# Do moons ever have moons of their own? **URL:** \ **Category:** Factual Questions\ **Created:** [November 30, 2003, 12:10pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049 "2003-11-30T12:10:18Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Dunderman](https://avatars.discourse-cdn.com/v4/letter/d/ecb155/32.png) [@Dunderman](https://boards.straightdope.com/u/Dunderman)\ **Post date:** [November 30, 2003, 12:10pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/1 "2003-11-30T12:10:18Z")
Planets circle stars, moons circle planets. Surely it must be possible for moons to have satellites of their own? Since it as far as I know doesn’t exist in our solar system I assume the phenomenon isn’t empirically known, but is there any reason it couldn’t happen? What would these satellites be called? ---
**Author:** ![leander](https://avatars.discourse-cdn.com/v4/letter/l/bcef8e/32.png) [@leander](https://boards.straightdope.com/u/leander)\ **Post date:** [November 30, 2003, 12:37pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/2 "2003-11-30T12:37:56Z")
According to [MadSci](http://www.madsci.org/posts/archives/nov98/911298108.As.r.html), moons may have moons. > [@](#): > > Certainly a moon can have moons. Just as our Moon orbits the Earth which in turn orbits the Sun there can be still smaller bodies orbiting the Moon. According to [Wikipedia](http://en.wikipedia.org/wiki/Natural_satellite), they cannot. > [@](#): > > No moons have moons of their own; the tidal effects of their primaries make orbits around them unstable. Does that help? 🙂 ---
**Author:** ![Dunderman](https://avatars.discourse-cdn.com/v4/letter/d/ecb155/32.png) [@Dunderman](https://boards.straightdope.com/u/Dunderman)\ **Post date:** [November 30, 2003, 12:39pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/3 "2003-11-30T12:39:31Z")
I’d much rather trust MadSci than Wikipedia. Besides, Wikipedia’s answer makes little sense. It’s not like the size of a planet or moon is a fixed thing. ---
**Author:** ![Tusculan](https://avatars.discourse-cdn.com/v4/letter/t/ecd19e/32.png) [@Tusculan](https://boards.straightdope.com/u/Tusculan)\ **Post date:** [November 30, 2003, 12:53pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/4 "2003-11-30T12:53:08Z")
Wikipedia’s answer gets confirmation from [this site](http://itss.raytheon.com/cafe/qadir/q332.html): > [@](#): > > It is very difficult for one object orbiting a planet to itself have a satellite because the planet would always win such gravitational tugs-of-war and convert the binary moon into just two independent moons orbiting the planet. The explanation makes sense to a layman like me. (I googled “moons have moons” and only got Wikipedia and this as useful links) ---
**Author:** ![Dunderman](https://avatars.discourse-cdn.com/v4/letter/d/ecb155/32.png) [@Dunderman](https://boards.straightdope.com/u/Dunderman)\ **Post date:** [November 30, 2003, 12:54pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/5 "2003-11-30T12:54:38Z")
> [@](#): > > \*Originally posted by Tusculan \* > \*\*The explanation makes sense to a layman like me. \*\* Then we’re two different kinds of laymen, 'cause it makes no sense to me. With that logic, why doesn’t the sun win the tug-of-war over the moon? ---
**Author:** ![Tusculan](https://avatars.discourse-cdn.com/v4/letter/t/ecd19e/32.png) [@Tusculan](https://boards.straightdope.com/u/Tusculan)\ **Post date:** [November 30, 2003, 12:59pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/6 "2003-11-30T12:59:26Z")
Hadn’t thought of that, good remark. Maybe it has to do with the longer distance. Remember that gravity drops with the square of the distance. This might mean those tidal effects that Wikipedia mentions don’t operate with the same strength? But then, I’m not really sure whether I know what those tidal effects are anyway. (#2 layman, the stupider kind 😉 ) ---
**Author:** ![Early\_Out](https://avatars.discourse-cdn.com/v4/letter/e/6f9a4e/32.png) [@Early\_Out](https://boards.straightdope.com/u/Early_Out)\ **Post date:** [November 30, 2003, 1:42pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/7 "2003-11-30T13:42:46Z")
I think it’s just a question of relative size. Our Moon, for example, could have a moon of its own, but it would have to be a very small one. Remember, when Apollo went to the Moon, the command module went into orbit around the Moon. It became, in effect, a moon of the Moon, and the Earth’s gravitational field certainly didn’t pull it out of Moon orbit. A much larger body, one that was, say 2/3 the mass of the Moon, wouldn’t be able to maintain an orbit around the Moon, because its attraction to the Earth would be too large, and it would simply become a second Earth moon. (#3 layman, taking a WAG) ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [November 30, 2003, 2:31pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/8 "2003-11-30T14:31:53Z")
> [@](#): > > Then we’re two different kinds of laymen, 'cause it makes no sense to me. With that logic, why doesn’t the sun win the tug-of-war over the moon? It does. If you graph the position of the Moon during the course of a year, you will see that at no point in its orbit does the Earth’s gravity pull t away from the Sun. The centre of mass for the Earth/Moon system travels on a smooth ellipse; the centre of the Moon’s mass is never travelling “backwards” in relation to the Sun. Here’s a site which explains this better, with pictures yet: [http://www.math.nus.edu.sg/aslaksen/teaching/convex.html](http://www.math.nus.edu.sg/aslaksen/teaching/convex.html) ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [November 30, 2003, 4:12pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/9 "2003-11-30T16:12:19Z")
Well, our Earth is a satellite of the Sun, and we have a satellite called the Moon, so a very massive main body seems to be required. ---
**Author:** ![myles](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@myles](https://boards.straightdope.com/u/myles)\ **Post date:** [November 30, 2003, 4:34pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/10 "2003-11-30T16:34:06Z")
To add something here, it was long thought that Pluto was actually a cast-off satellite of Nepture, or Uranus, Saturn, making Charon, therefore, retroactively a satellite of a satellite. However, I do not believe this theory of Pluto’s origins still holds. ---
**Author:** ![myles](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@myles](https://boards.straightdope.com/u/myles)\ **Post date:** [November 30, 2003, 4:35pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/11 "2003-11-30T16:35:54Z")
Why doesn’t Ganymede have a moon? If the size differential is considered a cause. Ganymede is larger than two of the solar system’s planets, but Pluto has a moon. Mercury is larger than Pluto, but Pluto has a moon, and Mercury doesn’t. ---
**Author:** ![myles](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@myles](https://boards.straightdope.com/u/myles)\ **Post date:** [November 30, 2003, 4:40pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/12 "2003-11-30T16:40:46Z")
Also, sorry to continually reply to myself, some asteroids have moons. Therefore, it is shown, that the presence or absence of a moon is almost certainly not a direct factor of size. To give my own theory, (warning, I am a history student, not an astronomy student), when asteroids were picked up by planetoids or planets for moons, the gravitational effects were probably enough to keep the asteroid as a separate object, but the orbit of the moon was probably too unstable, causing it to either be ejected into space, or to reabsorbed into either of the two larger bodies. ---
**Author:** ![Enderw24](https://avatars.discourse-cdn.com/v4/letter/e/ba9def/32.png) [@Enderw24](https://boards.straightdope.com/u/Enderw24)\ **Post date:** [November 30, 2003, 5:09pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/13 "2003-11-30T17:09:07Z")
> [@](#): > > \*Originally posted by myles \* > \*\*Why doesn’t Ganymede have a moon? > \*\* Because the universe isn’t a communist country where everyone is entitled to a moon. Sometimes it’s luck of the draw. Some planets are more equal than others. ---
**Author:** ![myles](https://avatars.discourse-cdn.com/v4/letter/m/779978/32.png) [@myles](https://boards.straightdope.com/u/myles)\ **Post date:** [November 30, 2003, 5:20pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/14 "2003-11-30T17:20:33Z")
I apologize Enderw24, that was a rhetorical question, meaning a question that need not be answered, I should have made myself more clear. The purpose of my question was not to ask a question, but to show that the relationship between the size of the astronomical body and the absence or presence of a moon was not a direct 1-1 correlation. I believe I proved my point. ---
**Author:** ![KP](https://avatars.discourse-cdn.com/v4/letter/k/a9adbd/32.png) [@KP](https://boards.straightdope.com/u/KP)\ **Post date:** [November 30, 2003, 6:07pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/15 "2003-11-30T18:07:54Z")
It’s as much a question of luck and timing as anything else. A moon can certainly have a moon – but for how long, and will we happen to be around to see it? Over a period of millions of years, Apollo’s orbit mught not have been stable (it was never designed for it) small periodic purturbations from the regular cyclical positions of the Sun and Earth might have moved it in, out or chaotically elliptical. There are some known solutions to the orbital mechanics, such as some of the LaGrange points of the Earth-Moon system, which would be stable of millions of years. Objects in or micro-orbiting those points would effectively orbit the moon. **There may indeed be _known_ moons of moons, though we don’t classify them as such.** One pair of Lagrange points lies in the orbit of the secondary body, 60 degrees ahead of and behind it. There are clusters of asteroids [Trojan asteroids] 60 degrees ahead of and behind Jupiter in its orbit, and though we describe them as ‘sharing Jupiter’s orbit’, over the course of a Jovian year they do complete a revolution around Jupiter. Yet we do not traditionally count them among the moons of Jupiter. We know of Trojans that co-orbit with moons (e.g. Saturn’s moon Tethys has two large enough that they were seen from Earth in 1980 - [Telesto and Calypso](http://www.solarviews.com/eng/telesto.htm) ), but whether we call them “moons of moons” is a matter of definition. And don’t even get me started on “quasi satellites” like “2002 AA29” ---
**Author:** ![Early\_Out](https://avatars.discourse-cdn.com/v4/letter/e/6f9a4e/32.png) [@Early\_Out](https://boards.straightdope.com/u/Early_Out)\ **Post date:** [November 30, 2003, 6:13pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/16 "2003-11-30T18:13:02Z")
**myles** , no one is saying that the absolute size of a body determines whether or not it has a moon. What some of us are saying is that the _relative_ size of two bodies is what matters. The Sun is huge. The Earth is a lot smaller. Our Moon is a lot smaller than that. The Earth couldn’t have a moon that was about the same size as the Earth. Because of the gravitational attraction of the Sun, an Earth-sized moon wouldn’t orbit the Earth for long; it would end up simply orbiting the Sun, as just another planet. ---
**Author:** ![KP](https://avatars.discourse-cdn.com/v4/letter/k/a9adbd/32.png) [@KP](https://boards.straightdope.com/u/KP)\ **Post date:** [November 30, 2003, 6:13pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/17 "2003-11-30T18:13:31Z")
Oops, I meant to clarify the similarity between 'moons of moons and Trojans by noting that any stable ‘moon of a moon’ (whatever its orbital radius) with an orbital period equal to that of the parent planet-moon system would automatically follow the orbit of its parent moon. I’m pretty sure that no such orbit would be stable, except at the LaGrange points. ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [November 30, 2003, 7:43pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/18 "2003-11-30T19:43:09Z")
_The Earth couldn’t have a moon that was about the same size as the Earth. Because of the gravitational attraction of the Sun, an Earth-sized moon wouldn’t orbit the Earth for long; it would end up simply orbiting the Sun, as just another planet._ If there was an Earth-sized object orbiting the Earth, they’d actually orbit their common center of mass, which would be a point between them, like two figure skaters holding hands and spinning around each other. You’d have to work out the physics for the specific individual situation to figure out it they’d both just become planets of the Sun or if they’d stay orbiting each other. You can, for instance, have a pair of equally-sized stars orbiting each other, and that pair orbiting a third star that is much larger and much farther away. ---
**Author:** ![Early\_Out](https://avatars.discourse-cdn.com/v4/letter/e/6f9a4e/32.png) [@Early\_Out](https://boards.straightdope.com/u/Early_Out)\ **Post date:** [November 30, 2003, 9:37pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/19 "2003-11-30T21:37:44Z")
> [@](#): > > \*Originally posted by Indefatigable \* > \*\*If there was an Earth-sized object orbiting the Earth, they’d actually orbit their common center of mass, which would be a point between them, like two figure skaters holding hands and spinning around each other. > > You’d have to work out the physics for the specific individual situation to figure out it they’d both just become planets of the Sun or if they’d stay orbiting each other. You can, for instance, have a pair of equally-sized stars orbiting each other, and that pair orbiting a third star that is much larger and much farther away. \*\* Yeah, I sort of realized that after I posted. I don’t pretend to remember enough of my college physics to work it out! Of course, if you have two roughly equally-sized bodies orbiting each other, could you call either one of them a “moon?” Physics and semantics collide, and it ain’t pretty. 😉 ---
**Author:** ![fortytwo](https://avatars.discourse-cdn.com/v4/letter/f/f9ae1b/32.png) [@fortytwo](https://boards.straightdope.com/u/fortytwo)\ **Post date:** [November 30, 2003, 10:04pm UTC](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049/20 "2003-11-30T22:04:15Z")
> [@](#): > > \*Originally posted by Early Out \* > \*\*Yeah, I sort of realized that after I posted. I don’t pretend to remember enough of my college physics to work it out! Of course, if you have two roughly equally-sized bodies orbiting each other, could you call either one of them a “moon?” Physics and semantics collide, and it ain’t pretty. 😉 \*\* [This](http://www.ast.cam.ac.uk/HST/press/pluto.html) might back up that statement. Charon is approx. half the size of Pluto. V [Next page](https://boards.straightdope.com/t/do-moons-ever-have-moons-of-their-own/216049.md?page=2)