Upon This Rock - David Marusek

The Prophecy family has demolished the entrance to their End Times mountain keep with explosives in order to isolate themselves from Governor Tetlin's army of Doomsday preppers. But the army has other things on its mind - The rogue planet Pipnonia, which is on a collision course with Earth, is being torn apart by cosmic forces in the sky overhead.

Deuteronomy Prophecy, outside the mountain keep and abandoned by her family, enlists the help of Ranger Kuliak in a last-ditch effort to rescue her youngest brothers and sisters via a narrow ventilation shaft. But even if they succeed, where can they find shelter when the sky is literally falling on their heads?