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New Approaches to Risk Assessment from Near-Earth Objects

Recent years have seen a downgrading of the threat assessment due to impacts from comets and asteroids. Data on the mass distribution of fireballs entering the Earth's atmosphere, coupled with the discovery rates of bodies from Near Earth Object surveys, have been used to estimate that Tunguska-like impacts (with energies greater than 10 megatons or so), caused by bodies in the 10-30m diameter range, occur on time-scales of order 1,000 years. Bodies of km size or greater strike the Earth, according to current estimates, on time-scales of order 600,000 years or more. Bill Napier, however, has been using new approaches to the problem and finds that the celestial hazard due to impacts may be a factor $\sim$5 higher these estimates.

First, the rate at which meteorites strike the Moon can be inferred from the number and age distribution of lunar meteorites found in Antarctica and elsewhere. Coupled with hydrodynamic code simulations of crater-forming impacts on the Moon, Napier finds that the small-body impact rate on the Earth is substantially higher than most current estimates, with a Tunguska-like impact expected roughly once every 100-200 years. One or more impacts in the thousand-megaton range may occur over time-scales $\sim$1,000-5,000 years. These impacts would eject a considerable mass of dust into the stratosphere and so be accompanied by brief, global coolings. They may constitute the greatest short-term celestial hazard to civilization.

Secondly, the case of multi-km bodies has been studied building on work by Bailey & Emel'yanenko, and Wickramasinghe & Hoyle. Dynamical models by Bailey & Emel'yanenko had previously shown that the population of comets captured from the Oort cloud and thrown into short-period orbits (orbital periods 20-200 years) should be about 100 times greater than is actually observed. They suggested that the missing comets may comprise a large, dormant population. If so, this population would constitute a significant hazard. However these hypothetical bodies would still be detected through ground-based telescopic surveys $\sim$50-100 times more often than they actually are: at least 400 should by now have been discovered, as against the handful actually found. Thus it has been argued that these dormant comets do not exist in significant numbers, and that the active comets have not become extinct but have instead simply disintegrated. However Napier, in collaboration with Chandra and Janaki Wickramasinghe at Cardiff University, has shown that the surfaces of inactive comets, if comprised of loose, fluffy organic material like the comet dust which enters the Earth's atmosphere, may developed reflectivities which are effectively zero in visible light. The NEO population may therefore include a substantial population of `stealth comets': multi-km bodies, undetectable with current surveys. These bodies may constitute the dominant impact hazard in the 100,000 megaton range.