How Cold Is Space? Why Space Is Freezing Even Though the Sun Is Hot

Short answer: deep space, far from any star, sits at about 2.7 kelvin, or −270 °C (−455 °F), just a few degrees above absolute zero. That temperature comes from the faint afterglow of the Big Bang, which fills the whole universe. But near the Sun, space is not simply "cold": an object in sunlight can heat up above 120 °C (250 °F) while its shaded side drops below −150 °C (−240 °F). Space is less a cold place than a place with almost nothing in it to carry heat.

That last point is the key to almost every question about temperature in space, from why astronauts need cooling suits to why the James Webb Space Telescope needs a sunshield the size of a tennis court.

What does "temperature" even mean in a vacuum?

On Earth, temperature is a measure of how fast the molecules of a substance are jiggling. Hot air is air whose molecules move fast; cold air is air whose molecules move slowly. When you touch something, those molecules collide with your skin and exchange energy with it.

Space is a vacuum, but not a perfect one. Between the stars of our galaxy there are on average about one atom per cubic centimetre, compared with roughly 25 billion billion molecules in a cubic centimetre of the air you breathe. With so few particles, there is almost nothing to collide with you. The temperature of the gas itself can be extreme, but it barely matters.

A striking example sits right above our heads. In the thermosphere, between about 90 and 600 km in altitude, the thin gas can reach 1,000 to 2,000 °C because it absorbs ultraviolet light from the Sun. Yet a thermometer there would read far below freezing. There are so few molecules that they cannot transfer meaningful heat to anything. Temperature describes how fast particles move; how much heat you feel depends on how many of them hit you.

Where does the 2.7 K come from?

Even the emptiest region of the universe is bathed in light: the cosmic microwave background, the radiation left over from the hot early universe. About 380,000 years after the Big Bang, the universe cooled enough for atoms to form, and the light of that moment has travelled freely ever since. As the universe expanded, that light was stretched to longer and longer wavelengths, and today it corresponds to a temperature of 2.725 K.

An object placed in intergalactic space, far from any star, would slowly radiate away its heat until it reached this temperature, and no lower: the background radiation keeps warming it by exactly as much as it loses. That is the "temperature of space" in the strictest sense.

Interestingly, there is a place colder than empty space. The Boomerang Nebula, a cloud of gas thrown out by a dying star about 5,000 light-years away, has cooled to about 1 K as it expanded rapidly, like the spray from an aerosol can getting cold. It is the coldest known natural place in the universe. Physicists in laboratories go much further, cooling atoms to within billionths of a degree of absolute zero. We explain why they can never reach it in The Big Chill: Why We Can't Ever Quite Reach the Bottom of the Universe.

Why is space cold if the Sun is so hot?

Heat can travel in three ways:

• Conduction: through direct contact, like a pan heating your hand.
• Convection: carried by a moving fluid, like warm air rising from a radiator.
• Radiation: carried by light, including infrared light, which needs no material at all.

In a vacuum, the first two are almost absent. The only way the Sun's energy reaches anything is by radiation, and the only way an object can lose heat is also by radiation. So the temperature of an object in space depends on a balance: how much sunlight it absorbs versus how much infrared light it emits.

Close to the Sun, the balance is hot. Far from it, or in shadow, it becomes very cold. The space between does not "have" a temperature in the everyday sense. It is mostly an empty corridor that light crosses.

Real temperatures in space

• International Space Station, sunlit side: about +120 °C (+250 °F)
• International Space Station, shaded side: about −150 °C (−240 °F)
• The Moon at noon (equator): about +120 °C (+250 °F)
• The Moon at night: about −130 °C (−200 °F)
• Permanently shadowed craters near the Moon's poles: below −240 °C (−400 °F), among the coldest places measured in the Solar System
• Deep intergalactic space: −270.4 °C (−454.8 °F)

The space station orbits the Earth every 90 minutes, moving from sunlight to shadow about 16 times a day. Its surfaces endure these swings constantly, which is why it is wrapped in insulating blankets and fitted with large radiators that pump excess heat away.

Is space cold for an astronaut?

Surprisingly, the bigger problem for astronauts is usually getting rid of heat, not keeping warm. A human body produces about 100 watts of heat at rest, and much more while working. On Earth, that heat leaves easily through the air and through sweat. In a vacuum, it can only leave by radiation, which is slow.

That is why spacesuits contain a liquid cooling garment: a layer of thin tubes filled with circulating water, worn against the skin. Without it, an astronaut on a spacewalk would overheat, even in the "freezing" vacuum.

Would you freeze instantly in space?

No, despite what many films show. Because heat leaves the body only by radiation, freezing would take hours, not seconds. The real dangers come much faster:

• Lack of oxygen: you would lose consciousness in about 10 to 15 seconds.
• Low pressure: water in the body's tissues starts to turn into vapour, causing swelling. Saliva on the tongue would bubble.
• Sunlight: without protection, intense ultraviolet light would burn exposed skin quickly.

Freezing would be among the last things to happen.

Why the James Webb Space Telescope needs to be so cold

Webb observes the universe in infrared light, which is essentially heat. A warm telescope would glow in infrared and blind itself, like trying to spot stars with a torch shining into your eyes. So Webb uses a five-layer sunshield about the size of a tennis court. The side facing the Sun reaches about +85 °C, while the telescope behind it stays below −230 °C (around 40 K). One of its instruments, MIRI, is cooled further still, to about 7 K, by a dedicated refrigerator.

Webb shows the principle of this article perfectly: in space, temperature is decided by sunlight and shade, not by the surrounding "air". Learn more in What the James Webb Space Telescope Really Sees.

Frequently asked questions

What is the coldest temperature in space?
The background temperature of the universe is 2.7 K (−270 °C). The coldest known natural place is the Boomerang Nebula, at about 1 K. Nothing can reach absolute zero, 0 K (−273.15 °C).

Is space hot or cold?
Both, depending on where you are. Space itself is nearly empty, so it does not hold heat. An object near a star in sunlight can become very hot, and the same object in shadow or far from any star becomes very cold.

Why don't astronauts freeze during spacewalks?
Their suits are insulated and heated where needed, and in a vacuum the body loses heat very slowly. In fact, spacesuits mainly need to cool the astronaut, not warm them.

Does space have a temperature at all?
The sparse gas in space has a temperature, sometimes extremely high, and space is filled with radiation that corresponds to 2.7 K. But because space is so empty, these temperatures say little about how hot or cold an object in it will become.

How cold is it on the Moon?
About −130 °C (−200 °F) during the lunar night, and below −240 °C in craters near the poles that never see sunlight. At noon, the surface reaches about +120 °C (+250 °F).

The bottom line

Space is cold in the sense that the universe as a whole has cooled to 2.7 K since the Big Bang. But "how cold is space?" has a more interesting answer: space is so empty that it barely has a temperature you could feel. What matters is light. In sunlight, things bake; in shadow, they freeze. Everything we send into space, from astronauts to telescopes, is designed around that simple and extreme balance.

Curious about what "empty" space really contains? Read The Physics of Nothing: Why Empty Space is Teeming with Energy.

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Written by Imad El Bojaddaini, university professor of physics. Physics Voice explains the strangest ideas of physics in plain language.