Space physiology is concerned with the structure and functioning of the body under the conditions encountered by space travelers. To date, these conditions have been confined to the environment of the spacecraft that houses the astronauts. In the future, however, as travel to other bodies in the solar system is undertaken, space physiology will include the atmospheric and gravitational conditions found on these planets, moons, or other stellar bodies.

Aside from the lunar missions of the 1960s, man's extraterrestrial voyages have been confined to orbital forays aboard space capsules or space stations. But even orbiting around Earth poses difficulties for the astronauts. The reduced gravity of a spacecraft makes it difficult for the body to distinguish "up" from "down." On Earth, such distinction by the vestibular organ of the inner ear is easy, because of the orienting power of gravity. In the space shuttle and the...