This is the profile of an extraterrestrial mass murderer: one whose existence was denied by scientific orthodoxy for nearly two decades, but has now been tracked down. 65 million years ago a 15km wide asteroid hit the Earth. In 1978 Walter Alvarez, a Nobel-prize-winning physicist, and his son Luis, first proposed the outrageous idea that a meteorite strike blasted the dinosaurs into extinction, taking with them half of life on the planet.