Bsrgvty

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?

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.

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.

"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.

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.