What's the Temperature of Outer Space?

The temperature of outer space is generally 2.73 Kelvin or -270.42 Celsius. This is actually the temperature of Cosmic Microwave Background Radiation, which is spread throughout the universe.

The sun-facing side of the International Space Station (ISS), for example, can reach scalding hot temperatures of about 121C, while thermometers on the dark side can plunge to a biting -157C. Thanks to the thermal conditioning in the ISS, astronauts are saved from getting torched or frozen due to extreme temperature fluctuations.

Barring asteroids, meteoroids, planets, moons, and other celestial bodies, most of space is a vacuum, wherein there is no matter. And whenever we say outer space, we generally mean vacuum.

A perfect vacuum has no temperature, as there is nothing in a vacuum whose temperature could be measured. So, in essence, you cannot technically measure the temperature of outer space.

Some parts of space are hot! Gas between stars, as well as the solar wind, both seem to be what we call "empty space," yet they can be more than a thousand degrees, even millions of degrees.

Most of the gas in space is too thin to warm anything up. There are not enough gas particles to "bump" into and transfer heat to an object. So if you were in space, but shielded from the sun, you would radiate away nearly all your heat pretty quickly and cool to the cosmic background temperature. Step (or float) into the sun, and you’d be warmed.

At the Earth's distance from the sun, a space thermometer with roughly half its surface is absorbing sunlight would register +7 degrees Сelsius.