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

TL;DR

This study evaluates the origin of asteroid (469219) Kamo'oalewa by weighing two sources: main-belt-derived NEAs and Giordano Bruno lunar ejecta. Using a Monte Carlo framework, it estimates the average fraction of Earth co-orbitals in quasi-satellite states ($F$) and applies NEA population models to predict how many Kamo'oalewa-like objects would arise from each channel, concluding that main-belt delivery yields about $N_{\text{K/NEA}}\approx1.23\pm0.13$ while Giordano Bruno ejecta provides only roughly $0.042$ objects on average. Survey simulations for CSS, Pan-STARRS, and the Vera Rubin Observatory indicate high discovery completeness for Kamo'oalewa-like objects ($\sim60-92\%$ depending on magnitude and survey) and show that current and upcoming surveys should be able to detect most such objects if they exist in the predicted numbers. The results favor a main-belt origin for Kamo'oalewa, with future in-situ Tianwen-2 measurements and JWST spectroscopy expected to provide decisive compositional tests against lunar ejecta scenarios.

Abstract

Asteroid Kamo`oalewa is the target of the Tianwen-2 sample-return mission by CNSA. Because of its orbit and its spectral properties, it was proposed that Kamo`oalewa originated from the Moon as impact ejecta, possibly from the Giordano Bruno crater. We aim at estimating 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. We first estimate the average fraction of quasi-satellite orbits at any given time. By using recently developed NEA population models, we extract the expected number of Earth co-orbitals of the same size of Kamo`oalewa, and then get 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 survey simulations to estimate their efficiency in the detection of Kamo`oalewa-like objects. We found that the main belt accounts for 1.23 \pm 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 order of magnitude smaller. We found a discovery efficiency of Earth quasi-satellites between 95% and 70% for absolute magnitude between 22 and 25 for the Pan-STARRS survey, and population models show that this is in agreement with the known population. The Vera Rubin Observatory should reach an efficiency of 92% down to absolute magnitude 25. These estimates show that population models of NEAs are capable to account for Kamo`oalewa-like objects, thus supporting the hypothesis that that Kamo'oalewa originated from the main belt. This will be further investigated by the in-situ exploration of the Tianwen-2 mission.

Figures (6)

• Figure 1: Time evolution of the resonant angle $\sigma$ (blue dots) of an example orbit which switches between horseshoe and quasi-satellite states. The green shaded areas are the quasi-satellite states identified with the running-window procedure.
• Figure 2: The running percentage of the average fraction of quasi-satellites, computed on an increasingly large sample of orbits.
• Figure 3: Cumulative size frequency distribution of NEAs between 30 m and 100 m in the Earth co-orbital region, extracted from the NEOMOD3 NEA population model. Error bars are computed assuming a Poisson statistics. The right y axis scales to the estimated number of Kamo'oalewa-like objects. Grey horizontal dashed lines highlight the cases with diameter of 30 and 40 m.
• Figure 4: Cumulative fraction of detected Kamo'oalewa-like quasi-satellites as a function of the year. The left panel shows the results obtained for the CSS survey (blue curve) and for the PanSTARRS survey (orange curve). The right panel shows the expected efficiency of the Vera Rubin Telescope during its 10 years operations period.
• Figure 5: Detected cumulative fraction of Kamo'oalewa-like objects obtained at the end of the simulations, as a function of the absolute magnitude $H$.