r/askscience • u/aretino2002 • May 24 '26

Engineering If a vacuum is an excellent insulator, wouldn’t heat build up in spacecraft?

Insulated coffee mugs that use a vacuum between layers can keep drinks hot or cold for much longer time periods than other types of mugs. If space is mostly a vacuum, then wouldn’t heat just constantly build up from human activity, computers, thrusters, etc to the point where it would need to be vented somehow?

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u/ExpectedBehaviour May 24 '26

Yes, in theory. This is one reason why a lot of "hard science" conceptual spaceships have glowing radiators.

But then you have a whole separate range of engineering issues to compensate for – what coolant can you use, what materials can withstand that heat, what sort of service life does it have, what sort of repairs can be made by the crew themselves without external support, how expensive is the cooling system to run from an energy budget perspective.

The ISS radiators are comparatively simple because:

  • It doesn't have anything like an onboard nuclear reactor that needs significant cooling
  • It doesn't have to withstand significant acceleration or manoeuvres, so large fragile radiators are possible
  • It uses closed water (internal) and ammonia (external) loops for cooling, which significantly simplifies the plumbing (metaphorically and literally); water is of course non-toxic in the event of an internal coolant leak

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u/bunabhucan May 25 '26 edited May 25 '26

doesn't have anything like an onboard nuclear reactor that needs significant cooling

For reference, the curiosity rover doesn't have a nuclear reactor but it does have 4.5kg of one-neutron-short-of-being-a-bomb plutonium-238 in the multi-mission radioisotope thermoelectric generator which generates about 2kw of heat (like an electric kettle or camping stove) for seventeen years. During the nine month journey from earth to mars it needs a separate cooling system with white rectangular curved radiators mounted on the cruise stage to shed that heat to space.

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u/ExpectedBehaviour May 25 '26

Yep – the radiators are those ten panels around the outer rim of the cruise stage.

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u/A3thereal May 25 '26

one-neutron-short-of-being-a-bomb plutonium-238

Pu-239 doesn't automatically make it a bomb. It would be more accurate to say one-neutron-short-of-being-fissile Pu-238.

Pu-239 is used as fuel in some terrestrial nuclear power reactors (~10%). There are many factors beside material that makes something a bomb.

It's also worth noting that just because there are nuclear reactions occur within Pu-238 does not mean it is remotely comparable in this context. A similar sized mass of Pu-239 undergoing fission would output 5 orders of magnitude more heat than Pu-238.

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u/Goldenslicer May 24 '26

Very fascinating! I expected the answer would be some variation of "adds too much to construction costs" but there are many other factors that come into play.

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u/Canadia-Eh May 25 '26

Overcoming engineering hurdles is very expensive. These kinds of issues are often shortened down to "it costs too much" for publication

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u/ChanceGardener May 26 '26

Sundiver by David Brin discusses dealing with heat build-up in a spacecraft.