The scientific community did not even accept the fact of asteroid collisions until the early 70's. Even on the moon, they thought most of the crater-work was from vulcanism. The theory of an asteroid wiping out the dinosaurs is a fairly recent one. Two pieces of compelling evidence keep it alive. First, the Chicxulub impact crater. It is a crater discovered on the edge of the Yucatan peninsula by radar topography which was found to be 112 miles in diameter and caused by an object at least 6 miles across. Also, the K-T boundary. This is in short, a layer of earth, fairly visible in many places on earth, where many species' fossils simply disappear. It is recognized by a dark band of sediment, usually riddled with iridium. Most asteroids are iridium rich, which implies that an asteroid strike at this time in the planet's history distributed this thin layer of the element planetwide.