While science fiction movies would have us believe that space is incredibly cold — even freezing cold — space itself isn’t exactly cold. In fact, it has no temperature at all.
Temperature is a measure of the speed at which particles move, and heat is how much energy an object’s particles have. So in a really empty region there would be no particles and radiation, meaning there would be no temperature either.
Of course, space is full of particles and radiation to produce heat and a temperature. So how cold is the room, is there any area that is really empty, and is there anywhere where the temperature drops to absolute zero?
How stars warm up space
The hottest regions of space are immediately nearby starswhich contain all the conditions to initiate nuclear fusion.
It only gets really warm when radiation from a star reaches a spot in space with many particles. This gives the radiation of stars like the Sun something to actually act on.
That’s why Soil is a lot warmer than the area between our planet and its star. The heat comes from particles in our atmosphere that vibrate with solar energy and then collide with each other and distribute this energy.
However, proximity to our star and possession of particles are no guarantee of heat. Mercury – closest to the sun – is sweltering hot during the day and freezing cold at night. Temperatures drop to a low of 95 Kelvin (-288 Fahrenheit/-178 ⁰Centigrade).
This is the result of a collision with an Earth-sized object early in its existence, causing Uranus to orbit the sun in an extremely oblique orbit, rendering it unable to retain its internal heat.
Far away from stars, particles are so dispersed that heat transfer via anything but radiation is impossible, causing the temperature to drop radically. This region is called the interstellar medium.
The coldest and densest molecular gas clouds in the interstellar medium can have temperatures of 10 K (-505 F/-263 ⁰C or ) while less dense clouds can have temperatures of up to 100 K (-279 ⁰F/-173 ⁰C).
What is cosmic microwave background radiation?
The universe is so vast and filled with such a multitude of objects, some sweltering hot, others unimaginably icy, that it would be impossible to give the room a single temperature.
Yet there is something that permeates the entire universe with a temperature equal to 1 part in 100,000. In fact, the difference is so insignificant that the change between a hot spot and a cold spot is only 0.000018 K.
This is known as the cosmic microwave background (CMB) and it has a uniform temperature of 2.7 K (-45⁰F/-270⁰C). Since 0 K is absolute zero, this is a temperature only 2.725 degrees above absolute zero.
The CMB is a remnant of an event that took place just 400,000 after the big bang called the last scattering. This was the point at which the universe ceased to be opaque after electrons bonded to protons forming hydrogen atoms, preventing electrons from scattering light endlessly and allowing photons to travel freely.
As such, this fossil remnant “frozen” in the universe represents the last point at which matter and photons aligned in temperature.
The photons that make up the CMB were not always so cold, it took about 13.8 billion years to reach us, the expansion of the universe has redshifted these photons to lower the energy level.
Originating when the universe was much denser and hotter than it is today, the initial temperature of the radiation that makes up the CMB is estimated to be about 3,000 K (5,000 °F/2,726⁰C).
As the universe continues to expand, it means that space is now colder than ever and getting colder.
What would happen if you were exposed to space?
If an astronaut floated alone room then exposure to the near-vacuum of space could not freeze an astronaut, as is often depicted in science fiction.
There are three ways to transfer heat: conduction, which occurs through touch, convection, which occurs when liquids transfer heat, and radiation, which occurs through radiation.
Conduction and convection cannot take place in empty space due to the lack of matter and heat transfer occurs slowly through only radiative processes. This means that heat is not quickly transferred into the room.
Because frostbite requires heat transfer, an exposed astronaut — who loses heat through radiation processes alone — would die much faster from decompression due to the lack of atmosphere than freezing to death.
For more information on the properties of space, see “Astrophysics for those in a hurry (opens in new tab)” by Neil deGrasse Tyson and “Origin of the Universe: The Cosmic Microwave Background and the Quest for Quantum Gravity (opens in new tab)by Keith Cooper.
- Harvard University, “The human body in space: distinguishing fact from fiction (opens in new tab)“, July 2013.
- nasa, “Fluctuations in the cosmic microwave background (opens in new tab)consulted in July 2022.
- nasa, “Cosmic Microwave Wallpaper (opens in new tab)“, July 2022.
- nasa, “Eta Carinae (opens in new tab)“, September 2020
- Paul Sutter, “You will not freeze to death in space (opens in new tab)Forbes, Apr 2019.