Astrometry, orbit determination, and thermal inertia of the Tianwen-2 target asteroid (469219) Kamo‘oalewa

¹ ESA ESRIN / PDO / NEO Coordination Centre, Largo Galileo Galilei, 1, 00044 Frascati (RM), Italy
² Deimos Italia s.r.l., Via Alcide De Gasperi, 24, 28060 San Pietro Mosezzo (NO), Italy
³ Department of Astronomy, Faculty of Mathematics, University of Belgrade, Studentski trg 16, Belgrade 11000, Serbia
⁴ Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang (Guizhou), PR China
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti, 93/3, 40129 Bologna, Italy
⁶ Starion Italia, Via di Grotte Portella, 28, 00044 Frascati (RM), Italy
⁷ ESA ESAC / PDO, Bajo del Castillo s/n, 28692 Villafranca del Castillo, Madrid, Spain
⁸ Deimos Space S.L.U., Ronda de Poniente, 19, 28760 Tres Cantos Madrid, Spain
⁹ ESA ESOC / PDO, Robert-Bosch-Straße 5, 64293 Darmstadt, Germany

^★ Corresponding author; marco.fenucci@ext.esa.int

Received: 29 November 2024
Accepted: 18 February 2025

Abstract

Context. (469219) Kamo‘oalewa is a small near-Earth asteroid (NEA) that is currently a quasi-satellite of the Earth. Light curve measurements have also revealed a rotation period of only about 30 minutes. This asteroid has been selected as the target of the Tianwen-2 sample-return mission of the China National Space Administration.

Aims. The first goal of this paper is to observe and improve the orbit determination of (469219) Kamo‘oalewa, and to better determine the Yarkovsky effect acting on it. The second goal is to estimate the thermal inertia of the asteroid, using an improved Yarkovsky effect determination.

Methods. Our observational campaign imaged the asteroid from the Loiano Astronomical Station and from the Calar Alto Observatory, in March 2024. We also accurately re-measured a precovery detection from the Sloan Digital Sky Survey from 2004. New astrometry was later used in a seven-dimensional (7D) orbit determination, aimed at estimating both the orbital elements and the Yarkovsky effect. The thermal inertia was later studied by using the ASTERIA, a new method suitable for estimating the thermal inertia of small asteroids.

Results. We detected a semi-major axis drift of (-67.35 ± 4.70) × 10⁻⁴ au My⁻¹ due to the Yarkovsky effect, with a high signal-tonoise ratio (S/N) of 14. The new orbit solution also significantly reduced the position uncertainty for the arrival of the Tianwen-2 spacecraft. By using different models for the physical parameters of Kamo‘oalewa, the ASTERIA model estimated the thermal inertia at Γ = 150₋₄₅⁺⁹⁰ J m⁻² K⁻¹ s^−1/2 or Γ = 181₋₆₀⁺⁹⁵ J m⁻² K⁻¹ s^−1/2.