I never thought about this until reading the comments below this answer. To radiate something like 150W (a very conservative lower limit of human + suit systems heat production - see @Uwe 's comment below) I think a radiator would have to be conspicuously visible, wouldn't it? I don't remember seeing heat sink fins sticking out of the astronauts packs while walking on the moon or doing ISS maintenance though. In fact I think everything was/is pretty much white all around - which minimizes heating from the sun but for essentially the same reason will not radiate efficiently.

How is the heat actually dissipated?

note: see the answer - the total heat is probably much larger than 150W I mention here.

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    $\begingroup$ Relevant on Youtube: youtube.com/watch?v=fJbztthNrVQ $\endgroup$ – user May 9 '16 at 12:52
  • $\begingroup$ @MichaelKjörling I'm enjoying it now - thanks for the link! $\endgroup$ – uhoh May 9 '16 at 13:07
  • $\begingroup$ @MichaelKjörling the footage at the end is great! $\endgroup$ – uhoh May 9 '16 at 13:47
  • $\begingroup$ Glad you enjoyed it @uhoh. $\endgroup$ – user May 9 '16 at 14:34
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    $\begingroup$ 150 W is enough for an astronaut at rest and suit system heat production. But a working astronaut producing 150 W of mechanical work generates about 450 W of heat. A suit with enough reserves for hard work should radiate up to 1 KW of heat. $\endgroup$ – Uwe Jul 1 '16 at 13:42

For Apollo, space suits used the Primary/Portable/Personal Life Support System during EVAs.

This is a schematic of the PLSS:

PLSS showing a sublimator that vents steam to the vacuum of space

The PLSS used sublimation cooling. Water was run over a heat exchanger, the heated water was then allowed to vent into the vacuum of space where it would sublimate.

The current Shuttle/ISS EMU suit uses the same principle:

the primary purpose of the water tanks is to feed water to the sublimator. The sublimator works on the principle of sublimation, that is, the process by which a solid turns directly into a vapor, bypassing the liquid phase. In this case, ice is formed on the sublimator evaporator sieve and is allowed to vaporize to space, removing heat with it. Air and cooling water are passed through fins in the sublimator, which extracts heat from each system.

  • $\begingroup$ Wow, that's amazing! Roughly speaking it's the specific heat and then the heat of vaporization of water that's the final step in the chain. It take a lot of work to pull all those sticky, polar water molecules apart. The PDF mentions a "metabolic rate" of 1600 to 2000 BTU/hour, which is about 470 to 580 Watts. I'm guessing most of that is actually everything else besides the metabolic rate - heat leakage as well as heat from all the systems within the pack. Is water sublimation cooling still used in spacewalks on the ISS? $\endgroup$ – uhoh May 9 '16 at 8:08
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    $\begingroup$ Modern suits use a seperate coiling circuit: en.wikipedia.org/wiki/… $\endgroup$ – SAnderka May 9 '16 at 8:41
  • $\begingroup$ @SAnderka that's incredible! OK so it really is direct ice-to-gas sublimating that removes most of the heat. I think the passive melt-flow-freeze-sublimate operation is fascinating! $\endgroup$ – uhoh May 9 '16 at 8:47
  • $\begingroup$ In the Wikipedia article mentioned by SAnderka there is an example from Apollo 12, the suit dissipated 894.4 BTU/hour, that is 262 W. It is a mean value for an EVA of 3 hours and 44 minutes. $\endgroup$ – Uwe Mar 18 '17 at 15:25
  • $\begingroup$ fyi I've just asked How were Apollo spacesuits cooled during simulations/rehearsals on Earth? $\endgroup$ – uhoh Jul 12 '19 at 8:55

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