### Can a comet orbit a planet?

• 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.

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.

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?

@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.

You really have to specify "a lot of water". If it's comparable to a large lake (like the great lakes in North America), we probably wouldn't notice much. If it's a significant fraction of the total liquid water on earth, we would burn up and die :P

If the comet actually orbits the Earth, then that is where the water ends up: in orbit. Since it's part of the comet.

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.

@RoryAlsop There's a strong hypothesis that Earth's water was delivered by comets. Clearly before they were all swept up by planets, the comets which have deliverer water to the planets would most likely have been the comets traveling slower and with closer orbits to the sun; that's just basic physics. The question I really want to get at is; what would that look like to man when it happened, if there was some major event? Would water be delivered unnoticed. Would there be some huge injection of superheated steam from the sky; if the comet settled into orbit, would water steadily fall as rain?

I know the hypothesis well. That is not the issue. A single comet is not going to give you something viewable. Either splat! and destruction, or millions of years of smaller bombardment (or both) - these scenarios don't really fit with the first half of your question - hence my request to split it out. The answers here give some good info on whether orbit is possible, so why not try that separate question on comet water?

@RobertFrost comets that impact the Earth look like fireballs. It looks like a great big kaboom in the sky or, if big enough, into the ground. The heat generated turns any water in the comet into steam and plasma, but since the remaining ingredients are still there, the Hydrogen and Oxygen from any water in the comet that impacts the Earth remains water on the Earth however, heating in the upper atmosphere likely leads to some atmospheric escape, so there's a trade-off. For more specifics, I'd do what Rory Alsop recommends and ask a new question.

• 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.

• 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.

I guess a small comet could orbit the moon. One way it could settle into an orbit is by means of impacting something, perhaps if it impacted the moon or another planet, or passed close enough to another planet to be slowed down and pulled into a planetary type orbit. Then over time it stands to reason it would move closer to a nearby planet.

I don't understand how a comet could impact another body such as the Moon, and then continue to orbit the Earth. Once it hits the Moon, its done. And it can't "pass close to another planet and be slowed down and pulled into a planetary type orbit". The physics just don't allow for such a scenario to possibly happen.

I'm specifically looking to determine where all the water would end up.

@RobertFrost If it impacts the Earth, nearly all of the water would stay on the Earth, unless it was large enough to hurl debris into space. As pointed out, a comet orbiting the Earth is hugely unlikely and if it improbably did happen, it's unlikely to be stable for long, but for the water to get to Earth it would either need to hit the Earth (this happens from time to time) orbit into the Earth's Hill Sphere and break apart, (unlikely), but if that happens, some water would be lost to the solar wind, some would fall to Earth, just as some of Saturn's rings falls onto Saturn as rain.

@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.

@zephyr: I don't know if that's true. Wasn't Shoemaker–Levy 9 designated a comet despite being discovered in orbit around Jupiter? Maybe it *shouldn't* have been so-designated, but nobody seemed to have any difficulty at the time with the idea that once something's a comet it remains a comet until it crashes...

@zephyr Clearly the reason there are no comets and asteroids in the inner solar system is that any of these smaller objects straying into the planetary zone have already been hoovered up by the planets. In fact a leading theory for where the Earth got its water, is that it came from a comet. This is one area of research in relation to What I'm really interested in is, suppose one way or another it is the Earth doing the hoovering on some occasion, and the comet is pretty large, what does that look like to us?

@SteveJessop I'm thinking there are big and small ones. Given the system in which they are steadily hoovered up by larger bodies, the law of probability dictates that the number of large ones will decrease over time. So it stands to reason that in the past there were more, and larger ones around.

@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.

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: https://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.

@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?

@zephyr *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?* Why two comets (which wouldn't interact noticeably anyway), why not, say, *the Moon*? Capturing a comet still is unlikely, but that's more due to the enormous momentum change involved.

@RobertFrost *suppose one way or another it is the Earth doing the hoovering on some occasion, and the comet is pretty large, what does that look like to us?* This sounds like you should ask a separate question. In any case, I'm unclear on exactly what you mean. Do you mean to ask how the comet would appear in the sky from our vantage on Earth?

@zephyr I mean what does the whole process look like... Comet approaches... settles into similar orbit around Sun... occasionally interacts with moon and earth... settles into orbit around Earth / moon... heats up due to tidal forces... turns to water / vapor... cloud of water/vapour orbits Earth & moon... slows down due to turbulence... starts to fall on to Earth and Moon as rain. What exactly is a realistic process ?

@zephyr the third body does not need to leave the system, it just needs to absorb the leftover momentum. The moon would speed up/slow down and change orbit a tiny bit, but otherwise won't budge. If capture always required the ejection of other bodies, it would be *exchange*.

@Chieron Sorry, you are correct.

• Yes. See Split comet Shoemaker-Levy 9 became a satellite of Jupiter Ríse hvezd, Vol. 74, p. 224-225

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).

• 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.

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?

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.

@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.

@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".

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.