Note, if you're easily depressed, read no further and hit the back browser or go to another page :o !
Anyways, here it is:
http://www.lpl.arizona.edu/impacteffects/ . If you can figure it out, it queries you on a bunch of scenarios and take forever to figure out, but it does figure it out. The good news, I picked this up off yahoo, and it mentioned that though no objects are calculated to have a significant chance of impact, if they were we would have decades or centuries of warning, so we could potentially gather our resources and find a way to prevent the object from harming us. Anyways, have fun (<--
)