Earth and moon observations by thermal infrared imager on Hayabusa2 and the application to detectability of asteroid 162173 Ryugu

Thermal Infrared Imager TIR on Hayabusa2 has proven its in-flight performance to detect celestial bodies during the cruise phase, especially by the observations of the Earth and the Moon before and after the Earth swing-by on 3rd of December 2015. The result indicates that the target C-type asteroid 162173 Ryugu will be detected from the distance of 3000 km at the beginning of the approach phase, and that a C-type small moon larger than 1 m will be detected from the home position, 20 km from the asteroid, if it orbits around the asteroid. •The very first images of the Earth and the Moon were taken by the TIR on Hayabusa2.•The detection limit of TIR was estimated using the Earth and the Moon thermal images.•Asteroid Ryugu will be detected by TIR even in the Approach Phase of Hayabusa2.•The moons larger than 1m diameter will be detected by TIR if they orbit around Ryugu.