[P-PS03] Solar System Small Bodies: A New Frontier Arising Hayabusa 2, OSIRIS-REx and Other Projects

[PPS03-03] Impact experiment on asteroid Ryugu by Small-Carry on impactor of Hayabusa-2 and observation of the impact ejecta by a Deployable CAMera-3

★Invited Papers

*Masahiko Arakawa¹, Takanao Saiki², Toshihiko Kadono³, Yasuhiko Takagi⁴, Koji Wada⁵, Yu-ichi Iijima², Hiroshi Imamura², Chisato Okamoto¹, Yuri Shimaki², Kei Shirai², Satoru Nakazawa², Masahiko Hayakawa², Naru Hirata⁶, Hajime Yano², Hirotaka Sawada², Kazunori Ogawa¹, Ko Ishibashi⁵, Hiroshi Kimura⁵, Masanori Kobayashi⁵, Naoya Sakatani², Hajime Hayakawa², Rie Honda⁷, Seiji Sugita⁸, Tomokatsu Morota⁹, Shingo Kameda¹⁰, ERI TATSUMI⁸, Chikatoshi Honda⁶, yokota yasuhiro², Toru Kouyama¹¹, Manabu Yamada⁵, Hidehiko Suzuki¹², Kazuo Yoshioka¹³, Yuichiro Cho⁸, Moe Matsuoka² (1.Graduate School of Science, Kobe University, 2.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3.University of Occupational and Environmental Health, 4.Aichi Toho University, 5.Planetary Exploration Research Center, Chiba Institute of Technology, 6.Dep. of Computer Science and Engineering, Univ. of Aizu, 7.Department of Science and Technology, System of Natual Science, Kochi University, 8.Department of Earth and Planetary Science, Graduate School of Science Sciece, The University of Tokyo, 9.Graduate School of Environmental Studies, Nagoya University, 10.School of Science, Rikkyo University, 11.National Institute of Advanced Industrial Science and Technology, 12.Department of physics, Meiji university, 13.Graduate School of frontier Science, The University of Tokyo)

Keywords:Asteroid Ryugu, Impact Experiment, Small carry-on impactor, Deployable camera3, Rubble pile body

A small carry-on impactor (SCI) equipped on Hayabusa2 is scheduled to be propelled toward to the surface of asteroid Ryugu this spring. Cratering process on Ryugu made by the impactor will simultaneously observed by a deployable camera 3 (DCAM3) detached from Hayabusa2. The mission objective of the impactor is to excavate the asteroid to expose a subsurface material as ejecta deposits around the crater. Thus, not only it enables us to give a good opportunity for obtaining a fresh or subsurface material by a sampler system, remote sensing instruments such as ONC, TIR, NIRS3 onboard Hayabusa2 will also have a good chance to observe the exposed subsurface material. Furthermore, this impact experiment on Ryugu is also a rare opportunity to verify the crater scaling law in the microgravity environment on the real asteroid materials, and is expected to enable us to improve the conventional scaling law, especially for the crater size and the ejecta velocity distribution.

After arrival at Ryugu, the surface morphology observation through remote sensing has turned out that a plenty of boulders cover throughout the surface, and many of the boulders are larger than 10 m. These boulders are distributed almost uniformly and the size frequency distribution of the boulders is a power law distribution with the power law index around -1, indicating relatively large boulders are dominant on the surface. Therefore, taking into account that the precision of an actual impact point toward an aiming point expands several 10 m, the impactor possibly collides with a large boulder, otherwise with small boulders having the power law size distribution, wherever the impactor would aim at on the surface of Ryugu. If the impactor collides with a large boulder, a crater should be formed in the strength regime, and if the impactor collides into a finer-boulder area, a crater will be formed in the gravity regime. The DCAM3 was designed for the observation of not only the impact cratering in the strength regime but also that in the gravity regime.

We believe that we will success to observe impact ejecta induced by the impactor when the SCI and DCAM3 operations are conducted as scheduled. We are planning to present a first report of the DCAM3 observation in this talk, and discuss how to search the impact point on the Ryugu surface using the DCAM3 images, beforehand the onboard imaging instruments. Moreover, the morphology of the ejecta curtain imaged by DCAM3 will bring us a lot of information about the crater formation process. We report the surface condition around the impact point, such as a large block or a small-boulders area, and an excavated area corresponding to the crater size. The DCAM3 will also observe individual dusts in the ejecta curtain to obtain information of the ejecta velocity distribution, so that we may have a chance to introduce these images. However, please note that this presentation strongly depends on the success of the SCI/DCAM3 operation.