Measurements of Ejecta Velocity Distribution from Regolith Targets in Oblique

Introduction: The velocity distribution of impact ejecta from regolith-like target has been investigated in laboratory for nearly three decades. For the case of vertical impact, the ejecta velocity distribution is determined experimentally and given by a power-law with an exponent of –1.22 [1]. There is an upper limit to the ejecta velocity (Hartmann cutoff velocity) that depends on impact velocity [2]. On the contrary, for the case of oblique impacts, it was reported that there exists a separate group of high-velocity ejecta component above the Hartmann cutoff velocity [3][4]. However, there are few studies concerning the velocity distribution of the high-velocity ejecta component for oblique impacts. More data and further investigation are obviously required to quantify the ejecta velocity distributions from regolith targets for various impact angles. Therefore we performed impact experiments on sand targets for various impact angles, in order to measure the ejecta velocity distribution of the highvelocity component above the Hartmann cutoff velocity. Experiments: Regolith targets were simulated by sodalime glass powders with density of 2.5 g cm-3. The mean diameter of the glass spheres was 229μm. Copper projectiles with mass of 0.27g were accelerated to 226 to 260 ms-1 by an electromagnetic gun. The electromagnetic gun can be tilted at various angles (q=15, 30, 45, 60, and 90O to the target). The ejecta were detected by secondary targets (thin aluminum foil set around the glass powders). The ejecta with sufficient velocities can penetrate the secondary targets, and then leave holes on the foil. The thicknesses f of the foil were 10, 15, 20, 25, 30, and 40μm. By using a transmission microscope, the number of holes penetrated by ejecta was measured. Figure 1 shows that the total number N of holes on the aluminum foil for each shot. The result clearly shows that N increases with decrease in q. It is noted that there is the critical thickness of the foil that ejecta can penetrate. For example, no ejecta can penetrate the aluminum foil with f =25μm or f=30μm at q=60O. There was no holes on the aluminum foil with f=40μm. 1 10 10