Although science fiction 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 amount of energy an object’s particles possess. So, in a truly empty region space, there would be no particles or radiation, which means there is no temperature either.
Of course, space is full of particles and radiation to produce heat and temperature. So how cold is space, is there a region that is truly empty, and is there a place 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 found immediately around stars, which contain all the conditions to start nuclear fusion.
Things really heat up when radiation from a star hits a point in space with lots of particles. This gives the radiance of stars as the Sun something to actually act on.
That is why Earth is much hotter than the region between our planet and its star. Heat comes from particles in our atmosphere vibrating with solar energy and then colliding with each other distributing that energy.
However, the proximity of our star and the possession of particles do not guarantee the heat. Mercury – closest to the sun – is extremely hot during the day and freezing at night. Its temperatures drop to a minimum of 95 Kelvins (-288 ⁰Fahrenheit/-178 ⁰Celsius).
Temperatures drop to -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, causing Uranus to orbit the sun at an extreme tilt, rendering it unable to hold on to its inner heat.
Far from the stars, the particles are so scattered that heat transfer via anything other than radiation is impossible, meaning temperatures drop drastically. 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 (-505⁰F/-263⁰C or ) while less dense clouds can have temperatures as high as 100 K (-279⁰F/-173⁰C).
What is cosmic background radiation?
The universe is so vast and filled with such a multitude of objects, some scorching hot, some unimaginably cold, that it should be impossible to give space a single temperature.
Yet there is something that pervades our entire universe with a uniform temperature of 1 in 100,000. In fact, the difference is so insignificant that the change between a hot spot and a cold spot is only 0 .000018K.
This is called the cosmic microwave background (CMB) and it has a uniform temperature of 2.7 K (-45⁰F/-270⁰C). Since 0 K is absolute zero, it is a temperature only 2.725 degrees above absolute zero.
The CMB is a remnant of an event that happened just 400,000 after the Big Bang called the last dispersal. This is when the universe ceased to be opaque after electrons bonded with protons forming hydrogen atoms, which stopped electrons from endlessly scattering light and allowed photons to move freely.
As such, this “frozen” fossil relic in the universe represents the last point where matter and photons were temperature-aligned.
The photons that make up the CMB were not always so cold, taking about 13.8 billion years to reach us, the expansion of the Universe has redshifted these photons to lower energy levels.
Originating when the universe was much denser and hotter than it is now, the starting 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 colder now than it’s ever been, and it’s getting colder.
What would happen if you were exposed to space?
If an astronaut were to drift alone in space then exposure to the near-vacuum of space could not freeze an astronaut as often portrayed in science fiction.
There are three ways to transfer heat: conduction, which occurs through touch, convection, which occurs when fluids transfer heat, and radiation, which occurs through radiation.
Conduction and convection cannot occur in empty space due to lack of matter and heat transfer occurs slowly by radiative processes alone. This means that heat does not transfer quickly through space.
Since freezing requires heat transfer, an exposed astronaut—losing heat only through radiative processes—would die of decompression due to lack of atmosphere much faster than he would freeze to death.
For more information on the properties of space, see “Astrophysics for people in a hurry (opens in a new tab)” by Neil deGrasse Tyson and “Origins of the Universe: The Cosmic Microwave Background and the Search for Quantum Gravity (opens in a new tab)by Keith Cooper.
- Harvard University, “The Human Body in Space: Distinguishing Fact from Fiction (opens in a new tab)“, July 2013.
- NASA, “Fluctuations of the Cosmic Microwave Background (opens in a new tab)“, accessed July 2022.
- NASA, “Cosmic Microwave Background (opens in a new tab)“, July 2022.
- NASA, “Eta Carinae (opens in a new tab)“, September 2020
- Paul Sutter, “You won’t freeze to death in space (opens in a new tab)“, Forbes, April 2019.