It’s more difficult than reading a thermometer
Our astronomer answers a question of interest to science-fiction writers.
Our astronomer writes:
It’s summer here in Alexandria, which means warm and humid weather. In these air-conditioned days it’s advisable to check the weather report before venturing outside for any length of time, not only in case an umbrella is indicated, but also to choose the right clothes and maybe bring along some water.
The current temperature and humidity are shown by a glance at the smartphone app. Well, current within the past hour, as measured at the airport a few miles away. How warm you actually get depends on things like whether you’re standing in the sun or shade, whether there’s a breeze off the river, whether your shirt is long-sleeved or short–many factors. The thermometer at the airport is only a guide. Even two types of fabric will come to different temperatures, though laid out in the same place at the same time.
Back to the question: how cold is space? One easy answer is about 3K, about -270C or -454F. That’s the temperature of the Cosmic Microwave Background, radiation left over from when the universe was much smaller than today (though still possibly infinite in size), also much denser and hotter. Pretty darn cold. But if you put something out in space, orbiting the Earth, that’s not the temperature it would reach. If it’s a blackbody, something that absorbs and radiates all sorts of energy equally, it would get much warmer, maybe 394K (121C, 250F). (Blackbodies are like ideal gases: they don’t exist, but they’re easy to calculate, and some things almost qualify.) That’s because it would absorb radiation from the Sun and heat up until it could radiate it away. Things that are not blackbodies can be either colder or hotter, depending on their details. The Earth is not a blackbody, in such a way as to be cooler.
Well, let’s get far away from the Sun, any star, out into the Galaxy. What’s the temperature there? As before, it depends on what is out there, what it’s made of. Interstellar gas is very thin, thinner than the best vacuum we can make on Earth, but there’s something there and starlight has an effect on it. In fact there are several components that absorb and emit radiation very differently, so the temperature of interstellar space has several values simultaneously: chiefly one about 50-100K, very cold by Earth standards, and another at 6-10,000K. On Earth that’s hard to do for long, because two substances in contact tend to exchange energy and come to a common temperature. But the components of the ISM are so thin that the particles can travel for years without encountering and interacting with each other.
The same sort of thing happens if you move far away from the Milky Way into intergalactic space. There’s still radiation, from galaxies as well as the CMB, and there is still a very thin intergalactic medium (IGM), about which we know a little. Some of it seems to be very hot; but how hot you’d be if placed out there, well, that’s another question.
So “How cold is space?” has no answer: space itself has no temperature. Or perhps it’s sort of half a question, something that needs another part before you can get an answer. You have to specify, not only where (including what stars and other energy sources are around), but what is out there that you’re taking the temperature of.
