Rodimus Prime wrote:If an astronaut is out in space, doing an EVA, and s/he removes his/her helmet, thus removing heat and pressure, will his/her head blow up from the inside pressure, or freeze from the cold of space first?
I don't know about the pressure thing, but the second part I can comment on.
Space isn't cold. Vaccum is an insulator. One of the most difficult things about being in space is dealing with excess heat.
Reference here:
http://tvtropes.org/pmwiki/pmwiki.php/Main/SpaceIsColdSpace Is Cold
Ah, Kirk, my old friend, do you know the Klingon proverb that tells us revenge is a dish that is best served cold? It is very cold... in space.
— Khan Noonien Singh, Star Trek II The Wrath Of Khan
Space is cold, right? We hear Speculative Fiction writers blather about "the cold depths of space" or "the freezing void." If you get thrown into space, you're going to freeze straight away, assuming you don't explode.
Actually, no.
An unfortunate inversion of Convection Schmonvection is the widely held assumption that space is objectively "cold." Although what little matter there is between the stars is extremely cold, it can only make other matter cold by coming in contact with it. There's so little matter floating around out there that the odds of hitting enough of it to actually absorb the heat from you are very, very low.
The physics behind this are quite simple. Heat transmission can occur in three basic ways: convection, conduction and radiation. In the near-perfect vacuum of space, convection and conduction are, if not completely out of the question, not going to happen nearly often enough to make a noticeable difference. This makes heat exchange vastly more difficult; observe vacuum flasks, which use vacua specifically for insulation. The biggest difficulty in designing modern spacecraft is in cooling them, not heating them, because there's (almost) nothing to transfer the extra heat to.
Changes in the temperature of an object in a vacuum depend on whether it radiates more energy than it absorbs from cosmic radiation. This means that a human body in interstellar space will eventually freeze, but it will take a very long time. Note that near a star (or other energy-emitting Negative Space Wedgie), an object in space is likely to gain far more heat than it loses.
However, the idea that anything exposed to space will instantly freeze has some basis in reality, though for different reasons. Since there is, for practical purposes, no air pressure in a vacuum, the boiling point of water will plummet, causing any water to immediately begin boiling. Since some extra energy is needed for water to change from liquid to gas (aside from the energy needed to reach the boiling point), the water robs the object of heat quickly. This evaporative cooling will likely cause some freezing on a person Thrown Out The Airlock — the eyes and mouth, for instance — but will just make their death slightly more horrible (and blurry), rather than instantly turning them into a Human Popsicle.
In addition, slightly more technically-inclined people may get confused by the so-called cosmic microwave background radiation, energy which permeates the known universe at a temperature of about 3 kelvin. A common simplification of this concept is to say that the temperature of space is 3 kelvin — which many take to mean that you'll freeze down to 3 kelvin if you go out there.
This is true in the long run; a person (or any other object) left in space for a prolonged period of time would cool down to the temperature of the surrounding space. There are however two caveats to keep in mind. First, this temperature will only be as low as 3 kelvin if there aren't any other radiation sources (like, say, stars) anywhere nearby: anyone exposed to sunlight in space is actually in danger of roasting to death, not freezing. Second, the "prolonged period of time" required for the human body to freeze in a totally dark vacuum is about an hour, not the seconds (and definitely not the instant flash-freezing) usually shown in movies.
However, any object found floating in space is likely to be very cold indeed; touching it would be a bad idea.
"Cold Space" is a near-universal trope in Speculative Fiction to the point that, when aversions appear, they meet with disbelief. 
Posted: Tue Jul 14, 2009 2:14 pm