Abstract: The hypervelocity impacts of space debris on bumper of spacecrafts were studied, and dimensions of holes in aluminum sheets were analyzed through 2017-T4 aluminum spheres impacting on thin 2A12 aluminum sheets with different thickness. The sphere diameters ranged from 3.18mm to 6.35mm and projectile diameter to bumper sheet thickness ratio, dp /t, ranged from 1.00 to 9.96. Impact velocities ranged from 1.50km/s to 6.98km/s. An empirical model developed from test data comprising of spherical aluminum projectiles impacting on aluminum target was proposed. The results show that hole diameter expand efficiency is dependence on projectile diameter, aluminum sheet thickness as well as impact velocity. If dp /t is held constant, as the impact velocity increases the hole diameter expand efficiency increases as well. When impact velocity is held constant the hole diameter expand efficiency changes nonlinearly with the dp /t, and as the dp /t increases, effects on the hole diameter expand efficiency weaken.