How cold is space?
Empty space far from stars has a background temperature of about minus 270 degrees Celsius (2.7 kelvin), just above absolute zero. But objects in sunlight can get extremely hot.
The temperature of deep space comes from the cosmic microwave background, leftover radiation from the Big Bang, which sets the baseline at about 2.7 kelvin. Absolute zero, 0 kelvin or minus 273.15 degrees Celsius, is the lowest possible temperature, when particles have the least possible energy.
In space near Earth, temperatures depend on sunlight. Because there is almost no air to carry heat, a surface in direct sunlight can heat to more than 120 degrees Celsius, while the shaded side can drop below minus 150. The International Space Station experiences these swings every orbit, so it uses insulation and radiators to stay comfortable.
Colder than space
The coldest known natural place is the Boomerang Nebula, a cloud of gas about 5,000 light-years away, at about 1 kelvin. Scientists in laboratories on Earth have cooled atoms to within billionths of a degree of absolute zero, far colder than anywhere in space.
Did You Know?
Without a spacesuit, a person in space would not freeze instantly, because space is a vacuum and heat escapes slowly without air to carry it away.