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Q: What color is a spaceship?

A: Black and white and (infra)red all over.

Several answers and comments on Why were Europe's first few satellites so stylish? Why the pronounced alternating white and black stripes? contain phrases like "thermal control".

This is back before fancy materials could have both high thermal infrared emissivity and high visible light albedo at the same time. (see answers to Why are RTGs different colors?)

But I can't see why the alternating high/low and low/high ($\epsilon/a$) stripes would be better than just slapping on a coat of gray paint. Did they simply not have gray "space paint" back then and so the stripes were a work-around? Or were there some thermal or other spaceflight or testing advantages that stripes had over a a solid gray?

$\epsilon_{ir}/a_{vis}$ would be thermal infrared emissivity and visible light albedo pairs, not meant to be as a simple ratio though. For an equation using these search this site for "spherical cow temperature".


Shamelessly cropped from images in this question and answer(s):

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On other satellites, such as Explorer 3, reflecting oxide stripes maintain the average inside temperature between reasonable limits; 30 and 100 degrees Fahrenheit.

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    $\begingroup$ I don't think I can make a useful answer to this question. Things have changed significantly in the last half century or so and the requirements on spacecraft coatings have become more complex. What I have done is add a paragraph about the new coating issues to my answer to the original question. $\endgroup$ – Vince 49 Mar 5 '20 at 5:09
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    $\begingroup$ Pure speculation, so not an answer, but: What if I have one paint that's less emissive than I need, and one that's more, and for some reason mixing them to get the emissivity I need won't work. Similarly, maybe I have the uncoated surface and one paint, but the same issue: One is more emissive than I need, and one is less. An actual answer along these lines would explain why the two paints couldn't be mixed, for example. I wish I knew, great question. $\endgroup$ – Wayne Conrad Mar 5 '20 at 19:12
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    $\begingroup$ Even if mixing them does get that intermediate emissivity, I can imagine it's easier to tweak emissivity during testing by adding more patches of white or black as needed, rather than having to remix a new batch of grey with a slightly different composition. $\endgroup$ – Anton Hengst Jun 24 '20 at 2:25
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    $\begingroup$ Check out the Mercury capsule retropack at the NASM, it's gloriously striped as well data4.primeportal.net/space/miles_lumbard/mercury_retro_pack/… $\endgroup$ – Organic Marble Jan 7 at 21:10
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    $\begingroup$ @OrganicMarble those are beautiful! zooming in on each is a reminder of what technology looked like back then (wires, connectors, etc.) and the "handcrafted details" done because everything was new and there were no standard manufacturing tools for space ships at the time. In data4.primeportal.net/space/miles_lumbard/mercury_retro_pack/… looking at the large circular hole in the center, it looks like the black stripes were spray-coated as the edges become diffuse in certain areas. I'll probably ask a question about the heat shield... $\endgroup$ – uhoh Jan 7 at 23:25
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Here's my view - I'd welcome further thoughts on anything I've got wrong.

Design practice

Choices in thermal design have to pay attention to how well known the material properties are. e.g. better to choose a well known, or stable, parameter rather than one whose nominal properties are just right.

I wouldn't be so bold as to state this is the criteria followed by early designers (60's/70's) though I encountered it so often in my earlier days (in the '90s) that it seems plausible.

Paint types

There are black paints (these days) that are very stable in terms of lifetime alpha/epsilon. Early white paints, at least to the 90's, degraded in alpha under UV illumination from 20% up to, I think, 40% ish. However I don't know how well known that was in the 60's. I don't know about grey paints though I can't see the idea of a different shade being mixed for each mission being attractive.

Film

Its not always paint! Sometimes it is film - terrestrial equivalent being sticky-backed-plastic. That aside I think similar pigment stability properties apply as for paints.

Synthesis

OK, this doesn't totally add up but it seems plausible that Black and White were chosen as they were available and believed to be well understood.

@Anton Hengst's comment about flexibility in test sounds plausible too. These days there is not much that is expected to be trimmed at the thermal vacuum test stage but it could well have been a more general expectation in the '60s.

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