Quote:
Originally Posted by higginsdj
2. Anything over 1km in size will most likely kill all life on earth. The debris raised into the outer atmosphere (and beyond) will rain back down over the entire surface at temperatures exceeding 1000c raising the ambient temperature of the earth atmosphere well beyond that capable of turning the earths surface into a cinder.
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I think you may be a little off here. Impacts of the 1 kilometer in size (asteroids and/or comets) occur on earth about once every million years (several sources on the internet will state this, including a NASA link
here).
A supercomputer simulation of a 1 kilometer impact can be
read here as well. While devastating, it most certainly won't kill off all life on earth.