface. A high- resolution camera revealed a host of new geologic features. These include layered sediments that may have been deposited in liquid water, and small gullies that appear to have been carved by water.

In April 2001, the United States launched the Mars Odyssey probe. The probe carried instruments to analyze the chemical composition of the Martian surface and the rocks just below the surface, to determine whether there is water ice on or beneath the surface, and to study the radiation near Mars. Mars Odyssey went into orbit around the planet in October 2001. In 2002, the probe discovered vast amounts of water ice beneath the surface. Most of the ice found is in the far southern part of the planet, south of 60 degrees south latitude. Scientists also suspect that there are large amounts of water ice north of 60 degrees north latitude. However, when the discovery was made, CO2 frost covered most of that area, preventing the probe from detecting underlying ice.

The water ice found in the south is in the upper 3 feet (1 meter) of soil. That soil is more than 50 percent water ice by volume. The total volume of the water ice discovered is

The Sojourner Rover examines a rock on Mars. The rover traveled from Earth aboard the Mars Pathfinder space probe, then rolled down a ramp to the surface. Sojourner is only 24 3/4 inches (63 centimeters) long. Image credit: NASA

Mars Global Surveyor studied the composition of the Martian surface, photographed the surface in detail, and measured its elevation. The space probe went into orbit around Mars in 1997. Image credit: NASA/JPL

roughly 2,500 cubic miles (10,400 cubic kilometers), more than enough to fill Lake Michigan twice.

The probe cannot detect evidence of water at depths greater than 3 feet. Thus, scientists cannot yet determine the total depth or the total volume of all the water ice on Mars.

Mars passed closer to Earth in August 2003 than it had in nearly 60,000 years. In that year, scientists launched three new probes. The European Space Agency's Mars Express mission included an orbiter that carried scientific instruments and a lander designed to analyze the planet's soil for evidence of life. The United States launched two rovers, nicknamed Spirit and Opportunity, to explore different regions of the planet's surface.

In December 2003, Mars Express went into orbit around the planet and released its lander, Beagle 2. Mars Express immediately began transmitting pictures and other information about the planet, but mission managers could not contact Beagle 2 and feared it was lost. In early January 2004, the U.S. rover Spirit landed safely in an area called Gusev Crater. The rover Opportunity landed later that month in an area called Meridiani Planum. The rovers transmitted detailed photographs of Martian ground features and began analyzing rocks and soil for evidence that large amounts of liquid water once existed on the planet's surface.

In March 2004, U.S. scientists announced that they had concluded that Meridiani Planum once held large amounts of liquid water. Their evidence came from an outcropping of Martian bedrock found in the small crater in which Opportunity landed. The rover's analysis showed that the rock contained large amounts of sulfate salts, which contain sulfur and oxygen. On Earth, such high concentrations of sulfate salts occur only in rocks that formed in water or were exposed to water for long periods. The outcropping's surface also bore tiny pits similar to those found on Earth where salt crystals formed in wet rock and later dissolved or eroded away.

The rover mission was scheduled to last only 90 days, but it was extended because Spirit and Opportunity continued to function well. In June 2004, Opportunity descended into a large crater that mission managers called Endurance and analyzed the layers of bedrock there. Also in June, Spirit arrived at a group of hills,

Mars was photographed by the Hubble Space Telescope in August 2003 as the planet passed closer to Earth than it had in nearly 60,000 years. The photographs captured many features of the Martian surface, including dark, circular impact craters and the bright ice of the southern polar cap. Image credit: NASA, J. Bell (Cornell U.) and M. Wolff (SSI)

The rover Spirit rests on Mars in a composite image made up of photographs taken by a camera mounted above the rover's body. Spirit landed on Mars in early January 2004. The pole at the lower left is one of the antennas Spirit uses to communicate with NASA controllers. Image credit: NASA

called Columbia Hills, after a drive of over 2 miles (3 kilometers). The rovers continued to explore these sites for several months.

Contributor: Steven W. Squyres, Ph.D., Professor of Astronomy, Cornell University.