was reading up on this story about the new giant crater discovered in Africa (a pretty good story I should probably post here), but I came across this catastrophe calculator. basically you enter information about an asteroid (size, composition, sepdd, etc.) and it'll tell you what will happen to you if you are such a distance from the impact (crater dimensions, thermal radiation, siesmic effects, ejecta, air blast).
http://www.lpl.arizona.edu/impacteffects/
I had some fun with it. I found it takes an asteroid at least 3573.29 miles in diameter to turn the earth into an asteroid belt between Venus and Mars
oh yeah, here's the article on the crater in Egypt
http://www.space.com/scienceastrono...big_crater.html