"It's hard to overestimate the devastation that could result from a serious bolide impact on Earth. The impact of fragments measuring between one and several hundred meters across can cause fiery, multimegaton explosions that destroy natural and cultural features across huge areas through fire blasts, earthquakes, and tidal waves (if the debris arrives over the sea). The danger in A.D. 400-600, concluded Bailey and colleagues, was of Earth running into a "cosmic swarm" of objects the size of the one that exploded over Tunguska, Siberia, in 1908. Some astronomers believe we can expect Tunguska-type impacts every 50 years on average, while an impact with explosive power in the 1,000- to 10,000-megaton range - a super Tunguska event - is likely in any 5,000-year period. Such impacts could trigger enormous global ecological catastrophe. Impacts between those two extremes might be expected often enough to account for these calamities. Direct evidence, however, is scanty. Associating craters to specific events is problematic at best; the Tunguska event left no significant crater at all, since the bolide exploded a few kilometers above the surface. Impacts in or over the ocean would not leave physical evidence.

We turned, then, to the written record and oral traditions. Comets were extraordinary objects that seemed rarely to escape written notice. Zachariah of Mitylene noted about A.D. 540 that - a great and terrible comet appeared in the sky at evening time for 100 days." Chinese texts about the same time say: "Dragons fought in the pond of the K'uh o. They went westward. ... In the places they passed, all the trees were broken." Similar descriptions are common throughout the Old World. Sixth-century events generally are well-dated. But with more ancient documents and traditions, dating usually is ambivalent at best. This is why similarly spaced events in the second millennium B.C. are so interesting. What are the chances of similarly spaced events in both Hebrew and Chinese histories, both with cometary associations, arising by chance? There is, I feel, a strong case for the contention that we do not inhabit a benign planet. This planet is bombarded relatively often. If this story is correct, we have been bombarded at least three times - and probably five times - since the birth of civilization some 5,000 years ago. And each time, the world was changed. "

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