Minimoons: Earth's temporary second moons

Earth has one Moon, and now and then a second one for a while. A minimoon is a small asteroid that Earth's gravity holds for weeks, months or a few years before it slips away and goes back to circling the Sun.^[1] Five have been found so far, and the latest, 2026 QQ8, was confirmed to be bound to Earth in October 2026.^[2]

What counts as a minimoon

The definition astronomers use has two parts, and no minimum stay. An object is temporarily captured while its energy relative to Earth is negative, so that it is gravitationally bound, and while it stays within about three Hill radii of Earth, roughly three times the distance out to which Earth's pull dominates the Sun's.^[1] In the three-body dance of Sun, Earth and asteroid, such a capture is always temporary, because the object can leave again through the gateways near the Lagrange points.^[1]

Within that definition there are two kinds. A temporarily captured orbiter stays long enough to complete at least one full revolution around Earth.^[1] A temporarily captured flyby is bound for a while but gone before it finishes a single loop.^[1] Flybys are about ten times as common as orbiters, and some stay bound for more than 200 days.^[1] The headlines usually reserve "second moon" for the orbiters, and the pages here say which kind each object is.

How small, how many, how long

Simulations of asteroids passing Earth predict that one or two objects of about a meter or more are in temporary orbit at any moment, with ever more numerous smaller ones.^[1] The average orbiter stays about nine months and makes almost three revolutions.^[1] 2020 CD3 stayed about two and a half years after a close pass by the Moon in September 2017, longer than average, which the models also allow for objects that pass the Moon closely.^[3][4]

None of them can be seen with the eye or a backyard telescope. 2020 CD3 was about 1.2 m across, among the smallest near-Earth objects ever found, and the telescope that found it could have detected it for only about 0.03% of the time it was captured.^[4] Objects this small are also harmless: an impact by a meter-wide minimoon would do no damage.^[3] About 0.1% of the meteoroids that hit Earth were minimoons first, and they arrive slower than most, at about 11.2 km/s against a typical 20 km/s, so their fireballs are fainter.^[1]

How they are found, and how they are checked

Minimoons turn up the way other asteroids do, as moving points of light in a survey telescope's images; the chain from first sighting to a named orbit is described in From discovery to defense. The 2020 discovery came from the NASA-funded Catalina Sky Survey, and 2026 QQ8 was found on 19 August 2026 at the Piszkéstető station in Hungary.^[5][2] When the orbit shows the object is bound, the Minor Planet Center says so in the circular that announces it: the 2026 QQ8 circular states that orbit integrations show it to be temporarily bound to Earth.^[2]

The next question is always whether the object is natural at all. Earth-like orbits are where old rocket stages end up, so each candidate's orbit is run backward and its motion checked for the push of sunlight, which moves a hollow booster far more than a rock.^[6] The lesson came in 2020: an object found in September that year by Pan-STARRS and held by Earth's gravity until March 2021 turned out to be the Centaur upper stage that launched NASA's Surveyor 2 toward the Moon in 1966, confirmed when its spectrum matched another Centaur booster still in orbit.^[7] The circular for 2026 QQ8 states that no link to a known artificial object has been found.^[2]

Where the natural ones come from is an open question. 2020 CD3 looks like a stony or basaltic asteroid, so capture from the near-Earth asteroid population is the likely main source.^[4] But 2024 PT5 reflects sunlight like Moon rock and not like any known asteroid type, and NASA scientists think it is probably a piece of the Moon knocked off by an impact a few thousand years ago.^[6]

What happens to them

A minimoon leaves the way it arrived, back into an orbit around the Sun that is almost the same as Earth's; 2020 CD3 crossed out of Earth's Hill sphere on 7 March 2020.^[3] Because the orbits stay Earth-like, the same object can come back. 2022 NX1 was briefly bound in January 1981 and again in June 2022, and is predicted to return in December 2051 and this time complete an orbit.^[8] 2024 PT5 is expected back in Earth's neighbourhood in 2055.^[9]

Quasi-moons: Earth's other companions

A quasi-satellite, or quasi-moon, is a different thing. It orbits the Sun, not Earth, in almost exactly a year, so from Earth it seems to circle us slowly at a great distance while never being bound to us.^[10] NASA's description of the best-known one: for half of each year it runs ahead of Earth, closer to the Sun, and for the other half it falls behind, farther out, in a leapfrog pattern that lasts for centuries.^[10]

Kamoʻoalewa (469219, also 2016 HO3) was found on 27 April 2016 by the Pan-STARRS 1 telescope on Haleakalā, Hawaii.^[11] It is between about 40 and 100 m across, never comes closer than about 38 times the Moon's distance and never strays farther than about 100 times it, and JPL's calculations show it has been a stable companion for almost a century and will stay one for centuries to come.^[10] Its name comes from the Hawaiian creation chant, the Kumulipo, and alludes to a celestial object that oscillates, as its path does when seen from Earth.^[11] Its reflected light is redder than any ordinary asteroid's and matches lunar soil best, so it may be a fragment of the Moon.^[12] It is the first quasi-moon a spacecraft has visited: China's Tianwen-2 sample-return mission, launched on 28 May 2025, is listed as arriving there on 4 July 2026.^[13]

2025 PN7 was discovered on 2 August 2025 by Pan-STARRS 1 at Haleakalā, and was identified as a quasi-satellite in a paper the following month.^[14] It is fainter and smaller than Kamoʻoalewa, with an absolute magnitude of 26.4 against 24.3, and it is the shortest-lived of Earth's quasi-moons: about 128 years in that state in all, against 381 for Kamoʻoalewa.^[14][11] JPL's orbit for it uses observations back to December 2013, so it had been photographed, unrecognised, for more than a decade before it was found.^[15] It joins a small club: with it, seven quasi-satellites of Earth are known.^[14]

The ones we know

In order of their capture:

• 2006 RH120: the first one found, captured in mid-2006, made a series of distant loops around Earth and left in mid-2007.^[5]
• 2020 CD3: bound from September 2017 to March 2020, discovered only in its last weeks; the best-studied minimoon.^[3]
• 2022 NX1: a flyby in June 2022 and before that in January 1981; due back as an orbiter in December 2051.^[8]
• 2024 PT5: a two-month flyby from 29 September to 25 November 2024, which NASA calls not quite a mini-moon; probably a piece of the Moon.^[9][16][6]
• 2026 QQ8: bound to Earth now, announced on 29 October 2026.^[2]

A sixth candidate is 1991 VG, which was bound for about 28 days in February 1992 without finishing a loop, the first documented capture of a satellite by Earth on a backward, or retrograde, path.^[17]

Questions people ask

Does Earth have a second moon right now?

Yes, in the loose sense. 2026 QQ8 is temporarily bound to Earth as of its announcement on 29 October 2026, and it will leave again.^[2] With an absolute magnitude of 31.3 it is about as faint as 2020 CD3, which NASA put at no more than 1 to 2 m across, so it is far too small to see.^[2][5] Whether it completes a full orbit, which is what makes an object a temporarily captured orbiter rather than a flyby, has not yet been published.^[1]

Can I see a minimoon?

No. 2020 CD3 was about 1.2 m across, and even a survey telescope could detect it for only a tiny fraction of its time in orbit.^[4] They are found by professional surveys and followed with large telescopes.

Could a minimoon hit Earth?

A meter-wide object that hits the atmosphere burns up as a fireball.^[3] 2020 CD3 has about a 1% chance of an impact between 2061 and 2120, and its size makes any such impact harmless.^[3] How impact odds are worked out for larger asteroids is covered in Impact risk.

Is Kamoʻoalewa a minimoon?

No. It orbits the Sun, not Earth, and is never gravitationally bound to Earth; it only appears to circle us because its orbit is so similar to ours.^[10] That makes it a quasi-satellite, described above.

Why was 2024 PT5 called a mini-moon if NASA says it was not one?

The two statements use different definitions. NASA's planetary defense office says 2024 PT5 stayed roughly nine times farther away than the Moon and was never captured by Earth's gravity.^[16] The astronomers who predicted its visit say it met the dynamical definition, bound to Earth from 29 September to 25 November 2024, without completing a revolution, which makes it a temporarily captured flyby.^[9] Both are stated on its page.