# Is it common for asteriods to have moons **URL:** \ **Category:** Factual Questions\ **Created:** [February 13, 2015, 7:56am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550 "2015-02-13T07:56:32Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![PlainJain](https://avatars.discourse-cdn.com/v4/letter/p/5daacb/32.png) [@PlainJain](https://boards.straightdope.com/u/PlainJain)\ **Post date:** [February 13, 2015, 7:56am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/1 "2015-02-13T07:56:32Z")
I was reading about this asteroid that just flew by us. It has a moon! Is that common? I’m sure it is but even though I realize earth is hurtling through space as well, it’s hard to picture a moon keeping up with a speeding asteroid. > **[Once-in-a-generation near miss for Asteroid with its own moon](https://newatlas.com/asteroid-moon/35786/)** > > You may not have noticed, but the Earth had a close shave on Monday as an asteroid with its own moon made a cosmic near miss. As it passed within 3.1 times the distance from the Earth to the Moon, NASA's 230-foot-wide (70-meter) Deep Space Network... ---
**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:** [February 13, 2015, 10:48am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/2 "2015-02-13T10:48:22Z")
Offhand I don’t know if we have bothered to look closely enough but I would be shocked if there were not a significant number of asteroids which are gravitationally locked with a neighbor. ---
**Author:** ![Keeve](https://avatars.discourse-cdn.com/v4/letter/k/f07891/32.png) [@Keeve](https://boards.straightdope.com/u/Keeve)\ **Post date:** [February 13, 2015, 11:20am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/3 "2015-02-13T11:20:38Z")
An asteroid’s gravity is so weak that it must be incredibly rare (but admittedly not impossible, and it is a big universe out there) for an asteroid to capture a passing rock. What I’d imagine to be more common is if when the asteroid got created, another nearby rock broke off as well, and they ended up moving together. The smaller one would be perceived as orbiting the larger, though technically they’re actually both orbiting each other (just as the sun and all the planets technically orbit each other). ---
**Author:** ![D18](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@D18](https://boards.straightdope.com/u/D18)\ **Post date:** [February 13, 2015, 12:13pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/4 "2015-02-13T12:13:09Z")
Your article has the answer to your question: 16% of asteroids larger than 200m are binaries. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 13, 2015, 12:28pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/5 "2015-02-13T12:28:00Z")
> [@PlainJain](#): > > … I realize earth is hurtling through space as well, it’s hard to picture a moon keeping up with a speeding asteroid. … This part makes no sense. Or more precisely, whatever mental model you apparently have of asteroids and planets’ motions is not anything like how the world really works. Everything in the Solar system is hurtling along at great speeds relative to the Sun. Right now the Earth is going around its orbit of the Sun at about 67,000 mph. The Moon “keeps up” just fine since it’s doing the same thing; orbiting the Sun at about the same distance as the Earth. At the same time the Moon is also doing about 2200 mph relative to the Earth as it goes around its orbit around the Earth. Asteroids are also traveling at great speeds relative to the Sun. But most of them are going more or less the same speed, since most of them are in more or less the same orbit out past Mars. That two asteroids traveling at about the same speed in about the same direction at about the same distance from the sun could get together and become in effect traveling partners is plausible. Given the billions of hunks of rock out there of various sizes, most tiny, it’s a statistical certainty to have happened a few times. Asteroids that pass near (or collide with) Earth aren’t really going _that_ much faster than the Earth is anyhow\*. They’re just going in a different direction so the combined speed of our passing them (or equivalently them passing us) is great. - simplifying to avoid confusion. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [February 14, 2015, 7:33am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/6 "2015-02-14T07:33:52Z")
Asteroid satellites are thought to form by the same mechanism that our Moon did: collision. Any debris from the collision that doesn’t achieve escape velocity (which is very low on an asteroid) but has enough sideways motion to avoid coming back to hit the asteroid will be in orbit. Various rocks that are in similar orbits will eventually collect together to form a single satellite. All asteroids have had numerous collisions during their existence, but because of the low escape velocity requirement, only a small fraction of them actually form satellites. Hence the low percentage of asteroids with satellites. ---
**Author:** ![erocked](https://avatars.discourse-cdn.com/v4/letter/e/f08c70/32.png) [@erocked](https://boards.straightdope.com/u/erocked)\ **Post date:** [February 14, 2015, 5:45pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/7 "2015-02-14T17:45:21Z")
> [@dtilque](#): > > Asteroid satellites are thought to form by the same mechanism that our Moon did: collision. Any debris from the collision that doesn’t achieve escape velocity (which is very low on an asteroid) but has enough sideways motion to avoid coming back to hit the asteroid will be in orbit. Various rocks that are in similar orbits will eventually collect together to form a single satellite. Agreed. An asteroid capturing a nearby object in its orbit is not likely, especially when the asteroid is already captured by the much larger gravitational force of our sun/solar system. If an asteroid were to capture a moon it would have to be way out in deep space and both the asteroid and the “new moon” would’ve had to be traveling in the same direction far enough out from any major gravitational forces. Collision would be the most probable way this asteroid aquired a moon. Just like earth’s moon. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 14, 2015, 6:44pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/8 "2015-02-14T18:44:46Z")
One situation where capture of a moon can definitely _not_ happen is “way out in deep space”. Capture is always at least a three-body process, and “deep space” means you’re lacking that third body. Whether the bodies in question are in free space or orbiting some other body, however, is completely immaterial, except insofar as they’ll usually be much more abundant in the vicinity of some larger body. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 15, 2015, 2:22am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/9 "2015-02-15T02:22:47Z")
Another factor is the Hill Sphere (the sphere within which it is possible for a primary object to hold onto a satellite against the perturbations due to the object that the primary orbits) [Hill sphere - Wikipedia](http://en.wikipedia.org/wiki/Hill_sphere); a small enough asteroid close enough to the Sun will can have a Hill sphere that is smaller than the asteroid itself, making it impossible for that asteroid to hold onto a satellite in the long term. ---
**Author:** ![erocked](https://avatars.discourse-cdn.com/v4/letter/e/f08c70/32.png) [@erocked](https://boards.straightdope.com/u/erocked)\ **Post date:** [February 15, 2015, 2:45am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/10 "2015-02-15T02:45:42Z")
Deep space meaning not in our solar system. I don’t think an asteroid would have the gravitational pull to capture anything in its orbit within our solar system. **Maybe** enough to cause a slight redirection of the other body, but not pull it into orbit. Either way, that scenerio wouldn’t be likely. Collision would be much more likely. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [February 15, 2015, 2:55am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/11 "2015-02-15T02:55:03Z")
Nothing is moving. Everything in space is standing still, until you compare it to a reference point. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 15, 2015, 2:52pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/12 "2015-02-15T14:52:28Z")
> [@Andy\_L](#): > > Another factor is the Hill Sphere (the sphere within which it is possible for a primary object to hold onto a satellite against the perturbations due to the object that the primary orbits) [Hill sphere - Wikipedia](http://en.wikipedia.org/wiki/Hill_sphere); a small enough asteroid close enough to the Sun will can have a Hill sphere that is smaller than the asteroid itself, making it impossible for that asteroid to hold onto a satellite in the long term. Your link is defective. Here’s the corrected version [Hill sphere - Wikipedia](http://en.wikipedia.org/wiki/Hill_sphere). ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [February 15, 2015, 2:55pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/13 "2015-02-15T14:55:39Z")
> [@LSLGuy](#): > > Your link is defective. Here’s the corrected version [Hill sphere - Wikipedia](http://en.wikipedia.org/wiki/Hill_sphere). Thank you. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 15, 2015, 10:42pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/14 "2015-02-15T22:42:31Z")
Again, a single asteroid (or any other celestial body) can never “pull something into orbit”. In a two-body system, if an orbit starts off open, it’ll forever remain open. You need interaction with some third body to turn an open orbit into a closed one. ---
**Author:** ![Lucas\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lucas_jackson/32/303_2.png) [@Lucas\_Jackson](https://boards.straightdope.com/u/Lucas_Jackson)\ **Post date:** [February 17, 2015, 4:28am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/15 "2015-02-17T04:28:02Z")
> [@Chronos](#): > > Again, a single asteroid (or any other celestial body) can never “pull something into orbit”. In a two-body system, if an orbit starts off open, it’ll forever remain open. You need interaction with some third body to turn an open orbit into a closed one. I admit I don’t know much about space stuff. Could you explain what the role of the third body is? ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [February 17, 2015, 8:17am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/16 "2015-02-17T08:17:21Z")
> [@White SIFL](#): > > I admit I don’t know much about space stuff. Could you explain what the role of the third body is? Since kinetic energy is conserved, any change in velocity/momentum of one object has to be matched by a change in the other object. For two objects that are not currently in orbit, this matching of changes means they will _never_ be in orbit. They best they could do is collide. But if there’s a third object, then objects 2 and 3 can exchange some velocity/momentum through their own gravitational interactions. This could mean that object 2 now has the right properties to fit into orbit around object 1. Object 3, with a changed velocity/momentum, winds up either fitting into a new orbit of its own, or is thrown out of the system and it carries the extra energy off. I was hoping I might find an animation of this, since its easier to see than to describe, but my Google-fu must be broken. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [February 17, 2015, 3:34pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/17 "2015-02-17T15:34:15Z")
> [@White SIFL](#): > > I admit I don’t know much about space stuff. Could you explain what the role of the third body is? In more simplified terms than \*\*dracoi \*\*used in his excellent explanation: Gravity is eternal. In free space with just 2 bodies, they’re either already on course to collide, OR they’re going to pass one another once and head away from each other forever, never to return, OR they’ve already passed and are heading apart, OR they’re already orbiting each other. There’s no fourth possibility. And possibilities 2 and 3 are really the same thing, just at different times versus their once-in-eternity moment of closest approach. Now if they are orbiting one another it might be like some of the stuff that’s orbiting with the Sun at great distance, where one orbit takes tens of thousands or even millions of years. But that’s still an orbit. So that’s two bodies in otherwise empty space. If you add a third body to the situation, it provides a source of change to the relationship between the first two. Nature always balances her books, so the net change of the relationship between the first two will also be reflected in a change between each of them and the third. ---
**Author:** ![lieu](https://avatars.discourse-cdn.com/v4/letter/l/c2a13f/32.png) [@lieu](https://boards.straightdope.com/u/lieu)\ **Post date:** [February 17, 2015, 4:12pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/18 "2015-02-17T16:12:15Z")
Does the third body requirement give us any useful information about the origin or orbit of those asteroids that do have a moon / orbiting body? For instance does it tell us that they have to have come from an orbit around the sun, or could smaller objects have provided that third object requirement? Also, with respect to the Hill Sphere, is there any evidence our moon ever had a moon of it’s own, even if just for a brief period? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 17, 2015, 5:58pm UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/19 "2015-02-17T17:58:29Z")
Sure, if you count the Apollo command modules as moons. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [February 18, 2015, 3:15am UTC](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550/20 "2015-02-18T03:15:32Z")
> [@lieu](#): > > Does the third body requirement give us any useful information about the origin or orbit of those asteroids that do have a moon / orbiting body? For instance does it tell us that they have to have come from an orbit around the sun, or could smaller objects have provided that third object requirement? The third body requirement plus the fact that the asteroid belt is (despite having hundreds of thousands of objects) really, really sparsely populated, tells us that the thrid-body capture virtually never happens there. To have a third-body capture, you have to have the third body in a fairly small area with respect to the other two bodies. The chances of there being three bodies being in exactly the right places is so small that it may never have happened or has happened only a very small number of times. > [@](#): > > Also, with respect to the Hill Sphere, is there any evidence our moon ever had a moon of it’s own, even if just for a brief period? Any naturally-formed moons of the Moon are going to have very brief careers. Orbits around planetary satellites are generally unstable due to perturbations by the planet and Sun. For our Moon things are even less stable due to the lumpiness of the Moon’s gravitational field. > [@Chronos](#): > > Sure, if you count the Apollo command modules as moons. There’ve been unmanned artificial satellites of the Moon as well. [Clementine](http://en.wikipedia.org/wiki/Clementine_(spacecraft)) and the [Lunar Reconnaissance Orbiter](http://en.wikipedia.org/wiki/Lunar_Reconnaissance_Orbiter) are but two of many. [Next page](https://boards.straightdope.com/t/is-it-common-for-asteriods-to-have-moons/712550.md?page=2)