Lets hope a 500km comet doesn't crash into earth any time soon.
And I wonder what they mean by "The evidence shows that this happened to earth 6 times before" or however they put it. What evidence? Source?
It's a good possibility!!! read the article linked!
http://www.sott.net/articles/show/151954-Meteorites-Asteroids-and-Com ets-Damages-Disasters-Injuries-Deaths-and-Very-Close-Calls