![]() We used four types of projectiles, as illustrated in Fig. Numerical simulation is done to support the experimental results. We investigated the sizes of the craters formed on the targets with strength to verify that, as in the gravity regime, crater size is scaled by a conventional scaling law when bulk density is set as projectile density. In gravity regime, as shown in the impact of the SCI projectile, the crater efficiency of hollow projectiles can be scaled by the pi-scaling law for crater formation when the bulk density of the projectile is used (Schultz and Gault 1985). ![]() Thus, in this study, we conducted impact experiments using projectiles of cylindrical or spherical shapes with hollow or solid interiors and targets with strength, basalt, and porous gypsum. Hence, the impact of projectiles with hollow interiors to larger boulders with strength could occur in future missions, and we should investigate the craters caused by projectiles with hollow interiors on targets with strength. The SCI projectile in the Hayabusa2 mission collided with smaller grains on the surface of Ryugu, but larger boulders than the SCI projectile exist near the impact point and it could have possibly collided with these boulders. In particular, there have been few impact experiments using complex-structured projectiles and targets with strength. 2017) and the LCROSS impact (Hermalyn et al. Hence, there have been a limited number of experiments in the field or in laboratories using projectiles with complex structures, such as the impact of hollow aluminum and nylon projectiles into sand and pumice targets in the context of clustered impacts (Schultz and Gault 1985) and the calibration experiments for the SCI impact (Saiki et al. However, to date, most impact experiments in laboratories have used solid projectiles, because the main purpose of these experiments has been simulating the impact of celestial bodies (e.g., Melosh 1989). Projectiles with such complex structures, including hollow interior, have been used not only for Hayabusa2 but also for other planetary explorations in recent years, such as DEEP IMPACT (A’Hearn et al. 2020), where the bulk density of the projectile 1.740 g/cm 3 is calculated by assuming that the projectile is a sphere with a diameter of 13 cm and a mass of 2 kg. An important outcome is that the size of the artificial crater produced on Ryugu is well scaled by a conventional scaling law in the gravity regime when the bulk density is simply set as the projectile density in the scaling law (Arakawa et al. This SCI instrument launched a copper projectile with a mass of 2 kg, in the shape of a spherical shell-a hollow ball with a thickness of approximately 5 mm and a diameter of 13 cm. ![]() Graphical abstractĪn impact experiment was performed on the surface of the C-type asteroid 162173 Ryugu using an instrument called the Small Carry-on Impactor (SCI), carried by JAXA spacecraft Hayabusa2 (Arakawa et al. Thus, we conclude that the size of the craters formed by the impact of projectiles with different shape and interior structure can be scaled using a conventional scaling law in the strength regime, using bulk density as projectile density. ![]() This is consistent with the experimental results. ![]() Results show that the distributions of the maximum (peak) pressure experienced and particle velocity in the targets were similar regardless of projectile shape and interior structure, implying that the dimensions of the final craters were almost identical. The numerical code iSALE was used to simulate the impact of projectiles of various shapes and interior structure with similar bulk densities. Using the bulk density of the projectile, the surface diameter and depth for basalt and the pit diameter and depth for porous gypsum were scaled using the pi-scaling law for crater formation in the strength regime. The surface diameter, inner (pit) diameter, and depth of the craters on basalt and porous gypsum targets were measured. Experiments on crater formation in the strength regime were conducted using projectiles of various shapes with an aspect ratio of ~ 1, including both solid and hollow interiors. ![]()
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