These papers make excellent fodder for the Tusk and I will put up a several of them in the next few weeks. I debated which I wanted to post first, and decided to separately post two of my favorites. First, below, from the indefatigable Bill Napier; and a second less familiar but welcome voice, anthropologist Terry Jones, PhD of the University of California Polytech State, San Luis Obispo, California.
There is now compelling evidence that an exceptionally large (50-100 km) comet entered a short-period, Earth-crossing orbit some time in the Upper Palaeolithic, and underwent a series of fragmentations. During this disintegration the Earth was probably subjected to occasional episodes of intense bombardment. Such an episode might constitute a sensible astronomical framework for understanding the postulated catastrophe at 12,900 BP. Concentrations of sub- kilometre bodies may still exist in meteor streams and constitute a significant hazard. Such bodies are difficult to detect, and current deflection and mitigation strategies do not seem adequate to deal with them. For larger bodies, a paradox exists in that the number of comets expected to be thrown into Halley-type orbits (periods 20-200 years) is at least two orders of magnitude greater than observed. The fate of these comets is unknown, raising the prospect that a significant population of dark Earth-crossing comets may exist and adding further uncertainty to impact hazard assessments. Discrete bombardment episodes are evident in the well-dated impact record of the past 250 Myr and several coincide with transitions between geological periods. There is evidence that these episodes have a ∼ 35-37 Myr periodicity, which may be connected to Galactic disturbances of the Oort comet cloud. The threshold for periodicity begins for impact craters >∼ 40 km in diameter; since this is also the threshold which impact ejecta create worldwide conflagration, it again implies that comets are a significant, if not dominant, component of the global impact hazard. W. M. Napier, Journal of Cosmology, November 2009