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The conference call with Bill Bottke (MP3 plus other documents) about "minimoons" presents an alternative to Asteroid Redirect first got me interested in orbital mechanics. The idea is that there are enough small asteroids that approach the earth-moon barycenter (EMB) that it seems likely there are frequent temporary captures of small asteroids into chaotic 3-body orbits in EMB system, that last for a few months to a few years.

Here is an example of one theoretical orbit from here or here:

enter image description here

One can read more in Granvik, Vaubaillon and Jedicke 2012 (The population of natural Earth satellites Icarus, 218 (1) March 2012, pp262-277) available without paywall in ArXiv.

Only one "minimoon" 2006 RH120 was known by 2012, but it seems there should be new ones regularly, according to Granvik et al. 2012.

Have there been others that were detected and reported?

enter image description here

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    $\begingroup$ One has just been discovered, but it looks like it's been here for at least 50 years, so far longer than the type of orbit you're referrring to. jpl.nasa.gov/news/news.php?feature=6537 $\endgroup$
    – Hobbes
    Commented Jun 17, 2016 at 7:33
  • $\begingroup$ @Hobbes that is great to know about! They call 2016 HO3 a "quasi-moon" because it's in orbit around the sun in a 1:1 resonance with earth. Watch here. They go around together, sort-of like a Trojan asteroid would. I'll have to read more - this is really interesting, at about 2.2 million miles a moon of earth would have a period of about 1 year, although that's if there were no sun. This one stays around 9 million miles away, so it's really in orbit around the sun, but in resonance with the earth. Curiouser and curiouser! $\endgroup$
    – uhoh
    Commented Jun 17, 2016 at 12:37
  • $\begingroup$ @Hobbes here's a crazy video of the trajectory, it just does a big figure-8 in a patch of the Celestial Sphere, bisecting Virgo and Leo The Trajectory of the Strange Object 2016 HO3. $\endgroup$
    – uhoh
    Commented Jun 17, 2016 at 13:04
  • $\begingroup$ Remember, that's just the view from Earth. Just like the planets and their weird retrograde loops. $\endgroup$
    – Hobbes
    Commented Jun 17, 2016 at 13:15
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    $\begingroup$ Not really an answer, but I would expect mini-moons to be commonplace around Earth. Because we have our big Moon, a body coming from interplanetary space effectively undergoes a three-body interaction when it encounters Earth. Energy can be transferred from an incoming body to the Earth-Moon pair, allowing the small body to be captured, and perhaps vice versa allowing the small body to later escape. $\endgroup$
    – Oscar Lanzi
    Commented Feb 27, 2020 at 22:11

2 Answers 2

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Possibly the earliest documented mini-moon was the one associated with the 1913 Great Meteor Procession.

After 2006 RH120, that you mentioned, another was identified on October 3, 2015 and designated WT1190F. It impacted Earth on November 13, 2015. It was probably space debris, and not natural.

There's only one other potential candidate that I know of so far: S509356. It was identified on April 8, 2016. It may yet be an artificial satellite, but there are none known that match its orbital period, and it has a color associated with S-type asteroids.

Update 5/16/2018:

S509356 turns out to be 2015-019C, 41929, upper stage from a Long March carrying a BeiDou satellite.

Update 2/27/2020:

Now we have an official confirmed mini-moon (temporary natural satellite): 2020 CD3. It was discovered February 15, 2020, and announced February 25. It is estimated to have been orbiting Earth since around 2017-2018, and it is expected to depart April 2020.

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  • $\begingroup$ The 1913 event makes for interesting reading, Chant 1913, O'keefe 1959, and O'keefe 1991 and Lapaz 1956 can all be read online. But it's an inference not an observation, so not really a documented minimoon. As you mention WT1990F is probably artificial (also see my question). S509356 is probably artificial according to this and there is an ephemeris for those interested. So it looks like the answer is really "probably not". $\endgroup$
    – uhoh
    Commented Jun 17, 2016 at 0:14
  • $\begingroup$ I've just asked Why did they think that S509356 was in orbit around the Earth? Where is the disconnect? $\endgroup$
    – uhoh
    Commented May 16, 2018 at 6:47
  • $\begingroup$ I've unaccepted due to recent news $\endgroup$
    – uhoh
    Commented Feb 26, 2020 at 22:35
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    $\begingroup$ @uhoh Updated. This should be the last update needed as it now definitively answers the question. It does not need to become a list of all future mini-moons. $\endgroup$
    – called2voyage ♦
    Commented Feb 27, 2020 at 13:28
  • $\begingroup$ I agree 100%, the answer to "Have there been any documented..." is now boolean, but the SE answer is a (boolean, url) tuple ;-) I've been on the edge of my seat ever since I heard the conference call with Bill Bottke mentioned in the beginning of the question, so for me this is closure. $\endgroup$
    – uhoh
    Commented Feb 27, 2020 at 13:31
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Yet another mini-moon, 2024 PT5, has come about for couple months (reported here).

The broader answer is that Earth often has these temprorary mini-moons, and likely we are seeing them more often simply because we have better instruments to detect them.

Catch and Release

Among neighborhood-clearing planets in our Solar System, Earth has a unique ability to create a three-body interaction with small objects (asteroids) by using its Moon. In theory, any planet with moons will give a multibody interaction, but Earth's Moon makes a relatively large constribution because the relatively large moon/planet mass ratio and also the Moon carrying most of the angular momentum of the planet-moon system (giant planets gave most if their system's angular momentum in their own rotation).

Thus we are likely to see a situation where an incoming asteriod interacts initially with the Earth+Moon to transfer some of its kinetic energy into the Earth-Moon binary, moving the Moon slightly away from Earth and allowing the asteroid to be captured. But eventually the same three-body process will transfer energy back to the captured asteroid, allowing it to escape. So break out your telescopes (apparently, you'll need professional equipment) and enjoy the latest third-body visitor while you have the chance!

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  • $\begingroup$ Indeed 2024 PT5's geocentric energy will drop below zero temporarily. Just curious, how close will it approach and how bright will it be then? I'm curious which telescope I need to break out. $\endgroup$
    – uhoh
    Commented Sep 28 at 23:57
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    $\begingroup$ @uhoh With only 32 feet (9.75 m) of length to work with, it looks like you'll need professional equipment. See fastcompany.com/91198752/…. $\endgroup$
    – Oscar Lanzi
    Commented Sep 29 at 0:01

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