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Published August 2025 | Published
Journal Article Open

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

Beniyama, Jin1, 2 ORCID icon
Bolin, Bryce T.3 ORCID icon
Sergeyev, Alexey V.1, 4 ORCID icon
Delbo, Marco1, 5
Abron, Laura-May6
Belyakov, Matthew7 ORCID icon
Sekiguchi, Tomohiko8 ORCID icon
Takagi, Seiko9 ORCID icon
  • 1. ROR icon Lagrange Laboratory
  • 2. ROR icon University of Tokyo
  • 3. ROR icon Eureka Scientific
  • 4. ROR icon V. N. Karazin Kharkiv National University
  • 5. ROR icon University of Leicester
  • 6. ROR icon Griffith Observatory
  • 7. ROR icon California Institute of Technology
  • 8. ROR icon Hokkaido University of Education
  • 9. ROR icon Hokkaido University

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, HV,HG, of 27.72 ± 0.09 and a slope parameter, GV, 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.

Copyright and License

© The Authors 2025. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Acknowledgement

We would like to thank Dr. Alessandro Morbidelli for insightful discussions and helpful comments. We thank Dr. Katsuhiro Murata for observing assistance using Seimei telescope. The authors are grateful to our reviewer Dr. Marcel Popescu for constructive comments on the manuscript. We acknowledge Dr. Robert Jedicke and Luis Langermann for the discussion regarding the steady state population of Earth’s mini-moons of lunar provenance and albedo of the Moon, respectively. J.B. thanks Dr. Theodore Kareta and Dr. Benjamin N. L. Sharkey for their constructive feedback on the spectra of 2024 PT5 and Kamo‘oalewa, respectively. J.B. is grateful to Dr. Max Mahlke for the development of, and valuable comments on, classy, a tool for exploring, downloading, analyzing, and classifying asteroid reflectance spectra. This work was supported by JSPS KAKENHI Grant Numbers JP23KJ0640 and 25H00665. This work was supported by the French government through the France 2030 investment plan managed by the National Research Agency (ANR), as part of the Initiative of Excellence Université Côte d’Azur under reference number ANR-15-IDEX-01. Some of the data presented herein were obtained at Keck Observatory, which is a private 501(c)3 non-profit organization operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Keck Observatory is located on Maunakea, land of the Kānaka Maoli people, and a mountain of considerable cultural, natural, and ecological significance to the indigenous Hawaiian people. The authors wish to acknowledge the importance and reverence of Maunakea and express gratitude for the opportunity to conduct observations from the mountain. This research has made use of LTE’s Miriade VO tool.

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August 25, 2025
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August 25, 2025