One way or another, it's on everyone's minds, living somewhere in the back of our collective consciousness. Hollywood knows it, and continues to plumb it for box office numbers. Sci-fi is rife with it. The fossil record shouts warnings across millennia about it. Even the dinosaurs developed a particular, albeit brief, loathing for it. The killer asteroid--the one that we might never even see coming--could end life on this planet and there would be nothing humans could do about it.