Would you freeze instantly in space?

When you go outside in winter, you don't lose heat because the cold "gets into" you, but because your body hands it to its surroundings. It does so in three ways: by conduction, touching something colder; by convection, when air or water warms up around you and carries that heat away; and by radiation, emitting infrared light.

In 10 °C water a person can become hypothermic in under an hour, because water conducts heat about 25 times better than air. In a vacuum the opposite happens: there are no molecules touching your skin, so conduction and convection vanish. Only radiation is left.

The power a body radiates follows the Stefan-Boltzmann law: it grows with the fourth power of absolute temperature. A person has about 1.8 m² of skin at roughly 33 °C. Surrounded by dark, empty space, they would radiate on the order of several hundred watts.

That sounds like a lot, but the human body is a big heat store: 70 kg of water-like material needs about 250,000 joules to drop a single degree. With a net loss of about 500 W, body temperature would fall by roughly a tenth of a degree per minute. It would take hours to reach severe hypothermia.

You can test the same idea with a metal object in the radiative cooling simulator: even starting at 500 °C, a 2 kg part takes about 40 minutes to reach room temperature.

The lack of pressure and oxygen. With no outside pressure, the air in your lungs expands and has to be let out to avoid damaging them. Oxygen in the blood starts escaping into the empty lungs, and consciousness is lost in about 10–15 seconds. Without help, the damage becomes fatal within a couple of minutes.

There is also an odd effect: at vacuum pressure, water boils at body temperature. Saliva and the moisture on the eyes and skin start to evaporate with bubbles. In 1966, NASA technician Jim LeBlanc suffered an accidental depressurization in a vacuum chamber during a suit test; he later said the last thing he noticed before passing out was the saliva bubbling on his tongue. He recovered fully because the chamber was repressurized in under half a minute.

That evaporation does cool the skin, just like sweat, and can form a little frost around the mouth or eyes. But it doesn't turn anyone into an ice statue.

An astronaut in a suit produces about 100 W of heat from metabolism alone, and much more while working. On top of that, at Earth's distance the Sun delivers about 1,361 W per square meter. Since the suit is insulated by the vacuum, that heat would build up inside. That is why suits include an undergarment laced with water tubes and a system that dumps heat by evaporating water into space.

The International Space Station faces the same problem on a big scale. Its equipment generates tens of kilowatts of heat, and the only way to get rid of it is to radiate it. For that it carries huge radiator panels, separate from the solar panels, with ammonia flowing through them.

Objects in space can end up very cold, but slowly. A switched-off satellite in shadow can fall below −100 °C, and the universe's microwave background is at about −270 °C. The key is the rate: a vacuum cools slowly, like a thermos. In fact, a thermos works exactly this way: there is a vacuum between its two walls so heat can only escape by radiation.

The radiation a body sheds heat with depends on its temperature. At about 33 °C, skin emits mostly infrared around 9–10 microns, invisible to the eye. It is the same light thermal cameras pick up. You can see where that wavelength falls on the interactive electromagnetic spectrum.

That is why space engineers care so much about the emissivity of surfaces: polished metal emits little infrared and holds on to heat, while special white paint reflects sunlight but radiates infrared very well. With layers, paints and radiators they control a spacecraft's temperature with no air around it.

Andy Weir's novel has several scenes where the main character works with very hot or very cold objects in a vacuum and has to work out how long they will take to change temperature. For the science behind those scenes in more detail, see our chapter guide.