# Thermal Imaging Performance of TIR Onboard the Hayabusa2 Spacecraft

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## Abstract

The thermal infrared imager (TIR) is a thermal infrared camera onboard the Hayabusa2 spacecraft. TIR will perform thermography of a C-type asteroid, 162173 Ryugu (1999 JU_{3}), and estimate its surface physical properties, such as surface thermal emissivity \(\epsilon \), surface roughness, and thermal inertia \(\varGamma \), through remote *in-situ* observations in 2018 and 2019. In prelaunch tests of TIR, detector calibrations and evaluations, along with imaging demonstrations, were performed. The present paper introduces the experimental results of a prelaunch test conducted using a large-aperture collimator in conjunction with TIR under atmospheric conditions. A blackbody source, controlled at constant temperature, was measured using TIR in order to construct a calibration curve for obtaining temperatures from observed digital data. As a known thermal emissivity target, a sandblasted black almite plate warmed from the back using a flexible heater was measured by TIR in order to evaluate the accuracy of the calibration curve. As an analog target of a C-type asteroid, carbonaceous chondrites (\(50~\mbox{mm} \times 2~\mbox{mm}\) in thickness) were also warmed from the back and measured using TIR in order to clarify the imaging performance of TIR. The calibration curve, which was fitted by a specific model of the Planck function, allowed for conversion to the target temperature within an error of 1 ^{∘}C (\(3\sigma \) standard deviation) for the temperature range of 30 to 100 ^{∘}C. The observed temperature of the black almite plate was consistent with the temperature measured using K-type thermocouples, within the accuracy of temperature conversion using the calibration curve when the temperature variation exhibited a random error of 0.3 ^{∘}C (\(1\sigma \)) for each pixel at a target temperature of 50 ^{∘}C. TIR can resolve the fine surface structure of meteorites, including cracks and pits with the specified field of view of 0.051^{∘} (\(328 \times 248~\mbox{pixels}\)). There were spatial distributions with a temperature variation of 3 ^{∘}C at the setting temperature of 50 ^{∘}C in the thermal images obtained by TIR. If the spatial distribution of the temperature is caused by the variation of the thermal emissivity, including the effects of the surface roughness, the difference of the thermal emissivity \(\Delta \epsilon \) is estimated to be approximately 0.08, as calculated by the Stefan-Boltzmann raw. Otherwise, if the distribution of temperature is caused by the variation of the thermal inertia, the difference of the thermal inertia \(\Delta \varGamma \) is calculated to be approximately \(150~\mbox{J}\,\mbox{m}^{-2}\,\mbox{s}^{0.5}\,\mbox{K}^{-1}\), based on a simulation using a 20-layer model of the heat balance equation. The imaging performance of TIR based on the results of the meteorite experiments indicates that TIR can resolve the spatial distribution of thermal emissivity and thermal inertia of the asteroid surface within accuracies of \(\Delta \epsilon \cong 0.02\) and \(\Delta \varGamma \cong 20~\mbox{J}\,\mbox{m}^{-2}\,\mbox{s}^{0.5}\,\mbox{K}^{-1}\), respectively. However, the effects of the thermal emissivity and thermal inertia will degenerate in thermal images of TIR. Therefore, TIR will observe the same areas of the asteroid surface numerous times (\({>}10\) times, in order to ensure statistical significance), which allows us to determine both the parameters of the surface thermal emissivity and the thermal inertia by least-squares fitting to a thermal model of Ryugu.

## Keywords

Hayabusa2 TIR Infrared camera Thermography Thermal emissivity Surface roughness Thermal inertia Ryugu 1999 JU_{3}Asteroid Carbonaceous chondrite

## Notes

### Acknowledgements

We thank T. Matsunaga, M. Matsuoka, and S. Matsuura for their comments and supports of the prelaunch experiments. We are grateful to all members of the Hayabusa2 team and thank NEC Inc. Co. who made the flight model of TIR.

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