[P-PS03] Small Solar System Bodies: New perspectives on the origin and evolution of the Solar System

[PPS03-16] Thermal infrared imaging of small bodies and operations of TIRI onboard Hera

★Invited Papers

*Naoya Sakatani¹, Tatsuaki Okada¹, Yuri Shimaki¹, Hiroki Senshu², Tomohiko Sekiguchi³, Takehiko Arai⁴, Takuya Ishizaki¹, Masanori Kanamaru⁵, Hirohide Demura⁶, Toru Kouyama⁷, Soichiro Furukawa⁵, Satoshi Tanaka¹ (1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.Planetary Exploration Center, Chiba Institute of Technology, 3.Hokkaido University of Education, 4.Maebashi Institute of Technology, 5.University of Tokyo, 6.University of Aizu, 7.National Institute of Advanced Industrial Science and Technology)

Thermal infrared observation is a unique method to reveal the surface physical properties of airless planetary bodies. Surface temperature relates to the thermal conductivity, and it varies depending on the surface physical condition such as grain size, porosity, and rock abundance. Thermal infrared imager TIR onboard Hayabusa2 first measured the global surface temperature of C-type asteroid, and a thermal inertia map of Ryugu was produced [1]. Boulder-rich surface and low thermal inertia suggest highly-porous boulders on Ryugu surface [2,3]. Diurnal temperature variation also shows that the surface roughness produces asymmetric thermal radiation, which affects the evolution of orbit and rotational state known as Yarkovsky and YORP effect.

Hera is a mission to binary asteroids Didymos-Dimorphos in 2027, and launched on October 2024. The new generation thermal infrared imager, named TIRI, is installed. TIRI is composed of microbolometer array (1024 x 768 pixels) with control electronics, optics with focal length of 75 mm, and 8-position filter wheels allowing multiband imaging. TIRI aims at investigating thermophysical properties of S-type asteroids, in comparison with those of C-type asteroid Ryugu and Bennu. Comparison of thermal properties and mid-infrared spectra between Didymos and Dimorphos will be important to investigate the formation of the binary. TIRI also contributes to understanding dynamics of the binary, orbital and rotation changes, by thermophysical modeling regarding Yarkovsky and YORP (B-YORP) effects. Furthermore, thermal imaging may find a DART impact crater or remnant crop and ejecta deposit as thermal anomalies.

Soon after the launch, Earth and Moon observations were performed to check the radiometric calibration based on the ground pre-flight test. On March 2025, Hera will perform Mars flyby, during which Mars and Deimos observations are planned. In this presentation, we will present the operation results of TIRI, and operation plan and scientific motivations of future Didymos-Dimorphos rendezvous.

References: [1] Shimaki et al. (2020) Icarus 348, 113835. [2] Okada et al. (2020) Nature 579, 518-522. [3] Sakatani et al. (2021) Nature Astron. 5, 766-774.