Why do astronauts wear space suits?

Above roughly 19 km (63,000 ft) — the Armstrong limit — ambient pressure is so low that body fluids begin to boil at normal body temperature. In orbit there is no ambient pressure at all. Unprotected exposure brings unconsciousness within seconds from oxygen starvation, along with swelling and ebullism; the body does not explode, as films like to suggest, but the outcome is fatal just as fast. Holding a survivable pressure around the crew member is therefore the suit's first and non-negotiable job.

An extravehicular suit is built in layers: a liquid-cooling garment threaded with water tubes, a pressure bladder with a restraint layer that stops it ballooning, and an outer thermal micrometeoroid garment of more than a dozen laminated layers. A backpack carries oxygen, carbon-dioxide scrubbing, cooling water, power and radio, while a gold-coated visor cuts unfiltered sunlight. The suit is kept at about 29.6 kPa (4.3 psi) of pure oxygen rather than cabin pressure, because a higher internal pressure would make the joints too stiff to work in. The price of that choice is a pre-breathe protocol to purge nitrogen from the blood and avoid decompression sickness.

The orange or white suits worn during launch and re-entry are not spacewalking suits. They are lighter, plugged into the seat for air and cooling, and normally unpressurised — insurance against a cabin leak rather than tools for working outside. Extravehicular suits are self-contained and far heavier, designed for hours of autonomy at the end of a tether. For the return to the Moon, new designs focus on abrasive dust, a wider range of motion and the ability to kneel and walk across terrain rather than drift along a handrail.