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

Abstract

The Japan Aerospace Exploration Agency's (JAXA) Hayabusa2 mission, following the successful return of samples of the near-Earth asteroid (NEA, 162173) Ryugu, in December 2020, has been extended to explore two more NEAs. These are (98943) 2001 CC21, which is scheduled for a flyby in 2026, and the fast-spinning 1998 KY26, for a rendez-vous on 2031. The extended mission has been named Hayabusa2#, where the # character stands for "SHARP" (Small Hazardous Asteroid Reconnaissance Probe). Several observing campaigns of these two targets have been and will be carried out to better understand their physical properties in support of the Hayabusa2# mission, and to optimize the observing strategy.In this work we present a new estimate of the size, albedo and rotational period of 2001 CC21. This is based on observations of 2001 CC21 spectral energy distribution in the thermal infrared obtained by NASA’s Spitzer Space Telescope and new ground-based photometric observations carried out at the 3.5m New Technology Telescope of ESO, at the 1.2m Haute Provence Observatory, and at the 0.7m Abastumani telescope. In the optical, we obtained three complete lightcurves in 2023-2024. The Spitzer observations of (98943) 2001 CC21 were obtained on November 20, 2005 from 10:17 to 12:26 UT with the Infrared Spectrograph (IRS). Data were acquired in low resolution mode covering the 5.2-38 micron range in 4 IRS long slit segments. The data were reduced starting from the basic calibrated data generated by the Spitzer Space Center automated pipeline, and  the sky background was removed by differencing two consecutive images taken at different nodding positions for each spectral segment. Finally, spectra were extracted using the Spitzer IRS Custom Extraction (SPICE) software. Data were modeled with the Near Earth Asteroid Thermal Model  to determine the asteroid size and albedo.From ground-based observations, we determine an absolute magnitude of H=18.94±0.05, and a rotational period of 5.02124±0.00001 hours, with a large lightcurve amplitude of ˜0.8 magnitude at a phase angle of 22o, indicating a very elongated shape with estimated a/b semiaxis ratio > 1.8, or a close-contact binary body.  The emissivity of 2001 CC21 is consistent with that of silicates, and its albedo is 21.6+1.1-1.0 %.  Finally, the spherical-equivalent diameter of 2001 CC21 is 465±15 m.The albedo value and emissivity here determined, coupled with results from polarimetry and spectroscopy from the literature, confirm that 2001 CC21 is an S-complex asteroid, and not a L-type, as 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 of 2001 CC21 by Hayabusa2 extended mission.