Unlike normal ships, which carried their propellant in tanks, Ymir was a big chunk of solid propellant—ice—inhabited by a sort of parasitical infestation of equipment whose purpose was to convert that propellant into thrust. Not knowing exactly what he would find on Comet Grigg-Skjellerup, Sean had come equipped with more than one alternative architecture for putting Ymir together. If the comet core had turned out to be a loose ball of ice-dust, then he’d have had to scoop out what he needed and pack it into something like a snowball, giving Ymir a spherical shape with the reactor embedded in its center. Another option would have been to fashion a long cylinder of ice and plant the reactor in one end of it, then "burn" it forward, consuming the ice en route, like a candle. What they were seeing now looked more like the third architecture, which was the shard. It suggested that, upon his rendezvous with Grigg-Skjellerup, Sean had found it made up of at least one fairly hard and solid crystal that could be relied on to hold itself together structurally during the maneuvers to come. He had split the shard off from the main body of the comet and planted the reactor system somewhere near its middle, then embedded the rest of his ship—the part where the humans lived—in what would become its nose. If the equipment had worked as planned, then executing the "burns"—i.e., pulling out the control blades to place the reactor into operation and make steam—had been a matter of sending signals to actuators embedded in the core: motors that would move the rods, valves that would control the flow of steam and water, and so forth.