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Given that moons commonly orbit planets, why do we never encounter a comet orbiting a planet?

What would happen (exactly, in detail) if a large one did settle into a stable orbit around the Earth?

Sorry... just making the question a bit clearer! The accepted answer will describe where all the water ends up.

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  • $\begingroup$ Partial answer here (not a duplicate question though): astronomy.stackexchange.com/questions/2357/… $\endgroup$ – Andy May 31 '16 at 15:31
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    $\begingroup$ If the planet is large enough it can happen easily. Shoemaker Levy-9 was a comet that was captured by Jupiter and orbited it at least once, perhaps more than once, but given the highly eccentric orbit, perhaps only once. $\endgroup$ – userLTK May 31 '16 at 18:18
  • $\begingroup$ Just for clarity, is your question essentially where would the water end up if a comet was to orbit the Earth? Which is an event that's unlikely to happen, but theoretically if it did happen, where would the water end up? $\endgroup$ – userLTK May 31 '16 at 18:44
  • $\begingroup$ @userLTK I'm working on the obviously true assumption that smaller orbital objects are routinely hoovered up by larger ones, including comets, although most of those in planetary orbits in our solar system have been sucked up by now. what I'm really interested in is how that looks to us if we were present as it happens. Where does the water go and what do we experience? Suppose it's a big one with a lot of water. $\endgroup$ – samerivertwice Jun 1 '16 at 7:14
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    $\begingroup$ I have downvoted as while I think the core of your question is ok, your focus on your final point and the comments you have made on the answers shows the misunderstanding you have around water from comets. I'd suggest removing this point, and perhaps asking another question on how much water from comments ends up on Earth, or something like that. $\endgroup$ – Rory Alsop Jun 1 '16 at 10:10
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Its very unlikely for a comet to become a satellite of an inner solar system planet. Much less likely than it is for an asteroid. Most asteroids are on fairly circular orbits, and so the relative velocity between asteroids and planets is quite low. In comparison comets have very elliptical orbits, and their relative velocities to the planets are much larger.

For an asteroid to be captured it must lose momentum. This is possible, though rare. For example, a binary asteroid can be captured if it is separated by tidal forces. For an comet with much more momentum, the chance of being captured is much much lower. Asteroids are captured by the Earth moon system, but not into stable orbits, they don't stay long.

If it did occur, the comet would still be active, with a coma of gas, which would be visible just like a very nearby comet. It wouldn't be particularly bright, since the surface brightness of a comet doesn't depend on distance from the Earth.

Over time the comet would run out of volatiles and become more or less indistinguishable from a captured asteroid. If it were in the Earth's orbit it probably wouldn't last that long, as there are not many orbits that are stable in the long term around the Earth, due to perturbations from the moon.

The dust and gas, including water vapour, will initally remain in orbit, forming a faint ring. It will, over time, be disrupted, and either end up in the atmosphere, on the moon, or ejected from the system. A comet doesn't contain enough water to make a difference to the Earth's ecosystem.

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Given that moons commonly orbit planets, why do we never encounter a comet orbiting a planet?

By definition, it would no longer be a comet, but rather a moon (or more properly a satellite). Comets are icy bodies that orbit the Sun, satellites are any body which orbits a larger body than itself, other than the Sun. There are many examples of asteroids and comets that have been captured and become satellites, although it is not easy to prove with 100% certainty that a satellite is indeed captured and did not form in place. Some familiar (potential) examples might be Phobos and Deimos, around Mars, or the various moons around Pluto such as Nix, Hydra, Kerberos, and Stix.

What would happen (exactly, in detail) if a large one did settle into a stable orbit around the Earth?

I have to take exception to this question by pointing out that it is impossible for a comet to "settle into a stable orbit" around the Earth. Even basic physics will tell you conservation of energy and momentum disallow capturing comets by them "settling". In order for one body to capture another, you need a third body in the system to conserve angular momentum. What are the chances that two comets come by Earth at the same time and with exactly the correct orbital properties such that they can interact and cause one of them to be captured? I'm not going to do the math but I can guarantee the chances are diminishingly small. The moon could play the role of the third body, but again, you need precise orbital characteristics which seems unlikely to me.

But we disregard that point and just say that somehow a comet has been put into orbit around the Earth, be it a nearly miraculous capture or else put there by human intervention or by some other effect. What would happen to it?

The answer to that depends on where its orbit is. It seems unlikely to me that a comet could survive for long in some orbits. If it was very close to the Earth, it would eventually fall onto the planet through friction with our atmosphere. If it was farther out, you'd have to consider how the Earth-Moon system affected its dynamics. Possibly there could be some stable resonant orbit between the Earth, Moon, and comet. It is hard to say what the long term results would be as an analytical solution to the three body problem is impossible. The only way to answer this is through a simulation.

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    $\begingroup$ I'm specifically looking to determine where all the water would end up. $\endgroup$ – samerivertwice May 31 '16 at 17:03
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    $\begingroup$ @RobertFrost: you say "all that water" -- the mass of comet 67P/Churyumov–Gerasimenko is within 10% of 10^13 kg. If was all water (it isn't), that'd be enough to cover the earth to a depth of 0.02mm. Put it all in one place and you have a lot of water, 1/50 of the smallest of the great lakes, Lake Erie. Scatter it around the Earth, and it's not much. $\endgroup$ – Steve Jessop Jun 1 '16 at 0:30
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    $\begingroup$ @RobertFrost: presuming that significant water was added to the early Earth from other bodies, then it was likely a series of impacts. Earth has too low an escape velocity, to capture anything that isn't in an orbit pretty similar to its own. Then others have explained that the Moon, being so large, prevents Earth from really having stable orbits, so anything like that which might have been captured early on is not going to last 4 billion years. $\endgroup$ – Steve Jessop Jun 1 '16 at 8:19
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    $\begingroup$ So what it "looks like to us", for large contributions of water, is a crust-melting impact and we die. Just as well this all happened before we were around. Sure, bodies in orbits close to Earth's can be captured: en.wikipedia.org/wiki/Claimed_moons_of_Earth, and small impacts are not catastrophic. Any water on it could be vaporised on impact and join Earth's water cycle, or could be part of hydrated minerals that make it to the surface. For a "just right" impact, the core of an icy meteorite could probably make it to the surface unmelted. $\endgroup$ – Steve Jessop Jun 1 '16 at 8:38
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    $\begingroup$ @RobertFrost Well, one of the competing theories is that a series of impacts brought water to Earth. Not necessarily comet impacts - in fact, there's significant evidence that the source corresponds to outer belt asteroids, rather than comets. But during extensive analysis of Moon samples, a better fitting theory shaped out - Earth had its water long before it finished forming, mostly in hydrated rocks and even in oceans (with temperatures ~200 °C). In any case, nobody suspects a single comet in orbit. Why do you focus on orbits so much, when a direct impact is much more likely? $\endgroup$ – Luaan Jun 1 '16 at 8:50
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Yes. See Split comet Shoemaker-Levy 9 became a satellite of Jupiter Ríse hvezd, Vol. 74, p. 224-225

and The Century of Space Science

comet Shoemaker-Levy 9 which was probably captured into a Jupiter-centric orbit and thus, temporarily, became a satellite of Jupiter before splitting into at least 21 piece and crashing into Jupiter's atmosphere

And according to Disintegration of fragments Q1 and Q2 of comet Shoemaker-Levy 9 during the fall to Jupiter Astronomy Letters, Volume 22, November 1996, pp.771-779:

The comet was captured by Jupiter and became its satellite in 1929 ± 9 according to Chodas and Yeomans (1995) and in 1959 according to the calculations Chemetenko and Medvedev (1994).

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It is absolutely possible for a planet to 'capture' a comet, Mars has already captured two asteroids and the only difference between the asteroids and comets are what they are made of. Plus, there are theories that believe that Saturn's rings were formed by a captured comet that strayed too close to the gas giant and thus, was ripped apart into millions of pieces of rock and ice (Assuming theory is accurate). Note that this is much more likely to happen to the outer planets due to their immense gravitational influence and their distance from the sun. Distance matters because comets generally go faster when they get nearer to the sun which makes it harder for a planet to capture comets.

Although, technically it wouldn't be a comet anymore but a satellite.

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  • $\begingroup$ I agree, what you say is obvious. I'm just wondering what happens when a big whopper with a lot of water gets hoovered up by the Earth. Where does the water end up and what does that look like to us? $\endgroup$ – samerivertwice Jun 1 '16 at 7:08
  • $\begingroup$ Mars' satellites were most likely already on an orbit very similar to Mars' own. That's completely different from a trans-neptunian object passing by - neither Mars nor Earth have anywhere near enough gravity to capture something with so much relative velocity. Yeah, planets can capture comets (at least temporarily), but as you noted, this is much more likely for an outer planet with lots of mass than for an inner planet with (relatively) tiny mass. $\endgroup$ – Luaan Jun 1 '16 at 8:53
  • $\begingroup$ @Luaan Thanks for your comment. I'm permitting in this question, the notion of some comet a long time ago which was perhaps not traveling as fast as those which remain nowadays. It would seem obvious that the solar system has been around a long time and consequently the slower ones have now all been filtered out by the planets. $\endgroup$ – samerivertwice Jun 1 '16 at 10:25
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    $\begingroup$ @RobertFrost I'm not sure that's true, though, assuming our theories of comet formation are correct. Comets are anything but stable, so it's not like there's a pool of comets that kept orbiting inside the Solar system for billions of years, slowly being culled by planetary interactions. Rather, it seems that the "injection" of a new comet is "accidental" - a result of complex gravitational interactions changing orbital characteristics of a body (that might have very well been on an extremely slow circular orbit at a very far distance) - less massive ones are more likely to be "injected". $\endgroup$ – Luaan Jun 1 '16 at 10:33
  • $\begingroup$ It has been suggested that Titan, the second largest moon, is a captured comet. It is still outgassing, but other volatiles than water. If it came close to the Sun it would have a tail. I suppose at Earth orbit, it would outgas quickly and the gasses would drift out with the solar wind until they can refreeze. $\endgroup$ – LocalFluff Jun 1 '16 at 15:57

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