Even when the sun betrayed them, they had each other.

21: Showstoppers

“So there it is,” Rose Delea said flatly. “You have two problems you can’t get over. Without the Chinese heavy-lift capability, you can’t finish the shield infrastructure on time. And even if you could, you don’t have a way to manufacture all the smartskin you need.” She sat back and stared out of her softscreen at Siobhan. “You’re fucked.”

Siobhan pressed the balls of her thumbs to her eyes, and tried to keep her temper. It was January 2039—six months after she had seen those first shield components stacking up on the Moon, already eighteen months since the June 9 event. Another Christmas had come and gone, a bleak and joyless festival, and little more than three years remained before the sunstorm was due to hit.

Save for Toby Pitt and the talking heads from space on the softscreens, Siobhan was alone here in the Royal Society Council Rooms, the location that had come to serve as her communications base. Toby’s job as the Society’s events manager had gradually evolved into his becoming her PA, amanuensis, and shoulder-to-cry-on. And she certainly felt like crying now.

“We’re fucked, Rose,” she said.

“What?”

“Rose, sometimes you sound like my plumber. You’re fucked is wrong. Language is crucial. It’s not my problem, it’s ours. We’re fucked.”

Bud Tooke, peering from another softscreen, laughed gently.

Rose glared. “Fucked is fucked, you stuck-up pom. I need a coffee.” And she pushed herself out of her chair and drifted out of shot.

“Here we go again,” Mikhail said grimly.

Despite her usual intrinsic anxiety about the schedule, before she had come into work this morning Siobhan had actually felt optimistic about the way things were going.

On the Moon, after months of stupendous effort by Bud and his people, the Sling was completed and operational. Even the construction of a second mass driver was under way. Not only that, but the glass manufacturing operations were proceeding apace: plants had been set up all over the bare soil of Clavius Crater, so that streams of components poured into the Sling’s launching bay by lunar day and night. Rose Delea, seconded from her helium-3 processing work, had proven a more than capable manager for that end of the project, despite her dour attitude.

Meanwhile Aurora2 had been safely brought back from Mars and was lodged at L1, the crucial Lagrangian point suspended between Earth and sun. With the Sling fully operational the first loads of lunar-glass buttresses and struts had been fired up to the assembly site, and construction of the shield itself had started. Bud Tooke was now in nominal charge of all the subprojects at L1, and, as Siobhan had always known he would, he was delivering quietly and efficiently. Soon, it was said, the proto-shield would be big enough to see with the naked eye from Earth—or would have been, were it not forever lost in the glare of the sun.

Even Siobhan’s personal life had been looking up, to general astonishment among friends and family. She hadn’t expected that her affair with Bud would deepen so smoothly and so quickly, especially since they spent almost all their time on separate worlds. In the toughest days of her life, the relationship had been a source of comfort and strength to her.

But now, in what should have been a routine weekly progress meeting, two showstopper problems had come looming out of nowhere.

On her screen Rose Delea reappeared with coffee that sloshed in a languid low-G way. The conversation resumed, and Siobhan tried to focus on the issues.

Mathematically, the positioning of an object at a Lagrangian point was simple. If the shield had been a point mass, it could have been poised neatly on the sweeping line joining Earth to sun at L1. But this project was no longer mathematics; it was engineering.

For one thing the L1 point wasn’t really stable at all, but only semi-stable: if you knocked that point mass out of position it would tend to drift back to its place along the line of the Earth—sun radius, but would happily float away from the line in any other direction. So you needed to add station-keeping mechanisms, such as rocket thrusters, to hold the shield in place.

And then, of course, the shield was not a point mass, but an extended object large enough eventually to shadow the whole Earth. Only the shield’s geometric center, intersecting the Earth—sun line, could be properly balanced at the L1 point. All other points were drawn toward the center, and given time the shield would have crumpled in on itself. Making it rigid would have raised the mass unfeasibly. The problem was to be overcome by giving the shield a slow rotation. The spinning was stately, at only four revolutions per year—“as if God is twirling His parasol,” as Mikhail described it—but enough to keep the shield rigid.

But the rotation created other problems. Docking with a spinning object in space, even one as slow moving as the shield, was a lot more tricky than with a stationary object. More seriously, by being spun up, the shield would become a huge gyroscope. As it followed its orbit between Earth and sun it would tend to keep the same orientation in space—and so, over a year, it would tip its face away from the sun—Earth line, making it useless as a parasol.

Meanwhile there were other forces to consider besides gravity. Sunlight itself, a rain of photons, exerts a pressure on every object it touches. It is too gentle a force for human senses to detect on an upraised hand, but it would be enough to drive a yacht with filmy kilometer-wide sails from world to world—and it was certainly enough to exert a significant force on an object as large as the shield. There were other complications too, such as perturbation by the gravity fields of the Moon and the other planets, and a tweaking by Earth’s own magnetic field.

To cope with all this, the shield’s surface was to be made adjustable. Panels could be opened and closed in careful patterns, so that the gentle pressure of sunlight could be harnessed to turn the shield. It was an elegant solution: sunlight itself would be used to keep the shield properly positioned.

But to maintain its station in this environment of multiple and constantly changing forces, the shield itself had to be smart enough to be aware of its position in space, and able to adjust itself dynamically. Ideally every square centimeter of the shield would know all about the forces acting on it and on the shield as a whole, and would be able to compute how it should position itself in response.

This distributed, interconnected intelligence was to be achieved by the manufacture of a “smartskin.” The shield’s epidermis, less than a micrometer thick, would not just be a reflective skin but would be packed with circuitry. The local smartness, interconnected, would of course add up to a total powerful intelligence. The completed shield would, it was thought, be the smartest single entity humankind had yet constructed—smarter even, probably, than Aristotle, the only uncertainty coming because nobody knew quite how smart Aristotle was.

So much for the design, complicated enough in itself. The implementation was something else.

The manufacture of the smartskin was one headache today; there weren’t enough nano-factories to turn it out in time. But even more serious was the problem caused by the pressure of sunlight. Although it could be used for active position control, its very existence caused a fundamental difficulty—which was the day’s second showstopper.

“Let’s go through it step by step,” Bud said. “The sunlight presses on the reflective face of the mirror. The light pressure acts against the sun’s gravity—so it’s as if the sun’s gravity is effectively reduced, and the L1 balance point is moved toward the sun along the Earth—sun line.