Napier and Asher talk about the threat from both asteroids and comets. Their preprint is available online.
"The Tunguska impact event and beyond"
Bill Napier and David Asher
Astronomy & Geophysics
Volume 50 Issue 1, Pages 1.18 - 1.26
Published Online: 16 Jan 2009
Abstract
Current strategies for dealing with the impact hazard are geared towards the detection and deflection of near-Earth asteroids, which typically have approach speeds ∼20 km s−1 and involve decades of warning. However, galactic signals in the age distribution of well-dated impact craters suggest that the globally destructive impactors (diameters between 1.5 and 2 km and upwards) ultimately derive from the Oort cloud. Warning times are then measured in months or days, and characteristic approach speeds are ∼55 km s−1. Concentrations of sub-kilometre debris in meteor streams may also be a significant regional hazard. Intersection with the debris of a large short-period comet may account for the widespread biological and cultural dislocation in North America around 12 900 BP.
Link: Preprint (PDF)
Link: Astronomy & Geophysics Citation
Link: COSMOS Magazine Article ("Earth under threat from dark comets," Monday, 16 February 2009, Heather Catchpole)
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