I think the discovery of a medium-sized asteroid en route to hit Earth in ~2 decades would be, ultimately, a positive event: in 2 decades, the asteroid would be deflected and there would exist a substantial space infrastructure.
What exactly would that achieve? Mars is DEAD. Inhabitable. If you want to build self sustaining bunkers where people are locked in anyway, you can do that on Earth as well. Even if a meteor the size of Texas hits us, Earth would still be a more pleasant environment than Mars could ever be. Besides that, instead of financing a colonization of Mars, they could as well finance a mission to destroy the meteor or change its course.
People still have a hard time dealing with living in the Antarctic. That would be like taking up life in a beach resort in the Maldives compared to setting up shop on Mars.
Probably wouldn't be enough time, still the politicians and their cronnies would sure try to get themselves off the planet. Maybe we could fake a on-coming rock and trick them into leaving?
Maybe that's what triggered the Fundamental Declarations of the Martian Colonies in the first place. They would surely forget all about Mars and concentrate on neutralizing the meteor threat, a monumentally simpler task than trying to establish a viable, self-sustaining colony on Mars.
Yes but what if we suffer one last gasp of flood basalt volcanism? You wouldn't be able to breathe here either. It is always good to have as many worlds populated as possible. Even Mars may not be far enough. We have DM 61+ 366(HIP 9481) Gilese 710 coming in: http://en.wikipedia.org/wiki/Gliese_710 According to the Starflight Handbook, DM 61 366 is supposed to come within .29 ly of us, with a one light year margin of error either side. That could mean a clean miss--or a When Worlds Collide scenario. Moreover, we have WR-104 perhaps aimed at the whole solar system: http://blogs.discovermagazine.com/badastronomy/2008/03/03/wr-104-a-nearby-gamma-ray-burst/