(469219) Kamoʻoalewa, A Space Weathering Matured Small NEA: Target of the Tianwen-2 Sample-Return Mission

Abstract Understanding the nature and evolution of near-Earth asteroids (NEAs) are of paramount importance to planetary science1 and security2. So far, three asteroid sample-return missions, Hayabusa, Hayabusa2, and OSIRIS-REx to asteroids have greatly increased our knowledge of several-hundred-meter rubble-pile NEAs. However, limited by ground-based observation spatial resolution, little is known about smaller NEAs. Recently, the China National Space Administration has proposed a new asteroid mission, Tianwen-2, which plans to first return a sample from an Earth quasi-satellite (469219) 2016 HO3 Kamoʻoalewa, and then orbit and characterize an active main-belt asteroid: 311P/PANSTARRS. Here we report that Kamoʻoalewa is an S-type, sub-hundred-meter-sized (69. 45 m × 58. 49 m × 51. 78 m), rapid-rotating (period is 27. 37 minutes) NEA developed with grain size < 2 cm regolith. We analyzed telescopic observations of Kamoʻoalewa, whose visible to near-infrared (VIS-NIR) reflectance spectrum shows a 0. 984 (+0. 003, -0. 004) μm absorption center, suggesting that it resembles LL ordinary chondrites in composition. Orbital dynamical calculations show that Kamoʻoalewa holds a 72 ± 5% probability of originating from the inner main belt ν6 secular resonance adjacent to the Flora family. Notably, Kamoʻoalewa exhibits an extremely red (positively steep) VIS-NIR spectral slope, making it a typical space weathering (SW) -matured object. Our spectral model calculations further suggest that the sub-microphase iron content in Kamoʻoalewa’s regolith is 0. 29 ± 0. 05 wt. % and the SW timescale is ~0. 5–1 × 10⁸ years. This means that Kamoʻoalewa is indeed a SW-matured object and the separation from its parent body likely took place in the inner main belt long before it evolved into an NEA. We ascribe Kamoʻoalewa’s extremely red spectral slope to the combined effects of SW, YORP spin-up, thermal degradation, low-frequency impacts, and non-rubble pile structure. We further predict that sub-hundred-meter, rapidly spinning silicate-rich NEAs with small perihelion may generally exhibit extremely red spectral slopes and SW-matured surfaces.