[M-ZZ45] Planetary defense - what should we do?

[MZZ45-P04] Observation of Earth-Approaching asteroids by Hera thermal infrared imager TIRI

*Tatsuaki Okada^1,2, Satoshi Tanaka¹, Naoya Sakatani¹, Yuri Shimaki¹, Takuya Ishizaki¹, Makoto Yoshikawa¹, Hiroshi Takeuchi¹, Yukio Yamamoto¹, Takehiko Arai³, Hiroki Senshu⁴, Hirohide Demura⁵, Tomohiko Sekiguchi⁶, Toru Kouyama⁷, Masanori Kanamaru² (1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.University of Tokyo, 3.Maebashi Institute of Technology, 4.Chiba Institute of Technology, 5.University of Aizu, 6.Hokkaido University of Education, 7.National Institute of Advanced Industrial Science and Technology)

Keywords:planetarry defense, Hera, Thermal infrared imaging

There are plenty of small bodies in the Solar System that still exist after the formation of planets from the dust particles in the solar nebula, and they have continuously impacted, fragmented, and accumulated each other. Some of them are along the Earth-approaching orbit and could make a tremendous disaster if they impact the Earth. In 1908, an ~50 meter diameter asteroid passing through the sky of Tunguska destroyed ~2000 km² area of Taiga forests, which might completely damage a whole megalopolis. In 2013, a fireball of 17 meter diameter falling at the suburbs of Chelyabinsk injured 1.5 thousands of people and damaged more than 4500 buildings. All the potentially hazardous asteroids in 1000 years are smaller than 100 meter in diameter, so that the impact deflection of spacecrafts should be a practical method to escape from the asteroid crisis. AIDA (Asteroid impact and deflection assessment) is the first international planetary defense demonstration mission and consists of NASA DART kinetic impactor mission and ESA Hera asteroid rendezvous mission. JAXA contributes to Hera with preparing the thermal imager TIRI and scientific studies inherited from Hayabusa2 mission.

DART was launched on 26 November 2021 and made a kinetic impact to Dimorphos, the moon of Didymos, on 26 September 2022. The images taken just before impact were downlinked to Earth and showed the surface geologic features and physical conditions. The dust ejection by the impact was observed from the ground observatories. After the velocity changed by the impact, the orbital period of Dimorphos around Didymos was shortened from 11h55m to 11h23m (delayed by 32 minutes).

Evaluation of DART impact deflection effect needs the information on the composition, density, strength, and porosity of the constituent materials, the bulk density of Dimorphos, as well as the dynamical motion of Didymos and Dimorphos. These values will be measured afterwards by Hera. The thermal imager TIRI onboard Hera will play a role of determining the thermophysical properties and composition of the binary asteroids with a wide band filter for thermal imaging and six narrow band filters for multiband imaging. TIRI will globally map the thermal inertia and constituent composition of both asteroids from 20-30 km distance, and locally observe the surface with higher spatial resolution from the low altitude, including the dimension of the excavated crater by the DART impact, compositional difference between the inside and the outside of the crater, and the sedimented dust ejecta around the crater. Along with the data by the other instruments, it will contribute to the evaluation of the impact deflection effect by the DART impact to Dimorphos for the purpose of planetary defense, as well as to the understandings of the origin and evolution of the Solar System through the first observations of the asteroid binary for the purpose of planetary science. The observation plan and the anticipated results will be presented.