“Yar,” Jao said, breathing hard. “Each thread is a single continuous molecule that reaches from here to the moon. My guess is that it’ll turn out to be mostly oxygen bonded to silica, magnesium, and aluminum, with a carbon backbone to help out with all the connections. It’d be harder than diamond and with a higher tensile strength than amorphous boron to start with, and then there’d be some kind of submolecular weaving between adjacent chains … And, oh yah, you won’t find where it’s anchored, because it reaches all the way down to the original core of this world, forty-five million miles under your feet. That’s because it’s only a guide thread—part of the warp and woof that held this world together while they were spinning it.”
“Slow down,” Bram said. “I can believe in your tied, down moon because I can see the evidence here with my own eyes—and by the way, you’d better radio Jun Davd right away and tell him that we’ve solved the mystery of why the moons are lower than they ought to be for synchronous orbit. I’ll even accept your endless molecule till a better explanation comes along. But how could it support another forty-five million miles of its own weight?”
“It’s the other way around,” Jao said smugly. “The idea isn’t to hold the moon down. The moon is what holds the world up.”
Bram looked around at an apparently solid landscape. They were near one edge of the rim here. The avenues of rubble stretched to the opposite side, fifty miles away. The rectilinear mounds were higher at the lunar longitude than they had been on the outskirts of the enormous complex—the buildings had been taller and more important here. It had not occurred to Bram to wonder why the moonropes were peripheral and not centered, because, after all, the entire surface of the diskworld was the “equator.”
“You’re getting too farfetched,” he said, and waited for the next dizzying supposition from Jao.
“Am I?” Jao retorted. “I’ll bet you anything you care to name that when we cross that plain to the other side, we’ll find another cable car station and another set of ropes. Making an equilateral triangle with a fifty-mile base and its apex on the moon. Wait a minute! Make that a very narrow tetragon! Why not? The angle of divergence is minuscule on that scale. You might as well have parallel tethers. No, wait again! How about spreading the moon terminal still farther apart? At an angle that converges at the disk’s core? With a little truing of curves, you could have a ninety-million-mile section of parabola for your. antenna. Bram, we’ve got to map the whole topography of this disk! I’ll bet it has a concave cross section. Hard to detect, but it would make this razor’s edge of a rim the widest part, except for the leftover bulge at the hub!”
“I’d have thought that even a few inches of overhang at a height of forty-five million miles would add up to insupportable stresses.”
“Don’t you see?” Jao’s voice exploded in Bram’s ear. “This world was built like a suspension roof! It had to be! Otherwise, with the spin needed to keep it from collapsing under its own weight, the synchronous orbital points would have been embedded somewhere below the surface! There wouldn’t be a stable surface! All the people and the buildings and the topsoil would be thrown off into space!”
“What’s a suspension roof?”
“It was an idea one of the Resurgist architects had for building our sports arena in the human compound. Arthe, his name was. You strung cables from supporting piers and laid roofing material over them. You kept the internal air pressure of the building higher, to bear some of the weight. It was a way to provide a larger interior space unobstructed by load-bearing pillars. Nothing ever came of it. The council was too conservative and decided to stick with a Nar-style shell.”
“A whole world built that way?”
“Why not? It makes sense. And it fits the facts. We know this world is lighter than air. About twice the weight of helium on average. And when that average includes a rocky surface and two apparently rigid faces, then we’re dealing with an artifact that for all its size is mostly a gossamer nothingness enclosing more mostly nothingness.”
“Yes, I remember that Smeth proposed a honeycomb structure or a membrane enclosing a gas.”
Jao snorted impatiently. “But how do you build up a honeycomb out of a gas giant’s mass without it collapsing into a sphere after the first hundred thousand miles? Even if your honeycomb were as light as hydrogen? Especially when you’re starting out that much closer to the center of gravity—not like out here on the rim, forty-five million miles away from it, where we’re down to about six decimal places worth of zeros with a one hung on the end of them, and we’re staying attached to the surface mainly by courtesy of the local gravity of the crust. As for a membrane, one meteor puncture and you’d have the deflated skin of a world.”
“It would take more than one,” Bram demurred.
“This world’s had more than one,” Ame said tartly.
“All right, gang up on me,” Bram said. “Go on, Jao. Lydis, are you recording this for Jun Davd?”
“Yes,” Lydis said. “I can always add it to the reserve air supply. After cooling it down.”
“Who told you it’d be risky to make a cislunar landing approach?” Jao reminded her. “That didn’t turn out to be hot air, did it?”
“Let him finish,” Enry’s voice broke in. “It fits in with my seismograph readings. The waves damp out a few miles down.”
“Thank you, Enry. I’m glad there’s one member of our expedition with a little vision.” Jao continued smugly. “You start with an unremarkable rocky-type body—maybe the core of the gas giant they poured into their parts bin. Next you tow your twelve moons into place, positioning them so that they occupy the same synchronous orbit in a stable dodecagonal configuration. If you don’t have twelve leftover moons handy, you fill in with a few hefty asteroids.”
“Skip the details for now,” Bram suggested, “or we’ll never get there.”
Jao made a pained sound. “Then you drop a line from each of your synchronous moons and anchor them at the planet’s equator. While you’re lowering the lines, of course, you have to keep judiciously raising the moons’ orbits, to keep the center of gravity in the right place. Even with the lightweight filament you’re using, the mass adds up as the line grows.”
“So far you’re describing the construction of an ordinary orbital elevator,” Bram said.
“You recognized that?” Jao sounded pleased. “Not many people knew about that project.”
“I worked on a part of the problem at the biocenter,” Bram said. “Ordinary viral monofilament—the kind the Nar used for the bubble car cable network—tested well within the breaking strength limits, but there were still some problems with prolonged ultraviolet exposure that needed to be worked out.”
“What’s an orbital elevator?” Ame asked.
“You’re looking at one,” Bram said, pointing at the plaited crystal tower that rose into the sky with its impaled passenger vehicle still hanging from it like a spitted egg. “The Nar had a scheme for building a number of space docking stations on the same principle. Eventually they looked forward to a whole ring of them around the equator of the Father World, linked together for stability. It was well within the limits of theoretical possibility. But the Nar thought in terms of thousands of years, and there was no particular sense of urgency, oxygen and biologically produced alcohol for shuttle fuel being as cheap as they were.”