The NASA video Juno Engineering: Precision Matters shows the Juno spacecraft on what look like it might be a spin balance table after about 02:50 and there one can see that much of the whole spacecraft is wrapped in a dull, reflective film.

According to @Dave's excellent answer the dish is covered not with aluminized or metallized foil, but with a germanium coated polymer film. The semiconductor is thin enough and has such a low carrier density that it is fairly transparent to microwaves and yet because it has an optical band-gap, still opaque to the Suns light (physics discussed more in this answer). This provides some thermal stabilization against temperature changes caused by sunlight.

Juno has several Microwave Radiometer antenna arrays on it's sides as well, and those also appear to be covered with the same germanium foil.

As far as I can tell many if not most recent deep-space spacecraft have their dishes and possibly other antennas covered.

But I don't think the early ones did. As far as I know the Voyagers' dishes were completely open, and I think (but I'm not sure) that Cassini's was open to space as well.

Question: When did deep-space probes start getting covered in germanium-coated films? Which one was first?

cropped from NASA video Juno Engineering: Precision Matters

Here's a photo of TESS, used in The appearance of TESS' dish seems to be evolving, what will be the final configuration? sourced from Solar Panels Opened on NASA’s TESS Satellite. Photo credit: NASA/Leif Heimbold

enter image description here

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    $\begingroup$ Interesting. Magellan certainly didn't solarsystem.nasa.gov/system/content_pages/main_images/… Galileo had that weird deployable mesh antenna that caused all kinds of problems. Not sure about Ulysses. $\endgroup$ – Organic Marble May 3 '19 at 16:12
  • $\begingroup$ In my opinion, I think that as the germanium foil is mostly used to resist high frequency radiation. So, we can assume that the coating started with missions closer to the sun, maybe starting with Venus projects. $\endgroup$ – Amar Apr 22 at 5:12

I found a reference on page 1 of this paper: 2010 Ishizawa Paper stating that,

"(b) Germanium coated polyimide: After a MIDORI-2 (ADEOS-2) malfunction caused by a discharging event, conductive materials should be used for thermal control of JAXA’s satellites. Germanium (Ge)coated black Kapton XC film (conductive) is selected for substitution of Beta-cloth (nonconductive) for the outermost layer of MLI (multi layer insulation). "

Looking up the date for this event lead me to this link EO Portal ADEOS-II Entry, which reported that the failure occurred on October 23, 2003.

It seems the failure investigation wasn't conclusive at first so it appears likely that it took several months or longer for the industry to conclude that the Ge coating was the way to go to prevent events like this in the future. I suspect the first spacecraft to carry the Ge coated MLI for this purpose, as a result of this finding began showing up around 2004, but I have not been able to find a definitive source. It is also possible some manufacturers were forward thinking and had begun using this type of coating earlier but it only gained widespread adoption after the ADEOS-2 incident.

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