Catastrophic disruptions revisited

We use a smooth particle hydrodynamics method to simulate colliding rocky a nd icy bodies from centimeter scale to hundreds of kilometers in diameter i n an effort to define self-consistently the threshold for catastrophic disr uption. Unlike previous efforts, this analysis incorporates the combined ef fects of material strength (using a brittle fragmentation model) and self-g ravitation, thereby providing results in the "strength regime" and the "gra vity regime," and in between. In each case, the structural properties of th e largest remnant are examined. Our main result is that gravity plays a dominant role in determining the ou tcome of collisions even involving relatively small targets. In the size ra nge considered here, the enhanced role of gravity is not due to fracture pr evention by gravitational compression, but rather to the difficulty of the fragments to escape their mutual gravitational attraction. Owing to the low efficiency of momentum transfer in collisions, the velocity of larger frag ments tends to be small, and more energetic collisions are needed to disper se them. We find that the weakest bodies in the Solar System, as far as impact disru ption is concerned, are about 300 m in diameter. Beyond this size, objects become more difficult to disperse even though they are still easily shatter ed. Thus, larger remnants of collisions involving targets larger than about 1 km in radius should essentially be self-gravitating aggregates of smalle r fragments. (C) 1999 Academic Press.