A minimoon will accompany Earth for decades to come

In July 2022, a group of amateur astronomers discovered a new object from a remote observatory in the Hakos Mountains of Namibia. At first, it was doubtful whether it was some kind of space junk or whether it was natural: perhaps an asteroid or a piece of the Moon.

“Its Earth-like orbit and its orbital evolution suggest that it could be of artificial origin, launched from our planet decades ago, or else a lunar fragment.” public the International Astronomical Union in its Minor Planet Electronic Circulars (MPEC), where it already officially appears under the name 2022 NX1.

The OSIRIS instrument on the Gran Telescopio Canarias confirmed that the asteroid or minimoon 2022 NX1 measures between 5 and 15 m

The following month, on August 6, astronomers from the Complutense University of Madrid (UCM) and the Instituto de Astrofísica de Canarias (IAC) observed the object with OSIRISa spectrograph and imager installed on the Gran Telescopio Canarias, on the island of La Palma.

The results, now published in the journal astronomy and astrophysics, confirm that it is an asteroid with surface characteristics incompatible with material of lunar origin. The albedo (reflected radiation) deduced from its spectrum indicates that its size is between 5 and 15 meters and, as for its composition, it can be either rocky or with a higher metal content.

Visible spectrum of 2022 NX1 (in gray) compared to two other asteroids. / R. de la Fuente Marcos and others.

“2022 NX1 follows a coorbital movement with our planet, that is, together they complete an orbit around the Sun in 365 days, traveling at the same speed”, explains the first author, Raúl de la Fuente Marcos, from UCM, to SINC , “ but, in general, it does not revolve around our planet like the Moon, but moves in a horseshoe trajectory.

This minimoon follows a strange horseshoe path through the same orbit as Earth, which temporarily captured it in 1981 and 2022, and will do so again in 2051.

There are dozens of near-Earth objects (NEOs) that come and go drawing this strange horseshoe in the same orbit as our planet. As they approach, they often turn around, but from time to time, some of them are temporarily captured by its gravity and turn into mini-moons.

Until now, three were known: 2006 RH120 and 2020 CD3, which have already completed several orbits around our planet, and 1991 VG, which has done so only partially: it does not make a complete turn. In the case of the fourth minimoon, 2022 NX1, the same thing happens, and it travels only one arc of its orbit around the Earth.

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negative gravitational energy

“For a few weeks the gravitational energy in relation to the Earth assumes a negative value when approaching it, with a very low speed in astronomical terms (of the order of 1 km/s or 3600 km/h) and temporarily becomes its satellite” , De la Fuente Marcos points out, “but it doesn’t make a complete turn like the Moon, but just an arc of the orbit”.

This minimoon has a certain probability of colliding with Earth from 2075 onwards, but its size is smaller than that of the asteroid that gave rise to the Chelyabinsk superbolide, so the consequences of a possible impact would be minimal.

Raul de la Fuente Marcos (UCM)

The authors’ calculations indicate that 2022 NX1 experienced a temporary capture of this type by our planet in 1981, had another in 2022 – when astronomers discovered it and took the opportunity to study it – and will last a third longer in the year 2051 .

“As a result of the close encounter at the end of that year or at the beginning of 2052, its subsequent trajectory could be altered in such a way that it would have a certain probability of colliding with Earth from December 2075 onwards”, warns De la Fuente Marcos, although reassures us: “Its size is smaller than that of the asteroid that gave rise to the Chelyabinsk superbolide over that Russian city in 2013, so the consequences of a possible impact would be minimal”.

Geocentric trajectories of 2022 NX1 during the periods it is captured as a minimoon from Earth: 1981 (orange), 2022 (black), and 2051 (blue). In red, the part of the trajectory with negative geocentric energy. The Earth is the blue dot and the axes are in astronomical units (1 AU = 150 million km). / De la Fuente et al./ Astronomy & Astrophysics


R. de la Fuente Marcos and others. “Horseshoe mini moons: A physical characterization of 2022 NX1 with OSIRIS on the 10.4 m Gran Telescopio Canarias”. Astronomy and Astrophysics, 2023

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