I am trying my hand at worldbuilding a sci-fi setting, specifically a partial/broken ringworld. Specifically I have trouble wrapping my head around gases. I have 3 scenarios, solutions of which will hopefully give me all the answers I need to move forward, so here we go:

A) Suppose there is a device in Geo Synchronous orbit around a planet. For simplicity lets say this device can ignore the laws of thermodynamics and is able to produce air mixture infinitely out of nothing. What will happen to the gas that is being created? How fast does it "spread"? I am mainly interested in whether there is any rate of gas creation that would provide about 20 - 25% density of Earth atmosphere in the orbit, or will it just fly away into space too fast for such ring to develop?

Hope I explained that clearly. I guess I can add illustrations if its not clear?

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    $\begingroup$ Hi Vern. I like this question, except it is 3 questions. Each of which I like but I am not the kind to be able to handle 3 at once. Maybe in years past, but now I am a one at a time WB stack exchanger type person, with dedicated love and care for each in turn. And this is in accord with the rules of the stack exchange. I put lines thru B and C but just so they are not forgotten and you can post them separately later. Right now let us have this question be the All A show. Welcome to WB stack! $\endgroup$
    – Willk
    Commented Jul 8, 2022 at 21:59
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    $\begingroup$ AlexP please make that an answer. I would think released gas moving at orbital speed would orbit, and if it slowed down most of it would fall into the gravitational well. But I dont get it. If you do, lay it out and edify us. Or me. Both. Yeah. $\endgroup$
    – Willk
    Commented Jul 8, 2022 at 23:29
  • $\begingroup$ Hi Vern, Willk is right. You are only allowed one question here. That's a hard SE rule. Please remove two of the three. The alternative is that the question is closed, which will be pretty swift. That would be a shame, as it looks fun. Welcome. $\endgroup$
    – user86462
    Commented Jul 9, 2022 at 6:19
  • $\begingroup$ @Willk: The center of mass of the released gas will orbit. But that is irrelevant. The gas will expand trying to fill the universe. (Unless of course that the assumption of the device being able to produce mass from nothing is to be taken literally it will eventually make a gas giant.) $\endgroup$
    – AlexP
    Commented Jul 9, 2022 at 10:28
  • $\begingroup$ Hello. I disagree that these are 3 separate questions. Question is clear, how does gas behave in orbit. As MolbOrg poitned out, I could ask this 3 times but: A) I would have to refer to the previous questions, which make no sense and B) the answers would be very similar. I was told to provide as much detail. The 3 scenarious are the details. Look at this question for example, why is this OK but mine not? worldbuilding.stackexchange.com/questions/188887/… $\endgroup$ Commented Jul 9, 2022 at 20:06

2 Answers 2


the gas will fly all over and form an athmosphere of a planet

From point of release air molecules, initially they will take all possible orbits in all directions and plus minus about 300m/s delta-v

  • not exact but it orbits from 22'000 km to 60'000km

  • ah even more that 22k km and 60k km, as I used average molecules starting speed of around 300m/s but there will be some with higher velocities, but it does not matter that much

  • it will form a ring but not for long, and the ring itself will be something like 600km thick.

Molecules in orbit will cross path with other moleculess and collide pushing spread of orbits further

So it will form some sort of a shell-like thing.

  • it hard to tell the dynamics, and specific shell configuration, as it just partial analysis of what happens to molecules, and why they won't hold in orbit.

It will evolve in some sort of shell from 22k km to 60k km ringish thing to more spherical in matter of days weeks, depends on density.

All elliptical orbits you can imagine will be eventually taken.

  • at this point it probably already felt on the planet, but we continue our partial analysis, and continue to have configuration which will not have place(probably) in reality, as all those processes will happen simultaneously

Once there is a shell(is it ring-ish or shell-ish it doesnot matter), an molecule with high ecliptical orbit, instead to raise to its apo-point(most distant point in orbit from center of mass) it will "hit" that shell, lose some energy the same way as airobraking works, but athmosphere is in other side in this case, taking a lower orbit (some may take higher orbits, or more circular ones, but we are interested in those who fall, because it those will eventually land the whole thing)

At lower orbit there will still go process of collision, a thin shell with circular orbits is not exsempt from those collisions, as two random orbits on that sphere have 2 points of intersection(nk matter ringish or shellish).

So the whole thing will be collisions collisions collisions and more collisions - which will lead to loss of energy as heat emission.

The thing will spread to some higher orbits(less probable, but some will), so as to lower orbits(more probable), a loss of energy is at any collision in form of electromagnetic waves - same as your regular air emits heat emission, same as it cools at nigth - this energy will be emitted in space, and the thing start to cool down, and we will observe it occupy lower and lower orbits.

At some point it starts to litobrake, hit the ground and then the process start to go even faster - loss of energy, transfer of impulse to the planet surface, etc

It would be interesting to look at potencial dynamics, which actual shapes it takes due the process and how it evolves in time, but...

As a result:

  • I am mainly interested in whether there is any rate of gas creation that would provide about 20 - 25% density of Earth atmosphere in the orbit, o...

Won't happen, no ring forming, for reasons I mention and more. Rings gas giant have are not made out of gas.

  • B)

It will get some, capture some gas, and also have athmosphere, and it will happen even faster if generator is in front of it. Not being able to hold gas over billions of years does not mean not being able to have athmosphere for some time, if escape velocity is higher than average molecules speed at that temperature, it may hold it for some time, more so if it constantly are supplied by something.

  • C)

Not sure I get 100% of it - "suck" the air out of orbit - there is no limit, it can become gas giant and a star eventually, if rate of supply is good enough.

Our moon can become gas giant in those conditions.

So as much as you supply, if it happensfaster than a loss of it

All in all - 20 - 25% density of Earth atmosphere - such a ring around a planet is not possible, and only handwavium can fix it. Everything will land on the surface of a planet, in similar way as it happened to our solar system, which also at some point was mixture of gas and particles cloud. With pressures you mention, and meaning coresponding rate of supply which comes from volume it require for it all - it will all happen much faster than it did with solar system, weeks, months, years - it depends on many factors, so as I just can't tell tnings at this level of details.


  • Can't comment so this way

There is no need to edit the q, strike out something or anything. Q in the title - behaviour of gas in orbit with geosync source/emitter of it. Scenarios are all miss, in this specific case - so there is no need to force OP to create additional two questions, which will have zero additional value for WB, for OP, for ppl of WB, for the Universe and which should have the same title and answers have to explain the same things.


Not a complete answer, but too long for a comment. I think the answer is complicated because of the solar wind and the your planets magnetic field.

Depending on how the gasses were released, if they were uncharged particles they would go into a higher or lower orbit, but the problem is that the solar wind has a flux of ions and protons and alpha particles, and electrons that move at high velocity. This would hit and ionize the molecules, and then after that it be comes more complicated depending on the magnetic field. Even in earths geosynchronous orbit there is enough field for it to matter.

The solar wind is significant, especially over time. I don’t know comet tails work, but I think you have molecules that become ionized and are pushed out more directly by the solar wind and the more curvy whiteish part of a comet tail are the heavier particles. The point of mentioning comments is that they start to have a tail pretty far out from the sun.

I suppose if you are thinking of mega structures you have the ability to set up magnetic fields to help somehow, or perhaps the mega structure could somehow block the solar wind producing shadows where the gasses wouldn’t get ionized.

If you look around there are some nice NASA videos on the solar wind and magnetic fields on YouTube. They sent out a set of satellites to specifically map out the magnetic fields around the earth and if I recall they also show how charged particles and solar wind behave. Closer to the earth there are also the Van Allen belts.


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