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Thread: Can moons have moons?

  1. #1
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    Can moons have moons?

    If I understand this correctly, yes, just barely, if everything goes right.
    https://arxiv.org/pdf/1810.03304.pdf
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    Would that situation be dynamically unstable?
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    Quote Originally Posted by Noclevername View Post
    Would that situation be dynamically unstable?
    I think as long as everything fits far inside each body's Hill sphere, it will work... but I think perturbations will pull the smaller sub-sub-sub-moons out of place first, destroying them or throwing them away, then the next set of moons, etc., until a long-term stability is reached. Nothing is permanent, but a dynamically stable situation would be reached without a lot of the "subs" remaining. My thoughts, anyway.
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    moonLIGHT

    Aside from the artificial satellites cirling our Moon, I know of no others. The outlook is dim.

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    Quote Originally Posted by John Mendenhall View Post
    Aside from the artificial satellites cirling our Moon, I know of no others. The outlook is dim.
    Agree.
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    There was just an arxiv preprint on this topic: Can Moons Have Moons?. Their study includes tidal dissipation affecting the submoons' orbits. Their results: in the solar system, our Moon, Callisto, Titan, and Iapetus could in principle host long-lasting submoons. However, the experience with our own Moon, where the lumpy mass distribution destabilizes even pretty high orbits quickly, make me think their criteria would need to have some factor for these effects added (not like we know that at all in exoplanetary systems).

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    Quote Originally Posted by ngc3314 View Post
    There was just an arxiv preprint on this topic: Can Moons Have Moons?.
    Yes, it's in post #1 ... but thanks for the arxiv entry link instead of straight to the pdf.
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    Is the solar system dynamically unstable? It all comes down to a question of scale.

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    Quote Originally Posted by thorkil2 View Post
    Is the solar system dynamically unstable? It all comes down to a question of scale.
    Then let's see the relative time frames between Moon loses its moonlet, and the Solar system dying.

    I'll bet the two are not similar numbers.
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    Where does the sub-moon go when its unstable orbit takes it away from its parent moon? Mostly a collision I suppose, but it can hardly escape the primary, so there are various other possibilities: co-orbitals, Trojan points, or just another orbit around the primary.

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    Quote Originally Posted by agingjb View Post
    Where does the sub-moon go when its unstable orbit takes it away from its parent moon? Mostly a collision I suppose, but it can hardly escape the primary, so there are various other possibilities: co-orbitals, Trojan points, or just another orbit around the primary.
    If it is still in orbit around its primary, isn't it still a moon?
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    Yes, my question is, supposing sub-moons that don't hit something become moons, are any of the moons in the solar system plausible candidates for this process of origin?

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    Quote Originally Posted by agingjb View Post
    Yes, my question is, supposing sub-moons that don't hit something become moons, are any of the moons in the solar system plausible candidates for this process of origin?

    How could we know? They'd just be moons.
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    Quote Originally Posted by Noclevername View Post
    How could we know? They'd just be moons.
    They would end up on somewhat suspicious, non-random orbits.

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    Incidentally is there a stable orbit around our Moon (at least decades long) for some spacecraft we might want there?

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    https://www.newscientist.com/article...led-moonmoons/

    I guess some people would prefer saying "moonmoon" instead of "submoon" which means the topic has already gotten silly.
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    Quote Originally Posted by chornedsnorkack View Post
    They would end up on somewhat suspicious, non-random orbits.
    Non random in what way?

    ADDED: Unless the "bobbling" happened very recently, orbital mechanics would put the former moonlet into a normal orbit. As with the moon-of-a-moon itself, the bizarre orbits are unstable over time.
    Last edited by Noclevername; 2018-Oct-10 at 05:09 PM.
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    Quote Originally Posted by agingjb View Post
    Where does the sub-moon go when its unstable orbit takes it away from its parent moon? Mostly a collision I suppose, but it can hardly escape the primary, so there are various other possibilities: co-orbitals, Trojan points, or just another orbit around the primary.
    There's also the possibility of inward migration until it hits the surface of the moon it's orbiting. That's probably quite a likely fate. As the moon is slowed towards synchronicity by tidal friction, the region in which submoons orbit faster than the moon rotates expands outwards towards the Hill sphere. Any submoon caught in that region will spiral inwards slowly, like Mars's moon Phobos, as it is braked by a lagging tidal bulge.

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    Here's a simulation of 20 moons around our Moon. They are equally spaced in SMA from 2000 km to 40,000 km.
    Although the close ones are very stable, in reality, lumpy gravity from mascons will probably doom them.

    This will run in your browser:
    http://orbitsimulator.com/gravitySim..._moonmoon.html

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    Quote Originally Posted by peteshimmon View Post
    Incidentally is there a stable orbit around our Moon (at least decades long) for some spacecraft we might want there?
    Spacecraft have structural integrity, so they are not as limited by the Roche Limit. They can orbit arbitrarily close to the Moon, as long as they strong enough to withstand the tidal force and are high enough to miss the mountain tops.

    BUT, the occupants will have to keep in mind that any loose objects outside the spacecraft will drift away from the ship on their own orbits. This would apply to a person on a space walk as well.

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    I was thinking of a science mission like a radio astronomy probe. It would be shielded from Earth behind the Moon and also from the Sun every full Moon. Then it could just listen to the whispers from the vast eternity. Sorry...just a bit of poetic whimsey there!

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    Quote Originally Posted by peteshimmon View Post
    I was thinking of a science mission like a radio astronomy probe. It would be shielded from Earth behind the Moon and also from the Sun every full Moon. Then it could just listen to the whispers from the vast eternity. Sorry...just a bit of poetic whimsey there!
    Whispers without FM talk radio blowing through the cosmos.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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  23. 2018-Oct-12, 04:16 AM

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    Not just FM band, but the auroral emission makes low-frequency radio astronomy impossible almost anywhere else within a good fraction of an AU. Hence the interest in radio cosmology from equipment that spends at least some time in orbit above the lunar farside (and landed there would be nice with appropriate infrastructure). Explorer 49 = Radio Astronomy Explorer B explored this with four 230-meter dipoles in lunar orbit in 1973. It produced this sky map at a frequency of 9 Mhz (although the annual resolution is charitably described as modest).
    Attached Images Attached Images

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    Nice to know someone has already had these thoughts and I am only 45 years behind. But the whole range of frequencies shoud have some advantage perhaps.

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    Quote Originally Posted by Noclevername View Post
    If it is still in orbit around its primary, isn't it still a moon?
    It would technically be renamed a planet if it could clear its new orbit of smaller bodies. So we might be waiting a few hundred million years.

    The exception is if it stayed in the same orbit with the planet it escaped. Thing is, the only way it could do that is if it set up a stable resonance with the planet (such as L4 or L5). But that means that it is still orbitally bound to the planet, which means it's still a moon of the planet.

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    Quote Originally Posted by DaveC426913 View Post
    It would technically be renamed a planet if it could clear its new orbit of smaller bodies. So we might be waiting a few hundred million years.

    The exception is if it stayed in the same orbit with the planet it escaped. Thing is, the only way it could do that is if it set up a stable resonance with the planet (such as L4 or L5). But that means that it is still orbitally bound to the planet, which means it's still a moon of the planet.
    I thought the planet was the primary of the moon, and the star was the primary of the planet. Am I using the term wrong?
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