Multi-epoch spectrophotometric characterization of the mini-moon 2024 PT₅ in the visible and near-infrared

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, Bd de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
² The Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
³ Eureka Scientific, Oakland, CA 94602, USA
⁴ Institute of Astronomy, V.N. Karazin Kharkiv National University, 35 Sumska Str., Kharkiv 61022, Ukraine
⁵ School of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
⁶ Griffith Observatory, Los Angeles, CA 90027, USA
⁷ Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
⁸ Asahikawa Campus, Hokkaido University of Education, Hokumon 9, Asahikawa, Hokkaido 070-8621, Japan
⁹ Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University, Kita-ku, Sapporo, Hokkaido 060-0810, Japan

^★ Corresponding author: jbeniyama@oca.eu

Received: 22 May 2025
Accepted: 13 July 2025

Abstract

Context. 2024 PT₅ is a tiny (D ≤ 10 m) near-Earth asteroid (NEA) discovered in August 2024. 2024 PT₅ was gravitationally bound to the Earth-Moon system from September to November 2024 and classified as a mini-moon. Several quick response observations suggest that 2024 PT₅ has a lunar ejecta origin, while the rotation state and albedo, essential properties for investigating its origin, are not well constrained.

Aims. We aim to characterize the spectrophotometric properties of 2024 PT₅ by ground-based observations to test its taxonomic classification and origin.

Methods. We performed visible to near-infrared multicolor photometry of 2024 PT₅ from data taken using the TriColor CMOS Camera and Spectrograph (TriCCS) on the Seimei 3.8 m telescope during January 4–10, 2025. The Seimei/TriCCS observations of 2024 PT₅ cover phase angles from 14 deg to 27 deg, and were obtained in the ɡ, r, i, and ɀ bands in the Pan-STARRS system. In addition, we analyzed Y, J, H, and K photometry taken with the Multi-Object Spectrograph for Infrared Exploration (MOSFIRE) on the Keck I 10-m telescope on January 16–17, 2025.

Results. Our light curves show brightness variations over time periods of several tens of minutes. We infer that 2024 PT₅ is in a tumbling state and has a light curve amplitude of about 0.3 mag. Visible and near-infrared color indices of 2024 PT₅, ɡ – r = 0.567 ± 0.044, r – i = 0.155 ± 0.009, r – ɀ = 0.147 ± 0.066, Y – J = 0.557 ± 0.046, J – H = 0.672 ± 0.078, and H – Ks = 0.148 ± 0.098, indicate that 2024 PT₅ is an S-complex asteroid, largely consistent with previous observations. Using the H–G model, we derived an absolute magnitude, H_V,HG, of 27.72 ± 0.09 and a slope parameter, G_V, of 0.223 ± 0.073 in the V band. A geometric albedo of 2024 PT₅ was derived to be 0.26 ± 0.07 from the slope of its photometric phase curve. This albedo value is typical of the S- and Q-type NEAs.

Conclusions. Using the albedo and absolute magnitude, the equivalent diameter of 2024 PT₅ is estimated to be 7.4 ± 1.0 m. The color properties of 2024 PT₅ derived from our observations match the ones of rock samples taken from the lunar surface, which agrees with previous studies.