Author: Li-Yong Zhou

School of Astronomy & Space Science, Nanjing University, Nanjing 210046, China

China’s first asteroid sample-return mission, Tianwen-2, is currently en route to the near-Earth object (469219) Kamoʻoalewa. Following its scheduled rendezvous with this Earth co-orbital in July 2026, the mission aims to deliver invaluable samples to Earth by the end of 2027, paving the way for groundbreaking discoveries. Prior to the scientific feast of analyzing the returned samples, the pre-return study of the asteroid itself serves as a vital precursor, offering a foretaste of the discoveries to come. The article by Wang et al (2026) from the Purple Mountain Observatory of the Chinese Academy of Sciences is just such a tasty appetizer.

Kamoʻoalewa (provisional designation 2016 HO₃), orbiting around the Sun in a 1:1 mean motion resonance with the Earth, maintains its position in the vicinity of the Earth, making it an ideal target for a sample-return mission. The asteroid is a small (~57 meters in diameter) fast rotator (~28 minutes in period), and previous spectroscopic observations suggest its spectral similarities with lunar samples, leading to the hypothesis that it might have originated from a lunar impact ejection event (Jiao et al 2024, Zhu et al 2025).

This research, however, suggests that this Earth quasi-satellite may not originate from the Moon, but could potentially come from the main asteroid belt between Mars and Jupiter. Three key candidate source regions within the main belt were selected and tested: the ν₆ secular resonance zone in the inner main belt, the 3:1 mean-motion resonance with Jupiter, and the Flora family. Numerous test particles were generated in these regions, and their orbital evolution was tracked over 100 million years through numerical simulations.

The simulations demonstrated that all three candidate regions could produce particles that evolve onto orbits similar to that of Kamoʻoalewa. Aided by the Yarkovsky effect, asteroids from these regions might be driven slowly into the 3:1 and/or ν₆ resonances, and then their orbits may be excited by resonances and/or encounters with terrestrial planets, which eventually deliver them into the near-Earth space.

If Kamoʻoalewa was really transported from the main belt to its current orbit in near-Earth space, it would be significant for the Tianwen-2 mission — obtaining a main-belt sample would be far more exciting than acquiring lunar ejecta. The true origin surely will be revealed after the samples are returned. 

References:

Wang, Y., Hu, S., Ji, J., et al. 2026, RAA, 26:041001

Jiao, Y., Cheng, B., Huang, Y., et al. 2024, NatAs, 8, 819

Zhu, M.-H., Morbidelli, A., Wei, Z., et al. 2025, The Innovation, 101183