5:15 PM - 7:15 PM
[PPS06-P17] Impact angle dependence of angular momentum transfer efficiency in porous asteroids; oblique impact experiments on a porous low-intensity targets
High-velocity collisions among planetary bodies are a universal phenomenon in the solar system, and asteroids are repeatedly destroyed and reaccreted in their histories. Then, the orbital angular momentum of the impactor is transferred to the asteroid, which may affect its rotational evolution. For example, in the gravity-dominated regime, the existence of spin barriers prevents the asteroid from shortening its rotation period beyond a certain period. On the other hand, in the strength-dominated regime, small bodies can rotate very fast. Therefore, angular momentum which is transferred to asteroids is an important parameter to understand their rotational evolution. In the previous studies, momentum transfer and angular momentum transfer efficiency decreased while the impact angle increased [1][2].
However, while previous studies have made experiments for the momentum transfer efficiency of oblique impacts on high-strength targets, few impact experiments have been conducted for low-strength targets that simulate porous asteroids such as Ryugu. Therefore, we conducted impact experiments using low-strength targets made of quartz sand mixed with plaster to study dependence of impact angle.
We used spherical targets with the diameter of 60mm made of a 20:1 mixture of quartz sand (grain size of 100μm)and plaster. The tensile strength is 59.3±22.0kPa, Young’s modulus is 1.4GPa, and Poisson’s ratio is 0.048. All experiments were performed using a two-stage light gas gun at Kobe University, at impact velocities of 1km/s. As impactors, we used a polycarbonate sphere with a diameter of 2mm. The target was hung in an acrylic box to recover the impact fragments after the shot, and the box was placed in a vacuum chamber. The chamber was evacuated up to about 20 Pa before the shot. The impact angle was varied from 2° (which is close to a head-on collision) to 79°. In all experiments, the collisions were recorded at 105 FPS from two directions (horizontal and vertical) using high-speed cameras.
The experimental results show that the momentum transfer efficiency in the direction of projectile trajectory decreased as the impact angle θ increased. As the impact angle increases, the momentum received by the target became less than that of the projectile. At all angles, the normal momentum at the impact point exceeded the momentum received from the projectile (calculated from the normal velocity component). This is because the ejecta which was generated during the crater formation ejected with the normal velocity component, and the target was accelerated as a result of reaction. On the other hand, the target was hardly accelerated in the tangential direction at the impact point. This suggests that the momentum transfer efficiency in the tangential direction was always lower than that in the normal direction. The angular velocity increases with increasing θ from head-on collision to oblique impact, and reaches a maximum value by 60°. As the angle increases further, it hardly rotates at all. As a result, the momentum transfer efficiency in the projectile direction is found to be about 1.4 times larger than that of the high-intensity target in the previous study by Yanagisawa et al. (1991) [2]. The angular momentum transfer efficiency was found to be about 20% lower than that of the high-intensity target of the previous studies.
[1] Masahisa Yanagisawa, Sunao Hasegawa, and Nobutoshi Shirogane (1996), Momentum and Angular Momentum Transfer in Oblique Impacts: Implications for Asteroid Rotations, ICARUS 123, 192-206, ARTICLE NO. 0149
[2] Masahisa Yanagisawa, Janusz Eluszkiewicz, and Thomas J. Ahrens (1991), Angular Momentum Transfer in Low Velocity Oblique Impacts: Implications for Asteroids,
ICARUS 94. 272-282 (1991)
However, while previous studies have made experiments for the momentum transfer efficiency of oblique impacts on high-strength targets, few impact experiments have been conducted for low-strength targets that simulate porous asteroids such as Ryugu. Therefore, we conducted impact experiments using low-strength targets made of quartz sand mixed with plaster to study dependence of impact angle.
We used spherical targets with the diameter of 60mm made of a 20:1 mixture of quartz sand (grain size of 100μm)and plaster. The tensile strength is 59.3±22.0kPa, Young’s modulus is 1.4GPa, and Poisson’s ratio is 0.048. All experiments were performed using a two-stage light gas gun at Kobe University, at impact velocities of 1km/s. As impactors, we used a polycarbonate sphere with a diameter of 2mm. The target was hung in an acrylic box to recover the impact fragments after the shot, and the box was placed in a vacuum chamber. The chamber was evacuated up to about 20 Pa before the shot. The impact angle was varied from 2° (which is close to a head-on collision) to 79°. In all experiments, the collisions were recorded at 105 FPS from two directions (horizontal and vertical) using high-speed cameras.
The experimental results show that the momentum transfer efficiency in the direction of projectile trajectory decreased as the impact angle θ increased. As the impact angle increases, the momentum received by the target became less than that of the projectile. At all angles, the normal momentum at the impact point exceeded the momentum received from the projectile (calculated from the normal velocity component). This is because the ejecta which was generated during the crater formation ejected with the normal velocity component, and the target was accelerated as a result of reaction. On the other hand, the target was hardly accelerated in the tangential direction at the impact point. This suggests that the momentum transfer efficiency in the tangential direction was always lower than that in the normal direction. The angular velocity increases with increasing θ from head-on collision to oblique impact, and reaches a maximum value by 60°. As the angle increases further, it hardly rotates at all. As a result, the momentum transfer efficiency in the projectile direction is found to be about 1.4 times larger than that of the high-intensity target in the previous study by Yanagisawa et al. (1991) [2]. The angular momentum transfer efficiency was found to be about 20% lower than that of the high-intensity target of the previous studies.
[1] Masahisa Yanagisawa, Sunao Hasegawa, and Nobutoshi Shirogane (1996), Momentum and Angular Momentum Transfer in Oblique Impacts: Implications for Asteroid Rotations, ICARUS 123, 192-206, ARTICLE NO. 0149
[2] Masahisa Yanagisawa, Janusz Eluszkiewicz, and Thomas J. Ahrens (1991), Angular Momentum Transfer in Low Velocity Oblique Impacts: Implications for Asteroids,
ICARUS 94. 272-282 (1991)