What happens if astronauts take off their helmets in space?

The body doesn't burst, nor does it freeze instantly. Skin and connective tissue can withstand a one-atmosphere pressure difference without tearing — the same difference a diver experiences at ten meters depth — and a vacuum is an excellent insulator: with no surrounding air, heat is lost only through radiation and the evaporation of surface moisture, a process that takes minutes, not seconds. What kills isn't the temperature — it's the lack of oxygen.

Once pressure disappears, the first thing that happens is that air rushes out of the lungs on its own; holding your breath is the worst possible move, since it tears lung tissue. Above roughly 19 km of altitude, ambient pressure drops below the vapor pressure of water at 37°C, so body fluids start boiling at low temperature — a condition called ebullism — and soft tissues swell. Consciousness doesn't last long: aviation physiology tables give nine to fifteen seconds of useful time at high altitude, which is the real window for repressurizing. After that comes unconsciousness, and within a couple of minutes, irreversible anoxia.

An extravehicular activity suit isn't clothing — it's a one-person vehicle that maintains pressure, supplies oxygen, removes carbon dioxide, and regulates temperature. Before going outside, crew members breathe pure oxygen for hours to purge nitrogen from their blood and avoid decompression sickness when interior pressure drops. In a NASA vacuum chamber test in the 1960s, a technician lost suit pressure and passed out in about fifteen seconds; he recalled the saliva on his tongue boiling. He survived because the chamber was repressurized immediately.