Outer space has a very different environment from that of Earth. The atmosphere, radiation, and gravity levels are so drastically varied that several adjustments are made to protect astronauts from the deadly effects of the space environment on the human body. In particular, gravitational effects are not well controlled and the effects of long-term exposure to microgravity are unknown.

Several experiments have already indicated that major biological changes begin in the human body within minutes of spaceflight. For example, when a person is exposed to microgravity, there is less blood volume in the legs and more in the upper body. This change makes the brain sense that there is too much fluid in the body and triggers an adaptive response such as increased urine production. This is similar to the sensation experienced by many people upon entering a swimming pool. Even the...