I saw a program on The Science Channel recently were a guy was rethinking the distinction between comets and asteroids. They tend to come from different starting points and asteroids tend to have more rock and metal, but overall they are more alike than not.
A bit nit-picky but hey, these are the internets and what else are they for?
stewartm0205
They forget a comet impact. Not exactly the same as an asteriod impact.