Tunguska-like events FROM ASTEROIDS once every 400 years
As you can imagine, I am real, real sorry to strip away your illusions of safety, but what this study means is that the other roughly 8 or so Tunguska class impacts per 1,000 years which have been observed for the last 5,000 years or so were caused by COMETS, not asteroids.
Since COMETS are far far more difficult to detect than ASTEROIDS, what this study also means is most of Weiler and Morrison's current plans for finding ASTEROIDS fail pretty completely to reduce the total impact danger. The only NASA plan which will work for finding COMETS is Dr. Mazanek's plan for man tended Moon based facilites. (PS - Larry J's orbiting telescopes won't do the job of finding COMETS either.)
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