Weightlessness
1. Weightlessness is the complete or near-complete absence of the sensation of weight. This is also termed zero-G, although the more correct term is "zero G-force". It occurs in the absence of any contact forces upon objects including the human body.
2. Weightlessness is the fact of having or appearing to have no weight, for example because there is no gravity.
The term zero gravity is often used to describe such a condition. Excluding spaceflight, true weightlessness can be experienced only briefly, as in an airplane following a ballistic (i.e., parabolic) path.
Crews of spacecraft are subject to the problems of weightlessness. It was learned from early Soviet and U.S. manned missions that there is a reduction in heart and respiration rates and a progressive loss of body weight and bone calcium during flights of relatively short duration. A reversal of most of these effects occurs, however, upon return to Earth.
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