WHAT IS MICROGRAVITY?   Microgravity is a term used to describe a very low-gravity environment; the prefix "micro" comes from a Greek word meaning small. Weightlessness is a similar term used to describe a (nearly) zero-gravity environment. An object can be subjected to microgravity by either placing it (1) at a location where gravity is naturally small, (2) between large masses where the gravitational attraction from each body is balanced, or (3) in free-fall. Gravity does not "disappear" in outer space, but reaches throughout space to hold planets in their orbits and the stars within galaxies. However, a location far removed from large masses, for example between stars (or even galaxies), would also have a weak gravitational field. Regardless of the distance from large masses (such as the Earth), an object may be put into microgravity by allowing it to freely fall without the influence of other forces. In this situation, the object will be weightless because weight is a measure of the object's resistance to gravity. While the object is freely falling, there is no resistance to gravity, and the object becomes weightless. If the object is at rest (or a constant velocity), there must be a force equal but opposite to that of gravity, based on Newton's laws. For a person climbing a rope, that opposing force results from the rope pulling the person upward, just as gravity pulls downward. If the rope breaks, the resistance disappears and the person will fall in a low-gravity condition; in other words, the person would be in microgravity. Gravity is such that objects fall at the same acceleration regardless of their mass. So the person and the broken end of rope would fall together, and relative to the falling person, the broken end of the rope would be floating. An accident where a painter fell of a roof inspired Einstein to develop the theory of general relativity, which explains (among other things) that an object in free-fall will experience the same microgravity conditions as an object in a weak gravitational field. He considered this to be his greatest scientific achievement [Wheeler, 1990].

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