The comet is carrying an alien life form that will find conditions on mars to be excellent. It will assimilate our rover and satellite technology and create a race of super-intelligent alien bears which will come to earth and try and wipe us out in order to take all our honey, picnic baskets and large breasted women for their evil experiments.
But we'll be our typically cagey human selves though and figure out a way to defeat them at the last possible moment. Possibly by giving them a virus, either biologically or electronically (or both) but not until they have really messed up our planet and wiped out a majority of it's inhabitants. We will however be able to unlock the secrets of their alien bearish technology which will be wicked awesome for those of us who survive.
And life goes on...
