chapter 6 · 2018-09-06 · 6.3 the overall layout of the solar system the planetary orbits are not...

CHAPTER 6The Solar System

Monday, March 1, 2010

6.1 An Inventory of the Solar System

The Greeks knew about 5 planets other than Earth

They also knew about two other objects that were not planets or stars: meteors and comets



Comets: appeared at long, wispy strands of light in the night sky that remained visible for up to several weeks and then faded from view



Meteors: “shooting stars”- sudden bright streaks of light that flash across the sky, usually lasting a second or two



The invention of the telescope made more detailed observations possible... Galileo Galilei



Telescopes enabled the discovery of Uranus, Neptune, many planetary moons, the first astroids, and the “asteroid belt”



Nonoptical astronomy (infrared and radio) along with spacecraft explorations have been vital to astronomy as we know it today



Our solar system currently contains: 1 star (the Sun), 8* planets, 63 moons, 6 asteroids larger than 300 km, 7000 smaller asteroids, myriad comets a few km in diameter, and countless 100m meteoroids



Comparative Planetology- comparing and contrasting the properties of diverse worlds we encounter to better understand the conditions under which planets form and evolve


6.2 Planetary Properties



The distance of each planet from the Sun is known from Kepler’s laws

A planet’s sidereal (or orbital period) is easily measurable from repeated observations of its location on the sky

The masses of the planets with moons may be calculated by Newton’s laws of gravity and motion

The sizes of the orbits are found by measuring their angular sizes and then applying elementary geometry



The masses of Mercury and Venus are measured by their influences on other planets or nearby objects



Today- masses are accurately measured through their gravitational interaction with artificial satellites and space probes launched from Earth


6.3 The Overall Layout of the Solar System

By our standards the solar system is HUGE... but by astronomical standards, even the distance to Pluto* is less than 1/1000 of a light year



All the planets orbit the Sun counterclockwise as seen from Earth’s North Pole

The planets also orbit in nearly an ecliptic plane (except Mercury- 7 degrees to the ecliptic)- our solar system is considered flat



The planetary orbits are not evenly spaced: the orbits get farther and farther apart as we move farther out from the Sun

There is still a certain regularity in the spacing known as the Titius-Bode law which seems to “predict” the radii of the planetary orbits


Interlude 6.1 The Titius-Bode “Law”

The spacing of the orbits increases more or less geometrically as we move out from the Sun: at any point in the list, the distance to the next planet out is about twice that to the next planet in

The is not an actual “law”, but more a rule for determining an approximate orbital semi-major axis

Start with 0.4 AU- the distance from the Sun to Mercury then add to it, successivley, 0.3 AU to arrive at Venus: semi-major axis of 0.7 AU; then Earth, 1.0 AU... etc.


6.4 Terrestrial and Jovian Planets

Terrestrial Planets: Mercury, Venus, Earth, Mars- small, dense and rocky

Jovian Planets: Jupiter, Saturn, Uranus, Neptune- large and gaseous



There are many differences among the terrestrial worlds

All have atmospheres, but the atmospheres are about as dissimilar as imaginable- ranging from a near vacuum (Mercury) to a hot, dense inferno (Venus); Earth is the only atmosphere with Oxygen

Present day surface conditions are also very different

Earth and Mars rotate about every 24 hours, but Venus and Mercury take months

Earth and Mars have moons and measurable magnetic fields, but Mercury and Venus do not



Terrestrial planets are similar compared to Jovian planets



Terrestrial Jovianclose to the Sun far from the Sun

closely spaced orbits widely spaced orbitssmall masses large massessmall radii large radii

predominately rocky predominately gaseoussolid surface no solid surfacehigh density low density

slower rotation faster rotationweak magnetic fields strong magnetic fields

few moons many moonsno rings many rings


6.5 Interplanetary Debris

Interplanetary matter: cosmic “debris” ranging in size from large asteroids to smaller comets to even smaller meteoroids to the smallest grains of planetary dust

Dust settles into the Sun or is swept away by the solar wind- a stream of energetic charged particles that continually flow outward from the sun and pervades the entire solar system


6.5 Interplanetary Debris

Asteroids and meteors are generally rocky and somewhat like the outer layers of the terrestrial planets- anything larger than 100m in diameter is an asteroid, anything smaller is a meteoroid


6.5 Interplanetary DebrisComets are generally icy rather than rocky and are typically 1-100 km

Their composition is similar to some of the moons of the outer planets

Comets striking Earth’s atmosphere do not reach the surface intact, so we do not have actual samples of cometary material- their chemical make-up is determined by spectroscopic studies of the radiation they give off before they are destroyed


6.6 Spacecraft Exploration of the Solar System

Since the 1960s dozen os unmanned space missions have visited all the planets except Pluto*


The MARINER 10 flybys of Mercury

1974- Mariner 10 came within 10,000km of Mercury, sending back high-resolution images of the planet

The spacecraft is in a 176-day orbit around the sun, aided by Venus’s gravitational pull- it revisits Mercury every 6 months- but after March 1975 the spacecraft’s fuel supply was exhausted and it has not returned any more data

45% of Mercury’s surface has been explored through over 4000 photographs


Exploration of VenusAbout 20 spacecrafts have visited Venus since the 1970s

U.S. Pioneer Venus in 1978 placed an orbiter 150 km about Venus’s surface and dispatched a probe of 5 instruments into the atmosphere where an hour later it reached the surface

Magellan probe in 1990 covered the entire surface with extreme clarity- theories about the planet’s surface were altered or abandoned because of Magellan


Exploration of MarsNASA and U.S.S.R. have exploration programs to Mars that began in the 1960s- due to political issues, the detailed data came from the U.S. unmanned probes

Pathfinder arrived at Mars in 1997 and parchuted an instrument package to the surface- a mini-rover, Sojourner, roamed Mars for 3 months collecting data


Missions to Outer PlanetsPioneer and Voyager missions in the 1970s traveled to Jupiter without colliding with debris

Pioneer 11 used Jupiter’s gravity to propel it along to Saturn

Voyager 1 and Voyager 2 also used Jupiter’s gravity to get to Saturn, and Saturn’s largest moon, Titan... Voyager 2 used Saturn’s gravity to reach Uranus and Neptune- it is still headed out of the solar system and still collecting data

Two missions current missions Galileo and Cassini are resolving issues about Saturn


chapter 6 · 2018-09-06 · 6.3 the overall layout of the solar system the planetary orbits are not...

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