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Many/most early artificial satellites were spheres with mirror-like polished outer surfaces.

I am guessing that for lower orbits the spherical shape minimized drag as well as simplified the analysis and interpretation of rate of decay, since this was critical to understand in the early days.

But the metal could have been roughened, anodized, or even painted, rather than having a mirror-like finish. Was this done for reasons of visibility and optical tracking? Or was it done for reasons of thermal management, where the emissivity in the visible determined heat absorption and in the IR determined radiation? Or another reason, or for no particular reason at all beyond aesthetics?

I'm looking for an answer supported with a source that addresses the design intention at the time.

Below Sputnik-1, Luna-1, and Vanguard-1.

Sputnik-1 Luna-1 Vanguard-1

Exploring the "aesthetics" angle, rendering of Vanguard-1 versus disco mirror ball:

rendering of Vanguard-1 disco mirror ball

uhoh
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1 Answers1

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Per the official NASA history of Vanguard it was for visibility and thermal control.

(Bolding mine)

Tousey had made some of the first calculations in the fall of 1955, contributing his knowledge of optics to ensure that protective coatings on the exterior of the booster and on the satellite shell would have sufficient reflec­tivity to permit telescopic observation of the course of the rocket as it rose and then optical acquisition and tracking of the satellite in space. Schach undertook the "thermal design," that is, the calculations of what tempera­tures to expect at various points in the satellite's orbit, in darkness and in daylight, the selection of the optimum thickness of coating materials to emphasize their emissiveness of solar heat radiation, and methods of keeping the satellite's surface free of contaminating substances such as soot which would ultimately raise the satellite's temperature. Hass worked out the techniques of applying the successive surface coatings-the gold plating, the chromium evaporated to vapor and deposited to serve as a primer, the silicon oxide to serve as a barrier, the thin layer of evaporated aluminum to give a mirror-like finish, and finally a film of silicon oxide to control emitted radiation. Drummeter and Schach were chiefly responsible for developing the sunlight simulator with carbon arcs as the source of high­ intensity light. Through windows in the large cylindrical vacuum tank in which the coated sphere sat for two or three days of testing, the simulated sunlight beat upon the satellite's surface and indirectly heated the inner pot of instruments. Measurements of the effects furnished means of determining the most desirable material and thickness of the layering required. LaGow acted as advisor and monitor on all these operations. Every man concerned with temperature control worked closely with every other.

Organic Marble
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    Holy granola! I had no idea that the surfaces were so complex; this is really fascinating stuff! I'm guessing that the final SiO2 on top of the evaporated aluminum works the same way as the back-side aluminized glass mirrors work on modern spacecraft radiators as mentioned in this insightful comment. – uhoh Jan 06 '18 at 01:50
  • The outer silicon oxide layer was deposited to protect the reflective coating. The coatings were actually deposited by Alan Bradford who worked for Dr. Hass at the time. – Vince 49 Jan 06 '18 at 06:12
  • @Vince49 so you believe the statement in the block quote "..and finally a film of silicon oxide to control emitted radiation." is wrong/false? Is that just a guess or can you source it? I'm thinking that it's possible at the time that spacecraft technology was considered somewhat sensitive, and the details may have been obfuscated. At the same time, a sputtered oxide layer may be too thin (physically, or optically) to behave the same way (from an emissive point of view) as those thick glass layers I've linked to. – uhoh Jan 06 '18 at 11:42
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    Hi uhoh. It's not a guess. Some members of Dr. Hass's group later worked for me. – Vince 49 Jan 06 '18 at 17:07
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    I was not able to complete the comment above due to the 5 minute edit rule.

    The coatings were made under the guidance of Dr. Hass, mostly at Ft. Belvoir, VA in an area that later became the Night Vision Laboratory. The quoted passage is correct in general, but not in some details. For example, the chromium layer is deposited before the gold for adhesion. Now days, usually titanium, rather than chromium, is deposited before a gold layer to improve adhesion.

     – Vince 49 Jan 06 '18 at 17:31
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    @uhoh, I'm working to get you information on the original intent of the coatings. One of the original players is still working part time and I just sent him an email. Please check back in a few days. – Vince 49 Jan 06 '18 at 18:46
  • Sounds like you should be able to post a very authoritative answer! Looking forward to it. – Organic Marble Jan 06 '18 at 19:14
  • @Vince49 Oh, it's definitely common for Cr or Ti to be used as an adhesion layer before Au. While it's not mentioned first, the paragraph indicates it's used as a "primer" which could mean (to me at least) before the gold. So while the wording is imperfect, I think could still be consistent with the Cr being applied before the Au. Anyway, thanks for the input! This must have been a really exciting time! :-) – uhoh Jan 06 '18 at 23:11
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    I too think is was an exciting time. From what they told me later, I believe the fellows involved saw that work as their magnum opus. And I would agree. Vanguard was the second US satellite launched and the first ever solar powered satellite! – Vince 49 Jan 07 '18 at 01:09