Thanks to a lot of ticklish work by robots in the last couple of years, the Hammerhead was now separated from the rest of Amalthea—99 percent of the asteroid’s bulk and mass—by walls of nickel-iron that were only about as thick as the palm of a person’s hand. This still made them extremely massive by the standards of space architecture—more than strong enough to hold in atmospheric pressure and to stop small bolides. But the additional tens of meters’ thickness of metal beyond those walls was now physically detached from the hand-thick walls, and could be pushed away by a puff of compressed air.