Bob Kobres has been talking about the danger posed by comet impacts since the early ‘80s. He’s got a better handle on the issue than most. And I’m of the opinion that just about everything he’s got to say about the subject should be required reading.
The following overview is mirrored from Bob’s web site. And the original can be viewed at: http://abob.libs.uga.edu/bobk/overview.html
A problem with conveying subject matter related to past encounters with Near-Earth-Objects (NEOs) is the novelty of this area of research. There is presently considerable divergence of opinion and not a small amount of mis-or-misleading information. This is especially true when discussion focuses on the recent past–say the last 12,000 years. In particular, contemporary findings DO NOT support the ideas of Immanuel Velikovsky. On the other hand, it is not correct to assert that there is no evidence of significant, recent, extraterrestrial influence. To aid readers who are new to this area of inquiry I offer this brief overview:
First, it is important to realize that the recent knowledge of numerous large NEOs has forced a shift of paradigm. The idea that conditions on Earth change only slowly (Gradualism) has been accepted and taught since the mid eighteen hundreds and only began to lose dominance in the late nineteen-sixties with the recognition of truly catastrophic floods. This quiescent view of the world was in part held necessary for biological evolution, which was also supposed to occur only through gradual change. Obviously this approved view of the world did not encourage investigation of evidence which suggested abrupt change–the tendency was to rationalize contrary observation into accord with the gradualist paradigm. What this means is that there are, in fact, no true experts in this realm–only a growing number of avid, qualified in their particular field, researchers.
Next, interaction with extraterrestrial material should be viewed as a continuous process with both long and short term components–similar to volcanism but harboring potential for greater affect on an existing order. In other words there is a continuum from dust input due to disintegration of NEOs–particularly comet type bodies–to discreet impacts with objects that are kilometers across. There is currently a tendency for the press as well as new to this field researchers to fixate on the latter due to the more dramatic aspects of large impact events. This focus can be misleading with regard to contemporary risk assessment.
Penultimately, the reader needs to be aware that there is considerable evidence for the recent (less than 30,000 years ago) arrival of a large (greater than 100 km in diameter) comet type NEO. These clues range from direct observation of a growing number of NEOs with above chance similarities of orbital parameters to stories of celestial events that have been passed down or archeologically recovered. One problem here is that folklore, symbolic artifacts, downturns of climate, the collapse of a civilization, and so on, have been studied with no appreciation of the potential for extraterrestrial influence. Whether one views this with dread or zest, there is need to reevaluate much potentially pertinent material.
Finally, there is reason to see this threat to the biosphere as an incredible opportunity to improve conditions on Earth. This can be accomplished by exploiting resource rich NEOs using energy directly from the Sun as the environmentally injurious processes of extracting the same elements from Earth are phased out.