That's why scuba divers wear wetsuits: to trap a layer of water and prevent it from carrying away that precious body heat. In a vacuum, there's no convection — and no conduction, either. That only leaves radiation. Every human is glowing, in the infrared spectrum, from radiating heat at about watts. A light bulb used to be the perfect analogy for the energy output of a person, until we all switched to CFLs and LEDs.
But you still get the idea. Usually we don't notice all this lost energy: swaddled in an insulating layer of air, and warmed by the sun above our heads and the ground beneath our feet, our thermal output is more than matched by the thermal input of our environment.
We can happily radiate all day long. In space there's nothing to insulate you, so eventually you'll freeze to death. But fortunately, that loss of watts of heat isn't all that much compared to the sheer mass of your body.
You ever notice how long it takes to boil a pan of water, or how long it takes for a pile of snow to melt? In the vacuum of space, you're not turning into a popsicle anytime soon.
What ultimately dooms you is your body's own traitorous circulatory system. There's no air in space it's kind of part of the definition , which means there's no oxygen. But your blood doesn't know that. The vacuum of space, however, is much more hostile. The most serious dangers of exposure to outer space are a lack of oxygen and ebullism. Ebullism is the formation of bubbles in body fluids due to a reduction in ambient pressure.
Your blood will also not boil. However, space is virtually vacuum , which means that any person unfortunate enough to find their self unshielded in such an environment will experience rapid internal decompression. The largest organ in the human body, the skin, is extremely flexible and tough.
The body will expand to roughly twice its volume because the water in the tissue will rapidly turn into water vapour. This action will also push against internal organs.
For instance, air expansion in the gut will push against the diaphragm and heart. As air is rapidly expelled out of the lungs, there might be some damage to the delicate tissue the lines the lungs and the airways. In fact, it is the loss of oxygen that will kill you first. If you do this, bubbles of air will be forced into the bloodstream, eventually arriving in the brain where they will cause a stroke.
Holding your breath will also expose the lungs to the force of atmospheric-pressure air against pure vacuum, likely rupturing quickly. This can happen to scuba divers if they descend too quickly into deep waters. That happens around the two-minute mark when all the other organs fail from oxygen deprivation. And it wraps us in a cocoon of lovely, lovely air pressure. In the vacuum of outer space, all that protection goes away.
The most serious danger is asphyxiation. After about fifteen seconds, your body has used up all the oxygen in your blood, and your brain loses consciousness. Now, you might be thinking - I can hold my breath for a minute! That might work underwater or under our atmosphere, but in outer space, there's no outside pressure. With no outside pressure air expands and can rupture the tissues in your lungs. Meanwhile, the water in your body turns into vapor beneath your skin, causing the mother of all bloating.
Youll swell to about twice your normal size. Your body wont explode like you see in some movies, but you will be in a world of hurt. After a few minutes, if the lack of oxygen hasn't killed you, the damage from depressurization will.
Outer space has other hazards - freezing temperatures, lethal radiation.
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