There's a pretty big assumption in there. That the comet is approaching earth and not going to miss it by half a parsec or that the comet is infact a bit of dirt on the lens of the telescope...
All of which need testing first before you start waving your hands around and start saying "WE'RE GOING TO FUCKING DIE - RUN!"
All of which need testing first before you start waving your hands around and start saying "WE'RE GOING TO FUCKING DIE - RUN!"