The vacuum of space can destroy an unprotected human body in a few seconds by explosive decompression. Energetic solar and cosmic radiations in space may also be fatal to an unshielded person who is not protected by the atmosphere of the earth. These environmental conditions can also affect the instruments and devices used in spacecraft, so the design and construction of these materials are dictated by the space environment. People can be protected against the space environment in several ways. At present, they are enclosed inside a hermetically sealed cabin or space suit, with a supply of pressurized air or oxygen to approximate conditions on earth.
Air conditioning controls the temperature and humidity.
Absorbing and reflecting surfaces on the outside of the spacecraft regulate the amount of heat radiation affecting the craft. Furthermore, space journeys are carefully planned to avoid the intense radiation belts around the earth. For lengthy space journeys, or for prolonged stays in an earth-orbiting satellite, the effects of weightlessness might be reduced by spinning the craft to provide artificial gravity. For this purpose, the spacecraft might be shaped like a large wheel that spins slowly around its own axis, or like a dumbbell, rotating end over end. History People dreamed of spaceflight for millennia before it became reality.