5 ms·

I'm not following the rejection of heat, can someone explain that to me? I thought it was cold out there and heat had to be generated?

minikites 12y ago
Temperature is more like "undefined" in a vacuum, since heat is the motion of matter and there isn't matter in a vacuum. Vacuum is a good insulator, which is why a Thermos is an effective storage vessel.
- smeyer 12y ago
  I don't think it's really fair to say there's no way to define a temperature in a vacuum. With radiation, it seems fair to call the temperature that to which a black body would be heated if placed in the vacuum.
  - arielby 12y ago
    Outer space is very, very far from thermal equilibrium. The temperature of the interplanetary plasma is different from the temperature of the redshifted-big-bang-light (CMB) which is different from the temperature of sunlight. The temperature of a black body depends on how much of it is exposed to sunlight vs. CMB.
    - smeyer 12y ago
      I'm well aware, but I was just talking about the easiest case (deep intergalactic space with the CMB and minimal other exposure to radiation). There's a lot more complication than that, as you indicated, but I still don't think it's fair to call it undefined due to the lack of matter (I'd grant calling it undefined in some areas due to the complication of the different sources of radiation and of the matter, but that's another story).
murbard2 12y ago
Heat conduction is very poor; you're practically in a vacuum, so you can only lose heat by radiating it. On earth, you can dump your heat into the atmosphere or the oceans, which are both gigantic heat sinks. The entire surface of the earth then radiates that heat out to space. In space, you only have your own surface to radiate the heat out.
JoeAltmaier 12y ago
Losing heat is the principle design issue for satellites. Easy to gain it -sunshine, power generation, circuits. Losing it requires deliberate effort, and radiation is your only pathway.
NamTaf 12y ago
Heat is transferred via 3 mechanisms: 1) Convection, i.e.: the mixing of bits of 'hot' material and 'cold' material such as the material itself mixes through. Think blowing an air conditioner in a room. 2) Conduction, i.e.: the flow of heat energy through matter that is touching. The matter itself doesn't mix, but the heat energy can transfer. Think putting your hand on a hot plate. 3) Radiation, i.e.: the energy turning into electromagnetic radiation and beaming through the vacuum of space until it strikes something. Think sunlight. 1 and 2 are out, as there's no matter (air, etc.) around the ISS for it to convect or conduct with. Thus, all heat must be radiated. The above list ranks them in order of efficiency, so heat rejection is in fact very difficult.
- pvaldes 12y ago
  mmh... interesting problem. I wonder if satelites could just put the heat in some not-more-needed stuff, gas or something and simply shoot or launch this stuff to the space. Yes I'm inventing the "farting satellite" :-). Another possibility could be to put "hair" (short metallic spikes) about lets say the 50% of the surfaces of the satellite. Several possible goals could be achieved with this idea, creating lots of dynamic shadows around the surface, protecting the real surface from small impacts (some bend spikes is better that having a hole in your roof, and maybe even to provide support to the ocassional human intervention out (you could substitute the spikes by a matrix of handles easy to grasp). Just some crazy ideas. If you need to loose heat look at how birds and mammals do this task with movile hairs and feathers.
  - cesarb 12y ago
    > I wonder if satelites could just put the heat in some not-more-needed stuff, gas or something and simply shoot or launch this stuff to the space. The astronauts actually did that on the moon! Their spacesuits used sublimation of water ice for cooling. Your limitation is coolant mass. The useful life of satellites is often linked to the amount of propellant they have left; you'd have the same problem with your proposed cooling system.
    - pvaldes 12y ago

minikites 12y ago

Temperature is more like "undefined" in a vacuum, since heat is the motion of matter and there isn't matter in a vacuum. Vacuum is a good insulator, which is why a Thermos is an effective storage vessel.

smeyer 12y ago
I don't think it's really fair to say there's no way to define a temperature in a vacuum. With radiation, it seems fair to call the temperature that to which a black body would be heated if placed in the vacuum.
- arielby 12y ago
  Outer space is very, very far from thermal equilibrium. The temperature of the interplanetary plasma is different from the temperature of the redshifted-big-bang-light (CMB) which is different from the temperature of sunlight. The temperature of a black body depends on how much of it is exposed to sunlight vs. CMB.
  - smeyer 12y ago
    I'm well aware, but I was just talking about the easiest case (deep intergalactic space with the CMB and minimal other exposure to radiation). There's a lot more complication than that, as you indicated, but I still don't think it's fair to call it undefined due to the lack of matter (I'd grant calling it undefined in some areas due to the complication of the different sources of radiation and of the matter, but that's another story).

murbard2 12y ago

Heat conduction is very poor; you're practically in a vacuum, so you can only lose heat by radiating it. On earth, you can dump your heat into the atmosphere or the oceans, which are both gigantic heat sinks. The entire surface of the earth then radiates that heat out to space. In space, you only have your own surface to radiate the heat out.

JoeAltmaier 12y ago

Losing heat is the principle design issue for satellites. Easy to gain it -sunshine, power generation, circuits. Losing it requires deliberate effort, and radiation is your only pathway.

NamTaf 12y ago

Heat is transferred via 3 mechanisms: 1) Convection, i.e.: the mixing of bits of 'hot' material and 'cold' material such as the material itself mixes through. Think blowing an air conditioner in a room. 2) Conduction, i.e.: the flow of heat energy through matter that is touching. The matter itself doesn't mix, but the heat energy can transfer. Think putting your hand on a hot plate. 3) Radiation, i.e.: the energy turning into electromagnetic radiation and beaming through the vacuum of space until it strikes something. Think sunlight. 1 and 2 are out, as there's no matter (air, etc.) around the ISS for it to convect or conduct with. Thus, all heat must be radiated. The above list ranks them in order of efficiency, so heat rejection is in fact very difficult.

pvaldes 12y ago
mmh... interesting problem. I wonder if satelites could just put the heat in some not-more-needed stuff, gas or something and simply shoot or launch this stuff to the space. Yes I'm inventing the "farting satellite" :-). Another possibility could be to put "hair" (short metallic spikes) about lets say the 50% of the surfaces of the satellite. Several possible goals could be achieved with this idea, creating lots of dynamic shadows around the surface, protecting the real surface from small impacts (some bend spikes is better that having a hole in your roof, and maybe even to provide support to the ocassional human intervention out (you could substitute the spikes by a matrix of handles easy to grasp). Just some crazy ideas. If you need to loose heat look at how birds and mammals do this task with movile hairs and feathers.
- cesarb 12y ago
  > I wonder if satelites could just put the heat in some not-more-needed stuff, gas or something and simply shoot or launch this stuff to the space. The astronauts actually did that on the moon! Their spacesuits used sublimation of water ice for cooling. Your limitation is coolant mass. The useful life of satellites is often linked to the amount of propellant they have left; you'd have the same problem with your proposed cooling system.
  - pvaldes 12y ago