Experimental investigation of impact close to the edge of boulders

Asteroid exploration has allowed detailed observations of boulders on the surface and measurements of craters on the boulders. We focused on the craters near the edges of the boulders and investigated the distance from the impact point to the side surfaces of finite-sized brittle targets when spallation of the side surfaces occurred. First, impact cratering data was compiled, including previous and newly conducted experiments on porous gypsum and less-porous basalt targets. When the distance from the edge to the impact point was shorter than approximately twice the crater radius, spallation of the side surfaces occurred, irrespective of the target material. Then, explosion experiments were conducted using porous gypsum targets to elucidate the physical mechanisms of this process. We investigated the relationship between the distance from the explosion point to the free surface and the incident angle of the stress waves relative to the free surface when spallation occurs. Experimental results suggest that spallation at the side surfaces occurs when the amplitude of the reflected wave caused by a stress wave incident perpendicular to the side surfaces is greater than that at the rim of the crater formed on the top surface. The quantitative relationships obtained in this study using both porous gypsum and less-porous basalt will help to constrain the history of cratered boulders with a wide variety of porosities on asteroids.