Sometimes their eyeballs freeze. Other times they explode. Often there is some element of grotesque inflation, other times instant unconsciousness. Movies have taken all sorts of contradictory stances on what happens to a human body in space. So: What really happens to a human body in space?
Actually, we don’t have to rely on speculation. Animal studies have been conducted quite extensively and, as we’ll see, we can even take advantage of a few human accidents to collect real data. Thankfully no astronaut has yet had a total suit breach in space, nor has any individual been stupid enough to actually take off their helmet. There have been smaller accidents though and — spoilers — the subjects have survived to tell us exactly what the process felt like.
First, temperature: Space is really cold, technically approaching absolute zero. However, there are two ways of having extremely low temperature. One is to slow the movement of atoms to the point that they basically do not move at all. The other is to remove atoms from the system until there is no atomic movement by default. Ask yourself: why do we use liquid helium and other such coolants in technologies like the Large Hadron Collider, when a simple vacuum can technically get even colder? Because when it comes to effective temperature, it’s all about heat transfer.
The vacuum of space is very cold, yes, but it is very bad at absorbing heat (or imparting cold). Since heat transfer happens through the physical collision of atoms and molecules, and a vacuum has virtually no free-floating atoms to do any such colliding, freezing is actually one of the lesser threats in exposure to a vacuum. Animals have shown rapid and total recovery after as long as 90 seconds in the cold of a vacuum.
In reality, rather than the cold, it’s this pressure that has the first effect. Upon sudden exposure to a vacuum your lungs will be immediately and totally drained of air. One of the most dangerous things you can do in this situation is hold you breath; like deep sea divers, astronauts know not to force their delicate lungs to deal will the huge pressure differentials. It’s likely a futile attempt anyway, as you will eventually be forced to evacuate your lungs or have them rupture.
In 1965, a leaking suit suddenly exposed a would-be space-farer in a pressure chamber to very close to vacuum — pressures lower than 1 psi — for under 20 seconds. This unfortunate individual later described the pressure effects, particularly the feeling of the moisture on his skin and tongue boiling away. He likened it to the feeling of fizzy soda in his mouth. The process caused no lasting damage, since the boiling was happening at roughly body temperature. He remained conscious for about 14 seconds — long enough for the completely oxygen-deprived blood from his empty lungs to reach his brain. Then he passed out. The brain can survive for some time without oxygen, and this impromptu test subject was resuscitated without any lasting injury.
By far the most dangerous aspect of exposure to space is another well known to divers: the benz. Nitrogen and other gasses in the blood will boil away, if given enough time. While the brain could hypothetically survive for several minutes without oxygen, this pressure effect acts much more quickly. It’s thought 90 seconds is about the upper limit of survivable time in the vacuum of space, and the benz is the main reason for this time limit.
In space, there are a lot of things that can kill you. Given enough time, the cold can get you. Unfiltered radiation from the sun can get you. Lack of oxygen can get you. However, you’ll probably be dead long before those become a big issue. The sequence of events, it seems, will be to have somewhere under 15 seconds of consciousness, during which time you won’t experience much pain. After extreme oxygen deprivation has forced you unconscious, the pressure effects on your blood and brain will do you in — and it’s only your corpse that will have to worry about things like frozen eyeballs.
Now, isn’t that comforting?