Outer space is a harsh and unforgiving environment. To get there, astronauts must ride atop complicated rockets that rely on controlled explosions to attain the terrific speeds required to achieve orbit. Out there, spacecraft and spacesuits must protect their occupants from wild temperature swings, a near perfect vacuum, and in some cases poisonous atmospheres and corrosive dusts. People must adjust to "weightlessness" and they may be exposed to potentially harmful doses of radiation. In addition, spacefarers must adjust to the psychological and social conditions of flight.

Acceleration

The first step in leaving Earth—achieving orbital velocity—requires high acceleration. In the 1830s, some people feared that the human body could not withstand the greater than 40-kilometer-per-hour (25-mile-per-hour) speed that might be achieved by railroad trains. Today we know that people are capable of accelerating to very high speeds as long as they are protected...