I take it you’ve all heard about asteroid 2004-MN4, which NASA have been having a field day over, because recent estimates have said that it’s chance of impact with the earth (on Friday 13th April 2029) could be as high as 1-in-37 (2.7%)… well; I’ve just found a well-hidden page on NASA‘s Near Earth Object Program that makes an impact estimate that’s terrifyingly higher… see a scary estimate (the server’s a little unreliable, you may need to try to connect a few times)…
Also, why not play with Arizona State University‘s Impact Effects Calculator, which estimates the damage that would be done by an asteroid impact with the parameters you provide.
That was funny. Made you look, didn’t I?