An extinction event, also known as: mass extinction, extinction-level event (ELE) or biotic crisis is a sharp decrease in the diversity and abundance of macroscopic life. They occur when the rate of extinction increases with respect to the rate of speciation. Because the majority of diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.
Over 99% of documented species are now extinct, but extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.
Since life began on Earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, Cretaceous-Tertiary extinction event, which occurred approximately 65.5 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time. In the past 540 million years there have been five major events when over 50% of animal species died.
There probably were mass extinctions in the Archean and Proterozoic Eons
but before the Phanerozoic there were no animals with hard body parts to leave a significant fossil record. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity.
COMET LEXELL (D/1770 L1) FLYBY IN 1770 AT 0,015 AU
D/1770 L1, popularly known as Lexell's Comet after its orbit computer Anders Johan Lexell, was a comet discovered by astronomer Charles Messier in June 1770. It is notable for having passed closer to the Earth than any other comet in recorded history, approaching to a distance of only 0.015 astronomical units, thats 2,200,000 kilometers or 1,370,000 mi.
The comet has not been seen since 1770 and is currently considered a "lost comet".
If this was the closest flyby ever take a look at NOVEMBER 9 2011 !
On November 9th 2011 (ELEven NINe) - The Comet/Astroid 2005 YU55 will pass the earth at just 0,0023 AU - That is 344.075,4 KM or just 213.798,542 MILES !!! This Comet/Astroid will "pass" between the Earth and the Moon and 2005 YU55 will be the closest ever to "Pass" earth. On the same date (Nov 9th) the Earth will pass the route of Elenin, if Elenin has a tail with debris, earth will be hit by rocks from the tail ... and not only the Earth but also 2005 YU55 will get hit, possibly pushing it in another direction ... OUR direction, that is why ELE-NIN is the name !