An interesting piece of perspective on the fragility of our position in the universe encountered recently is this: Consider a planetary imact on a scale to cause mass extinction—the KT-impact will do, this piece of information actually came from a programme about the Permian extinction. We're talking about a chunk of something (asteroid, comet—the jury's out) about 10 miles across. Although its overall effects are going to be devestating, a 10 mile diameter isn't really that much of the Earth's surface. It's a bit of a pebble compared with our rock.

At the moment the leading edge hits the surface of the Earth, the trailing edge is still in the stratosphere.