The term microgravity (mg) can be interpreted in a number of ways depending upon context. The prefix micro - (m) is derived from the original Greek mikros, meaning "small." By this definition, a microgravity environment is one that will impart to an object a net acceleration small compared with that produced by Earth at its surface. In practice, such accelerations will range from about one percent of Earth's gravitational acceleration (aboard aircraft in parabolic flight) to better than one part in a million (for example, aboard Earth-orbiting free flyers).
Another common usage of micro- is found in quantitative systems of measurement, such as the metric system, where micro- means one part in a million. By this second definition, the acceleration imparted to an object in microgravity will be one-millionth (10 -6 ) of that measured at Earth's surface.
The use of the term microgravity in this guide will correspond to the first definition: small gravity levels or low gravity. As we describe how low-acceleration environments can be produced, you will find that the fidelity (quality) of the microgravity environment will depend on the mechanism used to create it. For illustrative purposes only, we will provide a few simple quantitative examples using the second definition. The examples attempt to provide insight into what might be expected if the local acceleration environment would be reduced by six orders of magnitude from 1g to 10 -6 g.
If you stepped off a roof that was five meters high, it would take you just one second to reach the ground. In a microgravity environment equal to one percent of Earth's gravitational pull, the same drop would take 10 seconds. In a microgravity environment equal to one-millionth of Earth's gravitational pull, the same drop would take 1,000 seconds or about 17 minutes!
Microgravity can be created in two ways. Because gravitational pull
diminishes with distance, one way to create a microgravity environment is
to travel away from Earth. To reach a point where Earth's gravitational
pull is reduced to one-millionth of that at the surface, you would have to
travel into space a distance of 6.37 million kilometers from Earth (almost
17 times farther away than the Moon). This approach is impractical, except
for automated spacecraft, since humans have yet to travel farther away
from Earth than the distance to the Moon. However, a more practical
microgravity environment can be created through the act of free fall.
We will use a simple example to illustrate how free fall can achieve microgravity. Imagine riding in an elevator to the top floor of a very tall building. At the top, the cables supporting the car break, causing the car and you to fall to the ground. (In this example, we discount the effects of air friction on the falling car.) Since you and the elevator car are falling together, you will float inside the car. In other words, you and the elevator car are accelerating downward at the same rate. If a scale were present, your weight would not register because the scale would be falling too (Figure 1).