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This is an image of the radioisotope thermoelectric generator (RTG) for Cassini:

enter image description here

This one was for one left on the Moon:

enter image description here

And this one is for the Multi-Mission RTG, used by Curiosity on Mars:

enter image description here

One is black, one grey, and one white. Why such a difference in the color of these RTGs?

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  • $\begingroup$ I bet it has something to do with the thermal properties of the fins, but I haven't been able to find anything specific. I do know that some RTGs have beryllium housing/fins and some have aluminum housing/fins. $\endgroup$ – called2voyage Sep 7 '16 at 17:48
  • $\begingroup$ By the looks of it, that grey one might not be grey - it might just be covered in something, shall we say, fine and powdery. $\endgroup$ – corsiKa Sep 7 '16 at 20:47
  • $\begingroup$ I suppose that's possible. Hmmmm... $\endgroup$ – PearsonArtPhoto Sep 7 '16 at 21:13
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    $\begingroup$ The dust is on the pallet but not on the fins. The RTG and the 'soil' are both as black as asphalt. (photo is from Apollo 14) $\endgroup$ – amI Sep 7 '16 at 23:09
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    $\begingroup$ @gerrit: That looks to be an engineering mock-up or test article, rather than flight hardware. No need to work in a cleanroom environment unnecessarily--it just makes everything more difficult. $\endgroup$ – Drew Hall May 8 '18 at 6:23
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The big difference between the two darker RTG fins (Black and Grey) and the white RTG fins, is that the white fins were destined for use in an atmosphere (Mars). The presence of an atmosphere, even as diffuse as Martian air, would allow increased heat transfer from the RTG fins via convection and conduction, vs. the space based versions which would entirely rely on radiation to transfer heat.

Radiation heat transfer is affected by color and is likely the reason for the color differences.

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  • $\begingroup$ That makes sense. I knew there was something I had to be missing! $\endgroup$ – PearsonArtPhoto Sep 7 '16 at 18:00
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    $\begingroup$ I'd really like to see a technical link that explains this quantitatively. Ballparking: for convective heat transfer to (Earth) air, I see numbers like 5 W/m^2/K. If Mars is say 1% and it scales linearly with # of molecular collisions/time, then a 300K difference on a 0.5 m^2 area would be only about 10 Watts. $\endgroup$ – uhoh Sep 8 '16 at 13:26
  • $\begingroup$ Further - why not make it black and pick up the extra power via radiation? The atmosphere goes through wide daily variations in temperature and air speed which will affect the efficiency of the system. Adding radiative cooling would moderate those fluctuations, especially in "warm", windless spells. After poking around the internet and seeing that in every one of Curiosity's "selfies" on Mars the heat sink is indeed white, it does suggest that unless it's some kind of exotic material that the IR emissivity will be low. $\endgroup$ – uhoh Sep 8 '16 at 13:45
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    $\begingroup$ The white coating is probably something like Z93, which maintains high thermal emissivity while having low absorbtivity in much of the solar spectrum. It's used on the ISS radiators as one example. $\endgroup$ – Tristan Sep 8 '16 at 14:53
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    $\begingroup$ @Tristan now that makes sense! "...allowing only 14-16% of the solar radiation impinging on the spacecraft external surface to be absorbed through to the interior systems while emitting 89-93% of the internal heat generated to the cold vacuum of space." You'd want it "white" in the visible to minimize solar loading, but "black" in the IR to maximize radiation. If it were indeed the case here, it would suggest that this answer - roughly speaking "the color doesn't matter because theres a little bit of air" - is incomplete. Also thanks for linking to the Z93 - that's cool stuff! (punintended) $\endgroup$ – uhoh Sep 9 '16 at 3:36

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