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Hank answers a SciShow viewer's most pressing question about what happens if the human body gets exposed to space. Would your head really explode?

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References
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970603.html

http://blogs.discovermagazine.com/badastronomy/2012/01/31/qba-what-happens-if-you-are-exposed-to-the-vacuum-of-space/#.UMGD-pPjn0g
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Of all the exotic, horrible, and totally awesome ways to die that science fiction writers have dreamed up, some of the coolest involve being exposed to the vacuum of space. On the page or on the screen, you've seen it described somewhere. A spacesuit leaks or a ship's hull breaches, and suddenly some unwitting astronaut is frozen solid or their blood's boiling or they just start convulsing and screaming until... (head explodes). Totally fun to watch, but is that what would really happen?

Well, there have in fact been plenty of studies and tests and, yes, even a few near-misses with real-life astronauts to give us a sense of what would happen. And it's not what you might think.

First, you wouldn't explode. This was a popular theory for awhile because our bodies exert internal pressure to counteract the atmospheric pressure that's exerted on us from the outside. But in space, this pressure would be well contained by your connective tissue and your skin. As long as those things remained intact, the rest of you would too. 

Another scenario is that your blood would boil. Now sci-fi writers though this up because the lower the air pressure gets, the lower a liquid's boiling point is. So in space, where there's no pressure at all, most liquids as we know them would boil instantly. But if your live body was floating around in space, your blood would not boil because, again, it's part of an enclosed system-- in this case, your circulatory system-- so it wouldn't be directly exposed to the vacuum, it would be subjected to your body's same internal pressure.

So what about freezing to death? At first blush, this makes sense because the universe is like the coldest place in the universe: about 2.7 Kelvin, or -270 degrees Celsius. But in order for your body to lose heat, it needs something to transfer that heat to. This is known as thermal conduction; and in a vacuum, there's no air, there's no stuff around you, which means there's nothing to absorb your body heat. So you wouldn't freeze either.

So it's like "Will I die at all?" Well, yeah you would. You're not Superman. For starters, all of the air would rush out of your body really fast. No matter how tight you close all your orifices, you wouldn't be able to resist the pull of the vacuum. So every molecule of air would flow out of your lungs-- and also of your intestines, so if your underwear was still clean at this point, it wouldn't be any more.

In another 10 to 15 seconds, you would black out. That's the amount of time it takes for blood from your lungs to reach your brain, so with no air to breathe, your brain would start starving for oxygen.

The lack of pressure would cause all the blood vessels on the surface of your body to break, especially those in your eyes. And all of your body fluids that were directly 
exposed to space would in fact start to boil.

This is one of the few things that we actually have first-hand knowledge of because in 1965 during a test in a vacuum chamber at the Johnson Space Flight Center in Texas, a leaky spacesuit accidentally exposed a human subject to near-complete vacuum. He passed out in 14 seconds, and the chamber was re-pressurized 15 seconds later. When he regained consciousness, the subject said that the last thing he remembered was the saliva on his tongue beginning to boil. Because of the incredibly low pressure, the water in his mouth was able to boil at body temperature. 

In the end though, after blacking out, you would simply suffocate from lack of oxygen. So, keep all this in mind before your next extra-vehicular activity, and always double check your gear.

Oh! And if you were naked-- uh, I don't know why you would be-- but if you were naked, you would also get the worst sunburn ever, but you probably wouldn't mind that much considering how dead you'd be. 

Thank you to whoever asked us this question. Totally fun question to answer. We forgot who asked it though-- sorry about that. But if you have an interesting or weird question for SciShow to answer for you
, we're on Facebook and Twitter and also in the comments below. And if you want to keep getting smarter with us at here SciShow, you can go to youtube.com/scishow, and subscribe.

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