Although sci-fi movies would have us believe that space is incredibly cold – even freezing – 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 the energy that an object’s particles have. So in a truly empty space there would be no particles and radiation, which means there is no temperature either.
Of course, space is full of particles and radiation to create heat and a temperature. How cold is space, is there a region that is truly empty, and is there anywhere where the temperature drops to absolute zero?
Related: What is the coldest place in the universe?
How stars heat up space
The hottest regions of space are immediately around stars, which provide all the conditions for nuclear fusion to start.
It gets really warm when the radiation from a star reaches a point in space with many particles. This gives the radiation from stars like that Sun something to actually respond to.
That’s why Earth is much warmer than the region between our planet and its star. The heat comes from particles in our atmosphere vibrating with solar energy and then colliding and dispersing that energy.
However, the proximity to our star and the possession of particles are not a guarantee of heat. Mercury – closest to the Sun – is scorching hot during the day and freezing cold at night. Its temperatures drop to a low of 95 Kelvin (-288⁰Fahrenheit/-178⁰Celsius).
Temperatures peak at -371⁰F (-224⁰C). Uranuswhich makes it even colder than on the planet furthest from the sun, Neptunewhich has a still incredibly cold surface temperature of -353⁰F (-214⁰C).
This is the result of a collision with an Earth-sized object early in its existence that caused Uranus to orbit the Sun at an extreme tilt, making it unable to hold on to its internal heat.
Far from stars, the particles are so distributed that heat transfer is impossible by anything but radiation, meaning temperatures drop radically. This region is called the interstellar medium.
The coldest and densest molecular gas clouds in the interstellar medium can have temperatures as low as 10 K (-505 ⁰F/-263 ⁰C or ), while less dense clouds can have temperatures as high as 100 K (-279 ⁰F/-173 ⁰C).
What is cosmic background radiation?
The universe is so vast and filled with such a variety of objects, some scorching hot, others unimaginably cold, that it should be impossible to assign a single temperature to space.
Yet there is something that permeates our entire universe with a temperature that is uniform to 1 part in 100,000. In fact, the difference is so insignificant that the change between a hotspot and a coldspot is only 0.000018K.
This is known as the cosmic microwave background (CMB) and has a uniform temperature of 2.7 K (-45⁰F/-270⁰C). Since 0K is absolute zero, this is a temperature that is only 2.725 degrees above absolute zero.
The CMB is a remnant of an event that occurred just 400,000 after the Big Bang called the Last Scattering. This was the point at which the universe stopped being opaque, after electrons bonded to protons and formed hydrogen atoms, preventing electrons from endlessly scattering light and allowing photons to move freely.
As such, this fossil relic ‘frozen’ in the Universe represents the last point at which matter and photons were temperature aligned.
The photons that make up the CMB weren’t always that cold, taking about 13.8 billion years to reach us. The expansion of the universe has redshifted these photons to lower energy levels.
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 around 3,000 K (5,000 °F/2,726⁰C).
As the universe continues to expand, that means space is now colder than ever and it’s getting colder.
What would happen if you were exposed to space?
If an astronaut would drift in alone place then exposure to the near-vacuum of space could not freeze an astronaut, as is often portrayed in science fiction.
There are three types of heat transfer: conduction, which occurs through contact, 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 by radiative processes alone. This means that heat does not transfer quickly in space.
Because freezing requires heat transfer, an exposed astronaut—losing heat through radiative processes alone—would die from decompression much faster than freeze to death due to the lack of atmosphere.
For more information on the properties of space, see “Astrophysics for those in a hurry”. (opens in new tab)‘ by Neil deGrasse Tyson and ‘Origins of the Universe: The Cosmic Microwave Background and the Search for Quantum Gravity (opens in new tab)‘ by Keith Cooper.
- Harvard University, “The Human Body in Space: Separating Fact from Fiction (opens in new tab)“, July 2013.
- NASA, “Fluctuations in the Cosmic Microwave Background (opens in new tab)‘, accessed July 2022.
- NASA, “Cosmic Microwave Background (opens in new tab)“, July 2022.
- NASA, “Eta Carinae (opens in new tab)“, September 2020
- Paul Sutter: “You don’t freeze to death in space (opens in new tab)“, Forbes, April 2019.