The Non-Principal-Axis Rotation and Convex Shape Model of Earth Quasi-Satellite and the Target of China's Tianwen-2 Mission (469219) Kamo`oalewa

Abstract

(469219) Kamo`oalewa is the most stable Earth quasi-satellite and the target of China's Tianwen-2 asteroid sample return mission. Due to its small size, fast rotation, and the limited observing geometry accessible from the ground, many physical properties of Kamo`oalewa remain poorly constrained, including the rotational status and shape. We obtained three epochs of high-cadence, high signal-to-noise photometric lightcurves of Kamo`oalewa with the Gemini North Telescope from 2026 April to May, supplemented by one lightcurve from the Lowell Discovery Telescope in 2026 May. Our analysis suggests that Kamo`oalewa is in a non-principal-axis rotation with an elongated shape. Four possible solutions exist, including a long-axis mode (LAM) solution and a short-axis mode (SAM) solution, as well as their corresponding mirrored angular momentum directions. The most preferable solution has a LAM model with a precession period $P_{\ddot{I}}•=27.65\pm \text{min}$, and a rotational period $P_{\ddot{I}}ˆ=50.49\pm 0.08\text{min}$, and the angular momentum points to ecliptic coordinates $(\hat{I}»,{\hat{I}}^2)=(226^{\mathrm{o}}\pm 20^{\mathrm{o}},-39^{\mathrm{o}}\pm 15^{\mathrm{o}})$, although we cannot rule out other solutions or other close-by periods due to aliasing. We also derived a convex shape inversion for LAM with consistent rotational parameters but could not find a satisfactory inversion for SAM. The non-principal-axis rotation provides additional constraints on the dynamic history or the internal structure of Kamo`oalewa.

Submitted to ApJL