<p>I'm going to discuss only our moon, to keep this on topic.</p>

<p>This sort of orbit could be stable on the sort of timescales that you want.</p>

<p>Objects in Low Lunar Orbit will likely suffer from the irregular gravity of the moon. There are "<a href="https://en.wikipedia.org/wiki/Mass_concentration_(astronomy)" rel="noreferrer">mass concentrations</a>" as a result of ancient meteorite impacts. These "mascons" distort the orbits of objects in low orbit, and cause them to collide with the moon in a relatively short time. However at 24000km the gravity field is smoothed by your distance from the surface. You should not have problems with mascons at this distance, at least not over 100,000 years or so.</p>

<p>The <a href="http://curious.astro.cornell.edu/about-us/44-our-solar-system/the-moon/general-questions/104-can-moons-have-moons-intermediate" rel="noreferrer">Hill sphere of the moon extends to about 60000km</a>, so your orbit is well within the Hill radius, and so can be stable. </p>

<p>The moon spins slowly, so tidal effects would tend to cause your asteroid to lose altitude, and eventually hit the moon. The timescale of this would be longer than a few 100,000 years </p>

<p>Getting your asteroid in orbit is very tricky. You would need to hypothesise double asteroids transferring momentum between each other and the moon, and one being lost while the other ends up in lunar orbit. </p>

CC BY-SA 3.0
2017-05-07T15:26:48.170Z
2017-05-07T15:26:48.170Z
James K
<p>(Edited at James K's recommendation to stay on-topic for this SE.)</p>

<p>From <a href="https://astronomy.stackexchange.com/questions/856/do-moons-have-moons">my</a> <a href="https://astronomy.stackexchange.com/questions/18179/how-massive-must-a-rocky-body-be-to-liquify-adequately-to-eliminate-internal-mas">reading</a> on this SE and <a href="http://curious.astro.cornell.edu/about-us/44-our-solar-system/the-moon/general-questions/104-can-moons-have-moons-intermediate" rel="nofollow noreferrer">from</a> <a href="https://www.universetoday.com/109666/can-moons-have-moons/" rel="nofollow noreferrer">Googling</a> I've gotten conflicting reports on the viability of subsatellites this close to the Sun, but a naive application of the Hill Sphere equation suggests that if it got captured at all, a large asteroid could theoretically orbit the moon in a stable orbit.</p>

<p>What I don't know is this:</p>

<ol>
<li>For how long? Longer than a couple hundred thousand years, such that humans on Earth wouldn't have known anything else? Longer than a million years? Probably not longer than a billion, due to the tidal effects decaying the asteroid's orbit, but is that a bad assumption?</li>
<li>How eccentric would the asteroid's orbit be? Would it be unlikely for it to orbit with negligible eccentricity, whether on the ecliptic or not? If so, what kind of eccentricity/orbital inclination would be reasonable?</li>
</ol>

CC BY-SA 3.0
2017-05-07T15:27:32.450Z
I suggest that you edit this question to make it about our solar system, as hypothetical questions are off topic here.
James K
CC BY-SA 3.0
2017-05-07T18:41:58.263Z
Thanks for the heads-up, and for answering despite the question being off-topic. Am I right in assuming that in future questions that are about hypothetical systems should go to Worldbuilding?
Lielac
CC BY-SA 3.0
2017-05-07T21:35:01.703Z
@Lielac Worldbuilding is better for hypothetical and "how would this work" questions, provided they're not too broad or opinion based.  Your question was good for worldbuilding.
userLTK
<natural-satellites>
Could the Moon naturally capture a large asteroid and keep it as a subsatellite? For how long?
Lielac
# Could the Moon naturally capture a large asteroid and keep it as a subsatellite? For how long?

(Asked by: username_0 on 2017-05-06)

(Edited at James K's recommendation to stay on-topic for this SE.)

From [my](https://astronomy.stackexchange.com/questions/856/do-moons-have-moons) [reading](https://astronomy.stackexchange.com/questions/18179/how-massive-must-a-rocky-body-be-to-liquify-adequately-to-eliminate-internal-mas) on this SE and [from](http://curious.astro.cornell.edu/about-us/44-our-solar-system/the-moon/general-questions/104-can-moons-have-moons-intermediate) [Googling](https://www.universetoday.com/109666/can-moons-have-moons/) I've gotten conflicting reports on the viability of subsatellites this close to the Sun, but a naive application of the Hill Sphere equation suggests that if it got captured at all, a large asteroid could theoretically orbit the moon in a stable orbit.

What I don't know is this:

1. For how long? Longer than a couple hundred thousand years, such that humans on Earth wouldn't have known anything else? Longer than a million years? Probably not longer than a billion, due to the tidal effects decaying the asteroid's orbit, but is that a bad assumption?
2. How eccentric would the asteroid's orbit be? Would it be unlikely for it to orbit with negligible eccentricity, whether on the ecliptic or not? If so, what kind of eccentricity/orbital inclination would be reasonable?

**Tags:** natural-satellites
> **Comments:**
> **username_1**: I suggest that you edit this question to make it about our solar system, as hypothetical questions are off topic here.  
> **username_0**: Thanks for the heads-up, and for answering despite the question being off-topic. Am I right in assuming that in future questions that are about hypothetical systems should go to Worldbuilding?  
> **username_2**: @Lielac Worldbuilding is better for hypothetical and "how would this work" questions, provided they're not too broad or opinion based.  Your question was good for worldbuilding.  

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## 1 answers

### Answer by username_1 on 2017-05-07. Score: 8 | Accepted answer

I'm going to discuss only our moon, to keep this on topic.

This sort of orbit could be stable on the sort of timescales that you want.

Objects in Low Lunar Orbit will likely suffer from the irregular gravity of the moon. There are "[mass concentrations](https://en.wikipedia.org/wiki/Mass_concentration_(astronomy))" as a result of ancient meteorite impacts. These "mascons" distort the orbits of objects in low orbit, and cause them to collide with the moon in a relatively short time. However at 24000km the gravity field is smoothed by your distance from the surface. You should not have problems with mascons at this distance, at least not over 100,000 years or so.

The [Hill sphere of the moon extends to about 60000km](http://curious.astro.cornell.edu/about-us/44-our-solar-system/the-moon/general-questions/104-can-moons-have-moons-intermediate), so your orbit is well within the Hill radius, and so can be stable.

The moon spins slowly, so tidal effects would tend to cause your asteroid to lose altitude, and eventually hit the moon. The timescale of this would be longer than a few 100,000 years

Getting your asteroid in orbit is very tricky. You would need to hypothesise double asteroids transferring momentum between each other and the moon, and one being lost while the other ends up in lunar orbit.


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