If The Sun Heats The Earth, Why Is Space Cold?

As a science website, it’s easy to get bogged down in questions like is there life on Enceladus and where are all the aliens, when people are struggling with more basic (but fun!) questions like why can’t we power our cars with magnets and could people breathe the air on Mars.

One such question it appears people have is “if the sun is in the sky, why is there heat on earth but not in the sky?” asked in the Facebook group Physics is Fun (which it IS). 

First off, space is cold, with the background temperature being an average of 2.7 Kelvin (−270.45°C or −454.8°F). And the Sun is indeed hot, in confusing ways. The core reaches temperatures over 15 million °C (27 million °F) while the surface (the photosphere) drops off to around 5,500 °C (10,000 °F), while the Sun’s corona (outer atmosphere) gets hotter further away from the photosphere, reaching temperatures of  3.5 million °C (2 million °F).

So why isn’t space hot? Well, this seeming paradox probably comes from people thinking about the Sun intuitively as a hot fire, heating the planets like a toasty marshmallow near a bonfire. This isn’t how planets are warmed. The heat we feel on Earth is not direct heat energy from the Sun, but the result of solar radiation emitted from the Sun (wavelengths across the electromagnetic spectrum, including visible light) interacting with particles on Earth.

With fewer particles to interact with in the (almost) vacuum of space, there is not enough matter to heat via radiation. However, this doesn’t mean that you can send a spaceship close to the Sun and expect to experience very cold temperatures. When you put matter in the way of the Sun’s radiation, it is going to heat up. 

On its closest approach, NASA’s Parker Solar Probe’s solar shields for obvious example, will face temperatures of 1,400°C (2,600°F) as it tries to keep its payload at around room temperature. This is as it passes through the Sun’s corona at speeds human-made objects have never previously achieved.


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