- Main article: Weightlessness
A microgravity environment is one where gravity has little or no measurable effect. The only three methods of creating a microgravity environment are to travel far enough into deep space so as to reduce the effect of gravity by attenuation, by falling, and by orbiting a planet. The terms weightlessness and Zero-G refer to this same environment.
The first method is the simplest in conception, but requires you to travel an enormous distance, rendering it most impractical. Even during the missions to the Moon, the astronauts only experienced microgravity because they were orbiting the sun.
The second method, falling, is very common but approaches microgravity only when the fall is in a vacuum, as air resistance will provide some resistance to free fall acceleration. Also it is difficult to fall for long enough periods of time to do much experimentation or to support any commercial activity. There are also problems involving avoiding too sudden a stop at the end. However, it is still used as training for astronauts and for some experiments. Drop towers and airplanes (such as used by NASA's Reduced Gravity Research Program, aka the Vomit Comet) provide short term weightlessness.
The third is orbiting a planet, which is really just falling with sufficient forward (tangential) speed that you go all the way around the planet and end up back where you started (It's like falling, but you're going so fast forward that you keep "missing" where you were going to fall, i.e. that planet). This is the environment most people think of, with common examples being the Space Shuttle, International Space Station, Mir, etc. While this scenario is the most suitable for scientific experimentation and commercial exploitation, it is still quite expensive to operate in, mostly due to launch costs.
In a shot tower (now obsolete), molten metal (such as lead or steel), was dripped through a sieve into free fall. With sufficient height (several hundred feet), the metal would be solid enough to resist impact at the bottom of the tower. While the shot may have been slightly deformed by its passage through the air and by impact at the bottom, this method produced metal spheres of sufficient roundness to be used directly in shotgun shells or to be refined by further processing for applications requiring higher accuracy.
High quality crystals
While not yet a commercial application, there has been much interest in growing crystals in microgravity, as in a space station or automated artificial satellite, in an attempt to reduce crystal lattice defects. Such defect-free crystals may prove useful for certain microelectronic applications and also to produce crystals for subsequent X-ray crystallography.
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- Criticism of the terms "Zero Gravity" and "Microgravity", a persuasion to use terminology that reflects accurate physics (Sci.space post).