Origin of asteroid (469219) Kamo oalewa: The main asteroid belt or the Giordano Bruno crater on the Moon?

Abstract

Context. Asteroid (469219) Kamo'oalewa is the target of the Tianwen-2 sample-return mission by the China National Space Administration. Because of its orbit and its peculiar spectral properties, it was proposed that Kamo'oalewa originated from the Moon as impact ejecta, possibly from the Giordano Bruno crater. Aims. We aim to estimate the relative contribution of Kamo'oalewa-like objects originating from the general near-Earth asteroid (NEA) population, which originated in the main asteroid belt, and compare it with the relative contribution of Giordano Bruno ejecta. Methods. We first estimated the average fraction of quasi-satellite orbits in the Earth co-orbital space at any given time using numerical simulations. Using recently developed NEA population models, we extracted the expected number of Earth co-orbitals of the same size of Kamo'oalewa, and we obtain an estimate of the average number of Kamo'oalewa-like objects using the fraction computed before. Similarly, we obtain an estimate for the number of Kamo'oalewa-like objects that may originate as ejecta from the Giordano Bruno impact. We also performed simulations for the Catalina Sky Survey, Pan-STARRS, and Vera Rubin Observatory to estimate their efficiency in the detection of Kamo'oalewa-like objects. Results. Numerical simulations showed that 1.39% of the orbits in the Earth co-orbital space are quasi-satellite, on average. When combined with the expected number of Earth co-orbitals in the same size range as Kamo'oalewa from the NEA population models, we found that the main belt accounts for 1.23 ± 0.13 Kamo'oalewa-like objects on average. The expected number of Kamo'oalewa-like objects originated as Giordano Bruno ejecta is 0.042, which is more than an order of magnitude smaller. On the other hand, we found a discovery efficiency of Earth quasi-satellites between 95% and 70% for an absolute magnitude between 22 and 25 for the Pan-STARRS survey, and population models show that this is in agreement with the known population of Earth quasi-satellites. The Vera Rubin Observatory should reach a discovery efficiency of 92% down to absolute magnitude of 25. Conclusions. Quantitative estimates show that population models of NEAs based on the migration of objects from the main belt are capable of accounting for Kamo'oalewa-like objects. This relative contribution supports the hypothesis that (469219) Kamo'oalewa originated from the main belt, which will be further investigated by future observations and in situ exploration of the Tianwen-2 spacecraft.