In the new sci-fi film Sunshine, an astronaut named Mace must leave his spacecraft without a protective suit. He makes it through his exposure with only a case of frostbite. Could you really survive outer space without a suit?
Yes, for a very short time. The principal functions of a spacesuit are to create a pressurized, oxygenated atmosphere for astronauts, and to protect them from ultraviolet rays and extreme temperatures. Without it, a spacewalker would asphyxiate from the lack of breathable air and suffer from ebullism, in which a reduction in pressure causes the boiling point of bodily fluids to decrease below the body's normal temperature. Since it takes a bit of time for these things to kill you, it's possible to make it through a very quick stint in outer space.
At most, an astronaut without a suit would last about 15 seconds before losing conciousness from lack of oxygen. (That's how long it would take the body to use up the oxygen left in the blood.) Of course, on Earth, you could hold your breath for several minutes without passing out. But that's not going to help in a vacuum. In fact, attempting to hold your breath is a sure way to a quick death. To make it for even a few seconds, Sunshine's Mace must have expelled the air from his lungs before he ventured into the starry void. If he hadn't, the vacuum would have caused that oxygen to expand and rupture his lung tissue, forcing fatal air bubbles into his blood vessels, and ultimately his heart and brain. Scuba divers are also at risk for air embolism; they're instructed not to hold their breath as they ascend from the deep sea.
An astronaut who fell unconscious from lack of oxygen would last for a few minutes more before dying from asphyxiation or the effects of the pressure reduction. Ebullism would result in the formation of bubbles in the moisture found in the eyes, mouth, and skin tissue. One NASA test subject who survived a 1965 accident in which he was exposed to near-vacuum conditions felt the saliva on his tongue begin to boil before he lost consciousness after 14 seconds.
In the movie, Mace takes the precaution of wrapping himself in insulation torn from the walls of the spacecraft he's leaving. This might provide some protection against temperatures in space that can run from minus-200 to 200 degrees Fahrenheit. It might also ward off ultraviolet-related skin damage during a short jump through space.
What about the frostbite? That's actually the least plausible result of Sunshine's suitless spacewalk. The cold wouldn't cause Mace too much harm in just 15 seconds, even if he encountered the very lowest temperatures in space. That's because heat leaves the body very slowly in a vacuum. The more likely damage would be a "space hickey"—caused from the swelling and bursting of the skin's small blood vessels—which would look more like the effects of freeze-drying a wart than a case of frostbite.
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Explainer thanks Brandi Dean of NASA and Geoffrey Landis of the NASA John Glenn Research Center. Thanks also to reader Matt Burden for asking the question.