A condition of a body when it is an infinite distance from any other body. In practice the appearance of weightlessness occurs in space when the gravitational attraction of the earth on a body in space is equal to the centripetal force required by its orbital motion so that the body is effectively in free fall. Weightlessness can also be simulated for short periods in an aircraft flying a parabolic flight path, so that its occupants are again in free fall.
The apparent loss in weight of a body in free fall. Astronauts in an orbiting spacecraft do not feel any weight, because they are falling freely in the Earth's gravitational field (not because they are beyond the influence of Earth's gravity). The same phenomenon can be experienced in a falling lift or in an aircraft imitating the path of a freely falling object.
Astronaut Janice Voss experiencing weightlessness aboard a DC-9 aircraft. Parabolic trajectories are flown by aircraft such as this and the notorious KC-135 ‘vomit comet’, to produce weightless conditions for experimental and training purposes.
Credit: NASA
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