10:00 〜 10:15
[MZZ40-05] Ejection velocities and angles during kinetic impact events on flat and spherical targets investigated with hydrocode simulation
キーワード：小天体、天体衝突
NASA’s Double Asteroid Redirection Test (DART) mission was conducted on 26 September 2022 to investigate and demonstrate the changing of an asteroid’s motion through kinetic impact. The DART spacecraft collided with the asteroid Dimorphos, which is the satellite of the binary asteroid (65803) Didymos. The recoil from the impact ejecta generated by the kinetic impact changed the orbital period of Dimorphos (Daly et al., 2023). The momentum change of the asteroid due to the impact resulted from the combined momentum of the projectile and the resulting momentum of the ejected material. The β value, which is defined by the ratio of the momentum of the projectile to the total momentum of the system after the impact, has been estimated to be larger than two. Thus, the orbital change was caused mainly by the recoil of the impact ejection (Cheng et al., 2023). The origin of the large β value has been discussed (e.g., Stickle et al., 2023; Raducan et al., 2023). The crater size predicted by the π-group scaling laws with the parameters of “saturated soil” (Schmidt & Housen 1987) is larger than the radius of Dimorphos under the microgravity condition, indicating that the DART impact would significantly deform the shape of Dimorphos, suggesting that it is difficult to investigate the DART outcomes with such the analytical method. According to hydrocode simulations, the estimated β values for flat targets are overestimated by up to 30–40 % of those for spherical targets (Raducan et al., 2019; 2022).
Previous studies frequently determined “impact ejecta” as materials exceeding a certain altitude. However, we must carefully define the threshold height where the materials above the pre-impact surface are classified as “ejecta” because the materials in an excavation flow are continuously accelerated until the pressure gradient becomes zero. This phenomenon is called “the post-shock acceleration” (Kurosawa et al., 2018; Okamoto et al., 2020). Such acceleration would become significant under the microgravity environment of the Dimorphos surface. In this study, we investigated the flow fields in flat and spherical bodies driven by kinetic impactors with a 2-dimensional hydrocode in detail. The objective of this study is to propose a threshold height as the height where the materials are free from any pressures from adjacent fluid elements to evaluate how the target geometry affects ejection velocities and angles.
We used the iSALE2D-Dellen shock physics code (Amsden et al., 1980; Ivanov et al., 1997; Wunnemann et al., 2006; Collins et al., 2016). The projectile was an aluminum sphere with the radius of 0.37 m. The impact velocity was set to 6.1449 km s-1, which is the same as the DART impact. The target is a granite plane or a sphere with the radius of 17 m.
We showed that compressibility in an excavation flow should be treated carefully under microgravity conditions because the materials in an excavation flow are further accelerated until the pressure gradient becomes zero. We proposed appropriate criteria for impact ejection and evaluated how the target geometry affects ejection velocity and angle. We also found that the impact ejecta on curved surfaces tend to be faster and ejected at lower angles than flat targets at the given launch positions. Since the proposed method can be easily incorporated into grid- or particle-based hydrocodes, our criterion may be useful for evaluating impact ejection and its momentum under microgravity conditions.
Previous studies frequently determined “impact ejecta” as materials exceeding a certain altitude. However, we must carefully define the threshold height where the materials above the pre-impact surface are classified as “ejecta” because the materials in an excavation flow are continuously accelerated until the pressure gradient becomes zero. This phenomenon is called “the post-shock acceleration” (Kurosawa et al., 2018; Okamoto et al., 2020). Such acceleration would become significant under the microgravity environment of the Dimorphos surface. In this study, we investigated the flow fields in flat and spherical bodies driven by kinetic impactors with a 2-dimensional hydrocode in detail. The objective of this study is to propose a threshold height as the height where the materials are free from any pressures from adjacent fluid elements to evaluate how the target geometry affects ejection velocities and angles.
We used the iSALE2D-Dellen shock physics code (Amsden et al., 1980; Ivanov et al., 1997; Wunnemann et al., 2006; Collins et al., 2016). The projectile was an aluminum sphere with the radius of 0.37 m. The impact velocity was set to 6.1449 km s-1, which is the same as the DART impact. The target is a granite plane or a sphere with the radius of 17 m.
We showed that compressibility in an excavation flow should be treated carefully under microgravity conditions because the materials in an excavation flow are further accelerated until the pressure gradient becomes zero. We proposed appropriate criteria for impact ejection and evaluated how the target geometry affects ejection velocity and angle. We also found that the impact ejecta on curved surfaces tend to be faster and ejected at lower angles than flat targets at the given launch positions. Since the proposed method can be easily incorporated into grid- or particle-based hydrocodes, our criterion may be useful for evaluating impact ejection and its momentum under microgravity conditions.