How cold is space exactly, and how do we measure it? (2024)

If I were to ask you if space is cold or hot, you would probably say cold, right?

And if I were to ask you to guess what the temperature of space was, what would you say? Zero degrees? Absolute zero (-273.15 degrees)?

Well, the answer is a fair bit more complicated than that, and leads to some surprising results.

First, what do we mean by "temperature"?

Temperature is a way of measuring the heat of an object, or more precisely, measuring the average kinetic energy (you can think of this as movement or "jiggling") of the atoms/molecules of a substance.

When we add more heat, atoms jiggle more, which then raises the object's temperature.

In space, we don't have many particles, so in a sense it doesn't make sense to talk about the "temperature of space". But what about the temperature of the rocks and particles that are floating out beyond our atmosphere?

Well, let's start by looking at the satellites and other objects surrounding Earth. You would assume they'd get pretty frigid cold, right?

Well, in one sense you'd be right. If you were an astronaut on the International Space Station, when you were on the dark side of the Earth in the Earth's shadow, the temperature outside would be a chilly -150 degrees. Freezing! (Well, 150 degrees below freezing.)

But, while you are on the daytime side of the Earth, facing the sun, it would be a different story. Without the Earth's atmosphere to act as an insulating blanket, the temperature would shoot right up to 120 degrees! Enough to overboil your tea.

The International Space Station orbits around the Earth in 90 minutes, so the materials that make it and similar satellites have to be able to manage changes in temperature of more than 200 degrees every 45 minutes.

The Boomerang Nebula, located about 5000 light years from Earth, is the coldest known object in space. A dying star is shedding layers of gas out into space. The Atacama Large Millimeter/submillimeter Array has measured its temperature at a chilly -270 degrees.

But what of the average temperature of space away from the Earth?

Believe it or not, astronomers actually know this value quite well: an extreme -270.42 degrees (2.73 degrees above absolute zero). This temperature comes from a source of radiation known as the "cosmic microwave background", a remnant of the energy released by the Big Bang, and can be thought of as the average temperature of the present-day universe.

While this is absolutely freezing, we have measured temperatures even lower than this on our own planet (and no, I don't mean a Canberra winter).

The lowest recorded temperature is less than 100-billionths of a degree above absolute zero, and some of Canberra's researchers at the ANU Research School of Physics have achieved extremely close to this record.

So, if you ever find yourself wondering about the temperature of space, know that it is both cooler, and warmer, than you might think.