In-Flight Performance of Hera TIRI and its Future Operation Plan

Introduction: TIRI is an uncooled bolometer based thermal infrared imager with a filter wheel, developed by the Japan Aerospace Exploration Agency (JAXA) for the Hera mission led by the European Space Agency (ESA). Hera is a Planetary Defense mission to rendezvous with and explore the asteroid binary 65803 Didymos and its moon Dimorphos whose orbit around Didymos was deflected due to the kinetic impact by the Double Asteroid Redirection Test (DART) mission on 26 September 2022. The Hera spacecraft was launched on 7 October 2024, changed its trajectory to the asteroid binary by the gravity assist of Mars during the Mars Swing By on 12 March 2025, and will arrive at the targets in December 2026 and start the observations there. The current status and the future operation plan for TIRI will be briefly described.

 

In-Flight Performance of TIRI: We confirmed the instrument has been healthy after launch by having performed the initial function checks after launch, taken many sets of the Earth-Moon system images at various distances from the Earth, and a couple of Dark Sky imaging campaigns. The radiative calibration of TIRI has been carried out using Earth thermal images, even though their diameters are smaller than 0.5°, compared with those of the weather satellites as well as Moon thermal images compared with a thermal modeling of the Moon. The geometrical corrections of TIRI, i.e. alignments and distortions, has been conducted using the SPICE kernel provided by ESA Hera team. The calibrated TIRI data is almost ready now.

 

TIRI Observations during Mars Swing By: We have observed Mars and its moons Deimos and Phobos mainly for the purposes of TIRI calibration. Mars was a point source on 1 March but it was getting larger to cover the entire FOV of TIRI on 12 March, so that it is useful for the calibration of the size of source effect by comparing with the Mars thermal model and the observed data by EMIRS on UAE Mars Mission Hope and THEMIS on NASA Mars Odyssey. The comparison with the multiband filters will be also planned. Deimos has been imaged with all the bands of TIRI, so that the thermophysical properties and the constituent materials of its surface will be constrained. Crescent Phobos images of less than 10 pixel diameter have been taken during the Mars Swing By operation, which will be used for the relation of phase angle dependency of thermal emission to the surface geologic features.

 

Future Observation Plan for TIRI: We started to plan the asteroid phase operations. For TIRI, one-rotation thermal imaging (60-120 images in one rotation) of Didymos and Dimorphos as well as 8-band imaging (6-12 times in one rotation) will be planned from East, West, North, and South directions to investigate thermophysical properties, and during the Early Characterization Phase (ECP) and the Payload Deployment Phase (PDP) at 20-30 km distances for the first 8-9 weeks. Additionally, imaging at a higher frequency will be planned for tracing the shadows of Dimorphos is on Dimorphos and Dimorphos is in the shadow of Didymos. Similar observations but adding the imaging from the Noon direction will be planned during the Detailed Characterization Phase at 8-20 km distances for the next 4 weeks. After this phase, the spacecraft will perform flyby the asteroid binary at 4 km distances. During the close flyby, one-rotation thermal imaging of Didymos at a closest distance and at 20 km distances at high solar phase angles, as well as imaging of Dimorphos focusing on the DART impact area at higher spatial resolution at 1 m/pixel will be planned, during the Close-up Operation Phase (COP) for the next 6 weeks. Finally, much closer flyby observations to take images at higher spatial resolution will be planned at < 2 km distances during the Experimental Phase (EXP) for the last 6 weeks. Further detailed observation plan of TIRI will be reconsidered at a later stage of the mission.