Letter to the Editor

Size, albedo, and rotational period of the Hayabusa2# target (98943) 2001 CC21

¹ LESIA, Université Paris Cité, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, 5 place Jules Janssen, 92195 Meudon, France
e-mail: sonia.fornasier@obspm.fr
² Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
³ INAF-Osservatorio Astronomico di Roma, 00078 Monte Porzio Catone (Roma), Via Frascati 33, Italy
⁴ GEPI, Observatoire de Paris, Université PSL, CNRS, Place Jules Janssen, 92195 Meudon, France
⁵ Université Côte d’Azur, CNRS-Lagrange, Observatoire de la Côte d’Azur, CS 34229, 06304 Nice Cedex 4, France
⁶ School of Physics and Astronomy, University of Leicester, Leicester, UK
⁷ Institute of Astronomy, V.N. Karazin Kharkiv National University, 4 Svobody Sq., Kharkiv 61022, Ukraine
⁸ E. Kharadze Georgian National Astrophysical Observatory, Abastumani, Georgia
⁹ Samtskhe-Javakheti State University, Akhaltsikhe, Georgia
¹⁰ INAF-Astrophysical Observatory of Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹¹ IMCCE, Observatoire de Paris, CNRS, PSL Research University, 77 av. Denfert Rochereau, 75014 Paris Cedex, France
¹² Astronomical Institute of the Romanian Academy, 5 Cutitul de Argint, 040557, sector 4, Bucharest, Romania

Received: 19 April 2024
Accepted: 17 July 2024

Abstract

Aims. This study aims to determine the size, albedo, and rotational period of (98943) 2001 CC21, a target of the Hayabusa2 extended mission, using thermal data from the Spitzer Space telescope and ground-based observations.

Methods. The Spitzer data were acquired with the Infrared Spectrograph in the 6–38 μm range, reduced using the Spitzer pipeline, and modeled with the near-Earth asteroid thermal model to determine the asteroid size and albedo. The absolute magnitude and rotational period were determined thanks to new observations carried out at the 3.5 m New Technology Telescope, the 1.2 m Observatoire de Haute Provence, and the 0.7 m Abastumani telescope. Three complete light curves were obtained in 2023 and 2024 at the last-mentioned telescope.

Results. We determine an absolute magnitude of H = 18.94 ± 0.05 and a rotational period of 5.02124±0.00001 hours, with a large light curve amplitude of ∼0.8 mag. at a phase angle of 22°, indicating a very elongated shape with an estimated a/b semiaxis ratio ≥1.7, or a close-contact binary body. The emissivity of 2001 CC21 is consistent with that of silicates, and its albedo is 21.6±1.6%. Finally, the spherical-equivalent diameter of 2001 CC21 is 465±15 m.

Conclusions. The albedo value and emissivity determined here, coupled with results from polarimetry and spectroscopy from the literature, confirm that 2001 CC21 is an S-complex asteroid, and not an L-type one as was previously suggested. The size of 2001 CC21 is less than 500 m, which is smaller than its first size estimation (∼700 m). These results are relevant in preparation of the observing strategy for 2001 CC21 of the Hayabusa2 extended mission.