The Thermal InfraRed Imager on Hera

The Thermal InfraRed Imager TIRI onboard the ESA Hera spacecraft has been developed to explore S-type Near-Earth asteroid binary system, 65803 Didymos and its moon Dimorphos, for the purposes of planetary defense and planetary science. TIRI is a thermal imager based on a two-dimensional uncooled micro-bolometer array of 1024 × 768 pixels and equipped with a filter wheel to investigate the thermophysical properties as well as the composition of the surface constituent materials of these asteroids. TIRI is the successor of the thermal imager TIR onboard Hayabusa2, especially for its lessons-learned and the method of onboard data processing. TIRI is a one-component small instrument with a total mass of 3.94 kg. TIRI takes images of thermal infrared emission in 8 to 14 μm wavelength with a field of view of 13.0° × 9.9° and a spatial resolution of 0.013° per pixel. TIRI covers the target temperature range from 150 to 450 K (including the well calibrated range from 153 to 423 K or −120 to +150 °C). The accuracy of absolute temperature is within 3 K or better after integrating the images, and the accuracy of relative temperature (or noise equivalent temperature difference: NETD) is below 0.1 K at 300 K. TIRI has the 8-position filter wheel which includes a wide band filter at 8–14 μm for thermographic imaging, and six narrow bands centered at 7.8, 8.6, 9.6, 10.6, 11.6, and 13.0 μm for investigating the composition and the degree of alteration of materials with band ratios, as well as the final “closed” position used as the shutter for sun shield and the temperature reference plate. TIRI has an internal flash memory able to store 2039 thermal images of the observation targets and 8 images to store dark frame images for each of filters. The stored data can be compressed before packetization for telemetry. With this instrument, Didymos will be the first S-type asteroid to be imaged in thermal infrared wavelength for its whole surface. Dimorphos will be the smallest asteroid to be imaged in thermal infrared wavelength. The thermal properties and the composition of both asteroids will be compared for investigating their formation process. Thermophysical properties and composition of the DART impact remnant of Dimorphos allow us to study the internal structure of Dimorphos. Thermophysical models of Didymos and Dimorphos will be used to compare the long-term evolution of their orbit and rotation. Finally, a comparison with near-infrared and thermal infrared spectrum will help understand the space weathering process that occurred on the uppermost layer of asteroids. In this paper, the specification of TIRI instrument is introduced, and its mission objectives, the science cases, and the operation scenarios are briefly described.