nasa facts weightlessness

8/7/2019 NASA Facts Weightlessness

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WeightlessnessAn astronaut in orbit around the earth is said to bein a state of weightlessness, also called zero gravityor zero g.The condition called weightlessness may alsobe experienced, briefly, in fa m iI iar circumstanceson the ground. One example is i l lustrated in thesketch of a boy on a bicycle, firs t i n a normal grav-ity condition and then in an apparent weightlesscondition.In a playground swing, when you reach the high-est position and hang there for a split second be-fore descending, the sensation is that of apparentweightlessness. A pole vaulter seems to be weight-less a t the top of his leap.For an understanding of weightlessness, it is

a measure of the force exerted by the earths grav-ity. If you weigh 150 pounds, it means the earthspull upon your body is measured in a degree ex-pressed by the value 150 pounds.But in special circumstances such as those de-scribed above, it appears that you weigh nothing-even though the mass (the amount of matter) ofyour body has not changed, and the earths gravityis still there. This is what is meant when weigh t-lessness is defined as apparent lack of gravita-tional pull.Apparent is the key word in understanding thisdefinition. Gravity is present, but its force is notreacted upon by phFical contact between two ob-jects.necessary first to have in min d that the weight is As you stand on the floor, your body is attracted

Illustrated at l e f t is a boy riding on his bicycle under normal circumstances of gravity. The boys weight presses down-ward and the bicycle, supported by the ground, pushes upward with a force equal to his weight and supports him. A tright is an illustration of the boy and the bicycle in a free fall after riding off the ramp. For a brief time, although theboy and the bicycle both have weight, the bicycle is not mechanically supported and is consequently no t pushing uponthe boy; hence the boy experiences a brief weightlessness with respect to the bicycle.


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\ / ORBITIcGravity and th e motion of the spacecraft interact to holdthe spacecraft in orbit, where it is weightless.(pulled) by the earth, as a result of the earthsgravity. The sensation of b eing pulled downwardtoward the earth is experienced by certain bodysensors and as pressure against the soles of yourfeet. If you were weightless, the re would be nosuch pressure. The astronaut, weightless i n orb it,does not experience such pressure.The unit of measurement for gravity is g, whichrepresents the acceleration due to gravity, a t th eearths surface.Weightlessness is expressed by the te rm zero g,but th is does not mean that th e gravity of the earthhas disappeared, or that the object which is in aweightless con ditio n does not have mass. It means,rather, that the weightless object is subject notonly to gravity, b ut also to some other effect, whichbalances the force of gravity, thus producing ap-parent lack of gravitational pull, hence, zero g.In the case of the playground swing, you havevelocity w hich has been acquired by p ushing your-self in to the upward arc, bu t the pull of the earthsgra vity causes you to slow down. When you reachthe top of the swing you are suspended momen-tarily , in apparent weightlessness, before you startback down.The weightless state can be produced in an air-plane for a brief period (thirty to forty seconds) byputtin g the craft into a f l ight path vario usli calleda ballistic trajectory, a Keplerian trajectory or aparabolic arc (See sketch page 3). The pilot f irstnoses down to itkrease speed, then turns the nose

upward and directs it into an arc, as illustrateDuring this b r ief per iod no l i f t is produced on thwings, the airplane is not supported by the aand the occupants are not supported by the aplane. The result is zero gravity for the airplaand it s occupants. The procedure is used to stuweightlessness, and in the train ing of astronauts.To achieve orbit, the spacecraft is pushed uward, then gradually pitched over from the verticinto a horizontal path, and its speed acceleratto th e necessary rate by its launch vehicle. Tspacecraft separates from the launch vehicle agoes int o orbit.The spacecraft now i s trave ling with a velocwhich has been imparted by the launch vehicGravity and the motion of the spacecraft interato hold the spacecraft in orbit, where it is weigless. Gravity exerts centripetal force, ten ding pull the spacecraft toward the center of the eart(See sketch page 2) .The spacecraft, in motion, would go straigahead (dotted arrow) if its direction were naltered by the force of gravity. (Newtons first laof motion: A body remains a t rest or travels wconstant velocity in a straight line unless acteupon by an external force.) At orbital speed fthe alt i tude, the result is an orbital path for tspacecraft.Orbital flight sometimes is explained by sayithat the centripetal force of gravity, pull ing inwatoward the center of the earth, is balanced Centrifugal force exerted outward and crea tia balance which converts the movement of tobject into an orbit . This explanation is usefulit is realized that centrifugal force actually is teffect of inertia upon the moving body. It is nreally a force; a better term is centrifugal effecIts movement may be described as fallin g tdwathe earth, but with the curved surface of the eafall ing away from its path, so that it continues fall around the earth. Later, the retrorockets wfire, reducing speed in orbit, and gravity wil l brithe craft back to earth.Bllt while it is in orbi t it is weightless, and evething in the spacecraft also is weightless. Gravproduces the same change of direction on all ojects within the spacecraft as it does on the spacraft itself. They are motionless relative to eaother and produce no push or pull on one anothThe astronaut feels no pressure from his coucAny objects in the cabin not securely fastened w


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float.; John Glenn released his came ra in midair ,it remained there until he wanted it again andreached for it. Liquids do not fall, they float inlarge and small drops. Crumbs of food would floataround in the spacecrafts controlled atmosphereand become an inconvenience, or even a hazard, sofood fo r the astronauts is provided in special closedcontainers. Liquids are contained until taken intothe mouth. Solid foods are bite-sized to avoidcrumbs, or are in dehydrated form in plastic con-tainers, t o which water m ay be added, and thenthey may be squeezed into the mouth without let-tin g anything int o the cabin atmosphere. Once thefood, or water, or other liquid, has been taken intothe mouth, it can be swallowed normally; weight-lessness does not interfere with this muscularprocess.Anything wh ich separates from the spacecraft isstill in orbit, and still weightless. This was demon-strated during Gemini IV by Astronaut Edward H.White II in his experiment with EVA (extravehicu-lar activity), and by other astronauts on subsequentflights. Wearing a space suit especially designedfor the purpose, White emerged from Gemini IVand remained outside the spacecraft for more than20 minutes . Whites walk in space mo re precise-ly might be called floating in space, or maneuver-ing in space. A t one point he attempted to walkon the outside surface of the spacecraft; to do sohe had to pull on the tether which connected himto Gemini, to hold himself against the spacecraft.The pul l on the tether replaced the pull of gravitywhich he would have had on earth. With thi s assist-ance, he found it possible to take three or foursteps.The astronauts all report that the sensation ofweightlessness is a pleasant one. Astronaut White,during his time outside of Gemini IV, reported thatthere was no difficulty in sensing location and atti-tude. Im looking righ t down, he said, and itlooks like were coming up on the coast of Califor-nia, and Im going in slow rotation to the right.There is absolutely no disorientation.In the future of space exploration, when moreand more work of mo re and more kinds will have tobe accom plished in space-an example would bethe assembly of large space vehicles in space byputting together components launched separatelyfro m earth-weightlessness poses a special prob-lem in the use of tools.In a sta te of w eightlessness , when you put a

wrench on a nu t and turn, the result is not the sameas it would be in normal circumstances. To makethe nut turn, you need a firm base from which toexert the force which i s to be transmitted throughthe wrench to the nut. Lacking this, you may turninstead of the nut, or both you and the nut mayturn . So special tools are being developed whichwill provide the reaction required to exert force.Some of th e possible effects of weightlessness inrelation to liv ing organisms are still in question, andare being studied primarily on m an in the MannedSpace Flight program. On animals, cells and plants,the effects of weightlessness are being studied inNASAs Biosatellite program scheduled to beginthis year (1967). Biologists will send aloft livingorganisms ranging from bacteria to monkeys, withinstruments to measure effects and transmit datato the ground. What effect will weightlessness have

on the circulation of the animals blood, on hiscentral nervous system, the bones of the skeleton?The Biosatellite program will provide the answers.The Biosatellite program also is concerned withthe effects of radiation, and with the effects of re-moval from the earths rotation, which includes theday-night cycle, responsible for the daily rhythmexhibited by most living organisms on earth.Scientific study of weightlessness under con-trolled conditions on the ground is accomplishedwith d rop towers. The largest drop tower, orshaft, for these studies in the United States is theZero Gravity Facility recently completed at NASAsLewis Research Center, Cleveland, Ohio.Basic structure of the Zero Gravity Facility is ashaft that extends 510 feet into the ground. Theshaft is lined with an 18-i nch -thic k concrete casing28 feet in diameter. Inside th is is a welded steevacuum chamber 20 feet in diameter.Five seconds of weightlessness can be producedby releasing an experiment from the top of theshaft. The time is doubled when the experiment is

Diagram of an aircraft performing weightless flight. Follow ing .a power dive, the aircra ft, nosing upward, coaststhrough a no-l ift arc. During this period it is weightlessAstronauts in train ing float i n the cabin experiencing asensation of weightlessness.


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projected upward from the bottom of the chamber-by a high-pressure accelerator, since it is weight-less on the journey upward, and again as it fal lsback t o the bottom. A typical zero gravity experi-ment consists of a transparent tank containing testf luids. High-speed motion picture cameras recordth e f luids behavior during zero g operation.QUESTIONS:

1. When does man become weightless in space?Why?2. What special measures are necessary in prep-aration of food for the weightless man in space?

ACTIVITY:Tie a weight t o a string. Hold the weight sus-

pended, then swing it upward through a q uarter ofa circle. As the weight reaches its highest point,the str ing wil l become slack, and the weight i sweightless, l ike the occupant of the playgroundswing.REFERENCES:

The Physics of Space, by Richard M. Sutton (inthe Holt Library of Science, Space Science Series)Chapter 10.Sourcebook on the Space Sciences, by SamueGlasstone, Chapter 13.

Modern Space Science by Frederick E. Trinkleinand Charles M. Huffer, Chapter 23.Space, by Arthu r L. Costa (Volume 6 in lnvesti-gating Science with Children) Chapter 5.

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