“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.”
“The big difference here,” Jao rushed in, “is that Original Man’s skyhook used two tethers, anchored fifty miles apart. That was important. Even if they came together at the apex of an isoceles triangle twenty-odd thousand miles up, the slight lean from the true vertical wouldn’t have mattered that much. Not with the materials they had available.”
“Okay, you’ve got your twelve moons, bobbing up there like captive balloons, and connected to make a dodecagon—”
“To make a circle, Bram. The line bellies outward.”
“Okay, a circle. What next?”
“Next they started spinning the world. Faster and faster. At the same time, they began playing out more of their superfilament. They must have fed it out in a liquid form that instantly hardened—”
“There was a terrestrial animal called a spider that did that biologically,” Ame put in.
“Yah, I wouldn’t know about that. Anyway, they probably fed in the raw materials at the poles, from a couple of big gas clouds they had parked out in space. The oxygen and carbon could’ye come from carbon dioxide they siphoned off a hothouse-type planet—there’s one of those in almost every G-star system, same as gas giants. That must’ve been a sight to see—the two big vortices whirling from the poles millions of miles into space, while the tethered moons spun out farther and farther weaving a gossamer web between them.”
“You don’t think small, I’ll give you that.” Bram laughed.
“Now you start extruding your foam—mountains of it, oceans of it, following the network of filaments into orbit, the trapped bubbles of gases blowing the material up to two or three thousand times its volume before the molecules cross-link and become rigid. It maintains the disk shape, reaching a predetermined orbit at the limit of the spin force, with an excess material falling back to the surface. Foam that squeezes laterally through the web shears off, making a nice smooth face. Depending on the altitude, the excess stuff either slides outward or falls back to the hub. It’s exactly like the preliminary stages of the formation of a protostar … or … or a galaxy! The material settles more and more into a flat rotating disk! But now, with the growth of the disk, angular momentum is transferred and the spin slows down! So you get the stable situation we see here, with the moons traveling just a little faster—or lower—than they ought to be for synchronous orbit and maintaining the tension that helps hold the whole structure together.”
He stopped, out of breath.
“What’s happened to your rocky world at the center?” Bram asked.
“Oh, that? It’s smeared all over the faces of the disk. Some of it’s been flung out beyond the rim and fallen back, and we’re standing on it. And the rest of it’s a plug in the hole at the center of the disk, where the gravity is perpendicular to the disk surface. “It’s heavy there, that close to the center of gravity, I promise you!”
“What about it, Enry?” Bram said.
“It could be.” The geologist’s voice was muffled; he must have turned his head away from the helmet mike to check data. “I’ve got samples that could have come from the rocky core of a gas giant that broke up. Silicates that show signs of once having been under tremendous pressure. Millions of atmospheres worth—the kind of pressure that turns molecular hydrogen in
to metallic hydrogen. The paleomagnetism’s interesting. The orientation’s every which way. As if the samples originated elsewhere and were scattered all over the place.”
“What did I tell you?” Jao sounded smug.
“There’s something else,” Enry drawled.
“What?”
“The rocks show that there’s a steady leakage of gases from the interior of this … world. Oxygen, and carbon dioxide, and lighter gases like nitrogen and helium. The rim can’t hold on to an atmosphere, of course. But there could be a considerable amount of gas still trapped in the … cavities that Jao postulates.” There was a moment of silence with an unmistakable frown in it. “More than there ought to be, from the rate of leakage, after seventy million years.”
“The gases are subliming off the foamed surfaces,” Jao said quickly. “And maybe off the superfilament as well. Nitrogen, did you say? I’m going to have to rethink the chemistry of it. Plenty of oxygen, that’s for sure. Hey, we might be able to tap into it during our stay—take some of the load off Yggdrasil!”
“How far down would we have to drill to tap atmosphere?” Bram asked.
“I don’t know. It would take some pretty fancy mathematics to figure out the thickness of the crust, based on too many variables—the size of the original core, rate of spin-up, disk gravity gradients, surface friction affecting the dispersion of material … I think we’ll have to rely on Enry’s empirical methods. Theory falls in a case like this. It can’t be much, though.”
“Those bare paw prints,” Ame said. “Do you suppose…”
“Forget it, Ame. Any atmosphere belched out from the interior would instantly disperse. You’ll have to find another explanation.”
“There was life here within the last few million years—I’m certain of that,” she said stubbornly. “It’s…”
“What’s the matter, Ame?” Bram said.
She flashed her light around the base of the moonrope. “Look,” she said.
The tracks had been easy to miss in the permanent red twilight, especially when there was the awesome sight of that crystalline pillar drawing your eyes up instead of down. And the reflected light from directly above had tended to wash out the long, diffuse shadows cast by the horizon-filling sliver of the companion diskworld that rose above the brink.
But once you had noticed them, it was hard to see how the eye had skipped over them. Shallow as they were, they were perfectly plain, like inked thumbprints, a little smudged where the tiny paws had scrabbled in the dust. The myriad trails meandered a bit. But in the end pointed toward the soaring cable.
“They climbed it,” Ame said. “That’s where they went. They climbed to the moon.”
They had six hours to explore the central complex, Lydis told them. After that, they would begin to tax the walker’s ability to replenish their air supply. “I don’t want you to use your reserve bottles at all,” Lydis said. “That’s cutting it too close. I want you to come in with your reserve bottles intact. Understood?”
“Understood,” Bram told his daughter. He sighed. It had been five hundred years since she had been a little girl, but every once in a while it felt strange to be taking orders from her.
Ame was scrambling happily all over the mounds of rubble, leaving little electronic markers that would give off coded transponder signals when asked.
“Site number twenty,” she dictated to one of them. “Probable auditorium or lecture hall.” She followed with a series of dimensionless coordinates that would be fitted later into a triangulation grid by a computer.
“How can you tell it’s an auditorium?” Bram asked. “It’s just another dust pile, as far as I can see.”
“It’s fan-shaped,” she said, tossing him a grin over her shoulder. “We tend to think that public halls are supposed to be circular, with a stage in the center, because of the Nar influence. But this is a much more natural shape for human beings. And see, the focal point—abutting what might be classrooms or administrative offices—is at about the limit of distance from which a live lecturer or performer could be seen to any effect.”
Bram followed her around as best he could, climbing in and out of the walker, helping with the measurements, and operating the little portable thumper that located cavities beneath the surface.
“Libraries,” Ame exulted. “I’m sure of it. And museum warehouses and storehouses and depots and vaults for frozen samples. And all the support and recreational facilities you’d need for the population of millions that it would have taken to run this outpost—hydroponic farms, maybe even zoos! This will be a treasure trove for the archaeologists, Bram-tsu! And there’ll be middens—we’ll find seeds and organic refuse and bones…”
He didn’t like leaving her alone while he went off on his own side forays, and at one point he coaxed her into the walker for an excursion to the rim’s edge.
“Don’t go out any farther,” he warned. “I don’t know how secure this thing is.”
They were standing on the great skeleton arm of a gantry that extended out over the abyss—part of some sort of transport system that traveled an unknown distance down the face. Bram could see the stanchions that once might have supported an elevator or funicular dwindling with distance till they disappeared.
Hundreds—possibly thousands—of miles down the sheer face began a glittering fairy forest of tiny filaments that swept in a great arc until they could no longer be distinguished against the knife edge that cut the black night ninety million miles below.
“The feed array for the antenna system,” Bram said. “There must be others, equally spaced around the disk, aimed at a reflector at the hub.”
By this time they had worked themselves through to the opposite edge of the disk, facing the intergalactic night. The antenna complex was lit from above by ruddy moonlight.
The buried city, limned by mounded avenues of detritus, stretched all the way across the diskworld from rim to rim. And Jao had been right: There was another set of cables climbing to the moon on this side, too.
“So this,” Ame said, “was the voice of the human race?”
“Yes.” Bram dug through the centuries for an old memory. “My teacher, Voth, once said that humankind had learned to tame a sun’s power to shout across the gulf between the galaxies, but he couldn’t imagine how.”
He mused at the phased array, wondering at the scale that would allow its nearer ranks to be seen at such a distance. The elements must be miles high to be even remotely distinguishable—cantilevered or guyed against the topsy-turvy gravity. But that would have presented no problem to a race with moonrope at its disposal. And the gravity would be mild for the next few million miles, anyway. It would be a different story at the hub, where gravity would be crushing. Perhaps there were phase shifters installed at a safe radius. He would send an expedition down the face to see—in a space vehicle. And the physics would have to be carefully worked out so that the explorers would not find themselves slammed against a wall that had become a floor.
Bram shuddered at the thought of the mighty energies that once had been dispensed by that distant forest. In operation, it would have been a microwave inferno that would have sizzled a man to a crisp in milliseconds. No wonder a healthy stretch of no-man’s-land had been left—and not just to get past the gravitational edge effects.
Bram inched farther out on the gantry for a better look. Jao was going to insist on a full description. Too energetic a toe push sent him doing a handstand, and he walked a few steps on his hands before his boots settled down, holding on for dear life and being careful not to let go with one hand before he had a firm grip with the other. The asterioid-strength gravity was deceptive, he knew. He still had all his mass, and it was a long way to fall. Already, though he was no more than fifty yards over the edge, he knew that the horizontal component of the diskworld’s complex gravity was tugging every atom of his body toward itself in a complicated vector. He would have had to crawl out another million miles or so to feel anything, of course,
but it was there nevertheless. But if he were to fall past reach of a handhold, he would be accelerated inexorably—at the dreamlike rate of about one thirty-millionth of a foot per second to start—until, at an unknown fraction of the distance to the center, the reaching forces of the disk would slam him into the tilted wall-scape at a velocity sufficient to abrade him into a long, wet smear.
It didn’t help a whit to realize that he’d have been long dead of suffocation, thirst, or boredom before that happened.
Bram stopped his balloonlike four-limbed outward prowl and wrapped himself securely around a thick strut with an arm and a leg while he surveyed the cliff face from his improved vantage point.
There was movement beside him, and then Ame was pressed up next to him, peering past his shoulder into the abyss.
“I thought I told you to stay put,” Bram said.
“Don’t be silly, Bram-tsu. I’m perfectly able to take care of myself. What could possibly happen?” She leaned out alarmingly. “Do you think we have enough time to climb down there for a look at some of those caves?”
“No!” he said, hearing himself sputter. In a more reasonable tone of voice, he said, “We’ll come back later with ropes and proper climbing equipment and a team of trained outside workers. Maybe we’ll round up a few tame climbers from Yggdrasil’s vascular system and ride them. And the climbers will wear safety lines, too!”
“It was just an idea,” she said mildly.
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