space suits

To explore and work in space, human beings must take their environment with them because there is no atmospheric pressure and no oxygen to sustain life. Inside the spacecraft, the atmosphere can be controlled so that special clothing is not needed. But in order to work outside the spacecraft, humans need the protection of a spacesuit. Earth's atmosphere is 20 percent oxygen and 80 percent nitrogen from sea level to about 120 km. At 5,500 m, the atmosphere is half as dense as it is on the ground, and at altitudes above 12.000 m, air is so thin and the amount of oxygen so small that pressure oxygen masks no longer do the job. Above the 19,000 m threshold, humans must wear spacesuits that supply oxygen for breathing and that maintain a pressure around the body to keep body fluids in the liquid state. At this altitude the total air pressure is no longer sufficient to keep body fluids from boiling.
US spacesuits have been pressurized at 0.30 bar (30% earth sea level pressure), but because the gas in the suit is 100 percent oxygen instead of 20 percent, the person in a spacesuit actually has more oxygen to breathe than is available at an altitude of 3,000 m without the spacesuit. At the US suit pressure, before leaving to perform tasks in space, an astronaut has to spend several hours breathing pure oxygen before proceeding into space. This procedure is necessary to remove nitrogen dissolved in body fluids and thereby to prevent its release as gas bubbles when pressure is reduced; a condition commonly called "the bends". Russian and future NASA suits are pressurized to 0.56 bar, shortening the pure-breathing period to half an hour.
A spacesuit also shields the astronaut from bombardment by micro meteoroids and insulates the wearer from the temperature extremes of space. Without the Earth's atmosphere to filter the sunlight, the side of the suit facing the Sun may be heated to a temperature as high as 120 degrees C; the other side, exposed to darkness of deep space, may get as cold as -160 degrees C. Paradoxically, the suit's life support system has to remove the heat and moisture generated by the sweaty working astronaut. This is usually accomplished by circulating cool water through an undergarment worn next to the astronaut's skin. Heat overload of space suits caused several crises on the first space walks in the Voskhod and Gemini programs.
Early US space suits were adapted from pressure suits designed for pilots of high altitude military and experimental aircraft. The first suits designed from the beginning for use in space were the American A7L and Soviet Krechet suits. These were designed for walking on the moon during the space race of the 1960's. They provided the basis for those used aboard space station and shuttle missions.
The search for the perfect suit continues. It would seem the next major step will be suits suitable for use on the surface of Mars. These will have very different design criteria than those used in zero-G.