NEOWISE Data and Thermophysical Modeling of 98943 Torifune (2001 CC ₂₁ )

Abstract The Hayabusa2 ♯ flyby target 98943 Torifune (2001 CC 21 ) has an uncertain size based on an uncertain albedo and absolute magnitude. We have collected all the NEOWISE observations of 2001 CC 21 from 2021 November through 2024 February, a total of 132 frames, and analyzed these data to estimate an infrared radiometric diameter. We analyze the multiepoch 3.4 and 4.6 μ m NEOWISE data using an ellipsoidal, rotating, cratered thermophysical model (TPM) to obtain estimates for the diameter, rotation pole, shape, and thermal inertia. 2001 CC 21 is quite faint at 4.6 μ m when Δ ∼ 0.7 au, so the resulting diameter is substantially smaller than the 700 m derived from the H magnitude and L spectral type. Recent polarimetric data have 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>33</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>27</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>33</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> m (posterior median and central 68% confidence interval). Prograde rotation with an obliquity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">(</mml:mo> <mml:mn>2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>9</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo stretchy="false">)</mml:mo> <mml:mo>°</mml:mo> </mml:math> is preferred. We also applied this TPM to the Spitzer data presented by S. Fornasier et al. and obtained a diameter of 476% ± 9% m, which is consistent with the NEATM modeling presented by Fornasier et al. but with more realistic error bars. Finally, fitting the NEOWISE and Spitzer data together requires unexpectedly large thermal inertias and gives a bimodal posterior diameter distribution.