Forty-eight hours, give or take a few minutes, now remained before Earth would loom huge below them, and the nadir surface of the shard would sweat and steam as the radiant heat shining up from the incandescent air softened, melted, and vaporized the ice. It was then that they would have to pull out the control blades and execute Ymir’s next big burn. First they would have to spin the whole ship around so that she was flying “backward,” her nozzle bell pointed in the direction of movement. For the delta vee they needed was a negative one—a braking, as opposed to accelerating, burn.
For spin moves, all spaceships were equipped with thrusters, not powerful enough to impart big delta vees but capable of rotating the ship as a whole into the desired attitude so that the main engine was pointing in the right direction. As a rule the thrusters were more effective when they were situated out toward the “corners” of the vehicle, where they could exert more leverage and crowbar the thing around with minimal thrust. Not knowing what they were going to find at Grigg-Skjellerup, the mission planners for Ymir had packed aboard a collection of modular thruster assemblies that basically consisted of little rocket engines, propellant tanks, wireless control links, and hardware for anchoring them into ice. A cursory survey of Ymir and a look at the dead crew’s records made it clear that Sean and his crew had embedded those packages into the ice at suitable locations: one complex up at the nose with nozzles aiming in four perpendicular directions, and four more spaced around the fattest part of the shard.
Now that New Caird was docked, her engine could also be put to use in getting Ymir spun around. But this one maneuver—a 180-degree flip, which would have seemed comparatively simple in a small craft such as an arklet—was fraught with difficulties and complications in something as huge and asymmetrical as Ymir. Anticipating a need to use the thrusters, Dinah sent robots out to inspect them during that first “morning,” and Vyacheslav suited up and went out to do a bit of troubleshooting on a propellant line that had somehow become kinked. But so ponderous were the shard’s movements that the actual rotation, end-over-end, consumed eight hours, and tweaking it into precisely the right orientation then took another six.
Whereupon Markus announced that all of their assumptions were probably wrong anyway.
“The atmosphere is too big,” he said. He had been staring pensively, for a long time, at a string of emails from Izzy.
Dinah felt a spear go through her heart. After all that had happened in the last couple of years, it was remarkable that she still had it in her to react in that way to bad news. It seemed to be some kind of built-in psychological program, triggered by phrases like “your mother has cancer,” “there’s been an explosion in the mine,” or what Markus had just said.
They had known, from very early in the planning of the Cloud Ark, that the Hard Rain would heat up the air—all of the air, all over the world. When air got hotter, it expanded. The atmosphere had only one direction in which it could expand: out into space. So, whatever drag Izzy felt from the traces of air at its accustomed altitude of some four hundred kilometers was bound to get worse as the atmosphere reached upward. How hot the air would get, how much it would expand, and how heavy the drag would get were questions of colossal import that, however, simply could not be answered until the Hard Rain actually started. As Doob always put it, the experiment of blowing up the moon had never been attempted before. The most they could do was wait and perform observations. Which was exactly what they had been doing ever since the Hard Rain had begun. But Markus had been distracted for most of that time, and was only now absorbing the latest results.
For the Cloud Ark, of course, there were plans to cover various contingencies. In the easy case where the atmosphere didn’t expand that much and the drag wasn’t too bad, they didn’t have to do much. In the more difficult case—which was apparently the way the experiment was now shaping up—they had no choice other than to raise the orbit of every vessel they had—Izzy herself, and each arklet. The delta vees involved were not that large; three hundred meters per second sufficed to nearly double the orbital altitude and get them well clear of the danger zone. Each arklet had its own engine and enough of a propellant supply to accomplish that. For Izzy, matters were a little more complicated. If they were willing to ditch Amalthea they could get three hundred meters per second pretty easily. Bringing Amalthea along for the ride, however, increased the propellant requirements enormously. All of which had long ago been anticipated by mission planners. This was how the Dump and Run strategy had been dreamed up in the first place.