Abstract
Most Earth-crossing comets arrive from the Oort cloud and the transneptunian region by way of the centaur population, an unstable reservoir orbiting between the giant planets that feeds the Jupiter family and Earth-crossing populations. Fragmentation yields a power law distribution of mass with population index 1.7, from which interception of 10^14 g at 30 km/s may yield several impactors with energies up to 5,000 megatons, fully adequate for surface melting.
Current impact hazard assessments predict one such impact with recurrence time in excess of 50,000 y, but these assessments are based on the erroneous assumption of a steady-state comet population. The occasional injection of giant, short-period comets negates this assumption over timescales relevant to civilization.