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Why do EVA suits have legs?

Planetary exploration suits (PES) obviously need legs. Spacesuits with legs are “a classic icon of human space exploration” and a requirement for mobility. https://www.nasa.gov/feature/a-next-generation-spacesuit-for-the-artemis-generation-of-astronauts .

But do extravehicular activity suits (EVA) need legs? It has been reported that EVA suits have very little knee mobility compared with PES suits (unsubstantiated) Judging by photographs of EVAs, legs on EVA suits provide little utility:

enter image description here

If legs were not a requirement for EVA suits, it would provide more design latitude.

enter image description hereenter image description here Robonaut with mono-leg and bi-legs for comparison

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    $\begingroup$ Related space.stackexchange.com/q/18568/6944 $\endgroup$
    – Organic Marble
    Commented Jan 14, 2022 at 19:21
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    $\begingroup$ I don’t think astronauts can just chop off their legs for EVAs. Just doesn’t seem like too safe really. $\endgroup$
    – Topcode
    Commented Jan 14, 2022 at 19:25
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    $\begingroup$ Ok. Good point. They can keep their legs. But does the suit need legs? Why not a "monopant"? It would simplify design and add design options at the same time $\endgroup$
    – Woody
    Commented Jan 14, 2022 at 19:31
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    $\begingroup$ Related: space.stackexchange.com/q/26282/6944 $\endgroup$
    – Organic Marble
    Commented Jan 14, 2022 at 19:48
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    $\begingroup$ EVA suits have many layers - at least the inner cooling tubes layer will want to be close to the skin. The "monopant" design would create a hotzone between the legs? Maybe this is part of the reason? $\endgroup$
    – IronEagle
    Commented Jan 15, 2022 at 1:30

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Your question assumes that legs aren't used on EVAs, however legs are actually important for leverage. The first EVAs did not go well, astronauts were unable to complete tasks partly because they couldn't exert enough force. The assumption was that legs would be useless, but part of the solution was to anchor the legs on different parts of the spacecraft structure. Legs have some of the strongest muscles in the body, for them to be most effective they need to be apart some so that they can stabilize the hips and hold the lower body rigid. With feet together the body tends to flex. You can see this for yourself here on Earth, try holding your feet together while you exert a force on something, you'll find it's much harder then if you have your feet apart.

Also, having legs together limits mobility. Tie your legs together and then try and reach for something to the side, it doesn't work so well.

It's also important to remember that astronauts have to practice on Earth which does have gravity. Although neutral buoyancy tanks mimic zero G they still have to get to it, and walking is an advantage. As the point of training is supposed to be as realistic as possible the suits need to be the same as in space. I'm sure they could get past that if they wanted to, but there'd have to be a compelling reason.

It's hard to see what advantages there would be to a single leg 'mermaid suit' design, maybe they could save some piping but it isn't worth losing the mobility. If anything astronauts would like more leg mobility rather than taking away what little they've got.

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  • $\begingroup$ Do you have a reference for "there's other type of training on models where the astronauts need to be able to walk."? $\endgroup$
    – Organic Marble
    Commented Jan 15, 2022 at 14:22
  • $\begingroup$ I'm not sure how much still goes on these days @OrganicMarble, I'm thinking of the models that were used for Gemini, Apollo and Skylab where not all of it was done in the tank. $\endgroup$
    – GdD
    Commented Jan 15, 2022 at 15:03
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    $\begingroup$ I edited it out @OrganicMarble as I can't find what I'm looking for and it's not particularly important to the answer. Looking for EVA training outside the tank just brings more up about tank training. $\endgroup$
    – GdD
    Commented Jan 15, 2022 at 15:22
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    $\begingroup$ @OrganicMarble --- Nice photo. Is there a "map" of locations where footholds can be placed? And who places the footholds? It would be useful if hold points (hand or foot) were spaced regularly, like LEGO knobs so astronauts could anchor themselves at any location. $\endgroup$
    – Woody
    Commented Jan 16, 2022 at 6:44
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    $\begingroup$ @Woody they are called WIFs (Worksite InterFaces) and there are hundreds of them all over the ISS. Pick a random EVA picture (like this one preview.redd.it/…) and you'll see the little brass tubes. I highlighted one from the picture and show a list of the dozens just on the S6 truss segment here i.imgur.com/6i1Odgs.png Body stabilization is fundamental to EVA. $\endgroup$
    – Organic Marble
    Commented Jan 16, 2022 at 14:30

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