Gravity is such a common part of our daily lives that we are rarely conscious of it, even though it affects everything we do. Any time we drop or throw something and watch it fall to the ground, we see gravity at work. Although gravity is a universal force, there are times when it is not useful to conduct activities under its full influence. In these cases, space scientists and engineers perform their work in "microgravity"—a condition in which the effects of gravity are greatly decreased.

On Earth, brief periods of microgravity can be achieved by dropping objects from tall towers. Longer periods of microgravity, however, can be created only through the use of airplanes that fly special flight paths. The microgravity research aircraft of the National Aeronautics and Space Administration (NASA) is the KC-135, a four-engine turbojet, similar to the Boeing 707, which...