"I always thought space was dark and cold," he remarked vaguely.
"Forgotten the sun?" said Weston contemptuously.
— C.S. Lewis, Out of the Silent Planet
Outer space is really, really cold. -454.8 degrees Fahrenheit (-270 degrees Celsius) cold. Brr… Well, that's not completely true. Only matter can have a temperature, nothingness doesn't. And space is mostly a big empty void of nothingness. However, most random objects floating deep in space will be that temperature or very close to it.
Outer space lacks air, so heat is only transferred via infrared radiation1. This means that heat loss occurs very gradually. An object in deep space will eventually get to a few degrees Kelvin, but it's not the instant blood-freeze that's sometimes depicted in the movies. More like hours to freeze, and there's plenty of things to kill you before that. Something that's been floating in space for a long time will be really cold, too. Touching it would be horrible idea, as conduction would steal the heat out of you.
At the same time, solar winds can be really, really hot. The sun has a surface temperature of 9,980 °F (5,526 °C), and it radiates quite a bit of infrared. Likewise, interstellar gas clouds may be thousands of degrees.
The real nasty part here is the extremes of temperature, and the stresses they produce on objects outside of atmospheres and convection. At high earth orbit, the side of you that's in the sun will eventually reach 248 °F (120 °C). At the same time, the parts of you in the shade could eventually get as cold as -148 °F (-100 °C). The hot part is above boiling (212 °F / 100 °C), the cold part is below the harshest antarctic record (-128 °F / -89 °C). The human body doesn't take those temperatures very well, especially not at the same time2.
The temperature of other items will depend upon a number of factors: how reflective they are, how close they are to the sun and whether or not they're pointed at it, their shape and mass, how long they've been out there, etc. Polished aluminum pointed at the sun from roughly the same distance as the earth might heat up to 850°F. Something that's got an opaque coat of high-quality white paint might not get up above -40°F even if aimed at the sun.
Suffice it to say, you don't want to get stuck outside without your spacesuit.
Spacecraft in real life are given a slow rotation so that no part of the vessel remains in direct sunlight or in utter darkness for too long.
The boiling point of a liquid is not a constant - it depends on the pressure exerted on the liquid. This is why water boils more rapidly at high altitudes, where the air is thinner. Naturally the air can't get any thinner than "no air", so the boiling point of water is much lower outside an atmosphere.
In or near vacuum, the boiling point of water is below room temperature. This is part of why Space Exposure is so nasty - your blood boils in your veins. It's also why liquids are very rare in space - most matter in space is either a solid or a gas.
Game and Story Use
- Derelicts and space hulks may be brittle and unstable from temperature extremes.
- Environmental controls may have been compromised when the freezing water ruptured the reservoir.
- A species that lives in the void between stars must have incredible tolerances to temperature, radiation, and vacuum.
- Here's a wicked visual for you: Wreckage in high earth orbit includes a floating thermometer reading 45 °F, the mean temp as half of it is in light and half in shadow. Meanwhile, the spaced astronaut floating beside it is simultaneously burning and freezing.