Space is very cold. But interestingly, this would have little effect on you, even without wearing a spacesuit. Inside a suit, however, the real problem in space is in fact heat.

Header image: NASA astronaut Bruce McCandless II on an EVA about 320 feet away from the shuttle Challenger in 1984 (photo: NASA)

I’ve never thought about this until last night, when I did some research after reading something in a book, but there’s a misconception out there about what space is like that stubbornly clings to all kinds of fictional (and even non-fictional) stories about the subject. It is so stubborn in fact, that in the forty years of my life, I’ve read a lot about space exploration and travel and this only occurs to me now.

The misconception is that space is cold. Space is in fact very cold, it is almost as cold as anything can get, just a few grades Kelvin above the absolute minimum, which has been theorised as the heat left over from the Big Bang explosion that started the universe. Paradoxically, outer space is also very hot at the same time, as the precious few hydrogen atoms in it are moving around extremely fast (a concept called kinetic temperature) and heat up anything they hit quite a lot. For a single atom, that is. And as there’s only about one of them per cubic metre of space, the impact is very small.

Practically, though, to a spaceship or an astronaut, space isn’t cold at all. Since it is a near vacuum, you wouldn’t perceive it as cold. In fact, you could relatively easily take your space helmet off – as one character does in The Expanse TV show – and not feel any cold at all 1. That’s because vacuum is a very good insulator. Or in other words: Since there’s no air to cool the exposed parts of your body, there’s no way for you to be directly affected by the cold of space. The same goes for your spacesuit or a spaceship hull.

In fact, one of the biggest dangers in space is heat. Spaceships and astronaut EVA suits are well insulated mostly to prevent heat build-up from exposure to the sun’s rays, which can be hotter than a hundred degrees Celsius in orbit outside the Earth’s atmosphere. It’s hard enough to get rid of the heat generated by an astronaut’s body and the technology inside the suit or a spaceship as it is, without taking on more heat in the form of light from the sun – which is why spaceships, aside from being well insulated, are always painted white or covered in reflective foil.

There are only two ways of getting rid of heat in space. The first is radiation in the form of infrared light – basically directly sending your heat into the depths of space. The ISS, for example, is cooled in this way. It transports heat through a water cooling loop, exchanges it with a cooling loop filled with ammonia and this is circulated to thin white panels that are mounted next to the solar arrays the station uses to generate power. These radiator panels then lose some of the heat energy into space in the form of infrared light. This is a pretty slow process, though. It’s what would eventually cool your body to the median very low temperature of outer space if you drifted in the black for a few days. If you floated into sunlight, though, you’d also get heated at the same time. Astronaut EVA suits actually often get quite hot on the side facing the sun and cold on the side that radiates heat to the black of space. Hence they are insulated a lot so that the wearer doesn’t experience this.

The second way to lose heat in space is sublimation. This is the process by which a solid element becomes gaseous without ever experiencing the liquid state in between. This process is endothermic, meaning it needs heat energy in order for it to happen. You can experiment with this if you put very wet clothing out on a clothesline on a very cold but sunny day. The water in the clothes will freeze and when heated by the sun, the ice will disappear into thin air, drying the clothes. Astronaut EVA suits are cooled using this process. The suit actually has a device that forms a thin layer of ice which is then sublimated to gas that escapes out into the vacuum of space and, amazingly, takes heat with it.

The ice sublimator consists of sintered nickel plates with microscopic pores which are sized to permit the water to freeze in the plate without damaging it. When heat needs to be removed, the ice in the pores melts and the water passes through them to form a thin sheet which sublimates. When there is no need for heat to be removed, this water refreezes, sealing the plate. The rate of sublimation of the ice is directly proportional to the amount of heat needing to be removed, so the system is self-regulating and needs no moving parts. During EVA on the Moon, this system had an outlet gas temperature of 44 °F (7 °C). The pores eventually get clogged through contamination and the plates need to be replaced.

This process is much more efficient than direct radiation and thus removes more heat in a shorter amount of time.

With all the stuff that has been written about space exploration even since before NASA went to the moon, it is amazing to me how often people get this stuff wrong – or omit details like this. Even the educational books I read as a kid, which were packed full of all kinds of details about spaceflights, never explained this to me. There are so many misconceptions people have to this day on the topic of space. It’s quite amazing.

  1. You would suffocate very quickly, though, as the vacuum sucked all the oxygen out of your lungs and even blood. Interestingly, not much else would happen to you, as the pressure differential would only be one atmosphere. The saliva in your mouth would boil, but it wouldn’t hurt as it would still be at body temperature. All the stuff about your blood vessels exploding, you getting the bends or freezing instantly is more misconceptions brought to us by Hollywood. ↩︎