In the following image of Exploration Extravehicular Mobility Unit (xEMU) or Artemis Generation Space Suit, it can be seen the helmet contains double-pane of glass (inside the black rectangle)

enter image description here

What is in between the two glass layers - air, inert gases (Helium, Neon, Argon, Krypton, Xenon), or vacuum?

What are the advantages of having a double-pane space helmet? Some of the possible advantages that I could think are as follows:

  • For additional safety, to avoid accidental decompression in case of damage in the outermost glass layer.

  • For radiation protection (I can't think how this can happen)?

  • For thermal insulation (like the double-pane windows used in some houses)?

Or is that any other thing? Are the current generation space-helmets double-paned or single-paned (I can't see any difference clearly)?

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    $\begingroup$ I can't post an answer because I have no source, but the need for thermal insulation (if that turns out to be an answer) might be more to prevent condensation of water vapor (especially that exhaled directly on to the surface) than to prevent heat loss, since the suit has to go out of its way to get rid of excess heat (How have space suits dissipated the heat removed from astronauts?). Slightly related: Does sunlight warm an astronaut's face during a spacewalk? $\endgroup$
    – uhoh
    Oct 20 '19 at 5:40
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    $\begingroup$ If they have ended up with needing divergent properties they may be different materials, separated to avoid problems with thermal expansion. Or possibly to make it easier to service/swap parts. $\endgroup$ Oct 20 '19 at 5:44
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    $\begingroup$ @uhoh, Maybe this could help (See only What is a Double-Pane Window?). It is about double-pane windows, and I don't know whether it's reasonable to superpose the same concept here. $\endgroup$
    – Vishnu
    Oct 20 '19 at 5:48
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    $\begingroup$ @Intellex I think that comment means to point out that the visor may be providing several different functions at the same time, and some of those may be best served by glass and others by plastic (just for example). Since the materials have different coefficients of thermal expansion and temperatures can vary wildly in space during use, storage, and transportation, trying to laminate them into a single layer could cause trouble, whereas separating them allows each material to expand/contract on its own, stresses in a bonded bi-material visor could lead to cracks or de-lamination for example. $\endgroup$
    – uhoh
    Oct 20 '19 at 5:53
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    $\begingroup$ They are not currently double pane on the ISS US suits. $\endgroup$ Oct 20 '19 at 11:05

The outer pane is just a replaceable protection of the inner pane against scratches, dirt, and abrasion.

The helmet on the suits for Artemis missions will also feature a quick-swap protective visor. The clear protective visor is a sacrifi­cial shield that protects the pressurized bubble from any wear and tear or dents and scratches from the abrasive dirt of planetary bodies. The quick-swap function means that astronauts can replace the visor before or after a spacewalk instead of sending an entire helmet back to Earth for repairs.

From this page.

Within the pressurized airlock the gas between the panes is the breathing gas used in the airlock. During an EVA in space the gas would ooze into the vacuum. Using a noble gas between the panes does not make sense.

The outer pane does not protect against radiation and does not provide thermal insulation in a vacuum, but it increases safety by protecting the inner pane against damages.

By the way, I don't think the helmet's material would get dents.

  • $\begingroup$ "...breathing gas used in the airlock". Won't the Artemis programme make use of suit-ports to prevent lunar dust from entering the habitat? I think an airlock for astronauts will not be necessary. $\endgroup$
    – Vishnu
    Oct 20 '19 at 13:24
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    $\begingroup$ @Intellex I suspect that even with a suit port, it would be easier to put on a spacesuit if the pressure on the inside and outside is equal. And it would be safer, too. $\endgroup$ Oct 20 '19 at 13:37
  • $\begingroup$ @Intellex But how to replace the gloves of a suit if there is only a suit-port and no airlock? How to clean and lubricate rotatory joints of the arm parts of the suit? $\endgroup$
    – Uwe
    Oct 20 '19 at 15:07
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    $\begingroup$ @Dragongeek I think the added weight of carrying around that much water (in addition to the water used for drinking, because once you taint that water with lunar dust you probably don't want to drink it) would be prohibitively expensive. $\endgroup$ Oct 21 '19 at 14:20
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    $\begingroup$ @Uwe I suppose you could use suit-ports most of the time, and then just bring the suits into a full airlock when they need maintenance? Even if you have to bring them in once every five EVAs, that's still an 80% reduction in the number of opportunities for dust to get inside. $\endgroup$ Oct 21 '19 at 16:02

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