If outer space is a vacuum, how do we measure its temperature?

We're back for another instalment of Ask ARSE for your reading pleasure.

We received an interesting question from a follower in an email that reads:

"If outer space is a vacuum, how do they measure the temperature?"

Assuming he means us, so let's go ahead and Ask ARSE...

 

 

Temperature doesn't exist in space. 

But… 

If we had to answer: it’s very cold in space.

For context, you have to understand heat like scientists do: a measure of how wiggly atoms are. Cool things move slowly, hot things move fast. Absolute zero is when atoms come to a complete stop. The temperature in space is just a few Kelvin above that at an average temperature of 2.7 Kelvin (about -235°C).

However, it's reasonable to wonder what happens to a thermometer in space.

A body can exchange heat with its environment in three different ways: conduction, convection, and radiation. The first two don't work in space: there's nothing to conduct or convect. In space, there is no air or water, so the only way to lose heat is by radiation, where your warm and wiggly atoms release energy directly into space. 

Only radiation works in space.

As a result, if your thermometer is at room temperature, it will radiate heat into the empty space around it. But will it receive any heat in the form of radiation?

Yes.

If that thermometer is in the vicinity of the Earth, it will receive plenty of sunlight. It will also get a fair amount of heat from the Earth itself. So don't be surprised if you see the temperature of the thermometer soar.

But what if you take that thermometer into really, really, really deep space? 

If you found yourself in one of those incredible voids in between clusters of galaxies that are so far from everything you'd see nothing but pitch-black space, even the nearest galaxy is too far away for the human eye to see.

The thermometer will start to cool, radiating away its residual heat. 

It will cool all the way down to about 2.7 kelvin (that is, roughly -270°C). 

Why no further? 

Because at that low temperature, it will radiate the same amount of heat that it receives from the deep sky in the form of cosmic microwave background radiation. In short, the thermometer will be in thermal equilibrium with the sky.

So if we really were hard-pressed to assign (deep) space a temperature, this would be it: 2.7 K, the temperature of the cosmic microwave background radiation.

Does this answer the question, or is it more confusing?

Let us know in the comments.
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