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After about 08:52 in Destin's Smarter Every Day's Why Are there Holes in the James Webb Sunshield? (Explained by My Dad) - Smarter Every Day 270 (linked below) Destin's dad Daryl motions to a full scale simulator up of the JWST sunshield and says:

This is the sunny side, this is what the Sun will see, it will be purple like this. This is silicon on this side.

Question: Why is the sunny side of JWST's sunshield purple and covered with silicon?

Like Destin I'm curious why it's not aluminized and highly reflective.

note: The optical properties of the sunshield in visible light will also be important for answers to:

click for full size:

screenshot from Destin's Smarter Every Day's "Why Are there Holes in the James Webb Sunshield? Explained by My Dad Smarter Every Day 270" https://youtu.be/Pu97IiO_yDI screenshot from Destin's Smarter Every Day's "Why Are there Holes in the James Webb Sunshield? Explained by My Dad Smarter Every Day 270" https://youtu.be/Pu97IiO_yDI

screenshot from Destin's Smarter Every Day's "Why Are there Holes in the James Webb Sunshield? Explained by My Dad Smarter Every Day 270" https://youtu.be/Pu97IiO_yDI screenshot from Destin's Smarter Every Day's "Why Are there Holes in the James Webb Sunshield? Explained by My Dad Smarter Every Day 270" https://youtu.be/Pu97IiO_yDI

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From this presentation James Webb Space Telescope (JWST) Technology Discussion For APPEL Forum, dated August 7, 2008, I see this:

  • Low $\alpha_S$/$\epsilon_H$ to minimize absorbed solar heat load
  • Metallic coatings have $\alpha_S$/$\epsilon_H$ > 1

There's more detail in Technology Development for Large Deployable Sunshield to Achieve Cryogenic Environment, but I haven't found a free source for that one, so you'll just have to take my word for it. Here's a handy diagram from that paper:

Figure 10, basic theory of sunshield operation, showing layers and coatings

$\alpha_S$ is solar absorptance, and $\epsilon_H$ is the emissivity of the material. I can't perfectly identify the meaning of the H subscript, but "hemispherical" seems plausible.

If I've understood correctly, the low $\alpha_S/\epsilon_H$ value helps ensure a low steady-state temperature of the outermost layer, but the paper assumes that the reason for using such a coating is so obvious to its intended audience that it doesn't explain in any further detail. As I'm not an engineer or familiar with the issues of things warming up in sunlight I can't be certain.

Silicon was chosen due to its high emissivity and robustness to environmental conditions prior to launch. Germanium was also considered, but:

Germanium has been used in applications in the past and has the proper thermal properties. However, there are issues identified with property degradation due to humidity effects. The coating also has high electrical resistance causing a need for complicated supplemental grounding on a large membrane in space

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    $\begingroup$ Interesting! In this answer I talk about semiconductor films and explain that they are fairly opaque in the visible and infrared but tend towards transparent at microwaves. I don't address thermal IR there but the idea is that they should be very weak absorbers and so I'd expect it to have a fairly low thermal emissivity $\varepsilon_H$ at thermal infrared wavelengths. And yet if $a_S/\varepsilon_H$ is low it suggests $\varepsilon_H$ is not low. Okay I"ll go look at the paper now. Thanks! $\endgroup$
    – uhoh
    Mar 6, 2022 at 14:17
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    $\begingroup$ @uhoh it is quite difficult to find useful figures for absorptance/emissivity in thin films. FWIW, the paywalled Waldie paper quotes beginning-of-life solar absorptance of a 100nm silicon film as 0.4 and its emissivity as 0.74. The corresponding figures for germanium were 0.44 and 0.69. The absorptance of bulk aluminium across a similar spectrum is ~0.1, and the VDA layers on the sunshield presumably have an emissivity of <0.03. It doesn't define "solar absorptance" or go into detail about any spectra. $\endgroup$
    – Starfish Prime
    Mar 6, 2022 at 14:46
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    $\begingroup$ @StarfishPrime +1. Does the paywalled document actually state what the individual components of alpha and epsilon are for the sun-facing material coatings as distinct from their ratio? It might help to understand the context a bit better. $\endgroup$
    – Puffin
    Mar 6, 2022 at 19:06
  • $\begingroup$ These offer some ancillary insight but no quantitative information Solar Radiation Pressure Effects on the Orbital Motion at SEL2 for the James Webb Space Telescope and James Webb Space Telescope: large deployable cryogenic telescope in space (nice overview) and Cooling Technology for Large Space Telescopes $\endgroup$
    – uhoh
    Mar 6, 2022 at 22:04
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    $\begingroup$ @Puffin the only details they had I copied into the comment to uhoh just above yours. $\endgroup$
    – Starfish Prime
    Mar 9, 2022 at 9:24

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