The properties of fragments from catastrophic disruption events

We report and discuss the results from a series of catastrophic disruption experiments involving 21-cm spherical targets of alumina cement. These experiments were performed in the open air using a contact charge technique to simulate an impact at similar to 6 km/s, typical of collision velocities between asteroids in the main belt. The 1992 experiments reported here, the most recent in an extensive experimental program initiated by Giuseppe Martelli before his death in 1994, follow directly from those described in I. Giblin et al. (1994a, Icarus 110, 203-224), with a number of improvements to our instrumentation and analysis. By using two high-speed cameras at a mutual angle of 60 degrees we have made possible a three-dimensional analysis of fragment velocities alongside the standard size, shape, ejection angle and rotation rate measurements which can easily be made from appropriately oriented single film records. In this paper we report on the results of the 1992 experiments, together with various unpublished data from 1989. We make a comparison between these sets of data and between our data and those of other researchers in this field. Also, we compare our results to those of the most recent semi-empirical model (SEM) of P, Paolicchi et al. (1996, Icarus 121, 126-157) and to appropriate data concerning real asteroids, focusing on the dynamical families, which are believed to be remnants from the catastrophic disruption of precursor asteroids. A secondary purpose of this paper is to document our experiences and techniques in the implementation and analysis of these experiments. We find considerable variation in the slope of the fragment size distribution, even between closely similar experiments. Fragments are found to be slightly flatter and/or more elongated than those from some previous work, but in agreement with the previous study mentioned above. Fragment velocities are generally between 4 and 20 m/s with a few fast fragments observed up to 35 m/s. Only a weak correlation is found in either linear or angular velocity versus mass. We also test for the existence of a possible "radiant point" from which fragment velocities approximately originate and find that although it serves as a useful component in a model of the break-up, the location of such a point is not constant in our experiments. (C) 1998 Academic Press