How To Survive A Vacuum.


Another Wednesday, another science post. I happen to be in a particularly gruesome mood today, so let’s talk about what happens when you go into space without a spacesuit.

There are several popular misconceptions about human exposure to vacuum that have been debunked so many times I’m not even sure we can properly call them misconceptions any more. People do not explode in space. They do not freeze solid. Their blood does not start to boil. It is actually possible to survive – if rather uncomfortably – for a minute or two in a total vacuum, so it’s not an immediately lethal environment. Noted sci-fi hack Arthur C. Clarke wrote a key scene in 2001 where Bowman forgets his spacesuit helmet, gets locked outside the ship by Hal and has to get back inside by doing an unprotected EVA, and whatever else might be said about Clarke there’s nothing scientifically implausible about this scene at all. It hasn’t happened yet (as far as I know) but it could.

Dealing with those misconceptions in turn: people do not explode in space because the human skin does have some elastic strength to it, and is perfectly capable of containing the wet sack of internal organs that makes up the human body against a vacuum. You might experience some painful swelling (this has been reported by astronauts and high-altitude balloonists who experienced equipment faults that left various body parts exposed to vacuum), but you will not explode. You will not freeze solid either because vacuum is actually an incredibly good insulator; there’s simply nothing around you that can carry away your ambient body heat. Finally there’s the blood boiling thing, which is a misunderstanding of something called the Armstrong limit. When we say “boil” in this instance we mean “evaporate away” rather than “heat up”, and many liquids will boil away in a vacuum in the absence of any external pressure to keep them in their liquid state. This includes the saliva in your mouth and the tears in your eyes – but not the blood in your circulatory system. In order to keep the blood pumping around your body the circulatory system has to keep it under a certain amount of pressure by default, and this pressure is enough to stop it from immediately boiling away in a vacuum. So as long as you’re alive, your blood will not boil in space.


In fact, the issues of major concern to someone facing unprotected exposure to a vacuum are very similar to the ones that scuba divers spend a lot of time thinking about. This is understandable when you consider that both people are going to be spending time in an environment with a substantial pressure differential to the one the human body is designed to operate in. In the case of the diver his problems would start when he went from the high-pressure environment of the ocean to the low-pressure environment of the surface world. For the person being tossed out of the airlock, the problems are rather more immediate in that he’s going from a low-pressure atmosphere to a zero-pressure vacuum, but the general principles are the same.

By transitioning to an environment with a lower (or no) ambient pressure, anything inside your body that was previously held under pressure is going to expand. This includes gases in your lungs and in your bloodstream, so the fastest way to die upon being exposed to a vacuum is actually to hold your breath. The air in your lungs will expand quite literally to bursting point and cause catastrophic damage to your lung tissue, rupturing the lung, forcing its way into your bloodstream and from there on into your heart and brain, causing a swift death through pulmonary/cerebral embolism.

Divers are instructed not to hold their breath when going through decompression procedures for precisely this reason, but another thing they have to watch out for during decompression – and can’t necessarily avoid — is decompression sickness, also known as the “the bends”. The bends are caused by nitrogen gas which has been dissolved into the bloodstream expanding into a state of bubbly, frenzied liveliness thanks to the lower ambient pressure. This can produce a number of symptoms ranging from itching, to excruciating pain in the joints, to unconsciousness and death. Divers control decompression sickness by limiting the rate of their ascent (simulated or otherwise) to about ten metres per minute, but obviously this isn’t going to be an option for somebody experiencing a bracing vacuum environment, and so it can be assumed that anyone in a vacuum is going to experience a particularly severe variety of the bends.


This is somewhere near the bottom of their list of problems, though, since decompression sickness will only become an issue if they actually survive the vacuum. Achieving this will be especially challenging since they can’t hold their breath, which forces them to rely on the latent oxygen circulating around their bloodstream to support their higher brain functions. This will run out in about 10-15 seconds, after which our hapless victim is going to be rendered unconscious. If they can’t save themselves within this narrow time window and no outside help is available, they’re pretty screwed.

In light of that the other ill-effects of vacuum exposure seem rather trivial; for example, you’re going to be soaking up a lot of unfiltered radiation so you’d have the world’s worst sunburn afterwards, but you’re probably not going to be awake to notice that. You’re not going to be awake to notice any swelling that goes on, either. In fact you’d miss most of the fun stuff that goes on when you toss somebody into a vacuum; while the body deteriorates fairly rapidly and you would be dead within, say, two or three minutes, this is only going to matter if there’s somebody on hand to save you. If there is, and you survive, then you could expect a lengthy hospital stay but you’d probably recover. Mostly, anyway. If there isn’t, well, it is at least a relatively quick death, if only because you’re going to be unconscious for most of it.

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6 thoughts on “How To Survive A Vacuum.

  1. aosher says:

    Huh, that’s interesting. So what’ the minimum you’d need in order to survive in space for, say, ten minutes? Would some very high factor sunblock and a small portable oxygen cannister do it?

    • Hentzau says:

      No, because you literally cannot breathe in space. It is impossible; attempting to use the oxygen canister would simply result in the whole ruptured lungs thing. Even if you could the swelling would do horrible, horrible things to your blood flow and I suspect your cardiopulmonary system would simply shut down in protest after a while because so much blood is pooling in unfamiliar areas of your body.

      But really, nobody’s sure what ten minutes of vacuum exposure would do to someone. That 10-15 second unconsciousness number? That is derived from a single incident during the early space program where a man was accidentally subjected to a very low pressure environment of less than 1 psi. He passed out in fourteen seconds while the experimental team hurriedly tried to repressurise the chamber. That’s the sum total of our data for full exposure of a human to a vacuum environment, for the obvious reason that it is a very dangerous environment and we’re not going to take any unnecessary risks. We’ve done it with chimpanzees but the chimpanzees were obviously dead after two or three minutes, so we don’t know what ten minutes of exposure would do to a living one.

      • Gap Gen says:

        Didn’t the Nazis do various things like this to people, just to see what would happen? Or do people tend to disavow Nazi science as essentially a live action Saw movie?

  2. Gap Gen says:

    So this is nothing to do with Dyson spheres?

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