Feasibility and Accuracy of Thermophysical Estimation of Asteroid 162173 Ryugu (1999 JU3) from the Hayabusa2 Thermal Infrared Imager

We present the results of a numerical study to prepare for the remote sensing of asteroid 162173 Ryugu (1999 JU3) using the Hayabusa2 thermal infrared imager (TIR). We simulated the thermal characteristics of the asteroid with a thermophysical model (TPM) using an ideal body with a smooth and spherical surface, and investigated its feasibility to determine the thermophysical properties of the asteroid under two possible spin vectors; (λecl,βecl)=(73∘,−62∘)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$(\lambda_{\mathrm{ecl}}, \beta_{\mathrm{ecl}}) = (73^{\circ}, -62^{\circ})$\end{document} and (331∘,20∘)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$(331^{\circ}, 20^{\circ})$\end{document}. Each of the simulated snapshots taken at various local times during the 1.5-year proximity phase was analyzed to estimate uncertainties of the diurnal thermal phase delay to infer the thermal inertia of Ryugu. The temperature in a pixel was simulated based on the specification of the imager and the observing geometry. Moreover, we carried out a regression analysis to estimate albedo and thermal emissivity from the time variation of surface temperature. We also investigated the feasibility of determining thermal phase delay in a first attempt using realistic rough surfaces. We found that precise determination of the thermal phase delay would be difficult in the (331∘,20∘)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$(331^{\circ}, 20^{\circ})$\end{document} spin type unless the surface was nearly smooth. In contrast, the thermal phase delay is likely to be observable even if the surface topography is moderately rough in the other spin type. From the smooth-surface model, we obtained a less than 20% error of thermal inertia on observation opportunities under the likely range of thermal inertia ≤1000Jm−2s−1/2K−1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\leq 1000~\mbox{J}\,\mbox{m}^{-2}\,\mbox{s}^{-1/2}\, \mbox{K}^{-1}$\end{document}. The error of thermal inertia exceeded 50% under a realistic surface with roughness.