# FISH IN SPAAAAAACE!November 20, 2012 10:46 AM   Subscribe

Work in my office has ground to a halt over this question about the physics of fish in space.

So there are fish on the International Space Station. Further to this comical/distressing video about cats in space and these videos about the behavior of water in zero gravity, the question is this.

Say you're in a zero-gravity environment suitable for the survival of humans. You have a fish swimming in a bag/balloon full of water. You then pop the balloon, releasing the water to float freely as a single body. Without the normal constraints of gravity, would the fish be able to swim out of the sphere of water into the air, thus accidentally "drowning" itself?
posted by stuck on an island to Science & Nature (28 answers total) 17 users marked this as a favorite

Best answer: Yes. Swimming works by pushing water backwards. The fish gains leftward momentum, the water gains rightward momentum, and the two separate. The center of mass of the whole system remains constant.

(Fish:o<-<, water: *)
```    ****o<-<****
**o<-<******
o<-<********
o<-<     ********```
oh noes!
posted by 0xFCAF at 10:51 AM on November 20, 2012 [16 favorites]

The fish swimming at all would slowly disintegrate the sphere of water.
posted by Jehan at 10:52 AM on November 20, 2012 [1 favorite]

With little more than surface tension of the water-sphere holding it together, it's even possible that the mere motion of swimming would tear apart the water into different bubbles before it so much as hit the "surface."
posted by adipocere at 10:52 AM on November 20, 2012

The surface tension of the floating water ball > the force the fish would be able to generate within that ball.

Therefore it would not swim out of the water.
posted by 2bucksplus at 10:54 AM on November 20, 2012 [1 favorite]

Best answer: The force of the popping balloon is probably enough to scatter enough of the water, so the fish drowns.

The interesting question is how octopi would maneuver in a weightless space. Would it go chasing after globs of water to immerse itself in?
posted by Brandon Blatcher at 10:55 AM on November 20, 2012 [1 favorite]

You will see in this NASA video a ball of water remains stubbornly coherent.
posted by 2bucksplus at 10:57 AM on November 20, 2012 [2 favorites]

The globe of water is only held together by surface tension, which isn't very strong. The fish will be able to wiggle back and forth as it usually does (just as humans can in space--they can flex their bodies, say bend at the hip and then unbend). The fish can do so even better than a human in space since it has the water to push against.

However it will take very little of this wiggling inside the water globule to cause the entire water globule to break apart and disintegrate.

This is assuming a water globule not too much larger than the fish itself.

If we assume a globule far, far larger than the fish (either a very large globule or a very small fish) so that the fish can thrash about it in without causing the globule to immediately disintegrate, then the fish will be able to swim in much the same manner as it does on earth, as gravity doesn't play much of a role in that motion. The fish's forward motion is created by basically pushing against the inertia of the water, which is the same in space as it is on earth.
posted by flug at 10:57 AM on November 20, 2012 [1 favorite]

See this. And this.
posted by The 10th Regiment of Foot at 11:00 AM on November 20, 2012

Response by poster: NB: In the case of conflicting answers, my coworkers will accept the answer of the MeFite with the most impressive relevant credentials.
posted by stuck on an island at 11:00 AM on November 20, 2012

Best answer: Here's a video of a fish swimming around on a Vomit Comet. It's a bit different from your situation, in that the surface tension keeps the water attached to the sides of its container, but it shows how a fish behaves and demonstrates the surface tension. You can see how a void of air gets created, and how the fish has a hard time crossing the air-water barrier. It may have gotten across once.
posted by zsazsa at 11:06 AM on November 20, 2012 [7 favorites]

It doesn't look to me like the fish has much difficulty crossing the air-water interface. It's not like surface tension is stronger in zero G than in 1 G, and fish poke their heads out of water quite easily. (Signed jjwiseman, physics major for 6 months.)
posted by jjwiseman at 11:19 AM on November 20, 2012

Best answer: The surface tension of the floating water ball > the force the fish would be able to generate within that ball.

As far as the motion of the fish and the water goes, I would encourage you strongly to think in terms of Newton's laws and also the conservation of momentum.

The strength of the surface tension is neither here nor there in this calculation (in fact the surface tension probably does have some effect, but it will be a 2nd or even 3rd order effect, some orders of magnitude smaller than the Newton's law & momentum effects - it will have more of an effect on whether the globule can hold together than on whether the fish can move per se - and don't forget the surface tension exists on earth just as it does on space).

Assuming a decent amount of water around the fish, the fish is going to be able to get the same amount of forward thrust in space as it does on earth. That is because its thrust comes from the laws of motion (the fish propels water backwards and simultaneously, itself forwards). Newton's laws work exactly the same in space as on earth.

Since a fish can propel itself out of water on earth--defeating both surface tension and gravity--it will certainly be able to do so in space, where it need only defeat surface tension.
posted by flug at 11:22 AM on November 20, 2012 [3 favorites]

Since a fish can propel itself out of water on earth--defeating both surface tension and gravity--it will certainly be able to do so in space, where it need only defeat surface tension.

This is what I came in to say. The integrity of the ball of water is an interesting question, but it's a red herring. The fish would be able to leave the bubble regardless.
posted by no regrets, coyote at 11:29 AM on November 20, 2012 [3 favorites]

Here's a video of a fish swimming around on a Vomit Comet.

Yeah, that pretty much definitively settles it. The fish moves out into the air with just about exactly the same ease as it moves in any other direction.

You'll notice it does get caught up a bit in the surface tension at one point and is easily able to wriggle its way out of it and into the air--take a close look just at 1:06.
posted by flug at 11:29 AM on November 20, 2012 [1 favorite]

Of course the fish could swim out of the water. Simple action-reaction: fish pushes against water, fish moves one way, water moves the other, fish dies in air. The surface tension thing is a total red herring: fish are easily able to jump out of water on earth, and the surface tension would be no different in space.

This seems so patently obvious to me that I'm having a really hard time understanding why anyone would think it's not possible.

Credentials: Half of an aerospace engineering degree (before I switched to film school so there)
posted by ook at 11:31 AM on November 20, 2012 [2 favorites]

If a fish can jump out of water on Earth it can certainly break through the surface tension of a weightless waterball. There may exist fish that are so tiny that they can't do that, but I would imagine goldfish on up can do it without much trouble.

As a thought experiment, do you seriously think a shark can be stopped by surface tension?

If the underlying thought is that the fish couldn't propel itself at all, that is incorrect. Astronauts pushing against walls or moving along by grabbing a rail is the same physics, and for that matter so is how rockets work in space. One thing pushes on another, there is an equal and opposite force pushing back on the pusher, and the two things are propelled away from each other.
posted by philipy at 11:36 AM on November 20, 2012

Read Startide Rising. It's almost thirty years old now. Crap, I got old in a hurry.

While it deals mostly with mammals/dolphins rather than fish in space, he does a great job describing the issues involved with zero gravity and aquatic species.
posted by Sphinx at 11:58 AM on November 20, 2012

2bucksplus: The surface tension of the floating water ball > the force the fish would be able to generate within that ball.

Therefore it would not swim out of the water.
Since fish regularly do this on Earth, while also fighting against gravity, I can't imagine why you think this is true.
posted by IAmBroom at 12:06 PM on November 20, 2012 [1 favorite]

I suggest posing this question to Reddit's AskScience. While most reddit sections are just random people bs'ing on the Internet, the askscience sub is well moderated. (You can create a throwaway account instantly. The submit link is somewhat hidden down the right side menu.)
posted by justkevin at 12:17 PM on November 20, 2012

The fish moves out into the air with just about exactly the same ease as it moves in any other direction.

I thought that video was pretty clear at first, but then I realized that of course there is a layer of water on the front and back surfaces of the tank which are hard to distinguish from the air bubble. For example, at 1:06 the fishes head does seem to poke through the surface tension, but it doesn't actually appear to swim through it. In fact, to me it looks like the fish does a pretty good job of staying in the "water zone."

This isn't to say that a fish couldn't swim out of the bubble - I agree that it shouldn't be any harder than breaching the surface on earth (which fish can do) - just that the fish shown seems to be doing a pretty good job of not doing so.
posted by muddgirl at 12:18 PM on November 20, 2012

Everyone seems to be missing the second half of my two sentence answer.
The surface tension of the floating water ball > the force the fish would be able to generate within that ball.

The fish only has the mass of the unmoored ball of water to push against. This is different than a fish bowl or the ocean or any comparable situation on earth. Fish jump out of the water all the time on earth (and yes, the are overcoming both gravity and surface tension), but they are pushing against considerably more mass when they do so. I am arguing that the fish would not be able to generate much force from a balloon's worth of water. So little force, in fact, that it would be unable to break completely free from the meagre surface tension of a sphere of water.

The real answer is probably that it depends on the amount of water and the size of the fish.
posted by 2bucksplus at 1:08 PM on November 20, 2012

Best answer: From the book Colonies in Space "In a weightless space farm, it may be possible to raise fish without water. On Earth, when a fish is taken from water, gravity makes its gills collapse so that it cannot get oxygen. In weightless space these same fish might easily "swim" through an atmosphere of 100 percent humidity, keeping comfortably moist: hydroponic fish, if you will."
posted by Sophont at 1:22 PM on November 20, 2012 [4 favorites]

That's not what force is. Maybe you're thinking of reaction mass, but I think you're confused.

The fish generates the same amount of force with or without water. A fish with a drop of water on its tail can slap that drop away, gravity or not. Now scale up the drop of water to balloon size and imagine a fish's tail slapping it. Now imagine the fish's tail slapping it while inside the ball of water. It's all the same.
posted by jjwiseman at 1:25 PM on November 20, 2012

NB: In the case of conflicting answers, my coworkers will accept the answer of the MeFite with the most impressive relevant credentials.

FWIW - I have a Ph.D in physics and favorited the first answer
posted by doctord at 1:39 PM on November 20, 2012 [1 favorite]

I am entirely unqualified, but I favorited the answer of the MeFi with the Ph.D. in physics. Can I get partial credit? (And that, my friends, is how I got my humanities degree.)

On a more serious note, wouldn't the water with fish in it just... break apart because of the wiggling of the fish as they swim about? I can't imagine that mere surface tension would hold together a blob of floating water against wiggling fish tails.
posted by driley at 2:35 PM on November 20, 2012 [1 favorite]

2bucksplus, the second half of your sentence (which is wrong anyway, as jjwiseman points out) doesn't erase the fact that the surface tension of the floating water ball is pretty trivial, on the scale of a fish' tail force.

In or out of gravity. Fishtanks or none. Ants have problems penetrating that surface tension, but things much bigger do not.
posted by IAmBroom at 2:37 PM on November 20, 2012

Best answer: The fish generates the same amount of force with or without water. A fish with a drop of water on its tail can slap that drop away, gravity or not.

While the surface tension and the force generated by the fish are the same regardless of the gravity situation, I'm not sure a fish would be able to achieve the necessary momentum to break out of a small/balloon-sized volume of water (depending on the size and strength of your fish, of course). Given a larger free-floating volume, the fish would have no trouble swimming out into the air.
posted by pullayup at 7:44 PM on November 20, 2012 [1 favorite]