A freshly discovered train-sized rock that tags along with Earth as a constant companion orbiting the Sun is most likely a fragment of the Moon resulting from an ancient lunar impact.

469219 Kamo'oalewa – discovered by observers in Hawaii in 2016 – is about 41 metres in diameter and orbits the Sun in a trajectory not dissimilar from our own blue planet.

Although the nearest of Earth's quasi-satellites (don't worry, it's minimum orbital intersection distance* with Earth is five million km), very little is known about the rock's origins owing to its tiny size and habit of dwelling in the darkness of space.

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Yet scientists working on a University of Arizona-led project have been able to shed some light on the question. Their analysis of rays reflected from the surface of the space rock observed using the Large Binocular Telescope in the Pinaleno Mountains of southeastern Arizona, and the Lowell Discovery Telescope in the Coconino National Forest near Happy Jack, Arizona, strongly suggest it may have lunar origins.

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Graduate student Benjamin Sharkey and the team showed Kamo'oalewa has a red reflectance spectrum, very similar to that of minerals on the Moon's surface.

"This spectrum is indicative of a silicate-based composition, but with reddening beyond what is typically seen amongst asteroids in the inner solar system. We compare the spectrum to those of several material analogs and conclude that the best match is with lunar-like silicates. This interpretation implies extensive space weathering and raises the prospect that Kamo'oalewa could comprise lunar material," states the [4]paper published in Nature Communications Earth & Environment .

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Although the paper concedes that the conclusion is speculative, it seems more likely than other possibilities.

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Kamo'oalewa might have been captured in its Earth-like orbit from the general population of Near-Earth Objects (NEOs), but its "eccentricity and inclination are, however, rather atypical of such captured co-orbital states found in numerical simulations," the paper says.

Alternatively, it could originate from an as-yet-undiscovered population of Earth's Trojan asteroids, another group of solar system objects. It is an idea that could be tested with more observations of that system.

That leaves the possibility that Kamo'oalewa originates in the Earth-Moon system, perhaps coming from debris spewed out of an impact on the lunar surface. It could even have come from a tidal or rotational break up of another NEO.

An origin within the Earth-Moon system is also supported by the object's low relative velocity as it approaches Earth, being much lower than other NEOs.

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Kamo'oalewa's name originates from the Hawaiian words for fragment and oscillate. It sort of seems to orbit Earth yet doesn't. It orbits the Sun, but oscillates around the earth as its constant companion owing to a slightly different orbital period (366 days) and the inclination of its orbit compared with Earth's, as [12]this handy animation shows . ®

* While Kamo'oalewa's MOID is 5 million km, in real terms, that means it gets no closer than 14.5 million km from Earth.

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[4] https://www.nature.com/articles/s43247-021-00303-7

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[6] https://www.theregister.com/2021/11/10/nasa_delayed_artemis_crewed_moon/

[7] https://www.theregister.com/2021/10/26/blue_origin_orbital_reef/

[8] https://www.theregister.com/2021/10/01/uk_national_space_strategy/

[9] https://www.theregister.com/2021/10/01/blue_origin_sexism/

[10] https://www.theregister.com/2021/09/14/martian_blood_brick_plan/

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[12] https://en.wikipedia.org/wiki/File:Asteroid2016HO3-SunEarthOrbit.webm

[13] https://whitepapers.theregister.com/