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What Is the Temperature in Space?

By Brendan McGuigan
Updated May 21, 2024
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When calculating the temperature in space, it is important to understand that most estimates must take into account the varied makeup of space. Outer space is usually considered to be the portion of the universe that is almost entirely empty and, from the point of view of Earth, officially begins at an altitude of about 62 miles (100 kilometers) above sea level. In the void between planets, star systems and galaxies, the temperature in space is generally considered to be 2.725 Kelvin which is -454.72°F (-270.4°C). This is only a very small amount above absolute zero, the lowest temperature at which the movement of matter is believed to cease, at -459.67°F (-273.15°C).

Average Temperature

When scientists talk about 2.725 Kelvin as the temperature in space, they’re talking about an average temperature — actually, what is called the cosmic background radiation, which is the energy still left over from the Big Bang. Closer to Earth, such as just outside of Pluto’s orbit, the temperature is estimated to be closer to 35 or 40 Kelvin due to the effects of the distant Sun. This is still very cold, but nowhere near as cold as somewhere in deep space, far from any sunlight.

Measuring Temperature

Measuring the temperature in space is more complicated than just using a thermometer, since temperature is only a meaningful figure when heat can be efficiently transferred from one body to another. In space, while the temperature of particles can be very high, their density is very low so the ability to transfer heat is minimal. They could be at millions of degrees in the Kelvin range, but since they so rarely collide with one another, the actual phenomenon of temperature or heat exchange doesn't take place. The temperature in space therefore has to do with the movement and concentration of the molecules, which in turn determines how often they collide with one another to gain or lose energy.

Due to this, the temperature must be determined using Planck's law, which says that every object in the universe emits radiation according to its temperature. By looking at the radiation emitted from space and using this formula, scientists have found that the temperature is about 2.725 K. Different parts of space actually have different temperatures, and the Milky Way galaxy — where the Earth is located — is slightly warmer than many other areas.

Influencing Factors

It is important to remember that space is actually everywhere and that even human bodies are mostly composed of empty space. So when someone discusses temperatures in any particular region of space, he or she also has to include any objects that appear solid to the naked eye. Other factors that could eventually change experts' view on what the temperature in space is include the effects of dark matter and dark energy, which combined make up 96% of all the known universe.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By discographer — On Oct 30, 2015

@anon992783-- I don't think so. Kelvin's zero point is the absolute zero where matter has no movement. But I think he just wanted to identify that for his scale. I don't think that anything ever reaches absolute zero. Particles always have some motion.

By anon992783 — On Oct 02, 2015

But does the movement of matter ever cease completely?

By anon934952 — On Feb 22, 2014

Is it possible to assume that the varying temperature of space just outside our planet would also have an impact on our own temperature fluctuations on the surface of earth, and if so, are we monitoring the temperature on a daily basis to forecast dangerous levels based on an average?

By anon332803 — On May 01, 2013

If you look at the atom, it is mostly composed of empty space (without matter) so temperature is not solely dependent on whether matter is there or not to measure it. That conclusion is purely semantic in principle and totally false in nature because space is full of radiation, solar winds, the cosmic dust that started the universe in all and atomic particles not visible to the eye and not measurable by any technology we have yet. With quantum physics and string theory in conjunction with M-Theory, all things are infinitely possible.

By anon293030 — On Sep 23, 2012

Could someone please tell me how I would find out the expected temperature at a particular point in our solar system if a spacecraft were to be there? Specifically, the L4 Earth-Moon Liberation point.

By anon281262 — On Jul 23, 2012

For this discussion, all matter has three states: solid, liquid and gas. As a solid, such as frozen ice, when it heats up, the substance melts. With more heating, it evaporates. The reverse can also happen. Cool a gas made of iron and it will become a liquid. Cool it more and it freezes into a solid.

Temperature is a measure of the wobbling of matter. All matter vibrates back and forth. This is called Browningian Motion. As matter wobbles faster and faster (temperature rises), it changes states.

Light is made of high energy (high heat) particles in a short wavelength. When it hits matter, the energy is transferred to the matter. The matter then re-emits the energy in long wavelengths. If we are talking about planetary objects, this radiation is call Black Body Radiation. If the body has an atmosphere, Venus, for instance, it traps the heat radiated from the planet. The heat bounces back to the planet. Mercury is closer to the sun but it does not have an atmosphere. Therefore the re-emitted energy dissipates into space. This is why Venus is hotter than Mercury.

If Mr. Scott transported a person into space without a space suit, the person would not die of freezing or suffocation. His body would explode out into space because of the difference in pressure between the vacuum of space and of the human body. The person's body is not strong enough to stay together and the gravity his body produces is not strong enough to stay together. Matter tends go from high pressure to low pressure or in other words, to spread out from high density to low density.

By anon263724 — On Apr 25, 2012

Well there has to be heat in space because there is light from every direction because of the stars. Since there is light, there is heat, because light is a form of energy and energy causes heat.

By anon262505 — On Apr 20, 2012

@ Post 31: Energy isn't the only thing that causes change of state. Pressure does too.

By anon155970 — On Feb 25, 2011

Lots of questions about what will happen to your body if you are exposed to empty space.

Short answer: you'll suffocate to death, and possibly have time to feel a case of the bends. You won't freeze until long after you're dead.

Long answer: As far as freezing go, depending on where you are, you may actually heat up, or you may freeze, but that happens much slower than your agonizing suffocation.

If you are in earth orbit, and you have a direct view of the sun, you'll get the worst sunburn of your life. Exposed to the sun without the shielding effect of the atmosphere, you will be exposed to raw unfiltered sunlight. That will be worse than frying the beach on a cloudless day on the equator. You will definitely not freeze to death that way.

If you are in the shade however, or too far away from the sun, you'll slowly freeze, at about the rate things freeze on a cloudless night with no wind. Heat will radiate away from your body, but since there is no air around you, there is no convection that can take heat away from you.

But again, you won't freeze to death because you'll suffocate before that happens.

By anon144123 — On Jan 18, 2011

"If a body were to be submitted at -455 in deep space would the water in your brain boil or instantly freeze?"

Either way, death is quick -- within tenths of a second, faster than nerves can react thus quite a painless and humane death.

By anon144122 — On Jan 18, 2011

How does Voyager keep its circuits capable of broadcasting all these years in -455F?

By anon126392 — On Nov 12, 2010

If I fired a squirt gun in space, (a) would the water freeze or boil, (b) would I even be able to fire the squirt gun and (c) when I fired the squirt gun, would this propel me backward?

By anon123838 — On Nov 03, 2010

@31 - Exactly why you stated. It's the change in pressure that causes moisture to "boil" away not heat.

@28 - Gravity.

By anon115732 — On Oct 04, 2010

One poster has speculated that an object in deep space gets cold less quickly, because there are no molecules to transfer the energy to.

At first this did not sound right, but it just struck me that a thermos bottle with a near vacuum in its sealed chamber slows down the loss of heat energy from the inner lining and liquid, so the poster is right. Of course, he pointed out correctly that radiant heat loss would still take place.

I have learned more from this series of posts than I could ever have imagined.

By anon107677 — On Aug 31, 2010

If space is so cold, how does the change of pressure cause moisture to boil?

By anon103780 — On Aug 13, 2010

Air doesn't go flying off into space for the same reason you don't go flying off into space. Gravity holds it down.

By anon96585 — On Jul 16, 2010

If space is a total vacuum, why doesn't it suck all the air out of the earth's atmosphere? as we have pressure here.

By anon85886 — On May 22, 2010

I have heard from other sources, like NASA, that temperature in space has no meaning.

If you were to put a thermometer in space, it would measure the temperature of the thermometer, not space. There is no matter in empty space, so there cannot be any temperature.

The reason spacecrafts become cold is that heat is radiated off of them. Otherwise, there would be too much heat energy stored on the spacecraft and it could combust or cause sever damage to electrical components.

If there is no matter, there cannot be energy.

Even with the sun's heat and energy from other stars, there will only be heat were there is matter to collect it. This is why Venus is hotter than Mercury, even though Mercury is closer to the sun. Venus has an atmosphere to collect heat, whereas Mercury does not.

Temperature has no meaning in empty space. By the way, I am 15 years old.

By anon78162 — On Apr 17, 2010

Temperature is force where force = 3 x T in kelvins.

Temperature of space is 1.21 x 10^44 Kelvins.

By anon67311 — On Feb 24, 2010

I started wondering about the cold in space today. I have researched quite a bit and i have found a lot about what generates heat or "adds" heat into the cold space, thus makes the temperature rise.

However, i cannot find anything that explains to me why the space is cold in the first place? Why isn't it like, say room temperature?

Room temperature is an example only, and to reach room temperature, i need to turn up the heat in the winter, and turn on my air cooler in the summer.

Now, the reason i need to add heat to reach room temperature in the winter, is obviously because it's cold outside. It's cold outside because the sun is so low on the horizon.

As I've come to understand, the space itself wouldn't have a temperature unless there was mass there to measure the temperature of.

Lets say i had a ton of empty space, no suns, no remnants of a big bang, no other energy, just a total vacuum. Then put a brick in the middle of that. What temperature would i be able to measure on that rock, and why?

By anon66657 — On Feb 21, 2010

it has meaning Mr. emdegreg. This temperature helps us to conclude E=mc2, where a body must travel with a velocity of light. the temperature of empty space is required and space is not empty; it is filled by many gases. Even oxygen is also present in 0.1 percent. there is a meaning in the temperature of absolute empty space.

By anon58187 — On Dec 30, 2009

i will assume that friction /heat /light are pined together by energy or some type of the above given off by the sun is the source of particulate matter. at Avery very sub molecule base matter thus reacts differently by whatever molecule based matter it strikes /density mass structure opaque translucent depending on density of molecule structure whether it travels through or bounces off its subject.

By anon56485 — On Dec 15, 2009

I think this is a cool about how you know about the space temp. Can I assume that in order for temperature to be at all meaningful, there must be matter present which temperature can be measured, so that the temperature of absolutely empty space really has no meaning?

what is the temperature around the satellite if there are no temperature controls (i.e temperature of the part which is facing the sun and which is in the shadow region - with and without facing the sun).

By anon45699 — On Sep 19, 2009

If I remember correctly, it would take a person a while to freeze in space, as their body would have a substantial amount of energy, plus, heat transfer in space isn't as great as it is in a gaseous environment, so while you would feel cold, you'd stay quite warm for a little while, longer than you would say, in the siberian wilderness in the middle of January.

By anon42973 — On Aug 25, 2009

I want to know, asked if freezing requires water. freezing or loss of heat is decrease in molecular activity. In a warm body there is constant movement at the molecular level, as heat is removed the movement lessens, to the point of cessation, which is at about 455F.

By anon39333 — On Jul 31, 2009

@13: Actually, your blood pressure prevents your blood from boiling. The moisture in your mouth, nose, and eyes would however boil. This occurred in a NASA experiment in 1965, where a person was exposed to a near-vacuum for close to 15 second when a space suit ruptured.

By anon33797 — On Jun 12, 2009

The reason air would rush out into space through a hole in a space ship is the pressure difference. Because space has so few particles in it, the pressure, or lack thereof, creates a vacuum. If someone were to enter space without a pressurized space suit their blood would boil, amongst other things, before they froze.

By anon33401 — On Jun 05, 2009

Temperature is, in a sense, a measure of the sum of the energy present in the molecules that make up the object of interest. If no matter exists you have nothing of which to measure the temperature.

That does not mean that the potential for temperature increase does not exist. Radiation from stars has the ability to transfer its energy when it strikes matter thus causing an increase in the energy of that matter and a temperature increase. This increase is based on many factors including the type of radiation from the stars that reach the object, the intensity and density (related to proximity) as well as the molecular make up of the object being struck by the radiation, even the texture of the surface can have an influence on the energy absorbed by the object.

Since outer space is mostly empty there are very little molecules for which the radiation from stars to transfer its energy to, so the temperature of space is very low.

By anon32097 — On May 16, 2009

If space have no air at all, why is it cold? why isn't it just 0?

Also, if we are (for instance) in a space ship and a door opens, why would the air inside of it and in our body be vacuumed out?

By BruceBarstow — On Jan 22, 2009

I think it is perhaps easier to see the sun as not providing heat but rather providing a catalyst for heat. The sun produces radiation. The radiation affects molecules, causing them to speed up. This speeding up of molecules generates heat.

Friction is another catalyst but the heat is not caused directly by things rubbing together but rather from the way the molecules react to the friction. Again they speed up and end up producing heat as a result.

In the case of the sun, the more intense the radiation (proximity, density, angle) and the more direct contact, the more molecules react and the hotter they become.

Looking at a microwave, you can easily see that different types of molecules react differently to different catalysts.

By anon24601 — On Jan 14, 2009

Temperature is defined as the average kinetic energy of molecules. Most of the space is empty, no molecules, thus the concept of temperature becomes meaningless. The 3 degree K background radiation does not require atoms as most of the space is empty.

By anon23866 — On Jan 04, 2009

You say you'd freeze to death in outer space - surely you wouldn't actually lose temperature that quickly - I mean sure its cold, but there's no particles to pass your body's heat to, so the only way to lose your body's heat is through radiation, which would take a while. I'd be more worried about the lack of air...

By anon22595 — On Dec 06, 2008

H20 isn't required for anything to freeze, it simply has to be matter. Ice however forms as a result of frozen air or water. Mars for example, has plenty of ice, but no water! The ice on mars is thought to be many things, most hopefully frozen methane!

Electronics work optimally in a vacuum, so satellites don't have a problem, especially since they are heated using their solar panels! The slower molecules move, the higher the temperature, and vice versa.

By anon22548 — On Dec 05, 2008

Well... you'd probably freeze to death, cus your body is only like what, 75% H2O (Water), and it would probably be like very bad frost bite, and would just numb all pain away... Painless? maybe, maybe not... I just would like to make the suggestion that you should probably not choose to die like this. Just cause of the scenario of a small, fast moving, space chunk hitting you, and blasting you into a million pieces, hahaha

By wantoknow — On Nov 25, 2008

so if a body is in space it would freeze because of the cold temperature? But doesn't that require H2o?

By anon17862 — On Sep 09, 2008

what is the temperature around the satellite if there is no temperature controls (i.e temperature of the part which is facing the sun and which is in the shadow region - with and without facing the sun)

By emdrgreg — On Feb 08, 2008

Can I assume that in order for temperature to be at all meaningful, there must be matter present which temperature can be measured, so that the temperature of absolutely empty space really has no meaning?

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