# Heat in space

1. Oct 5, 2007

### bassplayer142

Heat is the the vibration of molecules in a medium. If this is so, then how is space cold if it is a vacuum. I'm aware that space has very few particles but I don't understand how heat is emitted. I'm guessing that it might have to do with radiation.

2. Oct 6, 2007

### G01

Yes heat can travel through a vacuum as radiation. The thermal energy of atoms and molecules (made of charged particles) causes then to vibrate, which causes them to give off energy as electromagnetic radiation. The electromagnetic radiation can travel through a vacuum, taking the energy with it. This is how thermal energy is transmitted through a vacuum.

Last edited: Oct 6, 2007
3. Oct 6, 2007

### Staff: Mentor

There are three mechanisms of heat transfer: conduction, convection, and radiation. Conduction and convection do not occur in space, but radiation does. The reason space is said to be cold is because it radiates like a black body at about 4 Kelvin.

4. Oct 6, 2007

### bassplayer142

Okay, so your saying that on earth as you radiate heat other things are radiating heat onto you. While in space you alone are radiating heat and nothing but distant stars radiate heat on you?

5. Oct 6, 2007

### DaveC426913

Correct, with one added detail: You are bombarded by microwave radiation at 4 degrees K.

6. Oct 6, 2007

### Staff: Mentor

Yes, and as long as your temperature is >4 K you will continually be radiating away more heat than space is radiating back on you.

7. Oct 6, 2007

### Repetit

What it the timescale of this radiation? I mean, if I jump into water at ~0 Celcius it will immediately feel extremely cold because heat is being rapidly removed from my body and transferred to the water. How will this feel in space? Im guessing that heat transfer by radiation is quite slow at temperatures of 37 C so that it won't feel very cold.

8. Oct 6, 2007

### Staff: Mentor

There is a nice explanation and calculator at http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html#c2

Using a human body-surface area of about 1.5 m^2 and an emissivity of about .7 you get something like 500 watts of heat loss. To put that in perspective, that is about 10000 food Calories per day, so you would have to eat at least 3 times the normal American diet just to get enough energy to compensate for heat loss, let alone the extra energy needed for other important things like breathing etc.

9. Oct 6, 2007

### Staff: Mentor

Why don't we expand on that a little - it seems to contradict what bassplayer said if not fully explained...

The microwave backtround radiation is left-over energy from the big-ban that is still flowing through the universe. It has been around so long and dissipated so much due to the expansion of the universe that it is now very "cold".

10. Oct 6, 2007

### Staff: Mentor

Very cool and good to know. At rest, a person dissipates somewhere around 70 watts. 500 is probably near the maximum that an in-shape athlete could sustain for much time.

So you would start to feel cold relatively quickly - just a couple of minutes.

11. Oct 6, 2007

### DaveC426913

I'm fairly certain that vacuum would be barkin' cold. Like frostbite cold instantly.

12. Oct 6, 2007

### DaveC426913

Actually, that phenomenon was fairly recent in time and limited in scope to the PF forum...

13. Oct 6, 2007

### Staff: Mentor

I don't see how....

14. Oct 6, 2007

### Loren Booda

While wearing an unheated, oxygen supplied, airtight suit of moderate albedo in 2.735o K space, one would lose less heat than if one were to fall into ice-cold salt water?

15. Oct 6, 2007

### Staff: Mentor

Even an unheated suit dramatically changes the thermal situation. Most importantly the body is thermally interacting with the suit rather than space, the 500 W radiated by the body could conceivably rapidly heat the suit to the point that it was radiating a significant amount of energy back to the body. Also, when wearing a suit conduction becomes a major mechanism of heat transfer.

I think the comparison that you really want here is simply to wave hands and dismiss the other lethal effects of vacuum and concentrate purely on the heat transfer. So the question is if ice-cold salt water removes heat faster than 500 W. I don't know the answer to that, but the mechanisms are very different. In space the heat transfer mechanism is limited to radiation, in the water you have less radiation, but you add both conduction and convection. Also, water has a very high thermal conductivity, much higher than air. If the water is flowing then the convection is even more problematic since the body will not be able to warm the nearby water. It is quite possible that ice water would remove more than 500W of heat.

16. Oct 6, 2007

### Loren Booda

Thank you, DaleSpam. An accurate synopsis.

17. Oct 6, 2007

### Mephisto

That was in one movie that I saw, but i cant remember which one now... Appolo13 or Armageddon or something? Someone's shield broke and he froze instantly in space. Then later on I read that that's actually wrong, and then I read a whole article from NASA web-page saying that if you were exposed to pure space vacuum, the thing that would kill you soonest would be the fact that you would suffocate from lack of oxygen... so I guess that implies cold is not the biggest issue at least for a couple of minutes.

18. Oct 7, 2007

### Huckleberry

How much radiative heating would a body in space recieve from the sun if it were in orbit around Earth?

19. Oct 7, 2007

### TVP45

You're generally correct in thinking heat transfer by liquid is quite efficient. But, there are two examples I can readily think of to show the sensation of cold by radiation: (1) go into the desert on a clear night and the sky feels cold; or (2) there is a classic demonstration in which you take 2 parabolic mirrors and place your eyeball at one focus and an icecube at the other and pretty quickly your eye feels cold.

20. Oct 7, 2007

### Staff: Mentor

1200w/m^2. There isn't a significant amount absorbed by our atmosphere.

21. Oct 7, 2007

### Staff: Mentor

The solar irradiance at 1 AU is usually a little under 1.4 kW/m^2. Of course, in this case it is not the total surface area of about 1.5 m^2 that is important, but only the cross-sectional area. That obviously depends on the orientation (i.e. much less cross-section in the feet-down position than in the face-down position), but it will always be less than half. Let's estimate it at one third in the face-down orientation. In that case the amount of solar irradiance at 1 AU is going to about the same as the amount of heat radiated away.

Of course, in these extreme environments (subject to 5800K radiation from one direction and 4K radiation from everywhere else) small deviations from "about the same" can lead to pretty large differences in terms of survivability. However, the bottom line is that 500 W plus or minus will not kill you before the vacuum will. All it will do is determine if your corpse will be freezer-burned or well-done.

Last edited: Oct 7, 2007
22. Oct 7, 2007

### Staff: Mentor

Very welcome!

23. Oct 7, 2007

### DaveC426913

Yes, I would agree on all counts.

A broken mask would not instantly freeze you - I wasn't suggesting that. Nor would cold be a big concern at that point - suffocation is your only real concern.

But that's sort of complicating the original question, which was simply about how cold is it.

Imagine a glove came off. I suspect it would be somewhat like Antarctica in a winter storm.

24. Oct 11, 2007

### phlegmy

lol i had an argument with a thermodynamics lecturer in the middle of a lecture over this.
he posed the question

"what is the temprature of space"
nobody answered so i offered
"that sir, is a nonsense question"
"what do you mean its a nonsense question"
"you may as well ask what is the voltage between a square and a circle"
"you're wrong, the temprature of space is aprox 4 kelvin"

i offered the argument that space is defined by an absence of matter and temprature was a property of matter. i suggested that perhaps an object in space would reach 4 kelvin but space itself could not have a temprature. he refused to conceed, as did i. but i still maintain he is wrong, in the wording of the question at least

25. Oct 11, 2007

### sneez

How can that be? In Antarctica the ambient air has temperature and hence radiates back to you. Space has no temperature, hence it should feel a lot worse ...? Assuming -80 C in storm conditions, you get back around 70 W m-2 from the air in radiation.

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