As we all know, space is a vacuum and doesn't support "temperature" too well. However, when a probe is placed away from a star, the temperature is quite enough to damage electronic components and outer bodies of interstellar payloads. So, how do long lasting satellites such as Voyager I keep from going below their operating temperature and damaging their inner and outer components?
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1$\begingroup$ I hope at least some of us know space is a vacuum and a vacuum has no temperature and does not conduct heat. The cosmic background radiation is about 2.7 K. $\endgroup$– UweCommented Jun 5 at 14:05
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1$\begingroup$ I mean ultimately the question is correct that objects far from a star will eventually drop to ~3 Kelvin and that will damage or disable a lot of components. I don't know that quibbling over the details of cooling in space is really relevant. $\endgroup$– Darth PseudonymCommented Jun 5 at 14:21
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$\begingroup$ dear, @Uwe, I accidentally wrote the question wrong over looking the fact that space is indeed a vacuum. Thank you for your observation. $\endgroup$– Aerospace_NerdCommented Jun 5 at 15:52
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1$\begingroup$ What exactly would "freeze" there? Only liquids can do that. $\endgroup$– MithoronCommented Jun 6 at 11:42
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1$\begingroup$ "However, when a probe is placed away from a star" We haven't placed any probes closed to being "away from a star". Spacecraft are designed for the thermal environment they are going to. $\endgroup$– tckosvicCommented Jun 6 at 19:42
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4 Answers
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Near the Sun, spaceprobes are primarily heated by direct illumination. To keep a good balance, probes often have radiators and sunshields, as well as carefully planned surface coatings and insulation to balance emissivity and reflectivity. There may also be coolant loops, to move heat away from instruments producing waste heat. For short duration events, such as satellites moving into the night side of the Earth, there may be small electrical heaters powered by on-board battery storage.
Further away from the Sun, pretty much the only long term source of heat is the decay of radioactive isotopes. These power sources come in two flavours:
- Radioisotope thermoelectric generators or RTGs, producing a small amount of electricity from a hot lump of radioactive material. The conversion efficiency is rather poor (~10%), so most of the output is just heat. The electrical power in turn becomes waste heat in whatever piece of equipment it is used in, which provides some flexibility in moving heat around.
- Radioisotope heater units. A hot piece of radioactive material, as simple as it gets. Stick them around where you want to keep your probe nice and toasty.
answered Jun 5 at 14:39
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1$\begingroup$ Also, their electric power sources deliver energy to the main body of the spacecraft, where the powered machinery dissipates the energy as heat. Often, getting rid of enough heat is more of a problem than losing too much $\endgroup$ Commented Jun 5 at 16:24
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$\begingroup$ In some spacecraft, for example the big Mars rovers, heat from the RTGs is also harvested in liquid circulation systems. See answers to Are there fluid disconnects that sever just before the Curiosity or Perseverance rovers separate from their cruise stages or landers? and for a photo of those heat-capturing fluid lines see Now that Perseverance is "hot" (RTG in place) and before it gets to deep space, how will it stay cool? $\endgroup$– uhohCommented Jun 6 at 7:16
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$\begingroup$ @uhoh those rovers can also dissipate heat into the martian surface and atmosphere, something a spacecraft in vacuum obviously can't do $\endgroup$– jwentingCommented Jun 6 at 11:55
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1$\begingroup$ @Peter-ReinstateMonica you are operating electronics outside their usual operating conditions or even outside their maximum rated conditions. All sort of things can go wrong. Extreme overclockers often encounter these issues and title them "cold bug". If your concern is only around non powered electronics, it's quite hard to get the thermal expansions fit good enough, so stresses appear and might lead to cracking of components. I guess this would be a question for EE.SE, maybe it's already on there. $\endgroup$– ArsenalCommented Jun 7 at 8:28
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1$\begingroup$ @Arsenal A couple of failure modes are discussed on EE SE here. The main problems seem to be (besides electrolyte issues in batteries and capacitors) (1) mechanical stress/cracking (2) Change of timing and resistivity and (3) below 50K or so, lack of charge carriers especially in silicium semiconductors. They all can be overcome (although (3) probably gets exponential worse when approaching 0K closely), but regular circuits will basically not work below 50K. $\endgroup$ Commented Jun 7 at 8:59
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Probes that are (approximately speaking) operating closer than the Asteroid Belt will be heated enough by the sun and by their own electrical waste heat that auxiliary heating is usually not necessary. Not coincidentally, this is also the zone where solar power is generally reasonable as an energy source. Depending on where the probe is, expelling heat to keep processors and other parts cool enough to function is often the larger problem than staying warm.
Once a probe starts getting out to Jupiter or beyond, or tries to operate on the dark side of a body, solar power becomes difficult or impossible to get, and solar heating is not able to maintain operational temperatures. The solution to both problems is usually radioactivity -- a radio-isotope thermal generator (RTG) uses the heat of decaying radioactive materials and a thermocouple (or other technology) to generate electricity, and also keeps the probe warm -- either directly by containing and transporting the waste heat from that process, or indirectly by running electric resistance heaters inside the probe's casing. The Pioneer and Voyager probes all used RTGs to generate power and heat.
Some probes use a little lump of radioactive material as a direct heat source without linking it to electrical generation. The Soviet Lunokhod moon rovers, for example, were solar powered, so they had to shut down during the long lunar night, but the heat from radioactive polonium kept them warm enough to avoid damage from the cold that could otherwise have prevented the wake-up process when the sun came up.
However, there are some probes that go a different direction. JUICE, the Jupiter Icy-Moons Explorer, has very large solar panels to produce enough power to operate at Jupiter without using an RTG, and it uses a 100 kg layer of insulation plus resistive heating to keep the internals warm despite the weak sunlight. That can only work to a point, though. Eventually there just isn't enough sunlight to generate power from, so once you're heading deep into the outer solar system, RTGs are the only reasonable answer we have so far.
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$\begingroup$ "There are a few probes, such as the Soviet Lunokhod moon rovers, that used a little lump of radioactive material as a direct heat source without linking it to electrical generation." Don't the Voyagers have several radioisotope heaters as well? I thought these were fairly common. cf. NASA's Radioisotope Heater Units $\endgroup$– uhohCommented Jun 8 at 3:01
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1$\begingroup$ I wasn't aware of that but sure, it looks like good information. RHUs may be more common that I realized. $\endgroup$ Commented Jun 8 at 3:30
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The vast majority of components in a satellite are already solid. So, they can't exactly "freeze". The main exception for this being any electrolytic capacitors. So, for the context of this answer, I'll take "freeze" to mean dropping below their acceptable operating temperature.
Electronic components generate heat.
This heat is generally enough to keep the probe at an acceptable temperature for operation. In the case that it isn't, probes can just add resistive heaters.
The vacuum of space is a very insulating environment, dissipating heat is often more of a challenge than keeping components warm. In fact, a lot of electronic components work better at lower temperatures.
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3$\begingroup$ Yes, vacuum is a great insulator. Ever heard of Thermos? $\endgroup$ Commented Jun 6 at 14:59
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However, when a probe is placed away from a star, the temperature is quite enough to damage electronic components and outer bodies of interstellar payloads.
As discussed in links within Is it bad if hydrazine freezes on a spacecraft? Is it always kept as liquid, or can it be safely allowed to freeze and then thawed when needed? and in answers there, we should remember that while freezing doesn't damage hydrazine, re-thawing hydrazine potentially damages spacecraft!
The solution here is to use the same technology that keeps other critical components sufficiently warm as discussed in other answers to keep the hydrazine lines and valves above the freezing point.
See also: