“And so that’s what we’re left with,” Bud said. “Igneous rocks make up ninety percent of the crust here. It’s as if we were learning to live on the slopes of Vesuvius. And there’s virtually no water, remember. Without water you can’t make concrete, for instance.”

“Hence glass.”

“Hence glass. Siobhan, glass grows naturally on the Moon. Wherever a meteor falls, the regolith fuses, and glass is splashed everywhere. So that’s what we use.

“And here’s the finished product.” With a showman’s flourish he pointed to glass components, some of them many times a person’s height, stacking up in a rudimentary store out in the vacuum. “There are no prototypes here, no test articles. Everything we make is intended to be launched; everything we build will wind up on the shield—everything you see here will fly. The designs they feed us from Earth keep changing, and we’re trying to optimize our manufacturing too, aiming for the minimum weight to provide a given structural strength. So the final shield will be a funny sort of hybrid, with the last components, five years younger, looking quite different from the first. But we’ll just have to cope with that.”

Siobhan gazed at the glass sections with genuine awe. They looked like nothing much, like buttresses for a fairground ride or a fancy trade-show exhibit. But these odd-looking struts of glass, and tens of thousands of others just like them, were to be shot into space, where they would be assembled to form the scaffolding of a mirror wider than the planet. Her wild back-of-the-envelope concept was already coming into physical actuality. She felt thrilled.

Bud was watching the workers beyond the window. “You know,” he said, “I think this could be the making of this crew. Before June 9 we were kind of playing up here, playing at being lunar colonists. Now we’ve got a sense of urgency, a specific goal, a schedule to fulfill. I believe this event will push forward the program of the colonization and exploitation of the Moon by decades, or more.”

That meant little to her, but she saw how important it was to Bud. “That’s wonderful.”

“Yes. But,” he added heavily, “sometimes I walk on eggshells.”

“Why?”

“Because this isn’t what these guys came here for. They’re mostly scientists, remember. Suddenly they’ve been drafted to work on an assembly line. Yes, there’s dynamism and adrenaline. But sometimes they remember their old lives, and they feel—”

“Resentful?”

“Well, I can take that. The worst thing is, they get bored. The disadvantage of overeducation. As long as I can keep them distracted we get along fine.” He peered out, the laugh lines around his eyes catching the light, and she thought he seemed very fond of his temperamental workers.

“Come on,” she said. “You haven’t shown me Hecate yet.”

As they walked on, she slipped her hand into his.

*********

______

Later he took her out of Clavius Base to see David’s Sling.

As they approached the site of the Sling, Siobhan stood up in the surface tractor’s bubble-dome pressure compartment to see better. Only three kilometers of the launcher had been completed, of a projected thirty. Even so, it was an astonishing sight: in the low sunlight, under a pitch-black sky and against a gray-brown backdrop of Moon dust, the launcher shone like a sword.

The engineers called it a mass driver, or an electromagnetic launcher—or, more simply, a space gun. The heart of it was an aluminum track standing on trestle legs, thin and light like all lunar constructions. Wrapped around the track was a coil of iron, a vast spiral that Bud called the solenoid. At the loading end spacesuited figures moved cautiously around a crane, which was hoisting a glistening pellet up onto the track. The track stretched away across the level floor of Clavius, soon passing out of sight beyond the Moon’s close horizon.

“The principle is simple,” Bud said. “It’s a cannon driven by electromagnetism. You wrap your cargo in a blanket of iron—which we can reuse, by the way. You put your cargo pellet on the rail. The magnetic field, generated from that blockhouse over there”—he pointed at a nondescript dome—“then pulses through the solenoid, and your pellet is pushed along the track.” The changing magnetic field induced electric currents in the iron blanket, and the currents then pushed against the magnetism: “It’s just the principle of the electric motor,” Bud said.

As he spoke he pressed his hand against the small of her back with a pleasing familiarity.

She prompted, “And after thirty kilometers of accelerating—”

“You have escape velocity, without the need for any of that messy business of rockets and launch pads and countdowns. And then you can go wherever you want—fall all the way down to Earth, even.”

“It’s really a fantastic conception,” she said.

“Yeah. But like most of what we do on the Moon, people figured it all out long before they had a chance to get here to build it. The idea of an electromagnetic launcher dates back to the 1950s, I think. A science fiction writer. Famous in his day …”

“Couldn’t you build a mass driver on Earth?”

“Yeah. In principle. But the air would be a problem. You would be flying at interplanetary speeds a meter above the ground. On Earth, at escape velocity, Mach 20 or 25, you’d burn up. But up here there’s no air, so no air resistance. Then we have our famous low gravity, so the speeds we need to acquire are much less than on Earth: down there you’d need a launcher twenty times as long as this one—maybe six hundred kilometers. As for power, all that lovely sunlight falls down from the sky for free. But the real economy comes from the fact that unlike with rocket technology, all our launch equipment stays bolted to the ground, where it belongs. With the Sling, we can get off this rock for pennies per kilogram.”

He started to wax enthusiastically about the opportunities the Sling and its more sophisticated successors would one day give to the Moon. “From here we can send heavy-lift components to the Lagrange points, or Earth orbit, or to the planets and beyond, for a fraction of the effort and cost of launching from Earth. Once people dreamed that the Moon would be the stepping-stone to opening up the solar system. Those dreams died when it was found that the Moon has only a trace of water. But this is how the dream will live again.”

She touched his arm a little wistfully. She relished his passion, his energy. But he was oddly like Eugene Mangles, in a way: as Eugene’s obsession was his work, so Bud’s was evidently the Moon and its future—to the exclusion of herself, she thought. “Bud,” she said. “You sold me. But for now, all I want the Moon to do is to save the Earth.”

“We’re working on it. Even though we all know it won’t be enough.”

The shield couldn’t provide perfect cover. It had had to be designed to block the sunstorm’s peak-energy bombardment in the visible light spectrum, but could do nothing about an anticipated accompaniment of X-rays, gamma rays, and other nasties, peripheral in terms of the storm’s total output, but potentially devastating for the Earth. “We couldn’t do it all,” she said.

“I know. I keep telling my folk that. But even so it doesn’t feel enough, whatever we do … Look. I think they’re ready for a test.”

The cargo pellet was in place on the gleaming track. The crane withdrew. She saw the pellet start to move: slowly at first, a ponderous start that told of its mass, and then more rapidly. That was all there was to it. There were no special effects: no flaring fire, no billowing smoke. But as the generators poured their energies into the launcher she felt a tingle in her gut, perhaps some biochemical response to the mighty currents flowing just a few hundred meters away.

The pellet, still accelerating, shot out of sight.

Bud clenched a fist. “Today all we can do is dig another hole in Clavius’s floor. But in six months tops we’ll be firing to orbit. Imagine riding that thing, riding the lightning across the face of the Moon!”