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 couldnt 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 Earthsun system. An object circling between Earth and the sun, such as Venus, follows its orbit more rapidly than Earth. But Earths gravity field tugs at Venus, though much more feebly than the suns. Put a satellite much closer to the Earthabout four times the distance to the Moonand Earths 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 sunEarth line, and the other two on the path of Earths own orbit, at sixty degrees from the Earthsun 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 Tobys baffled look she said, Like a football sitting at the summit of a mountain, rather than in a valley. The balls 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. Its 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 suns rays are parallel as they reach Earth. Then you can see you need a screen as large as the object youre 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.
Tobys jaw had dropped. But he pressed on doggedly. So were talking about a shield thirteen thousand kilometers across. To be built in space. Where the largest structure weve put up so far is
I suppose the International Space Station, Mikhail said. Much less than a kilometer.
Toby said, No wonder I didnt 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. Isnt 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.
Youd 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 dont have the heavy-lift capacity to get that amount of material off the Earth, even given years.
But we dont need to lift it off Earth, Mikhail said. Why dont 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: its twenty-two times easier to launch a payload into space from the Moon as from Earth. And were already building a mass driver! Theres no reason the Sling project couldnt be accelerated. Its launch capacity will be huge.
They factored an estimate of the Slings launch capacity into their back-of-the-envelope calculations. It was immediately clear that if they could launch the bulk of the shields 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 Siobhans 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?
Im afraid, she blurted.
Perdita glanced at her, troubled.
Of the storm? Maria asked.
Yes, of course. But I had to work hard to get the politicians to accept the idea of the shield.
And now
Now Alvarez is calling my bluff, in front of the whole world. Suddenly Ive got to deliver on my promises. And thats what frightens me. That I might fail.
Maria and Perdita walked over to her. Maria hugged her, and Perdita rested her head on her mothers shoulder. You wont fail, Mum, Perdita said. Anyhow you have us, remember.
Siobhan touched her daughters head.
On the softwall, the President continued to speak.
I offer you hope, but not false hope, Alvarez said. Even the shield alone cannot save us. But it will turn an event that would be nonsurvivable for any of us into a disaster survivable for some. Thats why we must build itand thats why we must build on the chance it gives us.
It goes without saying that this will be by far the most challenging space project ever undertaken, even dwarfing our colonization of the Moon and our first footsteps on Mars. Such a mighty project cannot be managed by one nation alonenot even America.
So we have asked all the nations and federations of the world to come together, to pool their resources and energies, and to cooperate in this most vital of space projects. I am delighted to say that we have had a virtually unanimous response.
Virtually unanimous my arse, Miriam Grec grumbled. Here in her Euro-needle office she sipped her whiskey and settled a little deeper into the sofa. How can you call it unanimous when the Chinese have refused to take part?
Nicolaus replied, The Chinese play a long game, Miriam. Weve always known that. No doubt they see this problem with the sun as just another geopolitical opportunity.
Maybe. But God alone knows what they are up to with all those taikonauts and Long March boosters
Surely they will come around, in the end.
She studied him. Even as he spoke, Nicolaus Korombel had one eye on the softscreen bearing Alvarezs image, the other on monitors that showed in a variety of ways the worlds response to Alvarezs unfolding message. Miriam had never met anybody with Nicolauss ability to parallel-process. It was just one of the reasons she valued him so much.