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Imagine you're an astronaut on the International Space Station and your fellow astronauts played a prank on you by taking all your clothes and putting you in the center of a module so that you cannot reach anything with either your hands or your feet.

What would be the most effective way to escape that situation if you're reluctant to start peeing?

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It is worth calculating (rather than just speculating) some of the methods described.

  1. Breathe in one way, and out the other. Your resting tidal volume is about 0.5 liter; take a deep breath and it can be 3-5 liters (thanks @Aaganrmu). That is about 4 gram of material. If you purse your lips to increase the velocity with which you expel the air, you can get it to around 10 m/s (I estimate this from the data in this paper which measured the velocity of air in a cough - 15 m/s - and spoken word - 4 m/s.). This gives a net momentum of 0.04 kg m/s, which means a 70 kg astronaut will get a reaction velocity of 0.05 mm/s, moving 1 cm every 20 seconds. But if you do this 10 times per minute, your acceleration will be about $10^{-4}~\rm{ m/s^2}$ and you will travel 2 m (to the nearest wall) in about 200 seconds. Feeling somewhat dizzy from the hyperventilation... Note that it's not necessary to turn your head: it's enough to breathe in slowly, and out rapidly.
  2. "swim" with your arms. If you can change the area of your arms by 20 cm$^2$ between the "forward" part of the stroke, and the "back" part of the stroke, and you can move the hand with a peak velocity of about 2 m/s for 50 cm, the approximate drag force will be $F = \frac12 \rho v^2 A C_D = 0.5*1*4*0.02*1.0 = 0.04~\rm{ N}$ - four times more than breathing out. And of course you can probably move your arms a great deal faster - let's say one complete stroke (two arms) per second, for an acceleration of $6\cdot 10^{-4}~\rm{m/s^2}$ and a time across the capsule of 80 seconds.

Combining the two techniques, the exercise with the arms will allow you to breathe more rapidly (at peak exercise, an adult male can move about 100 liters of air per minute - that is 10 times higher than the value I used). This should comfortably get you to the side of the capsule in under a minute.

If you can increase the mass you accelerate, you can greatly improve on these numbers. I briefly considered that spitting might be the answer, but your mouth will run dry pretty quickly. Other bodily fluids would greatly improve on the time - but given that you still have to live in that space after the fact, I think that spending a bit more time waving your arms about, then laugh at the videos your crew mates made, is the best approach here.

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    $\begingroup$ I wonder if what you had for dinner the night before (and how gassy it makes you) might be a factor in your net acceleration... $\endgroup$ – MikeTheLiar Aug 22 '16 at 13:54
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    $\begingroup$ @mikeTheLiar Do they even serve such meals in a space station with limited air and the necessity lo live together in confined space? $\endgroup$ – Hagen von Eitzen Aug 22 '16 at 17:53
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    $\begingroup$ Hurrah for actually mathing! $\endgroup$ – Dewi Morgan Aug 23 '16 at 6:06
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    $\begingroup$ @Aaganrmu - I have updated my calculation. Thanks for the pointer. $\endgroup$ – Floris Aug 23 '16 at 12:11
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    $\begingroup$ @uhoh - if you would like to create a Meta post, your point is best discussed there. But I'm afraid it's a topic that has been debated at length already... $\endgroup$ – Floris Sep 28 '16 at 13:10
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Since you have air around you, you can just take a deep breath and blow it out.

There's actually no need to turn you head while doing this, as some other answers suggest. Air has a high Reynolds number at human scales, and so the scallop theorem does not hold: even though the movements of inhaling and exhaling are reciprocal, the airflows they create are not.

(You can test this yourself by holding a hand in front of your mouth: you can easily feel the jet of air created by blowing out, even with your hand fully extended, but you can't produce a "reverse jet" no matter how hard you inhale.)

In practice, the momentum produced by inhaling is pretty negligible, as air flows in towards your mouth and nose from all sides, and so the only thing that matters is which way you exhale. By blowing air out of your mouth in one direction, you create a net airflow in that direction, and so, by conservation of momentum, propel yourself in the opposite direction. It works for squid and jellyfish (and scallops!), and it will work for you, too. Maybe not very efficiently, but surely enough to reach a wall in the tight confines of the ISS. Now, if we ever start building space stations with huge air-filled bubbles hundreds of meters across, then this might become a problem, but until then you should be fine.

Besides, you may not even need to resort to such huffing and puffing. Any real space station designed for human habitation in microgravity needs to have active air circulation fans anyway, both for heat distribution (important for both humans and equipment, since convection doesn't work in microgravity) and to keep exhaled air from accumulating around your body e.g. when you're sleeping. So in practice, the air around you will be moving slowly anyway, and you just need to wait until this ambient airflow pushes you close to a wall.

And of course, on the actual ISS, I doubt there's even any space big enough to properly pull off this prank. The largest open spaces on the ISS, like the Kibo pressurized module, are surrounded by ISPRs that are about 2 meters (6 ½ feet) wide, effectively making the interior cross-section a 2×2 meter square. Even if your crewmates somehow managed to position your body lengthwise along the center axis of an otherwise empty module so that you couldn't just reach out and grab a handhold, you'd just need to twist around like a cat (or, more likely, just flail around semi-randomly) until you managed to turn yourself 90° around, at which point either your toes or your hands should surely be able to reach a wall.

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    $\begingroup$ The problem is your head is quite far from your centre of gravity, so you're going to start spinning more than you start moving. $\endgroup$ – Stop Harming Monica Aug 22 '16 at 13:48
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    $\begingroup$ @OrangeDog tip you head back far enough and you can exhale approximately in line with your body (top to bottom). Your arms/legs can be extended as needed to shift your center of mass around a bit. $\endgroup$ – Dan Neely Aug 22 '16 at 13:51
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    $\begingroup$ @user151841 At least at that moment the prank would start reallylooking awkward $\endgroup$ – Hagen von Eitzen Aug 22 '16 at 17:49
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    $\begingroup$ For that matter, look up the Feynman question on reversing flow in a lawn sprinkler. Breathing in and breathing out are not inverses of each other. $\endgroup$ – Carl Witthoft Aug 22 '16 at 17:59
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    $\begingroup$ Obligatory Freefall. $\endgroup$ – Edward Aug 22 '16 at 19:07
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There are some obvious and boring solutions e.g. breathe in then turn your head 180 degrees and blow the air out. Repeat for long enough and you'll build up a net velocity.

More interesting is that if the gravitational field is not completely uniform you can actually swim in it. However this is such a small effect that you'd have died from old age (let along starvation) before you managed any significant change in position.

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    $\begingroup$ Does the breathing in and out thing actually practically work as a way to propel yourself? (I assume that a fair few astronauts have tried it, possibly when no-one was looking.) $\endgroup$ – Nathaniel Aug 22 '16 at 12:52
  • $\begingroup$ @Nathaniel: "No one was looking" is funny in this context. $\endgroup$ – Zaibis Aug 22 '16 at 12:53
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    $\begingroup$ You wouldn't even have to bend your neck to achieve net acceleration through breathing. Since the amount of momentum depends on the velocity of particles, it is enough to breathe in slowly and then blow out the air as hard as you can. That would be the solution to a much nastier prank, where your fellow astronauts cladded you in a whole-body-cast before placing you in the center of the module. However, reaching the side wouldn't to you any good in this case :-( $\endgroup$ – M.Herzkamp Aug 22 '16 at 12:58
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    $\begingroup$ My understanding is that the gravitational field on the ISS actually is sufficiently non-uniform that the middle of a module is an unstable equilibrium. You can't precisely match the orbit of the ISS's centre of gravity, so you'll drift either towards or away from Earth, along with other loose debris. Failing that, the ISS adjusts altitude on average once or twice a month, so try not to die of thirst before that and you'll hit the "floor" when it burns. $\endgroup$ – Steve Jessop Aug 22 '16 at 13:26
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    $\begingroup$ @M.Herzkamp: No need to even breathe in slowly; airflow at high Reynolds numbers is non-reversible, so just breathing in and out at a steady rate will create a net momentum transfer. (You can test this yourself by holding a hand in front of your mouth: you can easily feel the jet of air created by blowing out, but you can't produce a "reverse jet" no matter how hard you inhale.) $\endgroup$ – Ilmari Karonen Aug 22 '16 at 13:29
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For the experimental answer (with clothes on), watch this YouTube video titled "Astronauts enter the new Japanese module Kibo" which shows an astronaut demonstrating the ability to swim (slowly) through the air.

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    $\begingroup$ Link-only answers are actually discouraged for links can die any-time; better add the relevant content here or once you have sufficient reputations, you can add this as a comment. $\endgroup$ – user36790 Aug 22 '16 at 16:28
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    $\begingroup$ My answer is of course more the "tongue in cheek" version, since you can't actually see what causes the movement in the video (could be the "swimming", the breathing, air currents, some inital momentum or even movement of the ISS itself). But since the video depicts almost exactly the setup described by the OP, I thought I would include it for it's entertainment value. Besides it shows some very nifty movements that (of course) conserve momentum but change the astronauts orientation in a precise and controlled manner. $\endgroup$ – TToni Aug 23 '16 at 11:58
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Considering Newton's laws of motion the possibilities are very limited: You have to accelerate mass into the opposite direction of where you want to go. However, since air is the only material you can get hold of and you do not want to use body liquid, you have to use the air around you:

  • As John Rennie already suggested, breathe in and out in opposite directions.
  • Perform breast stroke as if you were in swimming in water. This will obviously not be as effective as in water, but air is also a fluid and will provide some propulsion.

Other theoretically possible propulsion techniques using relativistic gravitational effects or the momentum of photons will probably take so long to accelerate your body that you will have died long before you reach the walls of the ISS.

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    $\begingroup$ A small comment: its really not necessary to breathe in and out in opposite directions, as others have pointed out. $\endgroup$ – anon01 Aug 22 '16 at 16:47
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Not yet mentioned, to get some delta-v you can:

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