For the plane of the old moon—the place they had to go to find safe refuge in Cleft—was where all the rocks were. That was where the rocks had started out, at Zero, and for the most part that was where they had stayed. The ones that had fallen to Earth in the Hard Rain were only a tiny fraction of the lunar debris cloud: just a faint dusting compared to what remained up here. During most of Endurance’s journey, her pilots had, by choice, kept her in that angled Baikonur-compatible plane, well clear of the moon’s debris field. Otherwise they could never have survived for this long.

But the risk that they had to accept, in order to try for Cleft, was to fly through the debris cloud in which Cleft swam. Every time they had reached an apogee in the last few months, and burned their engines to bring their orbit closer to the plane of their destination, they had edged into dirtier and more dangerous space.

Their slowness was part of the problem. If the debris cloud was a fleet of cars roaring around a circular raceway at top speed, then Endurance was a child toddling out into traffic. That extreme disparity in speed would remain until the next apogee, ten days from now, when they would make their biggest and longest burn, expending all of Endurance’s remaining propellant to accelerate her to the same average speed as the debris cloud. In so doing, they’d convert the dual-hairpin orbit into a nearly perfect circle, remaining 384,512,933 meters from Earth forever. Having merged smoothly with the traffic on the circular raceway, they would go hunting for Cleft. Doob had spied it several times on his optical telescopes, gotten a fix on its params, knew how to find it.

This was his life’s work.

If he’d been asked several years ago, before Zero, he’d have said it was something else. But his life until Day 360 had been nothing more than preparation for the mission plan he had laid in and was now executing for Endurance. The day of the Break—the arrival of the propellant needed, the death of his friend and colleague Konrad, and the sundering of the Swarm—had made it clear what needed to be done, and who needed to do it. So he’d been doing it.

Ten days remained until they were swimming in the debris cloud. Perhaps a fortnight before they reached Cleft. He wondered if he would live to see it. Quite obviously, he had cancer. Diagnostic facilities were lacking, but the first undeniable symptoms had been in his digestive tract, and since then his liver had become swollen by metastases. Now he was feeling some weird stuff in his lungs. It had grown slowly. It might have been natural causes—something that had been seeded on Old Earth, before he had even come to space—or it might have been a piece of fallout that had made its way into his food and gotten caught in his gut. No matter. The main question on his mind was whether he would live to see Cleft. He actually didn’t feel that bad, and so the naive answer would be yes, of course; but cancer growth was something of an exponential phenomenon, and he knew how tricky those could be.

Bolor-Erdene was flying the ship, working in the Hammerhead—the deeply sheltered control room that they had built into the lee side of Amalthea. Or at least she was on the duty roster as the nominal pilot. Distinctions of rank and specialty had ceased to matter much. Everyone who had survived—nine men and nineteen women—knew how to do everything: fly the ship, fix an arklet engine, go on a space walk, program a robot. The Doob of a few years ago would have ridden it out in the Hammerhead with her, looking over her shoulder, checking the params, swapping witty remarks in the occasional moment of downtime. The Doob sitting in the Banana right now had seen it all before, thousands of times, and knew that it was as routine to Bo, or to any of the survivors, as driving to work would have been before Zero. Being there would only have gotten his stomach riled up. He needed to conserve his energy.

He realized that he had dozed off. Opening his eyes and focusing, with some effort, on the screen, he saw that nearly an hour had passed since apogee. They were falling toward Earth for the last time.

His phone rang. Held at arm’s length it was blurry, but some vestigial part of his brain could still recognize the smear of pixels as a snapshot of Bo, taken years ago. He swiped it on and answered it.

“We are being contacted by the Swarm,” Bo said.

“Are we really?” he answered. Suddenly he was awake. “What does J.B.F. want?”

“It’s not J.B.F. It’s someone named . . .” Bo paused. “A-ida. Or something. Two dots on the i.”

Doob tried to place the name. Aïda. He had a vague memory of her from his early days on the Cloud Ark. An Italian girl. Young. Arkie, not GPop. Socially a little weird. Hyperacute in a way that could be exhausting.

“It’s pronounced ‘I-yeeda,’” he told Bo.

“Anyway, they send congratulations on the successful completion of our maneuver, and request a parley. Should I wake up Ivy?”

“I’ll be there in a minute,” Doob said. “Let her sleep.”

He hated to think this way, but the Swarmers well knew what time it was, and which shift Ivy slept on, and that she was sleeping now. Rousting her out of bed would send the wrong message, making the crew of Endurance seem overeager.

Which might have been an excess of caution—a J.B.F.-style exercise of byzantine thinking—he reflected, as he pushed himself up the middle of the Stack. This had become a dingy place, sort of yellowed and shiny with human exhalations, condensed on its ice-cold walls and never really scrubbed off. He was glad he couldn’t see it very well.

They knew so little about the Swarm. From the straggler arklets they’d picked up over the last three years, they knew that J.B.F. had moved swiftly to consolidate her power, exploiting the crisis of the first coronal mass ejection—which had killed something like 10 percent of the population—to set up her own version of martial law. From there the trains had run more or less on time, albeit with a steadily dwindling population, until about a year ago, when some Arkies had begun to rebel and the Swarm had divided into two Swarms, coexisting with each other—as they had little choice—but not talking.

The people of Endurance had paid surprisingly little attention to matters Swarm related, because, in the end, it didn’t really matter that much. The die had been cast on the day of the Break. Not so much on the level of politics as on physics. Those who had stayed behind on Izzy had committed themselves to following Doob’s plan, his life’s work: the Big Ride. You were either aboard Endurance, simultaneously trapped and protected by her mass, or you weren’t. If you were, there was no getting off. If you weren’t, you had to find a way to survive as part of the Swarm, which meant moving to a completely different orbit and following a plan that was incompatible, on an orbital mechanics level, with the Big Ride. Once those orbits had diverged, the only way to reconnect was by effecting a big delta vee. That meant spending a lot of water that you were never going to get back. Less water meant less shielding from coronal mass ejections, limited food production, and hobbled maneuvering when bad rocks came at you. Getting a whole Swarm to agree on that course of action was impossible, and might actually have been a bad idea, since Endurance couldn’t accommodate a lot of refugees. Her mission plan was predicated on her ability to absorb significant bolide strikes without taking serious damage. A bunch of naked arklets following in her wake would soon get beaten to death. So on a physics level alone, the Break had been irrevocable, even had the two groups badly wanted to get together.

But apparently what was left of the Swarm had been watching Endurance. Biding their time, waiting to see whether she would win through.