Definition of "A Very Bad Day"
Asteroid Impact
A few years ago Discovery aired a "docu-drama" about the after-effects of a sizable impact, which I thought was pretty good. (I don't think this animation is from that particular show.) The way they depicted it, if you could survive both the actual impact and the 400 degree day that would follow, you stood a reasonable chance of being able to stick around for a while.
More recently, on "The Universe," I believe, it was postulated that if you wanted to extinguish all life on earth, you'd better have an impactor the size of Mars. Interesting, but doesn't that ignore e = 0.5mv^2?