Inspired by Are there types of animals that can't make the trip to space? (physiologically) and related to but not a duplicate of Can you swim in space?...

Swimming on the moon / in low gravity looks like fun. But can you swim in zero G / microgravity? - lets imagine a ball of water with a 5m radius floating in empty space within a suitably large space station. (would it hold together well enough or would you need a tank?)

I suspect that the act of swimming itself should be easy enough - you're simply displacing water to provide thrust, similar to how you would do so on Earth - it's the mass, not the weight of the water that allows you to do so.

I do wonder though about the problems associated with swimming however - how easy would it be to breach the surface and take a breath? How easy would it be to exit the water?

The latter question may have an answer of "just pick up enough speed and you'll just 'fly' out, but the way water likes to flow over surfaces might complicate that a little.

So, assuming we can afford the cost of getting a few tons of spare water into orbit, what problems would a swimmer encounter in microgravity - is it even feasible?

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    $\begingroup$ I love the swimming questions. Keep them coming so we can justify a tag! $\endgroup$ Sep 27 '19 at 10:24
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    $\begingroup$ Feel free to roll-back the edits if you don't approve. I know there are answers about the problems with waves in reduced gravity, but the only other question about zero gravity that I know about is Scuba diving in free fall, do I need to worry about the Bends? $\endgroup$
    – uhoh
    Sep 27 '19 at 10:44
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    $\begingroup$ The excellent John Varley story "Blue Champagne" covers this well. $\endgroup$ Sep 27 '19 at 13:51
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    $\begingroup$ You made me wonder about the opposite: How would swimming be in an increased gravity environment? We should build a super large cetrifuge that includes a pool. $\endgroup$
    – flawr
    Sep 28 '19 at 12:51
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    $\begingroup$ The movie Passengers has a scene of this: youtube.com/watch?v=Qt6LfsvbbiU $\endgroup$ Sep 30 '19 at 9:09

Two major problems present themselves right away. As the human body is almost neutrally bouyant with water, one might think that there are no issues with the actual movement in water. But this is only partially true.

Directional orientation in the water will be very difficult. On earth, when we swim, not only is our chest slightly more bouyant than our legs, but we can also feel the pressure gradient from increasing depth. In other words, even if pulled head-first into the water (e.g. heavy gold necklace), we can feel how the pressure lessens in the "up" direction and build a mental picture of our scenario based on that. There is no such pressure gradient in zero-g. On its own, this disorientation can be resolved by careful, rigorous training, similar to training used in the military to evacuate aircraft that have ditched into the water.

However a second issue arises that is associated with the geometry and material properties of our water sphere. As mentioned in other answers, water has significant surface tension, and in absence of a directional acceleration will clump into a wobbly sphere. It is reasonable to think the water will roughly hold together. However, we are disregarding physics of scale to use these assumptions to predict the behavior of water with our larger sphere.

When the radius of the sphere becomes large (on the order of meters), the internal motion of the water in the sphere (and this includes currents induced by swimming and pushing the water around) has much greater energy than the surface tension holding the sphere together. (This comparison is called the Weber number, which is important in determining whether liquids will "splash" and determining droplet size.) Not only would our sphere come apart, but large air pockets will be dispersed throughout. This is bad for swimming, as the effective density of the water is lowered through the inclusion of these air pockets. This effect is similar to swimming over a bubble generator. Combined with the fact that water still likes to stick to everything, including our face, we have a recipe for panic, suffocation and severe water inhalation, of which the latter two can be (obviously) fatal.

As the sphere gets huge, it may take time to create the air pocket inclusions, and swimming may be semi-normal. That is, until you need to break the surface for air. But as time goes on and the sphere disperses due to the aforementioned water currents, your situation will look more and more like the one mentioned in the previous paragraph.

Feasible: Yes for a little while

Dangerous: Also Yes

Cool Factor: Not as cool as the pool on the moon.

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    $\begingroup$ Most of these are only issues if you have a room which contains both water and air. If you have a large sealed room containing only water (and obviously give the swimmer some sort of breathing apparatus), it might be a different story. I believe there have been some experiments done with small fish in space that might apply. Getting enough water into orbit to allow a human to swim might be more challenging though. $\endgroup$ Sep 27 '19 at 19:01
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    $\begingroup$ @DarrelHoffman excellent point! A sealed room would remove the air mixing issues. I suspect that then this whole process becomes a lot like cave diving, where orientation management is a matter of training and experience, but can definitely be handled. $\endgroup$
    – Quietghost
    Sep 27 '19 at 19:05
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    $\begingroup$ I was thinking of the sealed room, but doesn't that raise the issue of how the swimmer gets into it? Would the surface tension at the airlock be enough to keep the water in the room? $\endgroup$
    – Barmar
    Sep 27 '19 at 22:18
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    $\begingroup$ @Baldrickk how do you get the water out of the airlock or vice-versa? On earth you can pump water out and air in because the water stays at the bottom and air at the top... here it would break up, float around and be hard to get out. $\endgroup$
    – user46053
    Sep 28 '19 at 2:35
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    $\begingroup$ @user46053 - getting the water out of the airlock would be easy if you're make it a three way one, just vent to space ;-) . More practically the water is valuable so, I suspect you could do something with a suction hose running through a centrifugal dryer before blowing back into the chamber. Squeezing through some sort of rubber portal might help with the initial quantity. $\endgroup$ Sep 29 '19 at 1:57

Off the top of my head, two issues for free swimming (no breathing gear) I can think of:

  • Absent a sense of "up" and "down", it would be very easy to become disoriented and lose track of where the nearest surface is to take a breath.
  • Surface tension will become the dominant force governing water flow as you come up for breath. In particular, the water will cling to your face, making getting a clear path to breathe difficult.
  • $\begingroup$ +1 for your second point. It's probably very difficult to get out of such a water ball. Plus that breathing thing is pretty important! $\endgroup$ Sep 27 '19 at 18:49
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    $\begingroup$ If you could manage to give your body a decent spin while coming out of the water, that should take care of the cling-to-your-face issue. Failing that, I'm not sure whether just vigorously shaking your head (and/or your whole body, like a dog shaking its hair dry) might be sufficient to clear it of water. $\endgroup$ Sep 27 '19 at 19:26

Mixed water and air environments could indeed be quite dangerous in microgravity. But an all-water environment should be possible. For a human to experience sustained microgravity already requires a breathing apparatus; often this is habitat or capsule sized, but personal units exist as well.

Several have commented on the challenge of moving between all-air and all-water habitat volumes by an air lock.

Existing practice for cleaning up liquid spills is the use of a vacum hose (likely venting to space, which would be wasteful of mass at air lock volume) and towels. A chamber could be designed to blow in air and pump out water (initially through vents, later via a handheld hose for catching globs) and separate the suctioned air and water via a centrifugal dryer and possibly a refrigeration-type dehumidifier at the final stages.

However, there's also another earth technology which could be quite applicable to initial separation: air bladder water tanks. An astronaut wanting to move from the air environment into the water one would enter the airlock, don the breathing apparatus and zip themselves into a flexible bladder connected to the water side door. The air pressure in the surrounding chamber would then be slightly raised, while that inside the bladder lowered, effectively "vacum packing" the astronaut (but only weakly, so that they can continue to breathe). Once most of the air has been removed from the bladder, the external pressure is slightly lowered and water pumped into the bladder. They are then free to open the water door, and enter the water environment.

To exit the water environment, they pass through the door into the bladder and close the door behind them. The external pressure is again slightly increased, and the water pumped out. Once most of the water has been removed from the bladder, they unzip, and use the previously mentioned recirculating blower, suction hose, and towels to clean up.

Finally, while orientation could indeed be a challenge, it is not likely to be more challenging in a microgravity water environment than it already is in a microgravity air one.

If there is a unique danger, it would be with things like water aspiration resulting from partial mis-operation of the breathing apparatus. Things an earth-bound diver can do will probably not be safe. Its quite possible the astronaut should effectively wear a drysuit during their time in the water habitat.


"lets imagine a ball of water with a 5m radius floating in empty space within a suitably large space station"

The sphere of water would break apart from the turbulence caused by the swimmer. Then the swimmer would drown inhaling the floating soup of water fragments.

If you enclose the water in a tank, then you can "swim underwater" - but getting in and out the water might be complicated.

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    $\begingroup$ Also - put flippers on your hands and feet and you can swim in air in zero-g. Much safer! $\endgroup$
    – Ags1
    Sep 29 '19 at 14:28

To swim in water you need to breathe air with only a small content of water droplets. You need to know when it is possible to open your mouth and take a deep breath of pure air. If you inhale too much and too often water instead of air you are in danger of drowning.

In zero gravity and under the influence of swimming, there will be many water droplets floating around in the air and air bubbles within the water. There is no force removing the air bubbles and water droplets like we are used to when swimming on Earth. If two droplets hit themselves at low speed they may unite, but for larger speed the result will be even more droplets in air.

So I think swimming in zero gravity is not possible. But an experimental proof would be extreamly expensive.

  • $\begingroup$ Yeah, I used to swim competitively. Even without the issues of water sticking to the face, you had to make sure you were clear of the water when breathing. I could have sworn I wrote something about using scuba gear when trying, but probably took it out to leave that as something to be answered. $\endgroup$
    – Baldrickk
    Sep 27 '19 at 22:59
  • $\begingroup$ You mean it's not possible without a breathing apparatus. But you need a breathing apparatus to experience sustained microgravity anyway, the only question is if it is personal or habitat scale. $\endgroup$ Sep 29 '19 at 1:59
  • $\begingroup$ @ChrisStratton The point is you will need a personal breathing apparatus to safely venture into water in zero-g as you have no means of not inhaling water otherwise. $\endgroup$ Oct 1 '19 at 14:39

Yes, you can.

This subject is well covered in the book The Integral Trees (1984) by Larry Niven

The majority of Smoke Ring animals have evolved to fly on at least an occasional basis—even the fish. The Smoke Ring contains numerous "ponds," globs of water of various sizes which float free like everything else. While there are aquatic and amphibious animals in the Smoke Ring that live the majority of their lives in such ponds, these animals may find their habitat unsuitable at any moment. Source

There are number of scenarios played out, and they all seem to be well balanced in science.

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    $\begingroup$ Niven may have overestimated surface tension for large globs of water. @Quietghost's answer makes the excellent point that energy of currents can get much larger than surface tension with creatures swimming aggressively. (A square-cube issue, since surface tension scales with the area of the near-sphere while the mass of potentially-moving water scales with the volume of creatures swimming in it) $\endgroup$ Sep 27 '19 at 16:50
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    $\begingroup$ While I love Niven's writing, and consider that "The Integral Trees" is one of his best and most original works, I do not think that anything in a book of fiction can be considered definitive when it comes to answering a question such as this. $\endgroup$ Sep 28 '19 at 19:16

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