Thermal infrared imaging experiment of S-type binary asteroids in the Hera mission

The thermal infrared imager TIRI onboard the ESA Hera spacecraft is being developed to investigate thermophysical properties of the S-type asteroid 65803 Didymos and its moon Dimorphos by mapping thermal inertia and compositional variations of them. TIRI is based on an uncooled micro-bolometer array of 1024 x 768 effective pixels and covers the field of view of 13.3° x 10.0°, with the resolution of 0.23 mrad per pixel. TIRI has an eight-position filter wheel to be used as one wide bandpath at 8-14 µm for thermal imaging, six narrow bands peaked at 7.8, 8.6, 9.6, 10.6, 11.65, and 13.1 µm for compositional information, and one closed plate both for a shutter and a temperature reference.

TIRI will be mounted on the top panel of the Hera spacecraft to point the target asteroids in the same direction with other instruments AFC, PALT, and Hyperscout-H, for the simultaneous observations. The asteroid surface temperature will change day and night according to thermal inertia and roughness of the surface layer, which will be consequently derived from the diurnal temperature profile. The maximum temperature in a day will also change according to the solar distance of the asteroid from ~1 to ~2 au at the beginning to the end of the nominal mission. During the early characterization phase (ECP) at 20 to 30 km from the asteroid, TIRI will take images from large solar phase angles from 50° to 70° with the spatial resolution of ~4.6 to 6.9 m per pixel to construct the asteroid shape model even in the night side and map the thermal inertia and composition. During the detailed characterization phase (DCP) at 10 to 20 km from the asteroid, TIRI will take images from the noon with the spatial resolution of ~2.3 m per pixel for more detailed thermal properties and compositional mapping. During the close-up operation phase (COP) at < 5 km from the asteroid, TIRI will take images from the noon with the spatial resolution of ~1 m per pixel. Higher spatial resolution will be achieved during the further close observations.

In the Hayabusa2 mission, thermal imaging has revealed the highly porous nature of C-type asteroid from global to local scales (Okada et al, 2020; Shimaki et al, 2020), but nobody knows the surface properties of S-type asteroids so that this is a unique opportunity to investigate the S-type asteroid Didymos in comparison with the C-type asteroid Ryugu. For the moon Dimorphos, it will be the smallest asteroid ever explored so that it is also a unique opportunity to investigate the small-sized asteroid, especially for the strength and porosity. TIRI will contribute to verifying Yarkovsky and YORP (B-YORP) effects, orbital and rotational evolution in relation to thermophysical modeling. The temperature profile and compositional difference between the inside and outside of the artificial crater formed by the kinetic impact of the NASA DART spacecraft will be the important target both for the purpose of planetary defense and science.