The phrase "The Never Ending Story" comes to mind... big asteroid... evacuate the planet. Start all over again... :-)
Basically, if there's an asteroid that big headed toward the planet you're going to WANT to leave the planet no matter what. Something of that size will shatter the planets crust for miles and will likely cause a massive fire storm that would destroy most surface structures as well as make the planet uninhabitable for months, possibly years, decades or even centuries (depending on the internal structure of the planet, its volcanic potential and at what angle/speed the asteroid impacts).
Basically, you have a huge asteroid coming in, you'd better run for your life. A large asteroid that was split into a 9-mile and a 6-mile piece caused enough damage on Earth to force an evacuation attempt... anything larger will just be a catastrophe.
Better get another ship built before that things comes down and gives everybody a headache.