2023A&A...670L..10D

Context. The near-Earth orbital space is shared by natural objects and space debris that can be temporarily captured in geocentric orbits. Short-term natural satellites are often called mini-moons. Reflectance spectroscopy can determine the true nature of transient satellites because the spectral signatures of spacecraft materials and near-Earth asteroids (NEAs) are different. The recently discovered object 2022 NX₁ follows an Earth-like orbit that turns it into a recurrent but ephemeral Earth companion. It has been suggested that 2022 NX₁ could have an artificial origin or be lunar ejecta.
Aims. Here, we use reflectance spectroscopy and N-body simulations to determine the nature and actual origin of 2022 NX₁.
Methods. We carried out an observational study of 2022 NX₁, using the OSIRIS camera spectrograph at the 10.4 m Gran Telescopio Canarias, to derive its spectral class. N-body simulations were also performed to investigate how it reached NEA space.
Results. The reflectance spectrum of 2022 NX₁ is neither compatible with an artificial origin nor lunar ejecta; it is also different from the V type of the only other mini-moon with available spectroscopy, 2020 CD₃. The visible spectrum of 2022 NX₁ is consistent with that of a K-type asteroid, although it could also be classified as an Xk type. Considering typical values of the similar albedo of both K-type and Xk-type asteroids and its absolute magnitude, 2022 NX₁ may have a size range of 5 to 15 m. We confirm that 2022 NX₁ inhabits the rim of Earth's co-orbital space, the 1:1 mean-motion resonance, and experiences recurrent co-orbital engagements of the horseshoe-type and mini-moon events.
Conclusions. The discovery of 2022 NX₁ confirms that mini-moons can be larger than a few meters and also that they belong to a heterogeneous population in terms of surface composition.