COLLISION RATES AND IMPACT VELOCITIES IN THE TROJAN ASTEROID SWARMS

Collision rates and impact velocities for Trojan asteroids have been e stimated by a robust numerical approach, based on a parallel computati on of orbits over time spans of approximate to 10(4) year and the subs equent statistical analysis of the resulting close encounter data base s. The average intrinsic collision probabilities for the two Trojan sw arms are 6.46 x 10(-18) km(-2) year(-1) (L(4)) and 5.30 x 10(-18) km(- 2) year(-1) (L(5)), namely about twice the corresponding value for the main asteroid belt. Collision speeds range from approximate to 1 to 1 4 km/sec, with a mean value close to 5 km/sec. These findings suggest that the intensity of the collisional process is about the same in the Trojan swarms as in the main belt, and are consistent with the detect ion of dynamical families among Trojans. The collisional flux in the T rojan regions is very anisotropic, with a dominant component perpendic ular to the invariable plane. Subtle dynamical mechanisms are likely t o somewhat affect the Trojan collision rates, but their understanding requires further work on the very longterm evolution of Trojan orbits. (C) 1996 Academic Press,Inc.