THE PROVENANCE AND EVOLUTION OF COMETS

The process of comet formation through the hierarchical aggregation of originally submicron-sized interstellar grains to form micron-sized p articles and then larger bodies in the protoplanetary disc, culminatin g in the formation of planetesimals in the disc extending from Jupiter to beyond Neptune, is briefly reviewed. The 'planetesimal' theory for the origin of comets implies the existence of distinct cometary reser voirs, with implications for the immediate provenance of observed come ts (both long-period and short-period) and their evolution as a result of planetary perturbations and physical decay, for example splitting and sublimation. The principal mode of cometary decay and collisional interaction with the terrestrial planets is through the formation and evolution of streams of cometary debris and hitherto undiscovered 'fam ilies' of cometary asteroids. Recent dynamical results, in particular the sungrazing and sun-colliding end-state for short-period comet and asteroid orbits, are briefly discussed.