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Heat in space

  1. Oct 5, 2007 #1
    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.
     
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  3. Oct 6, 2007 #2

    G01

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    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
  4. Oct 6, 2007 #3

    Dale

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    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.
     
  5. Oct 6, 2007 #4
    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?
     
  6. Oct 6, 2007 #5

    DaveC426913

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    Correct, with one added detail: You are bombarded by microwave radiation at 4 degrees K.
     
  7. Oct 6, 2007 #6

    Dale

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    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.
     
  8. Oct 6, 2007 #7
    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.
     
  9. Oct 6, 2007 #8

    Dale

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    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.
     
  10. Oct 6, 2007 #9

    russ_watters

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    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".
     
  11. Oct 6, 2007 #10

    russ_watters

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    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.
     
  12. Oct 6, 2007 #11

    DaveC426913

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    I'm fairly certain that vacuum would be barkin' cold. Like frostbite cold instantly.
     
  13. Oct 6, 2007 #12

    DaveC426913

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    Actually, that phenomenon was fairly recent in time and limited in scope to the PF forum...:biggrin:
     
  14. Oct 6, 2007 #13

    russ_watters

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    I don't see how....
     
  15. Oct 6, 2007 #14
    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?
     
  16. Oct 6, 2007 #15

    Dale

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    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.
     
  17. Oct 6, 2007 #16
    Thank you, DaleSpam. An accurate synopsis.
     
  18. Oct 6, 2007 #17
    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.
     
  19. Oct 7, 2007 #18
    How much radiative heating would a body in space recieve from the sun if it were in orbit around Earth?
     
  20. Oct 7, 2007 #19
    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.
     
  21. Oct 7, 2007 #20

    russ_watters

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