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Alright, so we have all seen the movies where someone gets blasted out of the airlock on their starship, or their suit decompresses while on a space walk. The poor schmoe usually either decompresses so violently that blood is oozing out of every orifice in their body, or they freeze instantly.

From this I have two questions:

  1. Would the decompression really be that violent?
    1. Clearly the drastic difference in pressure from a normal "earth" like environment to space would be bad, but would it be that devastating.
    2. I vaguely remember that standard atmospheric pressure was something like 15 psi, which doesn't seem like enough to mess you up that bad.
  2. Would you actually freeze instantly in space?
    1. Heat, or lack thereof is a measure of internal energy, but in a vacuum there wouldn't be anything to have internal energy, so does space even have a temperature?
    2. Wouldn't some form of matter have to be present in order to cool off? If there were no matter besides yourself and a few stray particles here and there, it seems like it would take a very long time to cool off.
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Most of what is shown in sci-fi on this is believed to be false. You do not freeze to death and your blood does not boil.

Provided you do not try to hold your breath there are not likely to any ill effects for at least 30 seconds or so. If you do try to hold your breath as the de-pressurisation takes place you can suffer a "burst lung" i.e. an embolism, just as a diver would if he holds his breath during an ascent from depth.

After some time in space the lack of oxygen will be the most damaging. A few minutes will lead to death.

You can find more details at NASA's Ask an Astrophysicist page, under How would the unprotected human body react to the vacuum of outer space?.

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    $\begingroup$ remember some SF story (I think it was something by Clarke) where a large number of people go into space for a brief period without a proper suit because they have to quickly evacuate a spaceship harmed in battle. They have a kind of space war veteran explaining them that the usual stories are bogus, and how they can make it across to the rescue vessel (it's just some hundred metres). It's very much in accordance with that article. $\endgroup$
    – Hanno Fietz
    Commented Jan 3, 2013 at 21:58
  • $\begingroup$ Unless you're near a hot object, you will eventually freeze, but it takes hours for all your body heat to radiate away. $\endgroup$
    – endolith
    Commented Jul 13, 2013 at 6:04
  • $\begingroup$ In agreement that "most of what is shown in sci-fi on this is believed to be false", I also want to mention the animated movie "Titan A.E." as an example of "more realistic" scifi. This movie has probably the most realistic depiction of people getting jettisoned to space of all scifi I've seen. The main character is specifically told to exhale prior to being sucked out into space. $\endgroup$
    – NeutronStar
    Commented Mar 12, 2016 at 22:10
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I saw a video last year of a person testing a pressure suit in a vacuum chamber. This occurred back in 1966. The video has been removed unfortunately. What happened is a pressure hose detached and the air pressure ran out of his suit. He actually stood up conscious for about 30 seconds, but then passed out. He fell off of a stage, which frankly looked painful. The crew outside repressurized the chamber and within a minute the guy was alert and stood up. According to this man he could feel the saliva in his mouth start to boil as if he were drinking a soft drink. Otherwise there were no ill effects.

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    $\begingroup$ Wow, now I feel grateful that my experiments are not as scary. Do you have any more information about this event, I would have guessed that he dies pretty fast as water boils violently in a vacuum chamber. $\endgroup$
    – Alexander
    Commented Nov 18, 2011 at 2:04
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    $\begingroup$ For anyone stumbling across this, heres the video Lawrence probably means: youtube.com/watch?v=KO8L9tKR4CY $\endgroup$
    – Anedar
    Commented Dec 13, 2017 at 0:17
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Your circulatory system puts enough extra pressure on your blood to keep it from boiling in a vacuum, and space itself doesn't conduct heat, so all the heating/cooling would be due to radiation or evaporation.

I think your exposed skin would swell, and fluids would evaporate and dry out your tongue, throat, and lungs, and your eardrums might burst, which would be painful. If you tried to move around you might risk overheating, since there's no way to cool yourself.

On the other hand, there's a chance your sweat-glands might still work, since they cool with evaporation.

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  • $\begingroup$ Ear drums release pressure easily, it's the opposite that bursts an ear drum (diving and not equalizing your ears). $\endgroup$
    – Michael
    Commented Mar 29, 2016 at 23:08
  • $\begingroup$ The cooling due to black-body radiation is on a similar proportion as what you might feel from typical cooking temperatures (except opposite, and probably slightly less). The heat from a hot frying pan can be felt at a distance, a similar but instead cooling feeling would be felt in all directions. $\endgroup$
    – Michael
    Commented Mar 29, 2016 at 23:14
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Other answers have handled the oxygen issue quite well.

In regards to the temperature, space itself has no temperature because it's a vacuum.

Objects in space, however, do have a temperature.

If a human is exposed, unprotected, to space near the sun (or any other star), the temperature change in their body could very well be terminal.

Even near our planet, at the ISS, bare metal in the sunlight can reach 260 degrees Celsius. (source: https://www.universetoday.com/77070/how-cold-is-space/) Closer to the sun, not surprisingly, things get much hotter!

Similarly, if the exposure happened very far away from any source of heat radiation, the human would be exposed to temperatures approaching 2.7 degrees Kelvin (that's very close to absolute zero). I'm not aware of any tests that have been conducted at that temperature on humans (thankfully), but we can speculate that the thermal loss and dehydration at that temperature would be fatal rather quickly.

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  • $\begingroup$ How quickly? Faster than the 30 seconds you have until lack of oxygen becomes a problem? $\endgroup$
    – stannius
    Commented Jan 23, 2019 at 21:06
  • $\begingroup$ The discussion of temperature here seems incomplete and not based on clear physical principles. There are going to be various mechanisms of heat transports in and out of the person's body. They are likely to be slow. $\endgroup$
    – user4552
    Commented Apr 1, 2019 at 1:23
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When you apply the Stefan-Boltzman Law, you radiate with power 1000 W in deep infrared spectrum at zero degree kelvin (black body radiation with 20 degree celsius skin temperature and area of two squared meters). On earth you would gain radiatian by environment and radiate only 100 W. So you would lose ten times more heat by radiation in space in the shadown of earth compared to the netto radiation loss on earth. At rest the body produces approximately 100 W power, so you would eventually freeze but it would need hours. Additionally the skin becomes cold and radiates less because the temperature goes by fourth power in Stefan-Boltzman Law. Below there is a link to a online calculator for this law.

Stefan-Boltzman Law application

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Real uncontrolled decompression would probably be even more violent than what you've seen in movies. As explained in this excellent answer, the air rushing out of the airlock could exert a force similar to getting hit by a car - or greater, depending on the size of the airlock door. This had tragic effects in the Byford Dolphin decompression accident, which killed five divers and badly injured a sixth. Autopsies of the divers revealed massive internal injuries likely caused by decompression alone. So if the blast of wind isn't fatal, rapid decompression almost certainly will be.

Based on the 1966 space suit test involving Jim LeBlanc, exposure to the vacuum of space alone isn't necessarily fatal. A person most likely would dry out a little, pass out after a few seconds and asphyxiate after a few minutes. It's worth noting that Jim didn't experience explosive decompression. His suit depressurized over about 10 seconds.

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    $\begingroup$ That's an interesting link (the physics, not the tragedy), but it is worth pointing out that this decompression was eight times larger than 1atm to vacuum. The mechanical effects are correspondingly exaggerated, both from the added pressure and the added mass of air. The biochemical effects (denaturing of lipoproteins, and the consequent precipitation) are unlikely to have a simple relationship with the initial and final pressures. $\endgroup$
    – Emilio Pisanty
    Commented Mar 31, 2019 at 18:47
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If you remember the infamous teenage date malady, the hicky. I think you'd see something like that as the blood would begin to ooze through your skin. But, I think this effect takes several minutes, so you'd already have died from lack of Oxygen.

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    $\begingroup$ Joseph Kittinger the man who parachuted from space had a leak in his pressure suit glove which he said gave him a very impressive hickey - his hand swelled to twice it's normal size $\endgroup$
    – Martin Beckett
    Commented Apr 1, 2011 at 16:50

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