I gotta tell ya
This is waaaaay more important than a manned mission to Mars. And we should get started looking for Oort Cloud objects on million-year orbits that intersect the Earth. Unlike near-earth asteroids, we won't see so-called "non-periodic comets" coming. Look at what Shoemaker-Levy did to Jupiter! Holy crap.
On a side note, the technology needed to look for faint, distant, comets (~1000 meter diameter) at distances of 100 AU or more might help develop techniques to image small exoplanets in other star systems. They could call it "Exo-Planetesimal Imaging (EPI)" or "Extra-Solar Planetesimal Imaging (ESP)", or something.