How cold is it in space?

“If we put a thermometer in darkest space, with absolutely nothing around, it would first have to cool off. This might take a very very long time. Once it cooled off, it would read 2.7 Kelvin. This is because of the “3 degree microwave background radiation.” No matter where you go, you cannot escape it—it is always there.”

Jonathan Keohane
for Ask an Astrophysicist

That explains it pretty well thanks!

It helps to think in terms of convective heat and radiant heat.

Convective heat depends on contact; when something hot touches something cold, the motion of the molecules in the hot thing gets the molecules moving in the cold thing, so the cold thing heats up and the hot thing cools down. This doesn’t happen in space because there’s no contact.

Radiant heat doesn’t depend on contact. Objects that are warm will radiate that energy in the form of very long wavelength (infrared) electromagnetic waves. As this radiation leaves the object, the object cools. Our bodies do this all the time, but that lost heat energy is also constantly being replenished by heat from our metabolism and by heat—both radiant and convective—coming from our environment.

In space, we would continue to radiate heat but, unless we were receiving radiated heat from a nearby star, there would be no incoming heat to replace that lost energy.

It seems that you don’t cool off very quickly when exposed to space. What kills you is the decompression and lack of oxygen. This article lays it out in some detail, although it doesn’t offer any sources. Still, it sounds like a good story, and offers a couple of decompression examples that did not occur in outer space.

@marinelife has the correct statement. Any object above 3 Kelvin will radiate energy into space, thus cooling itself down. The energy from the background radiation will always be getting absorbed and trying to heat things up, but the effect won’t be noticeable until your object gets down to 3 K. Then it reaches a balance of energy in vs energy out, so your object doesn’t go colder than 3 K.

@thorninmud, re: “in incoming heat to replace that lost energy”, don’t forget the heat generated by our bodies’ metabolic processes.

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So… at what rate does the human body radiate its internal heat in space (let’s assume we’re floating opposite the dark side of the moon and that we’re impervious to decompression and hypoxia)? How long will a healthy, live human body take to freeze solid?

@robmandu ”...lost heat energy is also constantly being replenished by heat from our metabolism…”

We are surrounded by the warmth of air. Space is vacuum. There is nothing there to warm, like we have on Earth.

@robmandu – the rate of heat loss would be faster than the rate of heat generated. In your situation, the person would soon cool down and go into hypothermia and die, then cool quickly to a frozen state. Temperature after death would asymptotically approach 3 K, so might take a while.

Heat loss is proportional to T^4, so would cool down pretty fast at first. The human body is normally 37°C (T=300 K). 300^4 is pretty big, but tapers to 3^4 eventually.

[mod says] This is our Question of the Day!

Great Answer from @RocketGuy.‘s I agree that heat loss from the human body would be extremely fast in space since the average temperature of the universe is less than 3 degrees above absolute zero, and radiative heat loss goes as the 4th power of temperature difference.

Consider that normally you radiate your body heat (at 37C) into a room-temperature environment (at 22C), so the process is driven by a temperature difference of 15 degrees. Now go to the arctic tundra where it’s -40C and the temp diff increases to 77 degrees, about 5 times greater compared to room temp. But in deep space, at a temp of -270C, the temperature difference increases to 307, about 20 times greater compared to room temp.

These factors of 5x and 20x are before raising to the 4th power! You get the idea.

Not that astronauts haven’t done it, but you need more than what the ski lodge has to offer in cold protection before going into the black of space, which is black below infrared (“heat waves”) all the way down to microwave frequencies.

Areas of collapsing nebulae get extremely hot long before new stars are being born. We are talking millions of degrees.

@RocketGuy & @gasman – Are we sure those formulae for rate of heat loss are for vacuum conditions rather than in atmosphere?

@Zaku @RocketGuy designs multimillion dollar satellites for a livijng, so I would hope he’s sure!

Yes, the equation for radiative heat transfer in vacuum is simga*A*F*(T1^4-T2^4) as shown here: http://en.wikipedia.org/wiki/Thermal_radiation

Convective heat transfer is only proportional to T:
http://en.wikipedia.org/wiki/Heat_transfer

Magnitude of heat transfer is different, though. You can get a lot of convective heat transfer by moving a lot of cold liquid. That makes cold water very risky for hypothermia – struggling would only make it worse. You would last longer in space if you wore a poorly insulated space suit.

@RocketGuy Thanks! So, not having any idea what to plug in to the formula, it sounds like you are saying that a person in a space suit that prevented effects other than cold from killing them, but exposed them to cold, would die of cold, but not as quickly as someone would die of cold in freezing water?

Yes, you would get hypothermia sooner in 0 C (273 K) cold water than in 4 K space. However, you would only cool to 273 K in water. You would eventually get down to 4 K in space.

@RocketGuy tell them about the hot and cold differences depending on which side of the satellite is facing the sun.

Oh yeah, the shadow side of a spacecraft wants to cool to 4 K. For spacecraft in Earth orbit, the sun side wants to heat up to 120 C (400 K) or more, depending on how “shiny” the surface is. We use zinc oxide (also found in serious sunblock) paints or shiny mirrors to reflect away the sun light. And we have systems to redistribute heat from hot side to cold side so that temps don’t go up and down all day.