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

Abstract

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. The Spitzer data were acquired with the Infrared Spectrograph in the 6-38 mu m 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 3.5m New Technology Telescope, the 1.2m Observatoire de Haute Provence, and the 0.7m Abastumani telescope. Three complete light curves were obtained in 2023 and 2024 at the last-mentioned telescope. We determine an absolute magnitude of H=18.94pm 0.05 and a rotational period of 5.02124pm 0.00001 hours, with a large light curve amplitude of sim 0.8 mag. at a phase angle of 22$^o$, indicating a very elongated shape with an estimated a/b semiaxis ratio geq 1.7, or a close-contact binary body. The emissivity of 2001 CC21 is consistent with that of silicates, and its albedo is 21.6pm 1.6 <!PCT!>. Finally, the spherical-equivalent diameter of 2001 CC21 is 465pm 15 m. 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 (sim 700 m). These results are relevant in preparation of the observing strategy for 2001 CC21 of the Hayabusa2 extended mission.