After April 2042 the Earth would still be hanging up there, just as it always had. But he wondered how it would look then.

Eugene continued to watch the President’s broadcast. “She’s being vague about the date.”

“What do you mean?”

Eugene glanced at him. Today that lovely face was contorted by a stress Mikhail hadn’t seen before. “Why doesn’t she just say April 20? Everybody knows.”

Evidently not, Mikhail thought. Perhaps Alvarez had some psychological motivation in mind. Maybe overprecision would make the whole prospect too scary—it would start doomsday clocks ticking in people’s heads. “I don’t think it matters,” he said.

But to Eugene, author of the prediction, it evidently did.

Mikhail sat down. “Eugene, it must be very strange for you to hear the President herself speaking to the whole human race about something that you figured out.”

“Strange. Yes. Something like that,” Eugene said, his speech rapid and broken. He held his hands parallel before him. “You have the sun. And you have my model of the sun.” He locked his fingers together. “They are different entities, but they connect. My work contained predictions that were borne out. So my work is a valid map of reality. But it is only a map.”

“I think I understand,” Mikhail said. “There are categories of reality. Even though we can predict it to nine decimal places, we can’t imagine the sun’s peculiar behavior actually intervening in our cozy human world.”

“Something like that,” Eugene said uneasily. His big hands plucked at each other, a man’s hands but a child’s gesture. “Like the walls between model and reality are breaking down.”

“You know, you’re not the only person who feels this way, Eugene. You aren’t alone.”

“Of course I’m alone,” Eugene said. His expression closed up.

Mikhail longed to hold him, but knew he must not.

************

______

The President said, “We intend to build a shield in space. Made of the finest film, it will be a disk wider than the Earth itself. It will be so vast, in fact, that as it begins to take shape it will be visible from every home, every school, every workplace on Earth, for it will be a human-made structure as big in our sky as the sun or the Moon.

“I am told it may even be visible, to the naked eye, from Mars. We will indeed be stamping our mark on the solar system.” She smiled.

Siobhan thought back to the session with her “motley crew” in the Royal Society, where Aristotle had first come up with the notion.

The idea couldn’t have been simpler, in principle. On a sunny day, if the light was too strong, you put up a parasol. So, for protection from the storm, you would build a parasol in space, a mighty cover big enough to shield the whole Earth. And on that crucial day, humankind would shelter safely in the shadow of an artificial eclipse.

“Its center of gravity would be at L1,” Mikhail had said. “Between sun and Earth, co-orbiting.”

Toby asked, “And what is L1?”

L1 is the first Lagrange point of the Earth—sun system. An object circling between Earth and the sun, such as Venus, follows its orbit more rapidly than Earth. But Earth’s gravity field tugs at Venus, though much more feebly than the sun’s. Put a satellite much closer to the Earth—about four times the distance to the Moon—and Earth’s gravity is so strong that the satellite is dragged back just so by Earth, and made to orbit the sun at the same rate as the Earth.

This point of equilibrium is called L1, the first Lagrangian point, for the eighteenth-century French mathematician who discovered it. In fact there are five such Lagrangian points, three on the sun—Earth line, and the other two on the path of Earth’s own orbit, at sixty degrees from the Earth—sun radius.

“Ah,” Toby had said, nodding. “Earth and satellite co-rotate. As if both Earth and satellite were glued to a great rigid clock hand that sticks out from the sun.”

Siobhan said, “I thought L1 is a point of unstable equilibrium.” At Toby’s baffled look she said, “Like a football sitting at the summit of a mountain, rather than in a valley. The ball’s stationary, but liable to fall off in any direction.”

“Yes,” Mikhail said. “But we have placed satellites at such positions before. You can actually orbit the Lagrangian point, use a small amount of fuel to station-keep. It’s well within the envelope of experience: astronautically, not a problem.”

Toby had held his hand up to the ceiling light, experimentally shadowing his face. “Forgive a stupid question,” he said. “But how big would this shield have to be?”

Mikhail sighed. “For simplicity, assume the sun’s rays are parallel as they reach Earth. Then you can see you need a screen as large as the object you’re trying to shield.”

Toby said, “So the shield has to be a disk with at least the diameter of the Earth. Which is—”

“About thirteen thousand kilometers.”

Toby’s jaw had dropped. But he pressed on doggedly. “So we’re talking about a shield thirteen thousand kilometers across. To be built in space. Where the largest structure we’ve put up so far is—”

“I suppose the International Space Station,” Mikhail said. “Much less than a kilometer.”

Toby said, “No wonder I didn’t find this before. When I ran my own search for solutions, I screened out the obviously implausible. And this is obviously implausible.” He glanced at Siobhan. “Isn’t it?”

Of course it was. But the three of them had hammered at their softscreens to figure out more.

Toby said, “There have been studies of this sort of thing before. Hermann Oberth seems to have been the first to come up with the idea.”

“You’d use ultrathin materials, of course,” Mikhail said.

Siobhan said, “Everyday plastic wrap comes in at ten micrometers thick.”

“And you can get aluminum foil the same thickness,” Mikhail said. “But surely we can do better.”

Toby said, “So with an area density of less than a gram per square meter, say, and even adding an element for structural components, your weight could be as little as a few million tonnes.” He looked up. “Did I really just say as little as?”

Siobhan said, “We don’t have the heavy-lift capacity to get that amount of material off the Earth, even given years.”

“But we don’t need to lift it off Earth,” Mikhail said. “Why don’t we build it on the Moon?”

Toby stared at him. “Now that really is crazy.”

“Why so? On the Moon we already manufacture glass, process metals. And we have our low gravity, remember: it’s twenty-two times easier to launch a payload into space from the Moon as from Earth. And we’re already building a mass driver! There’s no reason the Sling project couldn’t be accelerated. Its launch capacity will be huge.”

They factored an estimate of the Sling’s launch capacity into their back-of-the-envelope calculations. It was immediately clear that if they could launch the bulk of the shield’s mass from the Moon, the energy savings would indeed be prodigious.

And there was still no obvious showstopper. Siobhan had felt frightened to breathe, as if she might break the spell, and they had worked on.

But now, sitting in her flat with her mother and daughter, listening to Alvarez announce this preposterous idea to the whole world, different emotions surged in her. Suddenly restless, she walked to the window.

It was nearly Christmas, 2037. Outside, kids were playing soccer. They were wearing T-shirts. While Santa Claus still bundled up on the Christmas cards, snow and frost were nostalgic dreams of Siobhan’s childhood; in England it was more than ten years since the temperature had dipped below freezing anywhere south of a line from the Severn to the Trent. She remembered her last Christmas with her father before his death, when he had railed about having to cut his lawn on Boxing Day. The world had changed hugely in her own lifetime, shaped by forces far beyond human control. How could she be so arrogant as to suppose she could manage an even greater change, in just a few years?