16:00 〜 16:15
[PPS03-20] Ejecta plume evolution observed in the Hayabusa2 impact experiment on Ryugu
キーワード：小惑星、衝突イジェクタ、スケーリング則
Impact cratering is one of the most fundamental geological processes in planetary formation. The resultant particle ejection contributes to either resurfacing through the accretion process or interplanetary material transport as a consequence of ballistic escape. A number of experimental and numerical studies have therefore been conducted to characterize the dynamical evolution of impact ejecta. Nevertheless, their validities have rarely been tested in actual space environments. Against this background, Hayabusa2 conducted an impact experiment on the asteroid Ryugu in 2019.
In this experiment, a small carry-on impactor (SCI) was released from the spacecraft, firing a 2-kg liner at a velocity of 2 km/s. Previous research found that an artificial crater with an apparent diameter of 14.5 m was formed in the gravity-dominated regime [1]. In this study, we expanded our research to reconstruct the evolution of the ejecta plume produced by the SCI impact, relying on optical images obtained by Hayabusa2’s deployable camera (DCAM3) [2]. The dynamics of impact ejecta was investigated via two different approaches: the shape estimation of the ejecta curtain and the trajectory estimation of ejected boulders.
In the former analysis, we determined the velocity distribution of SCI ejecta by comparing the observed profile of the ejecta curtain and the theoretical model for gravity scaling. The major finding was that the material constants associated with ejecta scaling laws coincide with those of the typical sand target, μ = 0.41 and C2 = 0.64 [3], within their error ranges. The ejection angle of ejecta particles increased from approximately 35 to 50 deg in accordance with the distance from the SCI impact point. Although the angle and distance correlation was opposite to that of many on-ground experiments, the discrepancy can be well explained by the effects of oblique impact [4] and/or surface undulation. Combining the obtained results and the mathematical model describing subsurface excavation flows (Z-model), we further constrained the total mass excavated by the SCI impact and the fraction of particles that escaped from the asteroid.
The latter analysis aimed to determine the trajectories of decimeter-sized boulders observed as individual bright spots in DCAM3 images [5]. We successfully estimated the three-dimensional trajectories of four ejected boulders from 5–10 images. These boulders were ejected in the northeast to southeast directions. The surface-relative launch velocities were estimated to be approximately 20–30 cm/s, which were small enough for the boulders to return to the surface of Ryugu. The ejection angles of these boulders lay between 35 and 55 deg and were close to those of fine particles composing the ejecta curtain.
The ejecta evolution models constructed through this research could serve as a reference for future on-ground experiments and other space missions such as DART/Hera.
[1] Arakawa et al. (2020) Science, 368, 67-71.
[2] Wada et al. (2021) Astronomy and Astrophysics, 647, A43.
[3] Housen & Holsapple (2011) Icarus, 211, 865-875.
[4] Richardson et al. (2007) Icarus, 191, 176-209.
[5] Kadono et al. (2020) The Astrophysical Journal Letters, 899, L22.
In this experiment, a small carry-on impactor (SCI) was released from the spacecraft, firing a 2-kg liner at a velocity of 2 km/s. Previous research found that an artificial crater with an apparent diameter of 14.5 m was formed in the gravity-dominated regime [1]. In this study, we expanded our research to reconstruct the evolution of the ejecta plume produced by the SCI impact, relying on optical images obtained by Hayabusa2’s deployable camera (DCAM3) [2]. The dynamics of impact ejecta was investigated via two different approaches: the shape estimation of the ejecta curtain and the trajectory estimation of ejected boulders.
In the former analysis, we determined the velocity distribution of SCI ejecta by comparing the observed profile of the ejecta curtain and the theoretical model for gravity scaling. The major finding was that the material constants associated with ejecta scaling laws coincide with those of the typical sand target, μ = 0.41 and C2 = 0.64 [3], within their error ranges. The ejection angle of ejecta particles increased from approximately 35 to 50 deg in accordance with the distance from the SCI impact point. Although the angle and distance correlation was opposite to that of many on-ground experiments, the discrepancy can be well explained by the effects of oblique impact [4] and/or surface undulation. Combining the obtained results and the mathematical model describing subsurface excavation flows (Z-model), we further constrained the total mass excavated by the SCI impact and the fraction of particles that escaped from the asteroid.
The latter analysis aimed to determine the trajectories of decimeter-sized boulders observed as individual bright spots in DCAM3 images [5]. We successfully estimated the three-dimensional trajectories of four ejected boulders from 5–10 images. These boulders were ejected in the northeast to southeast directions. The surface-relative launch velocities were estimated to be approximately 20–30 cm/s, which were small enough for the boulders to return to the surface of Ryugu. The ejection angles of these boulders lay between 35 and 55 deg and were close to those of fine particles composing the ejecta curtain.
The ejecta evolution models constructed through this research could serve as a reference for future on-ground experiments and other space missions such as DART/Hera.
[1] Arakawa et al. (2020) Science, 368, 67-71.
[2] Wada et al. (2021) Astronomy and Astrophysics, 647, A43.
[3] Housen & Holsapple (2011) Icarus, 211, 865-875.
[4] Richardson et al. (2007) Icarus, 191, 176-209.
[5] Kadono et al. (2020) The Astrophysical Journal Letters, 899, L22.