Check out some poking around I've been doing on comet and asteroid dynamics from various databases. Comets are bad for our health! Well, at least over the long term-- look at what just *one* fragmet of Shoemaker-Levy 9 (the comet which impacted Jupiter in July of 1994) would have done had it hit Michigan instead of Jupiter! ... Charles Morris' JPL Comet Page . Mike A'Hearn's Home Page . Casey Lisse's Home Page . Small Bodies Node ...