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If you could travel from world to world, from star to star, out into the gulfs of intergalactic space, you’d move away from the warmth of the stars into the vast and cold depths of the void.
Better pack a sweater, it’s going to get cold.
But, how cold? How cold is space?
Unlike your house, car, or swimming pool, the vacuum of space has no temperature.
So, how cold is space? That’s a nonsense question. It’s only when you put a thing in space, like a rock, or an astronaut, that you can measure temperature.
Remember there are three ways that heat can transfer: conduction, convection and radiation.
Heat up one side of a metal bar, and the other side will get hot too; that’s conduction. Circulating air can transfer heat from one side of the room to another; that’s convection. But out in the vacuum of space, the only way heat can transfer is radiation.
Photons of energy get absorbed by an object, warming it up. At the same time, photons are radiating away.
If the object is absorbing more photons than it emits, it heats up. And if it emits more photons than it absorbs, it cools down.
There is a theoretical point at which you can’t extract any more energy from an object, this minimum possible temperature is absolute zero. As we’ll see in a second, you can never get there.
Let’s look close to home, in orbit around the planet, at the International Space Station.
A piece of bare metal in space, under constant sunlight can get as hot as two-hundred-sixty (260) degrees Celsius. This is dangerous to astronauts who have to work outside the station.
If they need to handle bare metal, they wrap it in special coatings or blankets to protect themselves.
And yet, in the shade, an object will cool down to below -100 degrees Celsius.
Astronauts can experience vast differences in temperature between the side facing the Sun, and the side in shadow. Their spacesuits compensate for this using heaters and cooling systems.
Let’s talk a little further out. As you travel away from the Sun, the temperature of an object in space plummets.
The surface temperature of Pluto can get as low as -240 Celsius, just 33 degrees above absolute zero.
Clouds of gas and dust between the stars within our galaxy are only 10 to 20 degrees above absolute zero.
And if you travel out far away from everything in the Universe, you can never get lower than a minimum of just 2.7 Kelvin or -270.45 Celsius.
This is the temperature of the cosmic microwave background radiation, which permeates the entire Universe.
In space? It’s as cold as it can get.
Want more resources? We’ve recorded an episode of Astronomy Cast all about temperature.