# Swimming in a ball of water in space

What would I feel if I went swimming in a ball of water in space? Would I feel greater pressure as I went deeper into the sphere? What would it be like to swim in something like that?

Also, let's say that I don't need air to survive in this scenario.

Best answer is given from xkcd's what-if: https://what-if.xkcd.com/124/. It's not space, but it describes the fluid flow in lower gravity - such as how you could jump out of the pool just by performing aquadynamic maneuvers, or walk on the water. It is really a cool read.

As mentioned in the xkcd article - diving and floating, being primarily about differences of density and viscosity, wouldn't change - you'd be able to dive at about the same speed as before.

Due to the law of gravity, the ball of water would have some gravity, simply due to it's mass. By Newton's shell theorem, you would have maximum gravitational attraction near the surface, and while the pressure would go up as you dived down (proportional to the ball of water's gravity), the gravity would go down. However, this effect isn't very measurable (if you want to see why, keep reading - otherwise skip to the bottom).

Assuming an unprotected body, you wouldn't be able to go very deep, or you wouldn't have a ton of water to hold this ball of water together - you really can only go down about 50m - 100m, according to the world records

A 100m radius ball of water, as Wolfram points out, is only 3 thousandths of 1% of earth's gravity - essentially nothing. Wading through this ball would be pretty much like spacewalking, but with inertial drag to stop you.

Scale the radius up by 100000 times, and you'll feel 30% of earth's gravity, (and your ball of water would stay together) but you would feel none of the variation with depth, since you can't dive very deep.

The breaking point between these two points would be when your swim speed would reach the escape velocity of your ball of water. In this case, Wolfram helps me again to show that would be around a 2.68 km radius. Smaller than that, and when you swam out of the ball, you'd just float away. Bigger than that, and the ball would catch you and pull you back in.

So, the bottom line is swimming in a big ball of water pretty much feels like swimming very slowly in space - until the ball of water gets big enough (2.68 km). Then it just feels like swimming in a giant pool on a distant planet. For practicality, the ball of water doesn't work, but the lunar swimming pool is awesome.

Edited for clarity on the source of the gravity, while the poster mentioned 0g.

• The question mentions explicitly 0 g. No gravity. This answer assumes gravity. Correct? – Fermi paradox Jun 20 '15 at 13:36
• This answer uses only the gravity from the ball of water itself. That's why I mentioned Newton's shell theorem. The ball of water, if it was sufficiently large, would have measurable gravity. – Mark Jun 20 '15 at 14:19
• It's surprising how small a ball of water you need for its own gravity to hold it together. Since I was actually needing to do precisely this, thanks for the detailed answer! – David Given Nov 27 '15 at 22:45

Hydrostatic pressure inside a pool on earth is given by:
$p=ρgh +p_{atm}$, (g: gravity, h: depth, ρ: fluid density, $p_{atm}$: atmospheric pressure)

Assuming 0 gravity and no atmosphere, there would be no pressure. You would feel no pressure at all whether you are on the surface or in the center of the sphere.

Also, you wouldn't be able to float to the surface since buoyancy would be 0 as well. You would still be able to swim to the surface of the water if you wanted to. However, taking your head "outside" the surface of the water might be tricky; without gravity pulling the water away from your face I don't think it would be possible.

One final problem would be that water would start to boil. The lower the pressure gets the lower the boiling point. Here is a demo of what happens when pressure gets too low.

As a conclusion, you would be swimming in a bubbling sphere of water, feeling no pressure at all, having a bit less difficulty in moving your arms and legs since the water would be full of bubbles, however I am assuming it would be harder to move around for the very same reason.

• Hi Fermi. I loved your answer, but I'm confused as to why there would be no pressure within the giant ball. Don't fluids create pressure by themselves due to the random motion of particles within them? – Joshua Ronis Jul 4 at 16:44
• @JoshuaRonis if the ball was giant, there would be pressure (perhaps too small to feel or enough to crush us). It depends on what 'giant' means. For example, to us humans a 2km sphere might look giant, while the moon's diameter is 3500km, sun: 1,500,000km, biggest stars: more than 1,000,000,000km :) – Fermi paradox Jul 4 at 17:47
• Thanks Fermi, but why does the pressure have anything to do with how large the ball is? I'm not talking about pressure due to the gravitational effect of the water, I'm talking about pressure due to the random motion of water molecules themselves! Or, am I just wrong in how pressure works to begin with? (Very much a beginner into fluids here) – Joshua Ronis Jul 4 at 17:49
• @JoshuaRonis I was referring to hydrostatic pressure due to gravity. At the molecular level, I don't know. It's an interesting question. My assumption (and I am no expert) is that net force is 0: colliding molecules on one side of your hand should exert a force that is equal to the attractive forces between water and skin. Imagine a tiny droplet on your fingertip. You point it down and the droplet doesn't fall, despite molecules bouncing off of your skin and pushing the water away from your skin plus gravity pulling down. – Fermi paradox Jul 4 at 19:19
• @JoshuaRonis now imagine a small water sphere in space (under pressure) and a tiny air bubble in it. I am assuming there is an extremely small pressure "felt" by the bubble (imperceptible to humans) due to surface tension. But as i said, this is my guess :) Feel free to ask a related question, I'd like to see the answers as well. – Fermi paradox Jul 4 at 19:29

Let’s assume that by “swimming in space”, it is meant that there is no gravity whatsoever. A (basic) microscopic view of the pressure as the force exerted by the particles (e.g., molecules) of a fluid on a surface implies that still there will be a pressure on a macroscopic body in the fluid, regardless of external forces, like gravity — although in a static situation, the net force on the body will be zero.

If outside of the fluid, the pressure is zero, then it means if you leave the surface and go into the bulk (inside the fluid), you will feel more pressure. But the pressure will be (almost) the same throughout the bulk of the fluid.

Finally, remind that what I said above holds true only in a quasi-static case. As one swims through the fluid, one perturbs it a lot, and this means local pressure will be non-homogeneous in space and changing with time.

## protected by Qmechanic♦Jun 25 '15 at 19:28

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