eli5 – How is space cold?


How is space “cold” when it’s a vacuum? Isn’t temperature transferred between mass? If anything, shouldn’t you overheat when in a vacuum, because your body generates heat?

In: 1

Yes and yes. Space is cold in the sense that there’s very little to conduct heat, but objects in space are at a serious risk of overheating (because other types of radiation can be transformed into heat) as it can be so difficult to get rid of.

NASA has a page about this and the International Space Station:


Space is not a true vacuum, and it’s cold because the few molecules that are there don’t have a lot of energy, which is by definition cold.

That said, you are not wrong in that overheating is a real problem in space. While it might be cold, there’s not enough around to give off heat via convection faster than we build it up, so both space suits and the space station have systems built in to actively shed heat in other ways – like having big metal radiators on the side of the space station that radiate heat away.

Things can be very cold but almost not conduct heat. Touching metal at -100 °C can hurt a lot, and cause serious damage to the skin. Meanwhile, touching plastics is relatively fine; with aerogel, you possibly won’t even notice the cold. That is due to their low thermal conductivity, which your body can overcome. Stuff feels hot or cold when it transfers heat to or from you.

It’s the same with the vacuum of space. Stuff in space (this includes entire planets, astronauts, satellites, asteroids, you name it) cannot conduct heat away in any immediately notable manner.

However, there is another method for things to exchange temperature: light, thermal radiation in particular. Everything emits light, the hotter the more and brighter. You can feel the radiating heat of a campfire even upwind. It’s also what makes very hot objects such as molten steel, incandescent lightbulbs or the sun itself glow brightly. Even your body does so, just at a significantly lower intensity, and effectively all of it as light our eyes cannot see.

It typically takes much longer to radiate heat away as light as to conduct it. Hence your body will heat itself up. Even worse, any sunlight will also add to that. Your own light is just too weak to counter those; it only changes long after you would have boiled to death. Or you stay in the shadow of a planet, in which case stuff will slowly cool down to deep freeze.

Why do you think they have to be mutually exclusive? Space is cold and also you would overheat.

You overheat because space can’t absorb and take that heat from you.

Space is cold because, well, it can’t absorb heat.

So, heat is the transfer of energy, mainly infrared radiation. Radiation is energy that radiates from a source. Here on earth, most radiated heat comes from the sun, with some heat being thermal generated from volcanic activity or other seismic movement.

In space, the radiated heat from the sun does not travel relatively far. While the earth is like 90 million miles from the sun, that’s not very far compared to Jupiter, Saturn, Uranus, Neptune, or the other millions of small ice objects floating around out there.

Now space has lots of stuff in it, but there’s a lot of empty space between the stuff. Heat will transfer either by radiation like with the sun, or through molecular transfer, like with a metal pan handle.

This is why bodies in space might get hot when directly facing a near by star, like how hot murky gets on the day facing side, but without an atmosphere, the dark night facing side releases all the surface heat and quickly becomes ice cold.

Here on earth, or any planet with a thick atmosphere, heat gets trap because of the air molecules acting as an insulator, this is one reason why temperatures get colder as elevation increases, air thins, and insulation qualities decrease, not the only reason, just an example.

Well space lacks that thick insulation atmosphere so heat doesn’t get trapped. This is why if a person floats around in space, the sun’s radiation will burn and boil the unprotected facing side while the side facing away from the sun would be freezing.