... Stuart and Binzel Asher et al. year 1984 1998 entity collisions method historic comet encounters comet dynamics comet dynamics comet dynamics Spaceguard close encounters with NEAs close encounters with comets Spaceguard lunar meteorites close encounters with NEAs active comets 43 Myr (mostly 1 km) Halley-type comets "comparable with NEAs" Impact frequencies Estimates of Tunguska-like impact frequencies have varied enormously over the years. ... The actual number of active H T comets is ~25. ...

... What's New? Bad Astronomy . ... News . ... Bitesize Astronomy . ... This time, they are reporting -- as straight news -- a story that on May 25 of this year a comet fragment will impact the Earth . ... Comets are giant hunks of ice and rock , and when they get near the Sun the ice turns into a gas and creates the long tail. ... However, the fragments of Schwassmann-Wachmann are all still pretty close together. ... While that's close as comet passes go, that's still a long way. ... 2008 Phil Plait. ...