> Just breaking it up (at the very last minute, launched minutes or hours before impact) doesn't help so much if everything still hits Earth, because a planet-killer turns into hundreds of city-killers randomly spread out.

Wouldn't smaller objects burn up a greater proportion of their mass in the atmosphere (due to the increase surface area) and kick up less of a dust cloud once they impact? Could hundreds of city-killers leave some areas unscathed and still populated and habitable?

I think it would be a great help if a planet-killer could be downgraded to a civilization-killer or even a species-killer.