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ThatDamnedPanda -> RE: Comet Lulin (2/21/2009 11:45:51 AM)
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There's no question at all that Earth has been hit by countless comets (and asteroids), and will be hit by many more. Over 170 large-object impact craters have been identified on Earth, ranging in size from about a .15 miles in diameter to over 200 miles (the Vreedevort Crater in South Africa). Other, even larger, impact sites are suspected, but more difficult to positively identify because they're so old they've been almost obliterated by geologic forces over the millenia.
Most of these known impacts have produced effects that can only be described as catastrophic. The worst of them can cause what is known as an "extinction level event" (ELE), or mass extinction of terrestrial life. At least 5 such ELEs have been idenitified over the course of Earth's history, and possibly as many as around 20 - the disagreement is largely a due to lack of clearly-defined standards for what constitutes a mass extinction. It should be noted that not all ELEs are proven to have been caused by impact events, but at least some have been, and in the 5 universally accepted ELEs, as many of 70% of all species on the planet (and in the case of the Permian-Triassic extinction, as many as 90%) were extinguished in each event.
There's no question at all that Earth will be struck again by comets and asteroids, and that there will be more mass extinctions in the future. The only questions are when the next one will hit, and how big it will be. It's very unlikely that any ELE would extinguish all life on Earth, but even the most minor ELE would pretty much destroy modern human society and plunge us into a new Dark Age, and a larger ELE would quite probably wipe out the species altogether. Significant extinction events seem to occur on a cyclic basis, and if I recall correctly cycles of 26 to 30 million years , and 59 to 65 million years, have been postulated by various geologists. But my information is out of date, and it's possible that these two cycles have been discredited since I last read up on it - or confirmed, for all I know. I just don't feel like googling it right now because i'm hungry and i want to get to the beach to have some lunch.
But the point is, large-scale extinctions are regular occurences, seem to occur on a fixed cycle, and are known to at least sometimes be associated with cometary or asteroid impacts. The last major known catastrophic impact was 65 million years ago in the Yucatan Peninsula, and the next one may not happen again for tens, or even hundreds, of millions of years. Or, it could happen next week. We have no way of knowing, and no way of preventing it. So, keep one eye on the sky folks, and have a cheery day!
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