20K years ago, comet 2P/Encke (a hitherto long period or extra-solar body) was captured in the inner solar system and disintegrated, possibly resulting in periodic earth impacts on a 3K year timescale, associated with the November Taurid meteor shower.
For reference:
http://star.arm.ac.uk/~aac/zetataur.html
http://en.wikipedia.org/wiki/2P/Encke
http://en.wikipedia.org/wiki/Han_Dynasty_silk_comet_atlas
See also: "Evidence of comet impact 13000 years ago" [added 2007-05-22]
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simon
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