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Have you ever wondered how cold space is? The real answer to that is more nuanced than you think. If you think about how cold is space you need to understand certain parameters about what is space. We now define space as outer space the region beyond the atmospheres of most celestial bodies. The region is considered to be the closest thing to a natural vacuum. However it is not completely empty. It does have some gases such as hydrogen and cosmic dust. This is called the interstellar medium.
When we think about heat we think about a form of energy. However we forget that heat like most forms of energy such as sound needs a medium to travel through. In the case of heat it needs matter to conduct it. A piece of forged iron when quenched transfers its thermal energy to the liquid that quenches it. If you place a plate of heated food out on a table for a long period of time the process of convection transfers the heat from the food to the air molecules surrounding it. In space the rules differ. A perfect vacuum has no molecules to transfer heat to. This means it can’t properly conduct heat. This principle is used to store hot liquids in storage containers such as thermoses. Essentially even though you may think of space as cold it would actually allow hot objects to retain heat longer than if they were on Earth, as with the absence of air because there is nothing to reflect, absorb, and scatter the entering heat.
So, modules on the International Space Station — and even the Moon — and astronauts working outside must endure temperatures as high as 115°C (240°F) when they are in direct sunlight, and conversely, when they are in shadow temperatures as low as -180°C (-290°F), as there is nothing to hold in the heat on the “night” side. Compare this to the average day/night temperature of the Earth in the northern US (even though it is the same distance from the Sun as the ISS) during a typical June is 21°C/11°C (70°F/52°F) because of our life-friendly atmosphere.
But how cold it is in space where there is almost no source of heat nearby? If you were to measure the temperature of space with an accurate thermometer then you could say with certainty that space is cold and very cold at that. Of course, you would first have to wait the very long time necessary for all the ambient heat in the thermometer to radiate into space before you could get an accurate reading. The resulting temperature reading would be 2.73 Kelvin (-270 celsius) is the coldest naturally occurring temperature to Absolute Zero. The reason that it is not at this temperature is due to background radiation left over from the formation of the Universe.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.