I found this to be an intersting article. Thought others might enjoy
reading it. Cool use of military satellites.

"Medium-size asteroids that could flatten a city the size of New
York strike Earth less frequently than previously believed, possibly
only about once a millennium, according to a study aided by military
satellites."

http://www.salon.com/tech/wire/2002/11/20/asteroid_study/?x

TangoMan