What would the environmental temperature be for a 3u cubesat satellite at an altitude of 650 km? When I look this question up online, I am given many different temperature ranges, so I am not sure what is accurate or which to use. Thanks.
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3$\begingroup$ Even on earth we often struggle to figure out what the temperature 'really' is. Mount a thermometer outside your window in direct sunlight, and it's going to read absurdly high. The standard is usually something like "dry-bulb air temperature in the shade", which obviously isn't going to work in space. $\endgroup$– Darth PseudonymCommented Nov 12 at 21:58
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2 Answers
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It depends on your thermal design. A bit below zero Celsius is typical. If it's good at absorbing light, but not so good at radiating infrared, it'll run hotter. The opposite case will run colder. If it doesn't rotate, it'll tend to have a hot side and a cold side. For a good prediction, you need to calculate based on your detailed design and operations concept.
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$\begingroup$ I don't know if I agree with 0 C being typical. If a moderately reflective surface is in permanent direct sunlight near earth, it'll quickly reach ~150 C. Objects in permanent shadow (like the important bits of the JWST), will radiate until they get close to 40 Kelvin. Spinning to spread out the solar radiation (aka "rotisserie mode") might result in something close to 0 C, but that's making an awful lot of assumptions. $\endgroup$ Commented Nov 12 at 21:54
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1$\begingroup$ JWST is far from the warm Earth, not at 650 km. At 600 km, the HETE-2 soft x-ray system, shadowed from the Sun by the solar panels, operated at -40 C. The GPS receiver, facing the Sun, could reach +70 C if it was turned on (it was a bit of a power hog). That's a fairly extreme case in LEO: a spacecraft that always kept one face oriented to the Sun. $\endgroup$ Commented Nov 12 at 23:43
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There isn't really an environmental temperature, in the sense that we usually mean. On earth, the environmental temperature is the temperature of the air around you. Or the water, if you're under water, or even the ground, if you are talking about the environmental temperature of a buried object. But, in orbit, the surrounding medium is so tenuous that its temperature is irrelevant to a satellite passing through it, even though that temperature can be as high as 2500° C.
What matters is solar illumination, and, even then, the equilibrium temperature depends on how well the satellite absorbs and reflects different wavelengths. I have read that a block of polished aluminum in continuous sunlight in space would get quite hot, well above the boiling point of water, but I cannot find a source for that. See the second answer to this question for details about temperatures based on different assumptions about absorption (in the context of the JWST heat shield).
Also, I said solar illumination, but, as user2702772 points out, it's really total illumination that matters. The sun is a huge part of that, but the earth makes a big difference at 600 km because it takes up so much of the sky.
Note that I am agreeing 100% with what John Doty says. But I thought this would fill in some background.
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2$\begingroup$ Also factor in the illumination from the Earth. The lower your orbit, the greater the impact of having xx% of your sky filled with planet. $\endgroup$ Commented Nov 13 at 9:36