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What makes space so cold?

Category: Space
Published: December 19, 2012

Space is not always cold. It depends if you are facing the sun or not. And even if you are in shadow, space is not cold in the sense that it will cool you down quickly. The part of an astronaut facing the sun becomes blazing hot while the side in shadow remains a moderate temperature due to the suits internal machinery. Think of how you feel standing close to a campfire on a cold night, and you get some of idea of the temperatures effects in space. The dark parts of the moon reach -410° F while the sunlit parts reach 240° F, as measured by the Lunar Reconnaissance Orbiter. We don't experience these cold and hot extremes on earth because the atmosphere mixes around and evens out the temperatures. Planets without atmospheres, however, experience these extreme variations.

satellite in space
Space can be a very hot place when in direct sunlight. Public Domain Image, source: Christopher S. Baird.

A related question would be: what makes space so cold when in shadow? The answer is that the sun is the ultimate source of heat in our solar system, so if you are not in sunlight, and there is no air or ground (or portable heater) to carry sunlight's energy to you, then you have no source of heat. Cold is simply the absence of heat. Any spot in the universe that is sufficiently blocked from all heat sources will eventually cool down to freezing temperatures. But points in space removed from heat sources aren't cold in the sense that they would quickly make you cold. Quick heat transfer requires contact or air, both lacking in space. As a result objects cool very slowly through the much slower mechanism of thermal radiation. A human exposed to outer space in shadow without a space suit does not instantly freeze to a block of ice. It takes a lot longer than movies show for a non-heated body in space to cool down to deep space temperatures, which are about -280° F.

Topics: cold, cold space, temperature, thermal radiation, vacuum