Presentation Information
[PPS04-P07]Target strength dependence of angular momentum transfer efficiency in an oblique impact on a porous asteroid
*AYAKA OHNISHI1, Masahiko Arakawa1, Minami Yasui1, Hatsune Okawa1, Reia Kakinoki1, Haruna Toyoshima1, Ryosuke Kikukawa1, Miyu Sawa1 (1.Kobe University)
Keywords:
Asteroid,Impact experiments,Planetary science,Crater formation
Recently, many potentially hazardous asteroids have been discovered. To avoid collision risks with them, NASA carried out the DART mission to alter an object's trajectory by intentionally colliding a spacecraft into it. To apply this approach to other objects, it is crucial to investigate how asteroid orbits change under various impact conditions. Additionally, through the Yarkovsky effect, variations in an asteroid's rotational period and shape also affect its orbit.
While previous studies have conducted impact experiments using targets simulating stony asteroids to examine linear and angular momentum transfer efficiencies, experiments simulating low-strength objects like Ryugu have been scarce. Therefore, this study conducted high-velocity impact experiments using targets simulating low-strength porous asteroids to examine impact-induced translational and rotational motion, as well as shape changes due to crater formation, as a function of target strength.
The experiments were performed using the horizontal two-stage light gas gun at Kobe University, with 60mm sphere targets composed of 100μm quartz sand and gypsum (5 different types with mechanical strengths of 59kPa, 218kPa, 866kPa, 1.2MPa, and 2.1MPa). The projectile was a 2mm-diameter polycarbonate sphere with an impact velocity of 1km/s and impact angles varying from approximately head-on at 2° to oblique impacts at 80°. The spherical targets were suspended in an acrylic box installed within a vacuum chamber, which was evacuated to approximately 20 Pa. For all experiments, the impact was observed by using high-speed cameras recording at 105 fps from both horizontal and vertical perspectives.
Experimental results showed that the momentum transfer efficiency along the projectile's trajectory increased from 1.3 to 1.8 during head-on collisions as the target strength decreased. Even at oblique impacts up to 50°, the efficiency remained higher for low-strength targets. This results from the fact that lower-strength targets produce larger craters, resulting in greater ejecta volume and momentum transfer. Consequently, ejecta with greater momentum directed opposite to the impact direction generates stronger acceleration of the target along the projectile's trajectory.
Post-impact target angular velocity reached a peak at impact angles around 50°, with this peak angle shifting toward head-on impacts as target strength decreased. This is because low-strength targets exhibiting larger angle cones for the ejecta curtain's leading edge, causing a larger fraction of ejecta to be launched in the direction of the target's rotation at smaller impact angles, thus transferring more angular momentum.
Unlike momentum transfer efficiency, the angular momentum transfer efficiency decreased from 1.0 to 0.6 as target strength decreased at a low impact angle of 15°. Furthermore, angular momentum transfer efficiency monotonically decreased to nearly zero as impact angles increased from 15° to 80°. While angular momentum transfer efficiency generally decreased with reduced target strength, for highly oblique impacts, it approached zero across all target strengths.
After the experiments, targets were examined using X-ray CT to analyze the 3D morphology of craters. The results showed that craters formed on spherical targets increased in both radius and depth as target strength decreased. Additionally, deeper craters formed closer to head-on impacts. However, no clear dependence of crater radius on impact angle was observed for targets at 1.2 MPa and 2.1 MPa. For these two high-strength targets, both the velocity component normal to the target surface and the tangential velocity component played significant roles even during oblique impacts. In the case of the 59 kPa target, maximum crater radius appeared at approximately 24° of impact angle, larger than 0°. This complex dependence of crater radius on impact angle may be attributed to shear stresses generated during oblique impacts in crater formation.
While previous studies have conducted impact experiments using targets simulating stony asteroids to examine linear and angular momentum transfer efficiencies, experiments simulating low-strength objects like Ryugu have been scarce. Therefore, this study conducted high-velocity impact experiments using targets simulating low-strength porous asteroids to examine impact-induced translational and rotational motion, as well as shape changes due to crater formation, as a function of target strength.
The experiments were performed using the horizontal two-stage light gas gun at Kobe University, with 60mm sphere targets composed of 100μm quartz sand and gypsum (5 different types with mechanical strengths of 59kPa, 218kPa, 866kPa, 1.2MPa, and 2.1MPa). The projectile was a 2mm-diameter polycarbonate sphere with an impact velocity of 1km/s and impact angles varying from approximately head-on at 2° to oblique impacts at 80°. The spherical targets were suspended in an acrylic box installed within a vacuum chamber, which was evacuated to approximately 20 Pa. For all experiments, the impact was observed by using high-speed cameras recording at 105 fps from both horizontal and vertical perspectives.
Experimental results showed that the momentum transfer efficiency along the projectile's trajectory increased from 1.3 to 1.8 during head-on collisions as the target strength decreased. Even at oblique impacts up to 50°, the efficiency remained higher for low-strength targets. This results from the fact that lower-strength targets produce larger craters, resulting in greater ejecta volume and momentum transfer. Consequently, ejecta with greater momentum directed opposite to the impact direction generates stronger acceleration of the target along the projectile's trajectory.
Post-impact target angular velocity reached a peak at impact angles around 50°, with this peak angle shifting toward head-on impacts as target strength decreased. This is because low-strength targets exhibiting larger angle cones for the ejecta curtain's leading edge, causing a larger fraction of ejecta to be launched in the direction of the target's rotation at smaller impact angles, thus transferring more angular momentum.
Unlike momentum transfer efficiency, the angular momentum transfer efficiency decreased from 1.0 to 0.6 as target strength decreased at a low impact angle of 15°. Furthermore, angular momentum transfer efficiency monotonically decreased to nearly zero as impact angles increased from 15° to 80°. While angular momentum transfer efficiency generally decreased with reduced target strength, for highly oblique impacts, it approached zero across all target strengths.
After the experiments, targets were examined using X-ray CT to analyze the 3D morphology of craters. The results showed that craters formed on spherical targets increased in both radius and depth as target strength decreased. Additionally, deeper craters formed closer to head-on impacts. However, no clear dependence of crater radius on impact angle was observed for targets at 1.2 MPa and 2.1 MPa. For these two high-strength targets, both the velocity component normal to the target surface and the tangential velocity component played significant roles even during oblique impacts. In the case of the 59 kPa target, maximum crater radius appeared at approximately 24° of impact angle, larger than 0°. This complex dependence of crater radius on impact angle may be attributed to shear stresses generated during oblique impacts in crater formation.