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As I was watching Ad Astra (Great movie, but where does the gravity come from?), I had the idea that in a building or closed vehicle in a low gravity location, maybe air pressure could be used to imitate gravity. Blowing air directly from above towards the floor could maybe create a gravity-like effect without any need for e.g. magnetic boots or spinning for artificial gravity.

Any thoughts?

For example, on the Moon, Mars, or a spacecraft. Think of a room for example 7m x 8m, with the ceiling at 2m height. From everywhere on the ceiling there is an air current towards the floor where there are vents for recollecting the air and sending it through pipes back to the ceiling.

The air current from the ceiling could be either the same wherever the person is in the room, or it could follow the person to only push from directly above the person. It could even be adjustable by the room owner or the person theirself.

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    $\begingroup$ On another planet you don't need to replace gravity because every planet has gravity. Small bodies with very weak gravity are not called planet. $\endgroup$
    – Uwe
    Jun 4, 2022 at 8:51
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    $\begingroup$ IIRC the Space Shuttle toilet did use air as a replacement for gravity, although via underpressure. $\endgroup$
    – MSalters
    Jun 7, 2022 at 8:56

3 Answers 3

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in a building or closed vehicle on another planet, maybe air pressure could be used to imitate gravity

There is no need to imitate gravity on another planet, because planets have gravity. Of course, some of them have weaker gravity (e.g. Mars' gravity is about 1/3 of Earth's gravity), and you might want to make up for the difference.

If you are talking about a spacecraft in freefall (what we usually call "microgravity") such as the ISS or an Orion capsule, where there is indeed no (perceived) gravity, then using wind to create a force is indeed possible.

In fact, we are doing exactly that, here on Earth:

As you can see, this vertical tunnel is creating exactly enough force to create one g of acceleration. They just installed it the wrong way round!

However, you can also see a couple of things that make it not so suitable:

  • It is very loud.
  • It is very energy-intensive.
  • It only works in a confined space.
  • It is heavily dependent on body position. The instructor is standing there comfortably while the students are flying because he is presenting a lot less area towards the wind. What this means for your space station is that you always have to walk perfectly upright. As soon as you bend over to pick something up, the force on your body will increase dramatically and slam you into the ground pinning you against the floor unable to get up again. Watch closely how the students enter: they stand upright in the wind, and as soon as they lean forward, they start getting "picked up" by the wind. The same would happen in reverse on your space station.
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  • $\begingroup$ Great answer. I hadn't thought about that. While a question arises. In wind tunnel the needed force would be close to body weight, e.g. 80kg. In small confined space, like a spacecraft, would same kind of windforce be needed to have a decent simulation of gravity? $\endgroup$ Jun 4, 2022 at 10:13
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    $\begingroup$ Also blowing air will have little effect on the inside of the body; You won't be able to balance because your inner ear is telling your brain "Nope, we're still in free-fall." Most of the health problems from free-fall will still apply. $\endgroup$
    – notovny
    Jun 4, 2022 at 11:05
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    $\begingroup$ @AnttiRytsölä - Yes, the wind tunnel in the answer needs winds of about 200 km/h to simulate 1g gravity upwards to cancel weight. If you want to create 1g gravity downwards you will need the same wind speed. A gentle breeze may help you to slowly return to the floor after a light jump but nothing similar to Earthly gravity. $\endgroup$
    – Pere
    Jun 4, 2022 at 19:47
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    $\begingroup$ OP also seems to have failed to consider the effect of such a solution on any loose objects left in the "gravity room". Imagine, for instance, the effect on someone entering the room with a stack of papers... $\endgroup$ Jun 7, 2022 at 6:52
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    $\begingroup$ Another drawback: It applies force on the surface only, organs are not affected by the force $\endgroup$
    – Ferrybig
    Jun 7, 2022 at 13:44
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While not strong enough to produce "artificial gravity" in a microgravity environment such as an orbital space station (eg ISS), it should still be noted that air circulation, and specifically the blowing of air, can serve safety-critical purposes beyond providing life support. Notably:

  • Moving air can prevent pockets of unbreatheable gas building up. If, for example, an astronaut were asleep and there were no circulation of air, it would be possible for them to build up a "bubble" of exhaled CO2 around them that could cause negative health effects or even suffocation.

  • Moving air can prevent astronauts from becoming "stranded" in microgravity. If, for example, an astronaut were floating in a large space or intersection and unable to reach any walls, they might be stuck there for a while unless they have something heavy to throw to act as "propellant". Moving air can prevent such scenarios, because then any such "stranded" astronaut would be gently blown away from where they are stuck until they can reach a handle or something to push off against again.

  • Moving air can alert astronauts to developing dangers onboard. Human noses are--compared to electronic equivalents we have--incredibly good at detecting contaminants or other chemical compounds in the air. Smoke from a fire or the smell of overheating electronics are very distinctive and can be picked up easily even if the astronauts aren't specifically smelling the air for trouble.

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    $\begingroup$ Moving air can prevent pockets of unbreatheable gas building up. I'm not sure how true this is. Do you have any scientific studies or papers or articles on this topic? It seems to me that diffusion would cause the high concentrations of something like CO2 to naturally disperse and spread, no? $\endgroup$ Jun 4, 2022 at 23:55
  • $\begingroup$ @GabrielStaples convection is usually much more efficient at fluid transport than diffusion. The classic example is adding milk to coffee. If you don't stir it, the brown and white fluids mix very slowly. $\endgroup$
    – craq
    Jun 5, 2022 at 2:31
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    $\begingroup$ @craq, correct, but convection happens because of gravity: lower-density fluids rise to the top due to gravity exerting a larger force on higher-density fluids. In space in the absence of gravity, diffusion is all that's left. All I'm saying is I wonder if diffusion is sufficient in fluids in space to prevent pockets of certain types of gases from forming. It seems to me that it would be sufficient to prevent pockets of isolated gases from forming, and that, my hypothesis, directly contradicts with the first bullet of this answer. So, I question the validity of that bullet. $\endgroup$ Jun 5, 2022 at 3:06
  • $\begingroup$ @GabrielStaples convection can be driven by other things like temperature gradients, or by moving objects such as fans or humans. I expect that the temperature gradients in most space stations would be enough for convective transport to exceed diffusion by Brownian motion. $\endgroup$
    – craq
    Jun 5, 2022 at 5:08
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    $\begingroup$ @GabrielStaples Here's an actual astronaut talking about this topic: quora.com/… . I'm not sure if this is just superstition or if there's actual research that has been done on this topic, but even on Earth in non-airtight rooms, CO2 levels can still rise to unacceptable levels simply because exchange through the door cracks and windows isn't high enough and this leads to negative health effects. I don't find it unreasonable that a partially sealed sleeping cabin could have a similar issue $\endgroup$
    – Dragongeek
    Jun 5, 2022 at 10:56
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The only problem I see is body orientation. As the indoor skydiving videos show, the instructor remains grounded, while the student assumes a "Superman" position. So, the amount energy required to lift a man stand straight and tall is much more than lifting a man assuming that position. And, what would happen if the instructor bends over or radically changes his orientation? In other words, there is only a cautious stability with airflow. Not a good place to conduct surgery.

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    $\begingroup$ To add, that airflow changes depending on "terrain". That is, it might be straight "down" when blown from the ceiling, but would be deflected by objects. Thus, if you stood a tall vase on a table, it might well stay standing up, but if you walked past it, the deflected airflow from your body might knock it over (and once it starts to wobble, the airflow would likely "amplify" the problem). You could of course use this to your advantage: "pass over the remote" might be as simple and cupping your hand in just the right way to make the remote slide across the table ;-) $\endgroup$ Jun 7, 2022 at 8:35

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