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(469219) Kamo`oalewa is a small near-Earth asteroid, which is currently a quasi-satellite of Earth. Lightcurve measurements revealed a rotation period of about 30 minutes, while the spectrum is compatible with that of S-type asteroids. This object has been selected as the target of the sample-return Tianwen-2 mission of the China National Space Administration.
In early 2024, we performed an observational campaign to better determine the orbit of Kamo`oalewa. Astrometric measurements were taken from the Loiano Astronomical Station and from the Calar Alto Observatory. We also accurately re-measured two precovery detections from the Sloan Digital Sky Survey from 2004. This new astrometry was used in a 7-dimensional orbit determination, aimed at determining both the orbital elements and the Yarkovsky effect. This process was performed with the Aegis software [1] of the ESA NEO Coordination Centre. We detected a semi-major axis drift due to the Yarkovsky effect of -68.96 ± 3.9 au/My, with a high signal-to-noise of 17.5. The new orbit solution also significantly reduced the position uncertainty at the time of arrival of the Tianwen-2 spacecraft.
Thermal inertia is then studied by using ASTERIA [2], a new method suitable to estimate the thermal properties of small asteroids affected by the Yarkovsky effect. By using different models for the physical parameters of Kamo`oalewa, the ASTERIA model estimated the thermal inertia at $155^{+90}_{-45}$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$ or at $181^{+99}_{-60}$ J m$^{-2}$ K$^{-1}$ s$^{-1/2}$. Assuming that the low thermal inertia is given by the presence of a regolith layer on the surface, thermal conductivity models [3] predict a grain size of the order of 0.1 – 3 mm.
[1] Fenucci et al. (2024), The Aegis orbit determination and impact monitoring system and services of the NEOCC web portal, CMDA, 136, 58
[2] Novaković et al. (2024), ASTERIA – Asteroid Thermal Inertia Analyzer, PSJ, 5, 1
[3] Gundlach and Blum (2013), A new method to determine the grain size of planetary regolith, Icarus, 223, 1