Ejecta plume evolution in the Hayabusa2 impact experiment on asteroid Ryugu
Shota Kikuchi, Masahiko Arakawa, Koji Wada, Toshihiko Kadono, Seiji Sugita, Kei Shirai, Ko Ishibashi, Rie Honda, Yasuhiro Yokota, Yuri Shimaki, Naoya Sakatani, Kazunori Ogawa, Hirotaka Sawada, Takanao Saiki, Yuya Mimasu, Yuto Takei, Naru Hirata, Satoru Nakazawa, Makoto Yoshikawa, Satoshi Tanaka, Yuichi Tsuda
Icarus, In Press, Journal Pre-proof, Available online 2 March 2026
“Highlights
- Reconstructed SCI ejecta dynamics from Hayabusa2 deployable camera images.
- Calibrated the ejecta velocity scaling law, yielding the exponent 0.42+/-0.03.
- Ejection angles average 45 deg and correlate with local topography.
- Four decimeter-scale boulders flew for hours to days before reimpacting.
- Ejecta deposit thickness at the Hayabusa2 touchdown site is 2.8 cm.”
“In 2019, Hayabusa2 performed an artificial cratering experiment on asteroid Ryugu by using a small carry-on impactor (SCI). This study focuses on the impact ejecta plume captured by a deployable camera (DCAM3) and investigates its dynamical evolution. In contrast to previous direct kinetic impact missions, the rendezvous configuration enabled high-resolution and comprehensive observations, thereby allowing the empirical validation of impact-physics models under actual space conditions. The image-based analyses are divided into two components: (1) reconstruction of the shape of the ejecta curtain composed of fine particles and (2) estimation of the trajectories of decimeter-scale boulders detected as individual bright spots. In the former analysis, the material constants associated with the ejecta velocity scaling law for the gravity-dominated regime are estimated through numerical fitting to the observed curtain edge. The distance-dependent ejection angle is simultaneously derived, constraining the kinematics of the excavation flow. The latter analysis aims to determine the ballistic trajectories of four ejected boulders based on their image plane centroid locations and nonlinear orbital dynamics. We examine the consistency between the estimated ejection velocities and angles of the boulders and the corresponding representative values for fine particles inferred from the ejecta curtain. The analysis results enable further discussion of ejecta deposition at the spacecraft landing site and of surface modification caused by secondary impacts of ejected boulders. In this study, remote sensing data obtained before, during, and after the SCI impact event are integrated to advance our understanding of the dynamic processes of impact ejecta, from excavation to deposition. A key finding of this study is that the ejecta velocity scaling parameters are μ = 0.42 +- 0.03 and C2 =0.55 +- 0.05. Also, the ejecta launch angle averages approximately 45°, and its spatial variation is consistent with the surface topography. These results indicate that the SCI impact on Ryugu can be characterized as an impact on a typical sand target. Based on the Z-model for excavation with a best-fit streamline exponent of Z= 2.86, the ejecta deposit at the Hayabusa2 landing site was estimated to have a thickness of 2.8 cm and a maximum excavation depth of 1.2 m, suggesting that the returned asteroid samples likely contain subsurface material.”
