How to Defend Earth From Asteroids

"Only 13,000 years ago, the North American megabeasts -- including woolly mammoths, saber-toothed cats, giant sloths -- also had a very bad day. Well, at least a bad millennium. It was so bad that some scientists have mistakenly attributed that mass extinction to a 2 ½ mile-wide comet explosion. That idea has also been debunked by many scientists"

And it has NOT been debunked by many scientists...far from it. The Younger Dryas Boundary hypothesis (which proposes that a cosmic impact took place 12,900 years ago) is very well documented.

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http://www.ia.ucsb.edu/pa/display.aspx?pkey=2748

"An 18-member international team of researchers that includes James Kennett, professor of earth science at UC Santa Barbara, has discovered melt-glass material in a thin layer of sedimentary rock in Pennsylvania, South Carolina, and Syria. According to the researchers, the material –– which dates back nearly 13,000 years –– was formed at temperatures of 1,700 to 2,200 degrees Celsius (3,100 to 3,600 degrees Fahrenheit), and is the result of a cosmic body impacting Earth."