One day you walk down to the local deli for lunch and are on the way back to work when you find a piano has fallen on the sidewalk, right where you pass every day. Gosh, you think, ten minutes later and I would have been standing right there. Close one. You think that’s close? Imagine how the earth feels. In nineteen-eighty-seven an asteroid measuring a whopping half a mile in diameter came by, missing our planet by a measly six orbital hours. An orbital hour is simply a measure of how far the earth moves around the sun in one hour. So, had we been six hours ahead of where we are, or had the asteroid come by six hours later, hello enormous rock. And, quite possibly, goodbye everything else.