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There's no advantage to replacing the air with water.

• you're adding weight.


• you're adding resistance: when you move around, water will flow around the body and because you're proposing a thin layer of water, the flow is obstructed which means it takes energy to push the water around.


• you're replacing the air layer (which is a reasonably good thermal insulator) with water (which is an excellent conductor) which means you need more heating/cooling capacity (=heavier backpack) and/or better insulation (which means the suit gets thicker).


• you want some space between the astronaut and the suit, to prevent abrasion. Making the suit smaller makes it more difficult to design a comfortable suit.

...a thin layer of water that was held down by oxygen up at the head.

"Held down" doesn't work so well in weightlessness. Water would move around inside the suit, climb up the astronaut's neck and put them in danger.

This happened to ESA astronaut Luca Parmitano when doing a space walk at the ISS. See Space.com's Italian Astronaut Recounts Near-Drowning in Spacesuit (Video)

There has been active research into EVA suits where the body is protected from vacuum just from the suit itself being very tight, rather than being inflated with a fluid such as air or water. They are generally known as Mechanical Counterpressure Suits (MCS), or Space Activity Suits due to the promise of increased mobility.

Such suits are very promising. However, there are a few bugs to work out:

• Humans change shape.


• Armpits suck.


• The interface between the suit and the helmet is prone to failure.


• Can't wear diapers.

In order to be effective, a gasless pressure suit needs to provide constant pressure across the skin. Fortunately, they don't need to provide the pressure on every millimeter of skin; merely every few centimeters of skin. Early designs looked like rubber skin suits that were molded right on the wearer. Current designs, such as the MIT Bio-Suit, look like spandex wrapped with a mile of thin bungee cords, as in the image below.

(Credit: NASA [Public Domain])

The first problem with this design (and especially older MCSes that were made from molded rubber) is that the human body changes shape significantly in microgravity, because our bodies are evolved to move fluids upwards. Our chest and head get larger, while our legs get skinnier... and over time, our bodies lose about 20% of the total water volume due to overpressure of fluids in our upper bodies. We also grow taller over the first few weeks in microgravity.

The bungees of the MIT Bio-Suit are adjustable, but looking at some of the more complex shapes of the human body, it is very tedious to adjust all of the bungees. Instead of starting to don an EVA suit only 2 hours before leaving the airlock, you would have to take an entire day to custom fit it, THEN take the normal 2 hours the morning of the EVA.

There is one region on every human body that can not be fully protected from the vacuum of space with an MCS: The armpits.

Armpits are just too concave.

There are also regions of the male anatomy that suffer some other interesting topological problems.

The armpits can be mitigated with an inflatable bladder in each side. This will reduce mobility, but will keep the astronaut from some serious hickies. The other topological problems will have to be solved with tape. Perhaps the space programs could enlist the expert advice of drag performers in this regard, or not allow people with certain body parts to perform EVAs in an MCS.

The next issue is how to put a breathable atmosphere on someone's face without also filling up the rest of the suit.

The whole point of an MCS is to provide constant pressure to the skin of a flexible human. That means that the MCS can not be rigid. However, in order to let the astronaut breath and open their eyes (much less see through the helmet's visor), they need a rigid helmet to contain some air.

If you connect the helmet to the MCS at the shoulders, then any time the astronaut raises their arms, their shoulders flex, and the air tight seal fails.

If you connect the helmet at the neck, you have a rigid ring around your neck and can't swallow (this can get very dangerous in case of saliva buildup, and is very uncomfortable anyways). It may prevent the astronaut from talking if it interferes with their larynx, and could cut off circulation to the head, especially if their blood pressure starts climbing.

If you connect the helmet at the jaw, the astronaut will be unable to open their mouth without the air tight seal failing (or will just be unable to open their mouth at all). They will be unable to talk and may have a hard time swallowing.

If you try to connect the helmet above the mouth, they'll be at extreme risk of injury if they try to open their mouth... Definitely won't be able to talk in that case.

And the last problem: Astronauts on EVA wear diapers.

Looking at the MIT Bio-Suit, and the many bungees in that area, it won't be possible for an astronaut to wear a diaper. They'd have to have a catheter.

And no, you can't just hold it. EVAs typically last 8 hours. They are also working in harsh conditions, so need to stay hydrated.

The problem is how you would keep the water at the bottom and the oxygen at the top. In gravity environments such as Earth or Mars, the water would understandably tend to gather at the bottom and the oxygen at the top, due to buoyancy. This is fine, of course, and what you would expect. But what else would happen?

Your astronaut in such a suit can't even bend over without the water sloshing up into their helmet, let alone enter a weightless/free fall environment.

Could you compartmentalize the air and the water so that each is sealed from the other? Perhaps you could, but with what? I don't know any astronaut who wants a tight clamp choking their neck while they are doing important Science stuff.

If your interest is reducing form factor then you should look in to mechanical counterpressure suits. As the name implies, these apply mechanical pressure to the human body to prevent expansion.

One such suit is the MIT Bio-Suit, which may be used for the Mars mission.

MIT Bio Suit next to the Mars Mark III planetary hard suit.

Wouldn't that be a bit impractical as the astronaut would be swimming in water. Unless I'm missing something, also their fingers and toes would look like raisins after the space walk. Getting in and out of it would be a mess too, with the water escaping and getting everywhere.

Another consideration is that air is breathable and contains oxygen which is somewhat useful for respiration.

Having a suit filled with air will provide a small reserve of air in case the life support systems have problems.

Also, getting in and out will be a lot messier if the astronaut has to get into a "bath" rather than into a suit.

It can't be full of liquid either, à la: The Abyss. "Despite some recent advances in liquid ventilation, a standard mode of application has not yet been established."

A significant problem, however, arises from the high viscosity of the liquid and the corresponding reduction in its ability to remove CO2. All uses of liquid breathing for diving must involve total liquid ventilation (see [link]). Total liquid ventilation, however, has difficulty moving enough liquid to carry away CO2, because no matter how great the total pressure is, the amount of partial CO2 gas pressure available to dissolve CO2 into the breathing liquid can never be much more than the pressure at which CO2 exists in the blood (about 40 mm of mercury (Torr)).