Paul R. Weissman, 1990. "The cometary impactor flux at the Earth", Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality, Virgil L. Sharpton, Peter D. Ward
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The spacecraft flybys of Comet Halley in 1986, along with other new techniques for observing long- and short-period comets, have greatly increased our knowledge of cometary nuclei. In addition, new dynamical studies have expanded our understanding of the Oort cometary cloud and suggested the existence of a massive inner Oort cloud that can be the source of intense “cometary showers” into the terrestrial planets zone. This new information is used to obtain revised estimates for the current cometary cratering rate on the Earth, both from the steady-state flux of long- and short-period comets, and from random cometary showers caused by close stellar passages and encounters with giant molecular clouds. It is found that the cometary showers account for approximately 17 percent of terrestrial craters ≥10 km in diameter, versus the steady-state flux of long- and short-period comets, which provides about 12 percent of the cratering flux. The total current calculated cratering rate for the Earth, including Earth-crossing asteroids, is in good agreement with the terrestrial cratering rate estimated from counted craters on dated surfaces. No evidence is found for an enhanced cometary flux at this time, or for periodic cometary showers every 26 to 32 m.y.