# 0g pool: can I keep a big ball of water in space?

I have a super luxury space hotel. It has artificial gravity in the living and shopping area and 0g in some parts of the recreational area.

There is a 0g pool available for the guests. I want it to be a large ball of water 10m in diameter. Can it work?

I know water can be kept together by surface tension. I saw videos of water balls a few inches in diameter from the ISS, is there a limit to how big it can be?

Obviously some of the water will disperse because of the interaction with the swimmer at least, so there will be a recovery and recycling system in place. However I would like the water to just sit in the middle with no infrastructure touching it for most of the time. Is it possible?

• Comments are not for extended discussion; this conversation has been moved to chat. Sep 11, 2016 at 20:33
• No. You can't swim on a 'surface' in 0g because the whole concept of buoyancy becomes meaningless. Oct 19, 2017 at 22:35

There is a 0g pool available for the guests. I want it to be a large ball of water 10m in diameter. Can it work?

I don't think you can make this work as you've envisioned it. Let's look at some of the issues:

1. Newton's Third Law

When swimming, you push against the water and the water pushes back on you. You go forward, and the water goes backwards. Water's also pretty good at transmitting waves. The result is that in a spherical pool treading water forces water downward, and that force will travel in a wave down to the other end of the sphere and create a splash there (though if it's going 10m to the opposite end of the pool, it'll be fairly spread out). So every movement of every swimmer is going to cause a ripple somewhere in the sphere. This contributes to the second issue:

2. No way to keep the water free-floating

Assuming you don't want a hand-wavy solution like some kind of force field, we don't have a good way to keep the water where you want it. Water isn't magnetic, so you can't use a magnetic field, and if you tried to introduce something magnetic into the water it would end up getting concentrated at the center of the pool.

You can't use air jets to keep the water in the center - if you're pumping air in (through the air jets) either you need to pull air out or the air will get increasingly dense (until it's dense enough to overpower the air jets). Same thing specifically with the air right next to the water - air will be circulating. That means that there have to be places at the surface of the water where air is flowing away from the water, and water will try to go with it.

Don't count on surface tension to save you, either - it's trivial on Earth to create a splash in a pool, and that's overcoming both surface tension and gravity.

Also, conservation of momentum is not your friend here - if you dive into the pool and stop, that momentum must be transferred to the pool. So without intervention the pool would start drifting to one side of the room. Also, good luck trying to get the momentum perfectly cancelled out.

We generally don't think about this too much because on Earth gravity does a pretty good job of helping us get dry. But think about what happens when you get out of a pool - you're still wet. In microgravity, this is a significant issue because the water that's sticking to you has no force attempting to pull it off of you, so you'd come up out of the water and have your face still covered by it. This is an easier problem to solve, though - a little bit of hydrophobic cream on your face (especially around your nose and mouth), and you'll be able to come out of the water and breathe.

4. Disorientation

Have you ever gone underwater and then spun around? It's pretty easy to lose your sense of which way is up, though once you stop spinning gravity will tell you which way is up. In a zero-g pool, this won't be the case. Once you go under the water, you have no real sense of which way is "up". If the sphere is 10m in diameter, this could be a significant issue - unless you're a strong swimmer, there's a fair chance that you wouldn't be able to swim that far before you run out of air and start panicking. So unless you're okay with people drowning, you shouldn't be allowed to swim without a scuba tank.

There are probably a couple other things that I haven't covered here, but these should be enough to help you see that, as awesome as it sounds, a free-floating ball of water isn't a good idea for a swimming pool. I'd suggest instead that you have a tank of water (as large as you'd like it) that people can exit and enter through an airlock. You wear scuba gear when you swim in it, plus the hydrophobic cream I mentioned earlier. Finally, you need to make sure that the water filtration system is able to separate out the air that divers have exhaled and pump only water back in.

• Thanks! On your point 1) I don't think splashing would be such an issue. If you think about it, when you are swimming downward in the sea you don't produce splashes on the surface except when your feet are very close to it. Obviously the law stays so I'd expect "the ball" to move a bit in the opposite direction as I swim inside it, but not much due to the difference in mass. So as long as I have a decent number of people moving in and out of it from every direction those small movements should cancel out. Sep 8, 2016 at 20:14
• @SilverCookies I think you don't see upward splashes because they spread out and they're fighting against gravity the entire time. But now that I think about it, 10m is a lot of room for a small wave to dissipate. Trying to do a cannon ball into the pool would probably still get you a big splash on the other side, though. That would actually be really cool to see, I think. Sep 8, 2016 at 20:22
• for you to watch youtube.com/watch?v=jyqOTJOJSoU and yes that frog video with strong magnetic field - also nice to find - magnetic field could be used for keeping position of ball, as well as other less exotic methods. Sep 8, 2016 at 23:41
• @MolbOrg interesting, that's something I didn't know about. Although, something tells me that a magnetic field strong enough to contain the pool of water would not be pleasant for the people swimming in the pool, and would probably play havoc with the other parts of the hotel. Sep 9, 2016 at 0:11
• I think the air jets idea would work OK if they were computer controlled, with the airflow being constantly aimed at the places where the water deviates the most from where it's meant to be. This would require a bit of R&D but it's totally doable with current technology, unlike a hand-wavy forcefield type of solution. I think you're right about the disorientation worry though. Sep 12, 2016 at 7:12

Yes, this is possible. The ball of water could be kept in place with equally spaced air fans to nudge splashed water back from all directions.

However...

At 10 metres across (5 metres deep) there is a serious risk of drowning. In 0g, the adhesion of water and slight difference in density will cause suspended objects ("swimmers") to be enveloped and drawn to the middle of the ball.

For an example of how water envelops objects in microgravity, look to this example of Chris Hadfield and the wet washcloth. Note how the water clings to the cloth and begins to spread over his hands. Scaled up to 10 metres wide, the spreading water will be deep enough to drown in if you can't clear it in time under your own power.

Here is another video, with Mark Wiesgel a zero-G engineer, demonstrating how water behaves in microgravity – and confirming you would be drawn into the water.

Yet another video showing how water sticks to objects and draws them to the middle, this time, a Go Pro camera. Note how the water sticks to the astronaut's hands later – he was being "assimilated" by it. (Remember we're talking about a 10 metre-wide sphere of water too.)

Scuba gear might be required. Or guide ropes would need to intersect the ball of water, swimmers would need tethers, or it would need to be a "diving only" past-time (i.e. enough momentum to pass through the ball of water).

Another option would be to spin the ball, which would cause denser objects to be pushed outward to the surface, but there is still the problem of trying to escape the water. Here is a video demonstrating how air bubbles converge along the axis of rotation and tea leaves and other objects are pushed outward by the force. Yes, density matters in microgravity.

• @rek This seemd surprising to me, intuitively. I've never noticed the effects of surface tension on earth. The power of one's arms and legs displacing water overwhelms that effect by at least a few orders of magnitude. What about 0g makes this effect starkly more dangerous than it is at 1g? In fact, why would a difference in density affect anything at all without gravitational effects? Sep 8, 2016 at 16:16
• Exhausted by the effort or unsupervised, the water would draw them back down. - not true, and let see why. Water is water, gravity or not but surface tension is not affected by gravity - we experience it every day with water. In sea this tension is not enough to suck human to bottom of sea - few newtons is enough to keep us above. Problem with it - it forms layer of that water, it sticks to the body and there is no gravity to strip us from that water suite -- just use hydrophobic creme, this can solve that problem. Sep 8, 2016 at 16:28
• "Mandatory flotation devices would also help." Flotation devices are dependent on bouyancy, which does not exist in a 0g environment. Sep 8, 2016 at 18:00
• I don't see why there would be any greater risk of drowning than in a conventional earth pool.. it didn't take much force to push the go-pro through and completely out the other side, and human swimmers are able to push themselves through water well enough on earth. Come to think of it, treading water should be easier, if anything; you needn't fight against gravity Sep 9, 2016 at 20:58
• @gandalf3 Onm earth, raising your head from the water is enough to clear your airways, because gravity pulls the water down and away from you. We are conditioned from burth to expect this, and not to expect the upward motion of our heads to bring with it a current of water that rises up past our face. Even with a hydrophobic coating, our instinctive movements won't be the right ones for survival. Even on earth, people regularly drown in just a few inches if they lose their footing. A 10m diameter sphere suggests that breathing apparatus would be very wise. Sep 10, 2016 at 2:50

Other answers mentioned water splashing away, as a problem. Yes, there will be lots of water splashing away.

But really, this is an opportunity: The water which splashes away is sucked in by pumps in the walls once it reaches the walls, then cleaned, maybe the temperature regulated and ejected in jets back at the sphere from the appropriate direction to cancel the sphere's movement away from the center.

In contrast to air jets which won't work (where will the air go afterwards?), this should work for at least as long as there are splashy people inside the sphere.

The whole area would probably become splashier and splashier with more water droplets mixed in the air, so you would also want some air circulation with a system filtering the water out of the air and putting it into the sphere-replenishing system.

But, as said, 10 meters can be dangerous. Swimming should work as on Earth, but carelessly diving around in it without diving gear would not be a good idea. To keep people oriented, you would likely want strong lighting: Underwater they would swim towards the strongest light to get to the surface. You could also

• use smaller spheres
• Nothing keeps you from directing the jets so as to squish the sphere into another form. For example you could form it into a 5mx5mx2m (bounding box) rounded block kind of thing by mostly firing water jets at it from two opposing sides.
• or maybe a 2x10m cylinder? By having the water flow from one end of the cylinder to the other, it would be easy to keep centered, do away with the jets, water splashed sideways would just be added at the existing inflow.

Water would stick to your face when you get out, but shaking your head should generate almost as much centrifugal force as Earth's gravitational force (proof: shake your head with long hair. Hair can mount even higher than 45 degrees[1]), easily shaking it off.

[1]Diameter increases with longer hair, but still.

• upvoted for first part of answer. jets idea is good, not necessary wait for splashing, make this splashing happen by jets. it will make flows of water inside ball, free gas inclusion etc. Sep 8, 2016 at 23:46
• @MolbOrg I'm not quite sure how this would work, that's why I didn't include something like this. Sep 9, 2016 at 9:38
• it will youtube.com/watch?v=XnAKiwIBYpM . I mean water force is the force and all that probably it is just figuring out how to use it, to solve tasks in that case. Sep 9, 2016 at 12:27
• @MolbOrg Oh, sure, I personally know how much force there is in such a hose. :-) But what that force will do, exactly, once the jet reaches the sphere, that is beyond me, fluid dynamics, complicated. Also those firefighter's hoses really put out a huge lot of water when you turn them up fully, much more than you would need for this sphere. Sep 9, 2016 at 12:56
• yea fluid dynamics needed to answer is probably beyond op's intentions and our capabilities to answer. Clearly he wish it just look good and astonishing. I would place 10-20-30 submerged nozzles in that bubble which can suck and shoot out water - and call it good for today. it is enough to manipulate the bubble, frees from unnecessary problems, may still looks good with transparent hoses or other ways to make it fancy. Sep 9, 2016 at 13:39

I'm actually going to disagree with a lot of the previous answers and say that while you could possibly create a ball of water 10 m in diameter in a 0g environment, no one is going to be swimming in it.

Assuming a perfectly spherical droplet, the pressure difference between the outside and inside of the droplet is going to be $\Delta p = \frac{2\gamma}{R}$ from the Young-Laplace Equation, where $R$ is the radius of the of the droplet and $\gamma$ is the surface tension. So for a water droplet with a 5 m radius ($\gamma = 0.072 \textrm{ N/m}$), the pressure difference between the inside and outside will be $\Delta p = -0.0288 \textrm{ Pa}$ (the negative sign is because it's a liquid droplet, so the pressure inside the droplet will be lower than the pressure in the air.

What's important here is that this is small. Really small. An air jet moving at 1 m/s stopped by a surface will exert a pressure of $p = \frac{1}{2}\rho v^2 = 0.6 \textrm{ Pa}$. Because the droplet is not a solid surface, this will cause the surface of the droplet to distort locally. When the surface distorts, the local radius of curvature changes. Going back to the Young-Laplace Equation above, the liquid may want to stabilize, which will then cause small droplets to break off.

People swimming, or really even just hitting the surface, of the bubble will cause pressure fluctuations several orders of magnitude greater than those caused by a gentle air jet, which means that the bubble would be unlikely to hold together under the force of swimmers. Erin's answer addresses a way to handle this, but once you go to a shelled system like that, you're no longer working with a bubble made just of water.

• This needs more upvotes! I was (obviously wrongly in hindsight) thinking that scaling up the size of a ball of water might also scale up its holding-together abilities. I wasn't aware of calculating the extra pressure from surface tension as being a useful approach for saying anything about surface tension, so this was very helpful to get a quantitative handle on how easily the bubble would break up. Sep 11, 2016 at 2:07

What you're talking about is, of course, beyond the science of today. But if, in your story, the science of gravity is far beyond ours, then, why not?

I am going to come at this from a safety and logistical standpoint rather than a science standpoint, because really, you can do anything given the right tech in a fictional universe. I know, I know, this has a physics tag, but there are more barriers to this than just physics. You asked if it was possible, and I am going to come at it from a slightly different angle.

The outer edge of the ball would have to have considerable tension--enough that there shouldn't be dispersal from the swimmers if it's going to work at all. EDIT: Thinking about this, it should be multi-layered, in order for people to be able to swim: layer one is the outer "harder" layer from which nothing escapes, except for release points, maybe on the top and perhaps the bottom. Layer 2, the inner layer, takes care of the splash and release of bubbles and physics of swimming action, most of which bounces back off layer 1, if it's water.

I would recommend that swimmers have breathing apparatus, because once they are in the ball, they would be under water. It sounds as though you are wanting surface swimming to happen, which should not be possible in this model, unless you are willing to have the top part be flat, like regular water. Otherwise, you risk drowning people, because if it's held together in this way, pushing through the surface tension should be very difficult for swimmers. Even if it isn't, most people would have a very difficult time orienting themselves in the ball. I can see people drowning just because they "got lost" and were disoriented. There's also nothing to push off of, no bottom...If there's a way to die, people will find it...

There are some logistical problems with no structure touching it. How would the people actually get into the water? The surface of the the water would have to managed minutely, so if you are also planning for people to "float" through the air to get in, that might be a problem. There could be a diving board above, but once they are in, how do the poor sots get out?

Here's how I would have it set up: ball 'o water about 5-9 feet above a regular pool. So maybe they dive in above, and then push out, landing in the ordinary water below. The pool beneath would have to mostly be for exiting the ball and not for any other purpose--OR you could have an enormous regular pool where people can swim in the ordinary way, with a part sectioned off, used just for exiting the ball 'o water. EDIT: So there should be exit/entrance points on the top and the bottom of the ball in case of emergency. This definitely would be controlled, like any slightly dangerous activity such as rock climbing, caving--that sort of thing. I don't see it as something that everyone will want to do or be capable of doing. Time in the ball would be regulated and supervised necessarily.

For safety purposes I would

• Have several life guards inside the ball (They would also tell swimmers when to get out, before the breathers run out)
• Have each swimmer inside the ball be issued a re-breather of some kind that is difficult to lose, as well as goggles.

The people swimming in the ball would be of the adventurous sort--like the sort of people who do zip lining. It would be kids above a certain age, they would sign waivers. I see this as a sort of luxury resort "experience" like those rock climbing walls you find on cruise ships, except more unique. It also adds to the experience of the swimmers in the ordinary side of the pool, because they can look up and see this marvel.

• The question doesn't require tech well beyond ours - if you've got a space station that's rotating, you could have a central area that doesn't rotate. Boom. Artificial gravity in some places but not others. Sep 8, 2016 at 17:17
• It's beyond the science of today. But if in the story, it's far beyond today's science, it would be easy. If there isn't artificial gravity, that doesn't actually solve a lot of the problems, like water clinging to people/being attracted to them. The ball itself is ABSOLUTELY possible, but having swimmers in it, well, that takes it beyond, because the principles needed to make it work would prevent swimmers from having a good time/being able to escape the ball. No gravity DOES NOT = ball of water. Sep 8, 2016 at 18:04

I'll take a bit of a lateral approach to the question. You don't need water.

In terrestrial recreation, water pools are used to combat the gravity (our bodies are nearly as dense as water, so we float on the surface) and propel the swimmers. In your zero-gravity hotel, you already have swimmers able to go anywhere with no regard to gravity. You need to give them techniques to propel themselves (think pressurized cans of whipped cream for a sweet party), and also, possibly, re-create the extent of isolation and solitude we have while swimming underwater (nitrogen fog machines, sound absorbing wall materials).

• Thet's just the regular 0g experience. You can't propel yourself and you can't brake at all at recreational speeds with air as a medium. Even worse with no visibility. All the "swimmers" would be floating to the infirmary with head injuries in no time.
– user26967
Sep 10, 2016 at 3:56

Probably possible with today's technology. A key fact is that water is slightly diamagnetic. Combined with (1) surface tension and (2) computer controlled powerful electromagnets surrounding the room containing the "pool", I suspect it could be done.

Price of lifting water to orbit may be nearly zero since (frozen) water may be found just flying around out there, and plenty of heat (exposure on the sun side) is also freely available. We can probably figure that the related technology and methodology for comet mining will be available by the time we make space hotels featuring any decent zero gravity compensation (not necessarily "anti-gravity").

First thing I'd worry about would be being trapped inside of it. Water is wet and resists things breaking its surface. Once you're fully within it, it could be hard to get your mouth/nose exposed to air again. Water will want to stay spread across them and may "stretch" to keep them covered. There won't be much to push against to force your way through.

We don't really know how much (how little) force is needed. The water sphere will tend to keep some shape naturally. Basic inertia will tend to keep its position. Any external magnetic fields would only be used to counter motion of the entire sphere away from center. Magnetic field strengths could be small. (Would be fun to test!)

• How will these magnets influence the swimmers, assuming that swimmers are about 80% water and also diamagnetic? Sep 9, 2016 at 9:12
• @svavil It should tend to keep them close to the sphere also, though not quite as much. It's unlikely to be noticeable. If swimming in the "pool" is the objective, I'd suspect that a slight pressure inwards would be useful. Components such as fats in the body tend us towards floating on Earth due to density, so there'll also be a slight tendency to "float" in the pool, i.e., to stay nearer to the surface. The biggest effect of a controlled magnetic 'bubble' with the "pool" inside of it would be simply to influence the sphere to stay near the center, and splashes as well. Sep 9, 2016 at 9:34
• God help the swimmer if they have any metal inside their body, like fillings or internal prostheses. Sep 9, 2016 at 13:21
• @DougWarren: implanted metal isn't usually ferromagnetic. Or did you mean they'd have to avoid moving too fast and inducing currents that would heat up their metal? Sep 11, 2016 at 1:59
• @DougWarren Interesting thought, but needs experimentation. The diamagnetic effect may tend to neutralize the external magnetic fields. When actually in the water, swimmers might be almost totally shielded. Sep 11, 2016 at 4:22

Yes, a ball of water, no matter what size (larger is better) can be kept in space in spherical shape. The largest of these water-balls are known as Ocean Planets and they are really, really big.

Anyhow. Yes, you can do this. Water has strong enough cohesive forces to stay together. Also if you build the container of this bubble with a strong hydrophobic substance (such as the wax on leathery leaves), it will help keep it isolated.

• It would be extremely unlikely that any planetary body would be entirely composed of water. Also I seriously doubt OP would want his pool to be planet-sized nor would he want it to be coated in a hydrophobic shell. Sep 8, 2016 at 16:08
• @MozerShmozer OP was asking if there was any limit to how large a ball of water in space could be. This demonstrates that the answer is effectively "no." Sep 8, 2016 at 17:16
• Ocean Planets are entirely hypothetical, so we really can't say much about their characteristics. Sep 8, 2016 at 19:01
• A ball of water that is the gravitationally dominant object in its own neighborhood is very different from one that's floating in air on a ship or space station. Sep 9, 2016 at 0:48
• Most any planet-sized body of "water" would most likely be a water/ice planet. Sep 9, 2016 at 14:29

As per the question, artificial gravity is available in the shopping and living areas. If the pool room is also spherical in shape, it would simply need an evenly spaced array of these artificial gravity generators. As the ball drifts towards a wall, a gravity generator on the opposite side could be briefly turned on, pulling the ball back towards the center. If the gravity generators could generate negative gravity then even better - the wall being approached by the ball of water can repel too.

A hydrophobic coating on the walls would mean even if the water ball does hit a wall, it would be relatively easy to "refloat" it again using the gravity generators.

• The problem is that gravity force is in cubic inverse proportion with distance. So water that gets close to one wall's gravity generator is basically unaffected by other 5 generators on the other walls. You would end up with hollow sphere of water on walls of the room, which would imho be cool as well. Sep 9, 2016 at 12:55
• @TomášZato my vision was the gravity generator only turns on when needed. So the ball is in a 0g room to begin with. If it's artificial gravity, it hopefully has variable levels of strength - the faster the ball is moving towards a wall, the stronger the gravity pulls from the opposite side of the spherical room. I like the hollow sphere idea too though! Sep 9, 2016 at 13:04
• I know quite exactly what you want, but it's impossible without homogenous gravity field. Which is much more far-fetched idea than gravity generator itself. Sep 9, 2016 at 13:28
• At that point wouldn't it be easier to just put the gravity generator at the middle of the ball of water. Sep 9, 2016 at 14:33

Actually the problem is that people should swim on top of the swimming pool to breath. Even if you succeed to keep the water ball together, anyone who enters to this ball will not be able to go out because surface tension will take the person inside and finding a way to go outside will not be possible.

Wow I remembered a very good example now, you can search and see well known "ant in water" issue. Same will happen probably.

Good luck!

• You can still out-swim surface tension in 0g, because it's small compared to human strength. If your description was correct, people couldn't break the surface after swimming under water. There is a problem with water, 0g, and surface tension (that water sticks to your face), but being able to get your body as a whole out of water isn't the exact problem. @Rob's answer mentions the actual implications of surface tension. Again, it's a serious problem, but not exactly the way you describe it. Sep 11, 2016 at 1:22

The main point here is that in such a large body of water, surface tension will be trivial next to all the other forces propagating through it in the form of waves from any kind of activity. If no one were using the pool and you managed to stabilize the water somehow and keep the air around it completely still (which would be impossible anyway, as vibration from the activities elsewhere in the hotel would be transmitted through the walls) it might stay as a ball for a little while, but very soon it would begin to break up again into smaller balls, as a result of tidal forces (unless your hotel were somewhere deep, deep in interstellar space). And, of course, the moment someone interacted with the water in any way at all, waves would start to propagate through it in all directions, causing a rapid breakup of the medium into smaller and smaller agglomerations. The default state for such a system would be a lot of water balls of various sizes floating around randomly, interacting with each other and other objects, sticking to the walls and the people, and tending to break up more and more when any movement is present and coalesce a little more when there is none. Obviously, in such an environment, a swimmer without scuba gear would not survive, as there would be no known place where he/she could go and be able to breathe. Water could be in any place at any time, and breathing would depend on the lucky coincidence of just happening to have your nostrils in the middle of an air pocket, which, by the way, could close at any moment while you are inhaling (and in fact, WOULD probably do just that, because of the movement of air towards your nostrils). Most likely, nothing bigger than a bacterium could survive in such of an environment for very long -- or enjoy being in it for any longer than it could hold its breath...

The easiest way to do this (in fact, very easy to do today) is to introduce tension. In other words, rather than just a ball of water, make it water in a ball. This will keep the "pool" from splashing all over the place, and provide the necessary pressure to keep the water liquid if you choose to put the pool in "space", outside of the pressurized compartment.

However, do note that you will need breathing equipment anyway.

Similar to mccdyl001's response, surround the pool with gravity generators. However, instead of only turning them on when the sphere drifts toward a wall, they should be on all the time. Further, they need to be much more finely tuned so that gravitational force doesn't multiply towards the center, but that there be a definite center (perhaps two generations of technology beyond conventional linear gravity generators). Buoyancy should prevent anyone from being unable to reemerge. Strictly, it wouldn't be a 0g activity, but marketing shouldn't be deterred by that fact.

Since gravity would be applied equally on all sides, there would be a risk that a person or object that is naturally buoyant might be stuck in the exact center of the sphere. The gravity should therefore periodically shift a couple meters to the side to allow the buoyant person/object to float to the nearest surface.

Presumably, a society that has developed such technology will also have automated lifeguards to monitor swimmers' vitals and intervene when needed.