Weightlessness

Weightlessness is the experience (by people and objects) during freefall, of having no apparent weight. This condition is also known as microgravity (see below).

Microgravity

The term microgravity is also used because weightlessness in e.g. a spaceship or other container is not perfect. Causes include:

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• Gravity decreases 1 ppm for every 3m increase in height.
• In a spaceship in orbit the required centripetal force is higher at the upper side.
• Though very thin, there is some air at the level of the orbit, which causes deceleration due to friction.

The "weight" caused by the first two items (the tidal force) is directed vertically away from the spacecraft, i.e. vertically away from Earth in the portion which is farther from Earth (or the body it is in orbit around) than the center of gravity of the spacecraft and vertically toward Earth for the rest. For the last item it is forward.

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The microgravity symbol, µg, was used on the insignia of the Space Shuttle flight STS-107, because this flight was devoted to microgravity research (see picture in that article).

Related Topics:
Space Shuttle - STS-107

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