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How is heat transfered in space?

  1. Feb 10, 2009 #1
    Hi, I'm asking a somewhat boring question. I'm trying to understand how heat would transfer if an astronaut were to be so unlucky as to be stranded in space. I know that heat generally transfer in three ways: conduction, convection, and radiation. If I assume the space suit is designed not to conduct heat from the human body, that eliminates one method. And if i assume space to be mostly empty, convection is also eliminated. So the only possibility is through radiating it out of the body.

    My question then is, how long before his body reaches the temperature of space (nearly absolute zero)? Will he radiate so much heat out through his suit that it kills him in mere hours and his body hits nearly absolute zero shortly after? Will his suit bounce much of the radiation, and therefor he would die or thirst and hunger, before slowly radiating all the remaining heat out; making the time span a few weeks or months? Or something else?

    Also, would light from the sun be enough to keep the astronaut warm in his suit?

    Thanks in advance!
  2. jcsd
  3. Feb 10, 2009 #2


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    You're correct all the heat transfer is by radiation.
    So the space suit is facing almost a full sphere of very cold (3K - not quite absolute zero) space, except there is also a 6000K sun and a 300K planet.

    The biggest difficulty is regulating the temperature between being cooked by the sun and frozen by space, thats why spacesuits are shiny, to try and reduce the radiation both arriving and leaving.

    The moon which is in a similair position (although less shiny) experiences temperate swings of +100C for the part facing the sun to -155C for the side facing into space. If it wasn't for our atmosphere Earth would have the same temprature swings!

    An astronaut near earth experience slightly warmer temperatures because most of the time they are also facing a relatively warm planet Earth.
  4. Feb 10, 2009 #3
    Space being a vacuum has no temperature. When you hear about the temperature of space being close to absolute zero, the temperature being refered to is the temperature at which the heat a body radiates away is in equilibrium with the heat it absorbs in deep space.

    Things are much different with an astronaut in earth orbit. The astronaut's body, assuming he's alive, is producing heat and both the sun and earth are radiating heat to him. His problem is how to stay cool. My question is similar to yours.

    How does the ISS stay cool? It has solar panels supplying power to it's instruments and lights and the people inside provide additional warmth. Much of the time it has the sun on one side and the earth on the other. How does it get rid of its heat?
  5. Feb 10, 2009 #4
    Oh you know, you're so right! Since temperature itself is the kinetic energy of the atoms of a body, there is no way space could have a temperature without any atoms! And your question is alot more interesting than mine. I'm wondering the same thing too now, how does the space station regulate its temperature so that humans are able to live inside of it? Is there some kind of central air conditioning system or something? And is the ISS super duper reflective?
  6. Feb 10, 2009 #5


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    Not exactly. Due to the CMB, the temperature of space is approximately 2.7K.
  7. Feb 10, 2009 #6


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    Not quite, the CMB means that nothing in space can get to below 2.7K naturally, but is't not really useful to define a temperature for empty space. It's better the think of space as a vacuum flask with walls at 2.7K but nothing inside.

    The air conditioning is a tricky part - especially in orbit because there is no convection, hot air doesn't rise - this makes it incredibly difficult to cool electronics.

    To regulate the temperature from the sun and space you make the ISS modules superinsulated, in the same way that you do for cryogenic equipement on earth. But in addition to the heat from the sun, the solar panels supply (IIRC) about 30Kw of power which will all ultimately end up as heat, there is also a crew contributing 100-200W each.

    The ISS is cooled in the same way as your car, there is a set of pipes running through it which carry water and then a radiator outside that faces the cold of space.
    Last edited: Feb 10, 2009
  8. Feb 10, 2009 #7
    Wouldn't it be impossible to classify space as a vaccuum? do the various billions of stars not count? There are also plenty of stray atoms , primarily hydrogen, dancing around out in space. without these atoms, how would the idea of an ion drive engine ever work?
  9. Feb 10, 2009 #8


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    Spacecraft with ion propulsion carry their own fuel and when a gas gets thin enough, the individual molecules stop acting like they are interacting and both temperature and pressure become meaningless.
  10. Feb 11, 2009 #9


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    Radiative transfer is the issue. Photons are the answer. Heat is electromagnetic.
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