03-04-2012, 05:29 AM
Well, an astroid big enough to destroy civilization is most likely going to come at one time. It's probably happened before. The question is is whether it will come before we are able to colonize other planets. But as for the OP, I don't think one big enough to kill a big portion of the Earth. And like you said, it's only around 60 meters. Isn't that small enough to burn up before it reaches the ground?