Oh come on, you say. Would a collision really have been that bad? After all, the earth has a diameter of eight thousand miles. Being hit by a half-mile wide asteroid would be like being tapped by a B-B. In one sense, that is true. The earth itself would not be moved from its orbit, or anything that dramatic. But we who live here would definitely have noticed. Calculations have shown that the explosion would have been equivalent to one thousand million tons of TNT going off all at once. Dusty Conditions A cloud of dust and debris would have been thrown up into the stratosphere, gradually darkening the skies and chilling the planet for months or even years. Many scientists believe just this kind of thing wiped out the dinosaurs, by changing the environment rapidly.