A week ago I posted on  Death by a Gamma Ray Burst?   This week, I focus on another space invader, a large asteroid.  PBS has had several documentaries on this topic:   The Asteroid and the Dinosaur  and   Dinosaur Apocalypse . The scientific term for the above is  Impact Event .  The biggest Earth event might have been the  Big Splash , or the Theia Impact: This when our Moon was formed approximately 4.5 billion years ago. Theia, the mythical Greek Titan, was the colliding body, mother of Selene, the goddess of the Moon. The isotope ratios of lunar and terrestrial rock are identical. There are many theories, but one is that the Moon was formed by the tangential impact upon Earth of a body the size of Mars. Much of this giant-impact consensus emerged from a 1969 conference held at Kona, Hawaii.  Those who don't believe are called the agnostics. But our solar system is today quite different, and planet-sized impact neighbors are no longer present. 2 billion years ago an asteroid cal