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
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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.
~Bob Kobres
A Catastrophe of Comets 
I understand that the scientific lobby will deride any supporter of Velikovsky, but what evidence exists for saying he was wrong?
Why mention him at all?
Since I’m not the author of that essay, It would be disrespectful to change it. I mirrored it exactly as Bob wrote it.
But since you asked, I get a lot of emails from people assuming that the evidence I am studying is supportive of Velikovsky’s assumptions, it isn’t. However it is very highly supportive of the work on the Taurid Complex by astronomers Victor Clube, and Bill Napier. It is also highly supportive of the work on Airburst phenomena done by scientists like Horton Newsom, and Mark Boslough. And the work of Bob Kobres, in connecting all the pieces of the puzzle seems especially prescient from where I stand.
As for providing scientific evidence that Velikovsy was wrong, that’s in the complete lack of physical evidence on Earth that he was right. For example, if Venus, and Mars, had nearly collided with the Earth 3,500, and 2,700 years ago respectively, as Velikovsky claims they did, then there would be debris in the worlds ice caps that was deposited both during, and after, those near collisions. There would also be corresponding evidence in ocean sediments world-wide.
And in the time since Velikovsky wrote ‘Worlds In Collision’ there have been many cores taken; both from the Greenland Ice Sheet, and the Antarctic Ice Cap. There have also been many cores taken from the deep ocean bottoms. In the decades since Velikovsky, they’ve filled warehouses full of cores, taken from all over the world. And not one shred of supportive physical evidence has ever been brought to the surface that those near collisions happened.
And not one shred of supportive physical evidence has ever been brought to the surface that those near collisions happened.
Lets have a think…
There is no supporting evidence for Immanuel Velikovsky in the ice cores from Greenland or Antarctica.
The ice cores from Greenland are meant to cover 123,000 years.
The ice cores from Antarctica are meant to cover 800,000 years.
Therefore, there is no supporting evidence for Bob Kobres in the ice cores.
Therefore, there is no supporting evidence for Catastrophism in the ice cores.
See my line of reasoning:
Perhaps you are looking for evidence in the wrong place…
Perhaps the dating of the ice cores are wrong [very likely]
Bottom line:
The strength of Immanuel Velikovsky’s analysis lies in the vast array of historical threads that he weaves into his historical narrative. Some of his threads may be weak [or tenuous] but, overall, his analysis is still remarkably strong. Additionally, the evidence used to counter Immanuel Velikovsky is usually very weak or extremely tenuous.
There is an antidote for Velikovskian delusions like yours. It can be read here: (Link)
As for your statement that there is no evidence of catastrophe in the stratigraphic record; nothing could be further from the truth. See Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. And for supportive evidence of the stratigraphic work of R.B. Firestone et al, you can turn to A.V. Kurbatov et al who also reported the Discovery of a nanodiamond rich layer in the Greenland ice sheet
The significance of the nanodiamonds at the bottom of the Younger Dryas boundary layer, and in the corresponding layer in the Ice Sheets, is that they can only form in the extreme temperatures, and pressures, you might find in a nuclear explosion, or a major impact event. And they are found at the bottom of the YD layer all over the northern hemisphere.
Below that level you find fossils of many of the large animals that used to be on this continent. Including two species of elephant, the Mastodon, and the Wooley Mammoth, the Giant Sloth, a species of Camel, and a horse, the Sabre Toothed cats, the Short Faced bear; the full list of animals that can be found in the stratigraphic record below that level, but not above it is huge.
The Clovis culture vanished from this continent at that time too.
So, contrary to your silly statement that there is no evidence of catastrophe, there is much evidence in the stratigraphic record, and the Greenland Ice Sheet, that indicate a catastrophic extinction level impact event happened around 12,900 years ago. There’s just nothing in the stratigraphic record, or ice cores that’s supportive of Velikovsky’s delusional assumptions that Mars, and Venus nearly collided with the Earth.
Velikovsky claimed that Venus and Mars nearly collided with the Earth 3,500 and 2,700 years ago, respectively. But if the debris Venus deposited in Earth’s atmosphere was so massive it caused 40 years of darkness after the Exodus, where is it today? There is no sign of it in the world’s ice caps or on the ocean bottoms
“An even greater and more poignant disproof of Velikovsky, which has long been overlooked, is the following: If Venus got close enough during a near collision for its air to flow onto Earth, then, as Philip Plait explains in BAD Astronomy (Wiley, 2002), this "means that Venus would have to be closer than 1,000 kilometers (600 miles) from the surface of the Earth" (p. 181). Such an encounter would have sterilized Earth’s biosphere and flung the Moon into interplanetary space. Neither happened and, in fact, ancient lunar calendars and other records show that the Moon’s orbit has not changed significantly in the past 5,800 years.”
~Leroy Ellenberger
The debate as to the cause of the Younger Dryas Cooling, and the Megafaunal extinction event that killed more than 35 genera in the northern hemisphere at the beginning of the Younger Dryas Cooling continues to go around, and around. And the debate is far from over.
There is still much work to do. But a good selection of peer reviewed science on the subject from both sides of the debate can be read at George Howards blog called The Cosmic Tusk. Just look in the right hand column, and scroll down a bit for enough good peer reviewed literature from both sides of the debate to keep you reading for the next year. And none of the mainstream science done to date is supportive of the delusional assumptions of Immanuel Velikovsky.