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I have a few questions related to a crazy setting I'm working for my novel.

Let's say we have a planet with a thin atmosphere and low gravity. It's volcanic activity is so intense that the planet has turned into hell, basically—even entire volcanic belts go off with very (read VERY) powerful eruptions.

The questions are:

  • Considering we have a low gravity and a thin atmosphere, can these eruptions throw volcanic material into space (even large rocks)?
  • Considering that these eruptions happen very frequently, how feasible is that a cloud of debris could form around the planet (even completely covering it)? The material might plummet back to the surface, but get replenished by new eruptions.

I know it sounds kind of crazy, but I want to build a hell of a world for a chapter. Am I pushing the boundaries of believability too far?

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    $\begingroup$ I don't have the science to answer your question, but the idea of a daytime sky filled with visible-to-the-naked-eye asteroids hanging overhead is wonderful. And the nearly constant partial eclipses which would cast shadows across the scene makes an interesting plot element. This world doesn't have day and night. It has night and the sheds of a day, interrupted by short-lived and unexpected twilights. $\endgroup$ – Henry Taylor Dec 13 '15 at 5:57
  • $\begingroup$ I must admit I didn't thought of it that way. But it's an intriguing concept to explore. The most immediate issue I see is having a breathable atmosphere in such a planet. However, maybe the atmosphere composition could be breathable to other kinds of organisms—say aliens. I will explore this and see what I can come up with. $\endgroup$ – A.C Louis Dec 14 '15 at 6:29
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    $\begingroup$ You'll run into a bit of a problem with orbital mechanics: basically since orbits are closed, if your orbit starts on the ground it will have to come back to the ground, unless you can add energy to the rocks after they reach space. Maybe via collisions or volcanic gas jets? $\endgroup$ – 2012rcampion Dec 14 '15 at 10:45
  • $\begingroup$ @A. C. Louis, The atmosphere doesn't need to be breathable, nor free of re-entering rocks, to be beautiful and enjoyable for future tourists. As our video capture technologies grow to include input for more senses, and our full-immersion virtual reality technologies improve, people will be able to experience life on planets that we couldn't ever actually live on. Entire stories might unfold without any of the characters actually being present on the world where it is "happening". $\endgroup$ – Henry Taylor Dec 14 '15 at 13:35
  • $\begingroup$ @2012rcampion I wouldn't mind rocks re-entering the atmosphere and plummeting back to the surface. That would actually increase the drama factor, which it's desirable. Rocks thrown into the atmosphere orbir and re-enter, then volcanoes throw more rocks in a constant cycle, like rain. Rain in hell. $\endgroup$ – A.C Louis Dec 14 '15 at 19:05
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Maybe. You don't given any details of your planet re: how thin the atmosphere really is, and what is the required escape velocity (or the details that would allow its calculation).

Ejecta velocities, magma chamber pressure and kinetic energy associated with the 1968 eruption of Arenal Volcano suggest the eruption velocities could reach as much as 2 km/sec. This would be sufficient to achieve lunar orbit. If the moon has significant air drag, orbit would not be possible though.

You want high vulcanism, but presumably not enough to destroy to the planet. Yet you also need a low escape velocity and no (if possible) or very thin atmosphere.

Put your planet in near orbit of a gas giant to cause lots of tidal heating in the core, or you could just have a lot of uranium and thorium in the core compared to Earth. If you need a breathable atmosphere but thin as possible, make the atmosphere essentially pure oxygen with a trace of CO2 at about 0.2 atmosphere of pressure (your planet will be a little oxygen deficient compared to Earth, but quite breathable).

Now, make your vulcanism primarily due to an extremely powerful and very tall volcano or chain to get the "launch point" above most of the atmosphere. Due to the lower gravity, you may be able to make the volcano 30 km high, maybe even a little more. Granite flows downhill under sufficient pressure given time, so there is a limit to how tall a mountain can be.

At this point you can make the orbital volcano ejecta believable if not entirely realistic. Keeping a breathable atmosphere might be hard to explain - perhaps oxygen was unusually abundant originally and it is still seeping out from the interior. Still needs to be larger than the moon to keep an atmosphere, probably Mars size would be needed in addition to the ongoing replacement of oxygen.


When I read your question, I thought you were interested in a thick orbital cloud. Now I think you are talking about an atmospheric cloud.

Re: Having a thick volcanic clouds within the atmosphere it is not necessary at all. Large volcanic plume have been seen on Earth up to 50 miles above the surface. This is primarily a result of the warm air rising. To keep a consistent cloud, the vulcanism would have to be ongoing. Historical heavy eruptions on Earth only generate a cloud dense enough to obscure for a fairly short duration (weeks or months) and never world-wide. Even a super-volcano such as Yellowstone is not expected to cloud over the whole planet.

To keep cloud duration high you actually are better off with a thick atmosphere, which is very hard to justify on a small planet, and it makes orbital insertion impossible.

A possible solution to keeping a dark sky that is a result of vulcanism is to add biology into the mix. Say that these is a common bacteria adapted to using the volcanic material in the atmosphere. The bacteria consume this and eliminates the waste as microscopic particles of soot. This type of soot could remain in the atmosphere for many years if no rain, etc. washes it out. A number of earth bacteria are sulfur based, it is not inconceivable that a bacteria could consume sulphur and CO2 and end up expelling tiny soot particles.

Alternatively, you could have a high carbon planet, and carbon or hydrocarbons are a large component of the hot volcanic exhaust. If these burn incompletely you get soot. These soot particle will not generally be small enough to stay in the atmosphere for years, so this would be considerable less effective in generating permanent dark clouds.


Since you don't need a breathable atmosphere at all, and you apparently want the cloud to be orbital, you don't want any atmosphere at all. Any atmosphere will cause drag in low orbit, so get rid of the atmosphere.

Forgot to mention you want to give your planet a fast spin to that the ejecta has a relatively large angular momentum. Only a small fraction of the ejecta will be able to stay in orbit (as a result of secondary collisions) and it will tend to stay in concentrated in a band with a latitude roughly less than or equal to the latitude of the volcano. You minimum the orbital band thickness (along the north/south line) by placing your volcano near the equator. The natural tendency of the orbital rubble will be to form a ring, not a cloud. As the rubble interacts, it will naturally become of very thin orbital ring. This takes some time, and since your ejecta in continually renewing the ring, you could have essentially a ring with some thickness. There is no believable way to cover the whole planet, or even a large fraction of the planet.

Since you don't need atmosphere, making the planet smaller will reduce the necessary orbital velocity too. However, this will also tend to reduce vulcanism, so you need even more uranium/thorium or tidal stress to generate vulcanism.

Again, I am arguing that this answer is only plausible, not realistic. Your need to use as many of the circumstances I've mentioned to maximum the density of the orbital cloud and believability.

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  • $\begingroup$ I see. So, under the right conditions, it could plane possible. I'm not looking for a breathable atmosphere. In my novel, a crew sails through the clouds of such a planet and find a spacecraft wreckage containing some important information for the rest of the plot. Some people said it wasn't believable since such clouds are just impossible to form without the debris accreting into a disc, like Saturn. $\endgroup$ – A.C Louis Dec 14 '15 at 6:34
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    $\begingroup$ I did not say it was necessarily possible, I said it was believable. Updated my answer too. $\endgroup$ – Gary Walker Dec 14 '15 at 9:21
  • $\begingroup$ Yeah, I got it. What I was saying is that as long as there's a small chance of it happening somewhere in space, it makes the plot believable enough while incorporating things that are not yet discovered. Thanks a lot for your ellaborated answer. $\endgroup$ – A.C Louis Dec 14 '15 at 19:07
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I think you are pushing the boundary of believability a little too far, at least for someone who is somewhat familiar with orbital mechanics. (That may or may not be a problem for your intended audience.)

Space isn't high up, like:

Yay! Space!

Space is fast, like:

Space isn't high up, it's fast.

Now, in principle, you could get around this by having your volcanic eruptions take place at an angle somehow (not sure how realistic that is), but just tossing material straight up will have one of two outcomes:

  • It will reach escape velocity and leave the planet's sphere of influence. It may still remain in a relevant orbit, like how you can gain enough velocity to break free of Earth's gravity only to find yourself still stuck in the Sun's gravity well.
  • It will fail to reach escape velocity, and simply fall back down. In this case, the object will likely trace approximately a ballistic trajectory. Keep in mind that in low Earth orbit, despite apparent weightlessness, gravity is about 90% of that on the surface.

The only reason why a satellite doesn't simply fall down is that the launch has imparted a great (approximately 7,000 m/s or more, in the case of the Earth) forward velocity. This forward velocity exactly (to within small error) compensates for the downward pull effected by the gravity of the Earth, in any given orbit. Compare for example Why is geosynchronous orbit an altitude, rather than a velocity? over on Space Exploration SE, or more generally their orbital-mechanics tag.

As others have already pointed out, absent additional energy inputs, orbits are closed, which means you can't toss material into an orbit. However, with suitably lucky atmospheric skips and low enough gravity, it just might be possible to toss material at an angle that eventually results into a (most likely highly elliptic) orbit. Such material would likely need to be launched at angles that are almost parallell to the ground, and obviously with sufficient velocity to maintain orbital velocity at orbital altitudes.

An alternative solution:

However, this might be easy to work around without altering your setting too much. You stated in a comment that:

In my novel, a crew sails through the clouds of such a planet and find a spacecraft wreckage containing some important information for the rest of the plot. Some people said it wasn't believable since such clouds are just impossible to form without the debris accreting into a disc, like Saturn.

(This has the added problem that such a "cloud" would create drag, which will quickly cause anything caught in it to deorbit. Even the ISS experiences drag and needs periodic reboosts to maintain its approximately 400 km high orbit.)

Consider instead of making this a natural phenomenon, simply making it an extreme runaway case of space debris, known as the Kessler syndrome or effect. Have the timing reasonably right, and it's plausible for a spacecraft to be disabled by space debris but not yet destroyed by it when your crew comes along.

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  • $\begingroup$ Hah, I loved your drawings. Sometimes it's hard to put things in their actual size. So, now considering your points, what if gravity in the planet is a lot lower than that or Earth and the escape velocity required to leave the atmosphere is also lower? I know that in the case of Earth, leaving the planet is extremely difficult because of the required speed and angle. But without those limitations, material could leave with less required power, right? $\endgroup$ – A.C Louis Dec 14 '15 at 19:12
  • $\begingroup$ Additionally, what you said about debris creating drag opens my possibilities even more. Originally, the crew find a vessel into the debris cloud and need to board it. The pilot needed to remain in the crew's ship to keep up the speed with that of the vessel as the crew explored it—requiring them to return to their ship before fuel runs out. But then I thought "nah, there's no drag in space", so I kind of took that out. Do you think it would be safe to say that is they don't anchor to the vessel, their ship end up crashing with the orbiting rocks? $\endgroup$ – A.C Louis Dec 14 '15 at 19:14
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    $\begingroup$ @A.CLouis The images aren't mine; I borrowed them from the linked XKCD What-If post. If the gravity is lower, then all else equal, the escape velocity is also lower (which has implications for the planet's ability to maintain an atmosphere: less gravity means less dense atmosphere). Kessler syndrome is not about debris creating drag, it is about risk of collision with debris (which creates a cascade effect); see the linked Wikipedia article. Anchoring or tethering doesn't reduce the risk of debris impact, and to maintain close distance, "all" you need to do is to match orbit and velocity. $\endgroup$ – a CVn Dec 15 '15 at 8:38
  • $\begingroup$ Thanks for your elaborated answer. Hopefully, I have enough information to start. I just didn't wanted to start with an unbeliebable story, since it's Chapter 1. $\endgroup$ – A.C Louis Dec 15 '15 at 15:56
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No. Gary Walker missed a critical point. Under extreme conditions you might be able to get a volcano to eject material at orbital velocities from a small world. This does not mean it can toss debris into orbit, though. One basic truism of orbital mechanics is that (barring precession and the like) an orbit intersects the last point the object was accelerated.

Thus if your supervolcano throws a rock at orbital velocity it goes around the planet--but then smacks back into the volcano at the same angle as it left the volcano in the first place. If you have any atmosphere the problem is even worse, it must fall short. Edit: Argh, I got the sign backwards. It's the same angle all right--if it leaves at 10 degrees above the horizon it returns at -10 degrees above the horizon--that is, it falls short. The only way to even hit the volcano is with an ejection angle of zero.

Note that this does not say a volcano can't eject rocks at escape velocity. Those rocks won't come back and hit the world but neither will they stay in orbit around it.

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  • $\begingroup$ You overlook another point of orbital mechanics, when abundant material is in a variety of different trajectories, some of the material will collide, altering individual trajectories. Without such interactions, you can't explain the formation of the moon from the Theia impact either. Thanks for pointing this out. $\endgroup$ – Gary Walker Dec 14 '15 at 5:10
  • $\begingroup$ @GaryWalker No, with the Theia impact there was no debris cloud to cause the impacts. You can get a moon from Theia it was a glancing blow by a very large object--much of it was already at orbital height when Theia was destroyed. $\endgroup$ – Loren Pechtel Dec 15 '15 at 23:17
  • $\begingroup$ Actually, the modern version of the Theia impact theory strongly suggests that much of the moon is formed from the crust of the Earth and that most of the iron core of Theia merged with Earth. Those Earth crust pieces of necessity interacted with others pieces of the crust or other parts of Theia after launch in order to remain in orbit. The Theia impact is also thought to have been much more direct than a glancing blow in the earlier models. $\endgroup$ – Gary Walker Dec 16 '15 at 1:04
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The best place in the Solar System to see the effects you want would be Io, orbiting Jupiter. Io

Io is truly a hellish place, with the most active vulcanism in the entire Solar System, and being a small moon, the sulphur and sulphides ejected from the volcanic activity routinely makes it into space. Io orbits Jupiter surrounded by a torus of sulphur ions (Jupiter's massive radiation belts and the two trillion watt "flux tube" between Jupiter and Io see to that). A spacecraft operating in or near Io's orbit will probably be coated in sulphur dust very quickly, in addition to being irradiated by the Jovian belts.

Somewhat more benign environments can be found around ice moons orbiting Jupiter and Saturn, where giant geysers can eject water vapour and ice particles into the space surrounding the moon. for various reasons, this isn't as intense as Io's vulcanism, but if the moon was closer to the primary, the extra energy from gravitational "kneading" might supply the force needed to put a truly impressive amount of water into orbit (once again, a fairly diffuse torus once it is spread around the orbit of the moon, but still enough to interfere with spacecraft operations).

The debris in orbit will follow power law distribution, so the bulk of the material will be gasses and ions, followed by simple molecules, then dust and so on down the line. The amount of energy needed to throw large boulders into orbit would also result in really vast clouds of dust and even larger amounts of gasses being ejected into space, possibly enough to make spacecraft operation completely impossible (moving into a dust cloud at interplanetary velocity would probably destroy the spacecraft).

Enjoy the ride.

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Expanding upon part of Michael Kjorling's answer:

Kessler Syndrome requires a technological civilization putting a bunch of stuff in orbit to come apart and make the orbital space into a debris cloud.

However, for the purposes of providing a deadly orbital hazard there's another approach: Your planet recently ate a moon. This will happen when the moon has an orbital period of less than a day--it will slowly spiral in and eventually pass within the Roche limit of the planet. How much farther in it will survive depends on how tough it is. (Something very strong survives to atmosphere, a gravel pile starts coming apart as soon as it crosses the line.) Once it fails it comes apart into a bunch of debris something like the rings of Saturn. While this is only a ring and not the whole orbital space there's no safe orbit short of not being at those altitudes in the first place. (And, yes, this was a scary thing for Casssini when it had to make a passage through the rings. They aimed for a gap but they knew there was a chance they would lose the probe.)

Look to Mars. This will be the eventual fate of Phobos. It's spiraling in and already showing the strain. (The exact failure point can't be determined without knowing it's density and strength. If it were liquid it would already be a goner.)

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  • $\begingroup$ Might add that the retrograde orbit of Phobos is the reason it is spiraling inward due to tidal drag. Retrograde orbits spiral in, normal orbit recede of time. +1 for a nice plausible alternative. $\endgroup$ – Gary Walker Dec 16 '15 at 1:24
  • $\begingroup$ @GaryWalker I thought anything that orbited in less than a day (by local measure, not Earth days) was dragged down by the tide, anything over a day was pushed outward (like is happening to Luna.) Retrograde is an extreme case of this. $\endgroup$ – Loren Pechtel Dec 16 '15 at 4:02
  • $\begingroup$ OK, to be precise - considering tidal drag alone. Retrograde orbits spiral in. Posigrade orbits with a periods less than 1 day of the parent body spiral in. Posigrade orbits with a periods greater than 1 day of the parent body spiral out. Phobos is not in actual retrograde orbit, it just appears that way from the surface of Mars. From the perspective of the planet surface, the rule is as I stated. Sorry I did not state this more carefully. $\endgroup$ – Gary Walker Dec 16 '15 at 4:30

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