“We’re going to have to rely on fission,” he said at last.

Sargenti nodded. “Inefficient, but only one third the neutrino flux per unit of power. Now, if we can power down to one-hundredth our present consumption, that just may be enough.”

“That will dim our neutrino glow by a factor of three hundred. Enough?”

“Yes,” she said. “That should save our necks. We’ll be virtually invisible to them.”

“It’s going to get cramped down here.”

“It’s the only way,” she added.

“And the end of a dream,” Isak continued. “We’re going to,” he said, choking up, “stop all new ranch construction … evacuate most of the shells and shut them down, bringing maybe three or four families into each home….” He trailed off into thought, trying to remind himself not to dwell on the sheer weight of his responsibilities or on the many ways that he could fail, for in that direction lay paralysis, if not madness.

He remembered the story of astronaut Fred Haise, trapped aboard an Apollo moonship that had become crippled by an oxygen tank explosion after crossing the fail-safe point. He and his crew had to redesign and actually rebuild the lunar landing module in midflight, so that it could act as both lifeboat and tugboat for the return to Earth. It had occurred to Haise that the explosion, which had destroyed most of the ship’s systems, might also have damaged the heat shield, meaning that all their efforts to save themselves would come to nothing when they thundered down through Earth’s atmosphere. The thought had nearly crippled Haise, but he shrugged it off and continued to work; for the greater and more present danger was that if he allowed fears of cracked heat shields to obsess him, he would never get the necessary work done and he would die whether or not the heat shield had survived.

“Never dwell on cracked heat shields,” Haise later told young, academy-bound Isak. The lesson was as important now, in the shells of Ceres, as it had been in the little titanium shell of Apollo 13, and Isak found the resolve he needed with which to face his responsibilities to the nine thousand settlers of the asteroid. Most were simply people who loved farming, and they had brought with them the best that selfreplicating robot technology could offer. Starting with an initial investment of only thirty machines, and using as building materials the substance of Ceres herself, whole armies of “worms” and “caterpillars” had grown and spread like a viral infection throughout the worldlet. At the north and south poles, borers were pushing up great dunes of dust, like piles of worm feces grown out of control. The dunes were joining to become mountain ranges; and at the edge of one range, the dust spread glacierlike into lower latitudes, where the centrifugal acceleration flicked it skyward and created a dust storm in space—which threatened to become yet another signature of civilization.

More than twenty kilometers below the spreading dust fields, the first and largest of Ceres’s inner shells was nearly complete. It resembled a giant bicycle wheel embedded in the asteroid, and it provided each of the homesteaders with a ranch larger than some Earth counties.