We've all seen that movie: where some unfortunate guy gets sucked out of an airlock and is thrown from his spaceship into unknown space. What is the first thing he does? Struggle to breathe. This is followed by panic, while blood oozes out of his eyeballs and ears. Until finally, his slow death ends, and all we're left with is a frozen corpse. Yuck.
This cinematic depiction is mostly Hollywood and part truth. Most space movies–– save for Interstellar, who had the incomparable Kip Thorne on its production team–– are focused more on depicting drama than science. So, what exactly happens when your body is exposed to space? (Hint: Don't get sucked out of an airlock... or take off your helmet... just be careful, OK?)
You're in Space. Now what?
Uggghhh so, you got sucked out of an airlock. I told you to be careful! People never listen. Well since you didn’t do what I told you to do, the first thing you can expect is a quick death. When a spaceship or astronaut's suit is exposed to space it quickly depressurizes. The spaceship or suit depressurization is due to the vacuum space creates. If this does happen, your only hope is that you are hit and die quickly by the flying debris that is being pulled out into space from the air lock. If you don’t die, there's more, trust me.
So you didn't get hit by anything to kill you, but now you’re floating out in space, alone. This would be you’re ‘oh crap’ moment. This is when I would truly be panicking. Unless you can grab on to anything and get yourself back into the spaceship, you're kind of... doomed. I’m guessing that you didn’t grab on to anything, so this is the next thing you can expect when your body is exposed to space. You are going to slowly lose the air in your lungs due to space's vacuum. Similar to how scuba divers experience ‘the bends’, (not the song by Radiohead). It's actually the buildup of atmospheric pressure happening inside your lungs–– if there's any air left in them. As a result, say goodbye to your lungs–– they're about to be completely ruptured under pressure.
You Implode, not Explode
Now you are gasping for air... but there isn’t any. The next thing to expect is swelling up to a blueberry like Violet Beauregarde did in Willy Wonka. Well not a blueberry exactly. At this point you are not lucky enough to be rolled away through some candy factory by Umpa Lumpas. You’re in space silly! You would experience swelling, or ebullism to be more scientific. The process of ebullism is relatively simple: in everyone's blood there is nitrogen; and as that nitrogen dissolves (as a result of space's vacuum), it will begin to quickly form bubbles. LOTS of bubbles. So much so in fact, that they are all going to expand, starting at your organs, until your body gradually begins to physically expand.
As your body is composed of 70% water, when it is exposed to space, the water turns to vapor. This would result in violently rapid internal swelling throughout the body. Like being boiled alive. This formation of the combination of bubbles and water vapor in your body wouldn’t exactly make you explode, rather implode. It would most definitely make you uncomfortable.
You're going to get COLD. Very cold.
Now that you got all of that extra water weight, you will next experience a little coldness. Not a little really–– the coldest you’ve ever felt in your life times a billion. I guess you should have brought a sweater with you on your trip out the airlock. But if you got sucked out of an airlock, you don’t really deserve a sweater now do you? Since you are obviously not one of those kids that listened to your mother, here is the next thing to expect next when the body is exposed to space: the vacuum of space causes heat to be transferred away from your body. This is going to affect your eyes, lungs, and mouth, and any water vapor will very quickly evaporate. So, rather than feeling the same cold you experience in the winter (i.e., the air being cold, externally), the body has no air to breathe or water to insulate it, which results in your eventual freezing. But at least all that water-weight will disappear!
The worst Sunburn of your life
The next thing you would experience is a really painful instant sunburn. Hopefully you put some sunblock on before getting sucked out of that airlock... Here’s what's going to happen next: like I said, you get a really bad sunburn, the worst you’ve ever experienced. Even worse than that time you fell asleep on the beach. Much worse. Since you don’t have the protection of the Ozone layer on Earth, the Sun’s ultraviolet rays would fry your skin. The effects would be devastating to your body. Your only hope at this point is that you pass out from shock and die... because if you don’t, your eyeballs are next. The brightness of the sun would damage the light sensitive retinas in your eyes leading to blindness. It would be the worst. Your blind and sunburned, but by this time death would be a little bit better.
The End is Near
So, you’re freezing your ass off, skinny, blind, got a sickening tan, and oh yea, still alive. Since you’re not-dead-yet, here’s the next step in this long drawn out death: you will become brain dead. Finally some relief, am I right? When your body is exposed to space, the brain can shut down within 15 seconds. How does it shut down exactly, you ask? Well, remember how the liquid from your body is going to be quickly sucked out and form gasses from the nitrogen bubbles that force you to expand? A similar reaction is going to go down inside your skull, as in, all the oxygen that was helping you breathe and stay conscious is now going to disappear. After about 15 seconds you're going to pass out, with no way to save yourself. At least the panic and suffering would end.
Now you know what it's like for your body to be exposed to space. And finally, you are dead.