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

TL;DR

This study uses NEOWISE and Spitzer infrared data combined with thermophysical modeling to accurately estimate the size, shape, and thermal properties of asteroid 98943 Torifune, revealing a smaller diameter than previous estimates and insights into its rotation and thermal inertia.

Contribution

It provides a detailed thermophysical analysis of 98943 Torifune using multi-epoch infrared data, improving size estimates and thermal property understanding compared to prior methods.

Findings

Estimated diameter of 337 meters from NEOWISE data

Preferred prograde rotation with 24-degree obliquity

Large thermal inertias and bimodal diameter distribution from combined data

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.