Well, 99942 Apophis has about a 1 in 45k shot of hitting us. It's believed that even if it does hit, it won't be an extinction level hit. Will sandblast most of CA if it hits the Pacific though... Obviously, a Comet's orbit results in a much higher unpredictability rating due to the rocket effect of a comet vs an asteroid. On an extinction level, threat scales as the square of the object's speed. Jets or Gholston have much better odds. Extinction level asteroid hits us about once every 650,000 years. Object that will destroy a large city hits us about once every 6,000 years. In fact, one flew by us a few years ago and we didn't even know about it because it came from the "blind spot".
