For Arkies and members of the GPop alike, it was easy to underestimate the “distance” separating Izzy from an arklet that was only a few kilometers ahead of or behind it in orbit.
The difficulties entailed in moving from one arklet to another could be mitigated by physically docking arklets to a common structure, forcing them to fly in a rigid formation. Or so it might seem to people who had not been steeped in the laws of orbital mechanics. But the fact was that an arklet docked to the end of a truss far off to the port or the starboard wing of Izzy was not in a proper orbit at all. Left to its own devices—free, that is, of the constraints imposed, and the forces exerted, by the truss—it would converge on Izzy, cross through Izzy’s orbit, diverge from it, turn around, and converge again, on the same ninety-three-minute cycle that clocked Izzy’s orbits around the Earth. An arklet mounted “above” Izzy on the zenith would want to go “slower” and lag behind; one mounted “below” on the nadir would want to race ahead. To the extent that the truss structure prevented those things from happening—to the extent, in other words, that it succeeded in its basic function of holding all the modules and arklets in a fixed configuration—it was undergoing stress, exerting forces on those arklets to prevent them from doing what they wanted to do. Humans in those arklets would notice themselves drifting and bumping into the walls as their natural trajectories, as ordained by Sir Isaac Newton, were perturbed by the structure of Izzy. The larger Izzy grew, and the more arklets and modules were connected to her, the greater those forces became, and the closer she came to breaking.
There was another, even more compelling reason for limiting Izzy’s sprawl. She was taking shelter behind Amalthea.
The space station’s original orbit had been carefully chosen. Any lower, and the thicker air would make the orbit decay too fast. Any higher, and the danger from micrometeoroids would increase. That was because the rocks zipping around in space were subject to the same slow orbital decay as Izzy herself. Which was a good thing, since it dragged them down into the atmosphere and destroyed them, leaving clear space for Izzy to sail through. Her orbital altitude of four hundred kilometers was a Goldilocks compromise between “too much orbital decay for Izzy” and “enough orbital decay to sweep the sky of dangerous rocks.”
The attachment of Amalthea to Izzy’s forward end a few years ago had changed all of that for the better. Orbital decay was less of a problem because of the rock’s high ballistic coefficient, and micrometeoroids tended to get stopped by the massive nickel-iron cowcatcher.
The White Sky, however, was going to put many more rocks into their path. Big ones could be detected from a distance and avoided, but little ones could do a lot of damage, and so the most important parts of Izzy needed to take shelter in the lee of Amalthea, crowding up against her aft surface. Some rocks might still come in from unexpected directions, but in general there would be a “prevailing wind” in the drift of lunar debris. Amalthea was aimed into it.
But Amalthea couldn’t protect any parts of Izzy that projected beyond its silhouette. Dinah and the rest of the Arjuna Expeditions crew had made some progress in “embiggening” the asteroid, carving out slabs of metal and then elevating them like flaps on an airplane wing to extend the sheltered envelope, but it could only get so big. At some point it had been necessary to draw a line under the expansion and “fix the envelope,” meaning that Izzy took on a definite shape and size. This had occurred on A+1.233. Since then they had found ways to jam more modules inside that envelope, or, where that wasn’t possible, to pack bags and bladders of stored material into gaps. And they had tacked on additional storage in the unprotected volume outside of that envelope. But nothing had been added to her before or since. She couldn’t grow in the aft direction because Amalthea’s protective shadow only extended back so far—bolides could “wrap around” in any direction, since they weren’t in perfectly parallel trajectories. Anyway, boost engines were needed back there, and being in the path of rocket exhaust made standing in hellfire seem like a pleasant summer’s day by comparison.