The Mystery of Asteroid Kamo'oalewa: Why It's Not from the Moon express
Kamo'oalewa is Earth's partner in a peculiar gravitational dance: it orbits the Sun almost in unison with us, edging closer and then pulling away. Its color — the palette of reflected light — matches the lunar surface. Hence the initial thought: it's a chunk of the Moon from the Giordano Bruno crater.
But calculations comparing brightness and orbits wiped out the romantic hypothesis. Such dancers from the main asteroid belt between Mars and Jupiter arrive at an average of 1.23 per epoch, while from that very crater — only 0.042. A gap of almost 30 times! The probability of Kamo'oalewa's lunar origin is laughable. More likely, it's simply a belt native that accidentally copied the lunar attire. In a year or two, the Tianwen-2 probe will collect soil samples — and the dance will end with a solution. Meanwhile, the Vera Rubin Observatory telescope watches its pirouettes.
Original abstract
as written by the authors arXiv:2601.08923v1
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.
Kamo'oalewa spans 40 to 100 meters — about as large as a few football fields — but through a telescope, it appears only as a faint point of magnitude 22. To spot it, you need equipment that captures light tens of times dimmer than what the eye can see.