What if the atmospheric pressure onboard the ISS was 5 atm, 5 times the pressure on Earth and currently on the ISS, while maintaining the breathable oxygen level, e.g. if the additional atmosphere would be made up of helium only? There's this

video of an astronaut stuck in microgravity who with effort manages to "swim" backwards to be able to grab a bar in the ISS' Kibo module. If the ISS's air pressure was higher, would it be easier for astronauts to "swim" through the air? Is it a proposal space agencies should consider?

  • $\begingroup$ I imagine astronauts are trained to make sure this doesn't happen (or at least as little as possible), that's more efficient than redesigning every expensive capsules that are already in orbit. $\endgroup$ – Mast Jul 9 '20 at 8:38
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    $\begingroup$ Helium wouldn't help because what helps swimming is density, not pressure, and helium is very light. $\endgroup$ – Pere Jul 9 '20 at 9:30
  • $\begingroup$ @Pere And how much helium would be needed to make up for 5 times the current density? Usually "higher air pressure = higher air density". $\endgroup$ – LoveForChrist Jul 9 '20 at 9:32
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    $\begingroup$ @LoveForChrist - Helium is about 15 times lighter than air. To get the same density than air at 1 atm you need helium at 15 atm. For the density of air at 5 atm, you need helium at 75 atm. Helium doesn't seem to ease the engineering challenges of the question. $\endgroup$ – Pere Jul 9 '20 at 9:36
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    $\begingroup$ It would be far easier, safer, and cheaper just to have each person carry some sort of "swim-fin" accoutrement to use in such situations $\endgroup$ – Carl Witthoft Jul 9 '20 at 11:33

Would a higher air pressure on the ISS or elsewhere make it easier to “swim” in microgravity?


But what's really important is the density, so instead of pressuring "normal air" you can just make a denser atmospheric mixture and keep the pressure the same.

This answer says

If you want the air to be 5 times easier to swim, you can just replace the nitrogen with xenon and increase the density without increasing pressure.

and while it is pointed out that Xenon is expensive and has a narcotic effect (this guy complains of tingly fingers from Krypton before breathing Xenon), so what about this?

Wikipedia's Sulfur hexafluoride says:

Sulfur hexafluoride (SF6) is an inorganic, colorless, odorless, non-flammable, non-toxic but extremely potent greenhouse gas, and an excellent electrical insulator.

Consider using a normoxic mixture (normal oxygen fraction of about 21 %) of SF6 for a while, but not permanently!

From Effects of Sulphur Hexafluoride on Psychomotor Performance:

The narcotic influence of sulphur hexafluoride on mental and psychomotor performance has been studied in 9 subjects at normal atmospheric pressure. Control experiments were performed with air and with nitrous oxide. Psychomotor, perceptual and cognitive abilities were assessed using a computerized test battery. Subjects were exposed to air and six different normoxic gas mixtures: 13, 26, and 39% N2O, and 39, 59, and 79% SF6. Significant performance impairments were found with 13% N2O and gradual further impairment with 26, and 39% N2O. During exposure to 39, 59, and 79% SF6 over-all performance was impaired by 5, 10, and 18%, respectively. Impairment was significant with 59 and 79% SF6. The results indicate that the relative narcotic potency of SF6: N2O is about 1:4 in humans. It is concluded that a normoxic SF6-O2 mixture can be inhaled for lung function studies without any harmful effects and that the short-lasting narcotic effect, although detectable with a test battery, would not impair the ability of the subject to perform simple breathing procedures.

Also see Relative narcotic potency and mode of action of sulfur hexafluoride and nitrogen in humans


In microgravity the ability "swim" in an atmosphere comes from the aerodynamic drag force produced on the astronauts fast-moving arms which is approximately

$$F_D = \frac{1}{2} \rho v^2 C_D A$$

where $rho$ is the density of the atmosphere, $v$ is velocity, $C_D$ is the drag coefficient which contains all of the fluid dynamics but is usually somewhere between 0.5 and 1, and $A$ is the area considered.

Since arms pivot at the shoulder each part moves at a different speeds, let's say an area of 0.01 m^2 does most of the work, and it moves at about half of the world's record speed for a thrown ball of 22 m/s (from this answer to How hard do you have to throw something off the ISS to make it deorbit?). The density of a standard atmosphere is about 1.225 km/m^3 and let's use $C_D$ of 0.5 for a non-optimal flailing arm.

That makes the drag force about 1.5 Newtons! Assuming the double arm swings are underhand to keep the force near the center of mass, a total of 3 Newtons over a 50 cm arc. With work equal to force times distance, that's 1.5 Joules of kinetic energy.

The "delta-v" the astronaut receives from each double-armed underhanded flail is then

$$\Delta v = \sqrt{2E/m}$$

or about 0.2 m/sec. That seems much faster than what a single flail gives the astronaut in the videos (Astronaut gets stuck in the Kibo ISS model and It can be difficult to remove oneself from the Kibo ISS module) but it's the right order of magnitude.

And a factor of 4 if not 5 in density from an ~79% SF6 atmosphere would be a big boost!

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    $\begingroup$ I think your video is the same as mine. $\endgroup$ – LoveForChrist Jul 9 '20 at 6:16
  • $\begingroup$ @LoveForChrist your original link was to a popular news item that had the YouTube embedded in it, but over time that news site may be archived or loose the link and so I added a direct link to the YouTube to your question. However at some point in the future you may roll-back that edit, so I also included a link here so that even if that happened, a reader of this answer would have immediate access to the two YouTube videos. Let's call it redundancy, which is a very important concept in spaceflight :-) $\endgroup$ – uhoh Jul 9 '20 at 8:05
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    $\begingroup$ These videos make me nervous considering how much of a nasty greenhouse gas that stuff is. Is there a chance they caught most of it and safely disposed? $\endgroup$ – htmlcoderexe Jul 10 '20 at 12:57
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    $\begingroup$ When you randomly stumble over a reference to the only question I ever asked on Space Exploration, gotta love that $\endgroup$ – Bananenaffe Jul 10 '20 at 13:33
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    $\begingroup$ @Bananenaffe it was an excellent question! $\endgroup$ – uhoh Jul 10 '20 at 13:35

Partial answer to "Is it a proposal space agencies should consider?"

Unlikely. Increasing the differential pressure by a factor of 5 would mean that the modules would have to be quite a bit stronger and therefore presumably costlier and/or heavier. (As pointed out in this other answer)

If getting marooned in midair is a constant problem (AFAIK it isn't) 1 a much cheaper and lighter solution would be to string tethers down the long axes of the modules. Swimming in the air is not a design requirement.

1 This answer quotes early ISS astronaut Dan Barry as saying "It's not easy to get stranded - I had to have my friends help me get perfectly still."

  • $\begingroup$ Well, the astronauts are lucky that they aren't alone. If an astronaut is stuck in mid-air he/she can cry for help. $\endgroup$ – LoveForChrist Jul 9 '20 at 6:11
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    $\begingroup$ Or if we have a large volume with a single astronaut, we can provide them with a battery powered pocket fan - although I know I'm slipping from space exploration to worldbuilding. $\endgroup$ – Pere Jul 9 '20 at 9:32
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    $\begingroup$ @LoveForChrist Worst case, could they just blow some air (hyperventilating a bit) and then wait a few minutes? $\endgroup$ – user253751 Jul 9 '20 at 9:38
  • $\begingroup$ @user253751 That would work well only in vacuum when you let air off which will blow you away. Onboard a spacecraft there'd be air resistence so you'd have to hyperventilate permanently but that would lead to fainting. $\endgroup$ – LoveForChrist Jul 9 '20 at 9:56
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    $\begingroup$ or take off a shoe and throw it opposite to where you want to go :-) $\endgroup$ – Carl Witthoft Jul 9 '20 at 11:31

If you want the air to be 5 times easier to swim, you can just replace the nitrogen with xenon and increase the density without increasing pressure.

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    $\begingroup$ The narcotic effect of xenon is much higher than that of nitrogen. Even at only 1 bar. Besides that xenon is very expensive. $\endgroup$ – Uwe Jul 8 '20 at 18:16

To improve swimmability, we need to increase gas density, not gas pressure - although both are related, it would be ideal to increase the former without increasing the latter.

Density of fluids can be increased by solids in suspension, as can be shown by hot pyroclastic flows denser than colder clean air. In Earth solids in suspension tend to settle due to gravity, but in space anything floating in the station atmosphere keeps floating there. Then, we can suspend in air a lot of mass and keep pieces large enough to not interfere with breathing. Therefore, the solution is:

The big micro-gravity ball pit

We just need to let some thousands of solid rubber balls floating in the station. When swimming, astronauts will trow back a large mass of balls with a little mass of air.

To optimize the system, balls must be large enough not to be swallowed, as massive as possible, not very hard to avoid hitting hard the astronauts and elastic, so they bounce on the walls instead of setting against them. Solid rubber balls a few centimetres of diameter seem a good trade-off between those requirements.

Of course, with a couple of balls for litre of air visibility will be highly impaired, but that's just a secondary effect to bear on.

  • $\begingroup$ And what air is in the balls? The same as elsewhere on the ISS, just denser? $\endgroup$ – LoveForChrist Jul 10 '20 at 8:19
  • $\begingroup$ @LoveForChrist Just read the answer carefully: "Solid rubber balls a few centimetres of diameter seem a good trade-off between those requirements." $\endgroup$ – Uwe Jul 10 '20 at 8:34
  • $\begingroup$ The balls are filled with rubber. It could be debatable if partially filling them with something more massive (like lead) would be an improvement. $\endgroup$ – Pere Jul 10 '20 at 8:47
  • $\begingroup$ And just for clarification: what gets denser is air + rubber balls, compared with air alone. Air itself doesn't change. $\endgroup$ – Pere Jul 10 '20 at 8:51
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    $\begingroup$ @uhoh - You are right, but I think the main cause of balls setting will be air currents from ventilation system driving them to intakes. That problem could be addressed by periodically reversing ventilation or by using vibration grids to keep the balls moving. In fact, in busy corridors the problem may solve itself when passing astronauts move air and balls. I think we can engineer solutions to all those problems, although the system is so impractical that it will be easier to find another way to move around. $\endgroup$ – Pere Jul 11 '20 at 8:46

The astronauts would get nitrogen narcosis even worse than in 40 m deep water breathing air. In both cases the gas pressure is 5 bar, but under water the partial pressure of nitrogen is 3.95 bar but in the spaceship 4.79 bar. This is equivalent to about 50 m deep water breathing air. See Wikipedia for signs and symptoms of the narcosis. These symptoms would endanger the life of a diver or astronaut.

But the spaceship would get too heavy anyway when built for 5 instead of 1 bar.

To avoid decompression sickness during an EVA, a partial pressure of nitrogen of 4.79 bar can't be used. A space suit pressurized to 5 bar is totally useless, so pure oxygen with about 0.3 to 0.4 bar is used to keep the suit flexible. A very long decompression procedure (several days) would be needed to avoid decompression sickness during transfer from 5 bar to only 0.4 bar.

So to avoid all these problems, high pressure swimming is impossible.

  • $\begingroup$ Aw, well, nitrogen was just an example. Would any other gas be healthy? $\endgroup$ – LoveForChrist Jul 8 '20 at 15:00
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    $\begingroup$ When you replace nitrogen by helium to avoid the narcosis, the other problems remain. $\endgroup$ – Uwe Jul 8 '20 at 15:23
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    $\begingroup$ This answer lists reason why it would not be practical to build a space station to do this. But is doesn't answer OPs question of if it would work or not. $\endgroup$ – DarcyThomas Jul 8 '20 at 20:45

There is some misconception involved in the phrasing of the question. Take a look at the ideal gas law:

$$\frac{pV}{nT}=\rm constant$$

$p$: pressure; $V$: volume; $n$ amount of substance ("mass" of the gas); $T$: temperature

What you need to do in order to increase the swimability is to increase the density, which is the ratio $\frac{n}{V}$. Assuming the volume $V$ of the space station's modules remain constant, you'd need to increase $n$ by pumping more atmospheric gas into the station.

By that law, the pressure $p$ would inevitably rise, leading to problems stated in @Uwe's answer. Although our atmosphere is not ideal but a real gas, one can conclude:

Yes, but one would have to manage Nitrogen narcosis as discussed in @Uwe's answer.

If you insisted on increasing the pressure without increasing the mass, you could change the temperature. But this is just a theoretical answer, as a temperature of around 1500 K is necessary to reach a pressure of 5 atm. In such an environment, the astronauts would not be able to do anything but to evaporate.

  • $\begingroup$ It would be better to increase density without increasing pressure by making temperature drop, like in Titan. However, astronauts would freeze. $\endgroup$ – Pere Jul 10 '20 at 17:41
  • $\begingroup$ @Pere in addition, you'd need to increase the mass of the atmosphere $\endgroup$ – Everyday Astronaut Jul 11 '20 at 21:32
  • $\begingroup$ Yes, I mean increase density while keeping volume (of space station) constant, that is, increasing mass. In fact, the question about increasing pressure also involves increasing mass. $\endgroup$ – Pere Jul 11 '20 at 22:19

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