Collision rates and impact velocities in the Trojan asteroids swarms

Collision rates and impact velocities for Trojan asteroids have been estimated by a robust numerical approach, based on a parallel computation of orbits over time spans of ~10^4 year and the subsequent statistical analysis of the resulting close encounter data bases. The average intrinsic collision probabilities for the two Trojan swarms 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 ~1 to 14 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 detection of dynamical families among Trojans. The collisional flux in the Trojan regions is very anisotropic, with a dominant component perpendicular to the invariable plane. Subtle dynamical mechanisms are likely to somewhat affect the Trojan collision rates, but their understanding requires further work on the very long-term evolution of Trojan orbits.