"Combining these two points - that most species meet extinction not as a part of a mass extinction, and that only one out of four major (and none of many minor) extinctions can be definitely attributed to an impact - it appears that death by asteroid is not the most likely end in store for our specie. We should not be obsessed with it to the exclusion of other environmental and societal concerns."

Christ! This is not about mass extinction of human beings. Rocks which cause mass extinction events are on the time scale of 100s million years. The human species are on the scale millions of years and human culture on order of tens of thousands of years. Rather, it's about the history of life on this planet (billions of years) and geology (billions of years).

"By examining the size and frequency of meteor impact craters, Gene Shoemaker and others calculated that we might expect a K/T-sized impact every 100 million years. This frequency roughly fits the facts on Earth: there have been five major extinctions in the past 500 million years. But the K/T extinction is the only one undoubtedly caused by an impact. And a salient fact remains - the K/T asteroid came nowhere near wiping out all animals and plant species. We took this hit, reeled a bit, and got back to business relatively quickly."

So what don't you understand? If you were to look at any inner planet (or Moon) you will find a pattern of K/T-sized impact every 100 million years (roughly speaking). Earth, of course, has this same pattern. The details of exactly how big and exactly when & where have yet to be determined/proven. But these types of events *HAVE* occured AND they would have caused mass extinction events.

Oops out of time, I'll try to get back to rest of post.