Why is space cold?

05 July 2016


I'm teaching a bit of physics in my grade 4 class. We were having a lesson on heat transfer and when I was explaining 'radiation' I gave the example of the sun, even though it being so far away, the heat travels along way and can still warm us. Anther example was being warmed from a camp fire. Then a young girl asked, "Well, why is space cold then? If the sun is so hot, and we can feel it here on earth, why is it cold in space? As we move closer to the fire, we get warmer, the space between us an the fire is always hot and we can feel it all the time. Why isn't it like that in space? She's 9 - A budding scientist. Can you give me a simple explanation please? Thanks.


Chris Smith put this to the Open University's Andrew Norton...

Andrew - The thing you got to remember is the space between you and the fire is filled with air. That air gets heated up, the air molecules, the oxygen, the nitrogen are moving around rapidly. That rapid movement is what we mean by temperature. Things that are hotter, the atoms and molecules move around quicker. Now in space, it's pretty empty. There's a few molecules, maybe one atom per cubic meter or whatever it is. So, there's nothing there really to absorb that energy, that radiant energy from the Sun. So that's why space itself is pretty cold. If you had an astronaut in space, they would be absorbing that heat radiation from the Sun. so, the side of the astronaut facing the Sun would indeed get very, very hot. The side facing away from the Sun would be freezing cold. So, you need something there to absorb the heat, absorb the radiation to feel the effect of temperature.

Kat - That is a great question.

Andrew - It really is.

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