Ed White was the first American to perform a space walk. Photo courtesy of NASA

Why do Astronauts wear Spacesuits?

A spacesuit must protect the astronauts from the harsh environments of space. In space there are very few air molecules. Note that air pressure is the result of the movement and collisions of air molecules. This absence of molecules is referred to as a vacuum.
An astronaut's body fluids would fizz and bubble if he or she was wearing street clothes and was placed in a vacuum. Thus, one of the main purposes of the spacesuit is to provide a leak-proof bladder which stores air pressure to protect the astronaut's body. This function is so important that sometimes spacesuits are referred to as pressure suits.
In 1884, a Frenchman named Le Chatelier noted that: If a stress is applied to a system at equilibrium, the system tries to readjust itself to reduce the stress, if possible. The applied stress could be a change in temperature, a change in pressure, or some other external force. The readjustment results in a new equilibrium.
The Earth's atmosphere exerts a pressure against our bodies. This pressure is 14.7 pounds per square inch at sea level. This pressure helps keep our bodily fluids from escaping or changing into vapor, although some is lost through perspiration and other means. This air pressure thus places a stress on our bodies. Gravity also places a stress on bodies.
When this air pressure is removed, an astronaut's body changes to release this stress according to Le Chatelier's principle. This is achieved through the boiling of bodily fluids. The internal stress in the astronaut's body drives this change.
Astronaut Michael Collins described the suits the Gemini astronauts wore as follows.
When fully dressed and ready to go on a space walk, the astronaut wore the following from the inside out: (1) long cotton underwear, (2) a nylon comfort liner, (3) a pressure bladder of neoprene coated nylon, (4) a Link Net restraint layer, (5) one layer of felt, (6) seven layers of aluminized mylar superinsulation, and (7) a cover layer of high temperature nylon.
Reference: Michael Collins, Liftoff, Grove Press, New York, 1988.
Link Net is a fishnet-like, loosely woven fabric.
Also, note that some of the spacesuit layers were changed to be fire-resistant after the Apollo 1 fire which killed Gus Grissom and two other astronauts.
Space shuttle astronauts wear a launch/entry suit, as described in the following book:
B. Bondar and R. Bondar, On the Shuttle Eight Days in Space, Greey de Pencier Books, Toronto, 1993.
The purpose of this launch/entry suit is to provide the astronauts with protection against the heat and pressure changes during liftoff. It also provides an extra layer of protection from cold or fire in case of an emergency. It consists of an outer suit, long underwear, communications hat, helmet, parachute, flotation device, gloves, back pad, seat support, boots, gravity pants, socks, diaper, and diaper belt. Yes, each astronaut needs to wear a diaper because the liftoff acceleration puts pressure on the astronaut's bladder. Also, the astronauts must sit through a long countdown prior to launch, which may last three hours.
Once in orbit, the shuttle astronauts wear casual clothes. These clothes have velcro pads to hold items needed by the astronauts.

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