NEOWISE data and Thermophysical Modeling of 98943 Torifune (2001 CC21)

Abstract

The Hayabusa2# flyby target 98943 Torifune (2001 CC21) has an uncertain size based on an uncertain albedo and uncertain absolute magnitude. We have collected all the NEOWISE observations of 2001 CC21 from Nov 2021 through Feb 2024, a total of 132 frames, and analyzed this data to estimate an infrared radiometric diameter. We analyze the multi-epoch 3.4 and 4.6 micron NEOWISE data using an ellipsoidal rotating, cratered ThermoPhysical Model (TPM) to obtain estimates for the diameter, rotation pole, shape, and thermal inertia. 2001 CC21 is quite faint at 4.6 microns when Delta is about 0.7 AU, so the resulting diameter is substantially smaller than the 700 meters derived from the H magnitude and L spectral type. Recent polarimetric data has also suggested a smaller diameter, but not quite as small as the diameter derived from the thermal IR data. A fit to an ellipsoidal TPM model gives a volume equivalent sphere diameter of 337-27+33 meters [posterior median and central 68% confidence interval]. Prograde rotation with an obliquity of 24-9+6 deg is preferred. We also applied this TPM to the Spitzer data presented by Fornasier etal. (2024) and obtain a diameter of 476 +/- 9% meters which is consistent with the NEATM modeling presented by Fornasier etal. but with more realistic errorbars. Finally, fitting the NEOWISE and Spitzer data together requires unexpectedly large thermal inertias and gives a bimodal posterior diameter distribution.