What happens if you go into space without a spacesuit?

Films have settled on two endings for this, and both are wrong. In one the body swells and bursts. In the other it freezes solid in seconds, usually with a decorative layer of frost. Neither happens, and the real answer is stranger and much more specific.

You would not explode

Skin is elastic and the tissue underneath is porous, and together they are more than strong enough to hold a body at one atmosphere of internal pressure against nothing outside. There is no rupture.

What does happen is swelling. Water in the tissues turns to vapour at that pressure, which can bloat the body to as much as twice its normal size and slow the circulation. It is serious and it is reversible — the skin holds.

You would not freeze either

This is the surprising one, and it follows directly from why space is cold in the first place. Heat leaves an object three ways: conduction into what it touches, convection into moving air or water, and radiation. In vacuum the first two are unavailable, and radiation is slow.

So a body in space loses heat far more slowly than a body in cold water, or even in cold air. Freezing is not the thing that kills you. Something much faster is.

Fifteen seconds

The moment the pressure drops, oxygen stops crossing from the lungs into the blood — and worse, begins crossing back out. The blood already circulating still has oxygen in it, and the heart keeps pumping, so there is a brief grace period while the last oxygenated blood reaches the brain.

That takes about 9 to 12 seconds. After it, unconsciousness. Not gradually — the way a light goes out.

It has actually happened

In December 1966 a NASA technician named Jim LeBlanc was testing a spacesuit in a vacuum chamber in Houston when the hose feeding it pressure came loose. His suit fell from a normal 3.8 psi to roughly 0.1 psi in under ten seconds.

He stayed conscious for about fourteen seconds. What he remembered afterwards was the saliva on his tongue starting to bubble and boil — his own body fluids vaporising at that pressure, the effect called ebullism.

Colleagues began repressurising the chamber within seconds. He was exposed for roughly half a minute in total, came round during the repressurisation, and stood up almost immediately. He had sore ears. That was all.

The line where it starts

You do not need to be in space for this. At about 19 km up, air pressure falls low enough that water boils at human body temperature. That altitude is called the Armstrong limit, and above it a mask supplying pure oxygen is no longer enough — the water in your body will vaporise regardless of what you are breathing.

It is why high-altitude aircrew wear sealed pressure suits rather than oxygen masks, and it sits far below the 100 km line where space is usually said to begin.

How long you would have

• 0 seconds: pressure gone. Air rushes out of the lungs. Moisture on the tongue and eyes begins to vaporise.
• 9 to 12 seconds: the last oxygenated blood reaches the brain. Unconsciousness.
• Up to about 30 seconds: repressurisation at this point is unlikely to cause lasting damage.
• About 90 seconds: animal studies put the boundary here. Recovery before it is normal; beyond it, survival is not expected.

So the honest answer is that a spacesuit is not protecting you from cold, and not from bursting. It is holding pressure — and everything else it does, the oxygen and the cooling and the white outer layer, is secondary to that one job.