The sky was full of disks.

The nearest one, only a hundred million miles away, turned half the sky blind. It stood almost edge-on—seen only as a paper-thin rim faintly traced by light, sketching the partial outline of a tall ellipse whose shape could be inferred from the stars it blotted out.

It was immense. Unbelievably so. A planet would have been imperceptible against it, a sun a mere pinprick. Its diameter was, in fact, that of a planetary orbit.

Another disk, equally huge, bracketed the other side of the sky, showing as a somewhat fuller ellipse. But this one presented its inner face and was visible as a pale wash of refracted light.

Between them hung a whole collection of similar shapes, like paper cutouts all dangling at the same level from invisible threads. Directly ahead was a great illuminated circle on what must have been the opposite side of the hidden sun. A smaller circle was a black silhouette trying unsuccessfully to eclipse it. On either side of the smaller circle were attendant disks, canted inward to make narrow ovals. Their inner faces, closer to the unseen sun than the gigantic disk opposite, made brighter daubs against its inferior illumination. A bite had been taken out of the edge of one of them by the eclipsing circle.

Through the spaces between them could be seen a whole swarm of still smaller disks—if objects that were millions of miles in diameter could be called small. The glimpsed shapes were in a different plane than the outer disks; the ellipses they presented were horizontal, not vertical.

The tiny dot of a sun in the center of that bewildering arrangement had never peeped forth again in all the two years they had been traveling toward it. In view of the complicated schedule of eclipses, its brief emergence must have been an exceedingly rare event.

Bram stared over the heads of the crowd at the flat, queer shapes floating in the darkness. He swallowed hard. Reason said they could not exist. But they did.

“There can be no stranger sight in the universe,” Jun Davd said to no one in particular.

People jostled and crowded around him at the safety rail in front of the long, curving observation wall. This was a real view, not a holo. Naked space was on the other side of the transparent polycarbonate sheet, and people had been gravitating here even though Yggdrasil's slow rotation periodically turned the scenery on its head. The holo still ran at the opposite end of the lounge, but even though it showed close-ups, there was no added detail to make it worth watching.

“It works out to an ingeniously timed energy trap,” Jao was burbling to anyone who would listen. His burly form was at the center of a knot of people, the nearer ones in danger of getting clipped by his waving hands.

“Listen to him,” Smeth grumbled to Bram. “You'd think he was taking credit for it himself. It's nothing like the continuous bubble he theorized about."

“...though the timing's decayed somewhat after seventy-four million years,” Jao went on. “Otherwise, we wouldn't be seeing the disks by so much leaked light, and we never would have seen the star itself."

A pretty admirer who must have stretched Jao's uxoriousness to the limit spoke up. “I know you explained it before, Jao, but it's awfully confusing. It gives me a headache just to think about it."

“It's beautiful, beautiful!” Jao boomed. “Look, there are four shells of disks—an outer and inner shell in equatorial orbit, and an outer and inner shell in polar orbit. The polar shells are the itsy ones on the inside, and their main job is simply to reflect all radiation into the equatorial plane."

“I understand that, but..."

“Each shell consists of three disks whose diameters are equal to the radii of their distance from the sun. Actually, it's their centers of gravity that're in that orbit. But they don't swivel. Each one of them has exactly enough spin to make its day equal to its year, so that the flat side always faces sunward.” He frowned. “Except that one of the inner ones once got a nudge from something that messed up its synchronization—probably a solar flare. That's why the sun was able to pop out in the equatorial plane when we were in the cometary belt."

“But why don't they all just crash into one another?” his admirer wailed.

“Look—each set of three consists of disks whose centers of gravity are at the points of an equilateral triangle, thus occupying the same orbit in a state of equilibrium. It's a very stable arrangement. And the fact—now, get this—the fact that the diameter of each disk is equal to the radius of its orbit means that the zone of interception of the inner set is equal to the zone of interception of the outer set—so that when you project that cone, it's like having a solid fence of six disks, all tangent to each other.” He smiled benignly. “Except that you don't have to worry about them crashing together. To say nothing of all the mass you save."

“By using littler disks?"

He nodded. “Almost all the mass of the energy trap is in the three big ninety-million-mile disks. The next shell inward—the one that orbits at thirty-six million miles—contains only about a sixth as much mass. Call it four twenty-fifths. And the ratio holds as you keep diminishing—so that all of the inner disks put together add up to less than the mass of one more big disk. Original Man was very clever. He made his fence out of geometry instead of mass."

“It's a fence with a lot of gaps, though, isn't it?” said a smart aleck who looked as if he were the boyfriend of the girl or aspired to be.

“Not as much as you think,” Jao said indulgently. “Let's figure it out. Hey, Smeth, what's the formula for a hypocycloid—never mind, I'll graph it."

He grabbed for the touch pad he had dangling from a chain around his neck and poked at it with thick fingers. An electric-blue circle grew on its photoplastic surface, followed by a horizontal line that bisected it, then two curves with the same radius as the original circle that sprouted from the ends of the line and met at the top. Little boxes began to subdivide the resulting figures, getting smaller and smaller until the eye could no longer separate them. The negatively curved triangle in the center differentiated itself with a change of color. Jao's fingers asked the touch pad a couple of questions, and he read off the answer.

“Yah,” he said. “The equatorial fence intercepts about seventy percent of the solar energy that comes its way. So does the polar shell. Together they cover somewhat more than one-fifth of the surface of an imaginary sphere enclosing the sun at any radius. I guess that was good enough to do the job."

“What ever happened to Jao's Bubble?” his opponent asked maliciously.

Jao was totally bland. “Oh, yah, the idea of enclosing a star inside some kind of a continuous shell. It wouldn't work."

“I can't stand it!” Smeth groaned. “Now he's going to disown the whole idea."

“He has no shame.” Bram laughed. “Jao throws off ideas, but. he isn't attached to them."

Jao was lecturing the young man earnestly. “In the first place, there's no way to keep it from drifting off center,” he said. “You might start with the star perfectly centered, but once the slightest drift started, it would keep getting worse, because the attraction from all other directions would keep decreasing..."

“That's exactly what I told him,” Smeth complained to Bram.

“...and can you imagine the centrifugal force at the equator if the thing rotated at all? Your sphere would suffer from slump. Everything would tend to collect at the equatorial plane. No, my friend, your sphere's a picturesque idea, but you can't have it."

“How does he do it?” Smeth grumbled.

“And who needs it, anyway?” Jao said before his young antagonist could open his mouth to protest. “Let's figure out how much energy Original Man had at his disposal for his intergalactic beacon."

He reached for his pendant touch pad again and began tapping at it one-handed without looking.

“Look at him showing off,” Smeth said in disgust. “He hits wrong numbers all the time that way, but that doesn't stop him."

“Each one of those big disks has a surface area of ... hmm ... call it six point three six quadrillion square miles. A six followed by fifteen digits. On each side. If you're trying to make sense of a number like that, it means that—” He punched numbers again. “—one side of a disk has thirty-two million times the surface area of an ordinary planetary body like the Father World."

“Why one side?” his admirer queried.

“We're only concerned with the side that's soaking up energy.”

“Oh."

Jao continued. “We've already got a value for the solar constant at the distance of the cometary halo, a light-year out. Now let's crank it back according to the inverse-square law, and we've got—” His fingers busied themselves again. “At ninety million miles, it works out to ... hmm ... one point four kilowatts per square meter ... times sixteen times ten to the nineteen square meters—"

“Oh, for pity's sake!” Smeth burst out. “All that rigamarole! He wants to say that each of the three large diskworlds receives twenty-three times ten to the twenty-first kilowatts of energy!"

“Thank you, Smeth,” Jao said equably. “Twenty-three sextillion kilowatts. And we're not through yet. We multiply by six, and—"

“Multiply by three something, you mean,” interrupted his rival for the young woman's attention.

“No. Don't forget the three disks of the inner equatorial shell may be a lot smaller, but they're closer to the star, and they intercept exactly as much solar radiation as if they were big disks in the outer orbit."

“But—"

Jao sailed on serenely. “Which means that in the equatorial fence alone, Original Man had one hundred and thirty-eight sextillion kilowatts at his disposal to turn into radio waves.” He paused for effect. “That is ten trillion times as much power as Nar civilization produced on the Father World."

Bram caught his breath. The figure was staggering. He had never bothered to work it out himself, though he had known it must be very high.

“They thought big, those people,” he said to Smeth. “By now the human genetic code will have reached the Virgo cluster. I always assumed that was the target. But with that kind of power, I wonder if they were aiming beyond."

Smeth snorted. “Huh, don't let Jao's raving impress you too much. We don't know how efficient those ... constructions were at converting energy into the longer radio wavelengths and modulating them."

“It hardly matters, does it?” Bram responded. “The waves will keep spreading through the universe. If they were meant to reach Virgo, the limit of their detectability must lie many times beyond it."

Over by the view wall, Jao was unleashing one of his terrifying smiles on the girl. Though he was utterly faithful to Ang, he didn't mind going through the motions.

Jao's young competitor glowered and made another try at impressing the girl. “If most of the mass of the system is in the big disks,” he said belligerently, “how come the sun hasn't drifted toward one or another of them over a period of time—just as it would toward one side of a ‘Jao's Bubble'? Excuse me—'Jao's shell.' The same thing would apply—the attraction of the other two disks would decrease with distance, and it would keep getting worse!"

He must have been a physics, apprentice. He stole a glance at the girl and went on in a classroom voice. “In a synthetic system like this one, which is essentially three big masses mutually revolving at the points of an equilaterial triangle, a mass occupying the center can't move above or below the equatorial plane because of the combined pull of the three major components.” He stared a challenge. “But it can and will move within the plane!"

“I didn't say most of the mass of the system was in the big disks,” Jao said kindly. “I said the big disks contain most of the planetary mass. Actually, they only mass about the same as a good-size gas giant—maybe a few tenths of one percent of the mass of the G-type star in the middle. So they're in orbit around it in the normal way."

“But that would mean—"

“Right. We know they're very thin—maybe as little as fifty miles across the rim. But even so, with a diameter measured in orbital distances, that would give them a volume of maybe four thousand times the volume of your run-of-the-mill gas giant. So they're lighter than they have any right to be."

The boy did some quick figuring in his head. “Four thou—but that would make them lighter than air!"

“Correct. About three and a third times lighter. In fact, they have a density of only about twice that of helium, on average."

“But that's impossible."

“I said on average."

Smeth bustled over. “One might posulate that they're hollow, or honeycombed, or a gas enclosed by a membrane. Or made of a rigid, infinite-length polymer with properties we can't imagine."

“What could be that light—and strong enough to maintain its shape over interplanetary distances?” the boy said.

Jao stared out the window at the strange floating circles that had taken the place of most of the sky. His face was flushed with excitement.

When he finally spoke, it was in Bram's direction. “We'll have to land on one of them to find out, won't we, Captain?"

Bram kissed Mim good-bye, feeling self-conscious in front of all the spectators. A crowd of about two thousand was jammed into the cavernous hangar, waiting to see the takeoff, and the rest of the population of the tree must have been watching on their holo sets. Bram could see the camera crew perched high on the spidery platform of an interbranch shuttle vehicle, where they had an overall view.

“Be careful,” Mim said, pressing herself against the tough hide of his vacuum suit. “I wish you weren't going this trip."

He embraced her one-armed, his bubble helmet tucked under the other arm. “The year-captain's expected to lead the way,” he said. “That's why they elect him. But don't worry. Lydis is the best landing craft pilot we have—and it's not going to be like landing, anyway. It'll be more like docking with a nonrotating branch. She's practiced it in the simulator a hundred times."

“But it's spinning."

“So slowly at the rim that it makes practically no difference. You're thinking in terms of a body like Yggdrasil, with a diameter only a few hundred miles across. In this case, the spin isn't there to provide gravity. It cancels it. So when we match for it, we'll touch down as lightly as a leaf."

“I'd still feel a whole lot better if I knew you were landing on the flat side."

A few feet away, under the skeletal arch of a landing leg, Jao left off nuzzling a clinging Ang and looked across her golden head toward them.

“That'd be a lot trickier, Mim, even though it looks simpler,” Jao boomed past Ang's ear, making her wince. “Your normal instincts don't apply on a body as bizarre as that. Neither do your first mathematical assumptions about up and down. Landing anywhere between the hub and the rim on a disk-shaped body would give Lydis some complicated gravitational gradients to cope with. The vertical component and the horizontal component don't behave the same way in relation to the center of gravity. And then there'd be the added factor of centrifugal force tending to make us slide outward, though we don't think it'd exceed the diagonal gravitational vector tending to pin us down. To say nothing of all sorts of unpredictable edge effects to get past before we could cross to those interesting structures on the rim. No, Mim, this is the simplest way. We've got it all worked out."

Bram had felt Mim stiffen at Jao's mention of “sliding outward” and “edge effects.” He turned it into a joke. “What Jao's really worried about is having to hike across a ninety-million-mile plain to get to where we're going."

She smiled gamely. “I guess I don't understand physics."

“That's all right, Mim, I don't understand Bach,” Jao said.

“Don't worry, we'll be very careful,” Bram told her, “and we'll be locked in to Jun Davd and his computer the whole time.

The third member of the exploration team, a dour geologist, named Enry, pushed his way through the well-wishers and said apologetically to Bram, “Lydis says she's about ready. Says it's time to get these people out of here and climb aboard."

Enry stood there, stolidly waiting. He was a blocky, square-jawed man who long ago had been a touch associate with a geology touch group on the Father World. Though the Father World no longer existed, Enry had never given up his speciality; he pored over the old Nar records in the library and published a monograph every quarter century or so. He was the nearest thing to an expert the tree possessed, and he handled himself well physically in the null-gravity sports at the trunk's center. Bram had thought of him immediately when choosing the exploration team.

“All right,” Bram said. He gave Mim a final peck that turned into something more as their lips touched again, then went with Enry to pry Jao loose from Ang.

A warning blast came from a two-tone bass whistle. Exasperated monitors wearing headbadges rushed back and forth, trying to shoo lingering spectators out of the drop area.

“Behind the ropes, behind the ropes! Everybody behind the ropes! Other side of the air curtain track!” The crowd moved as sluggishly as sap. “Keep it moving, keep it moving, unless you want to breathe vacuum!"

Bram got Enry and Jao started up the landing leg ladder with their gear and was preparing to climb it himself when he became aware of a disturbance at the fringe of the retreating crowd. A small, agile figure was darting past the monitors, getting chased by them, and darting back into the forbidden area. The interloper evaded a pursuer and made a beeline for the base of the ladder.

Bram saw corn-yellow hair flying and green eyes on either side of an upturned nose and recognized his great-great-great-granddaughter. “Ame!” he exclaimed. “What are you doing here?"

“I'm going with you,” she announced. “Here, take this."

She unslung a lumpy shoulder bag and thrust it at him. The clinking sound of some kind of equipment came from within.

“You can't,” he said. “We don't know what we may run into. Anyway, it's only a scouting trip. You'll have plenty of opportunities to go along on the later landings, like everybody else."

“That's the point, Bram-tsu. You ought to have a palentogist along on your first survey, and I'm the only authentic specimen you've got.” She grinned engagingly at him. “Besides, I've turned myself into a pretty fair geologist, so I'll carry my weight."

“We've got a geologist, a good one."

“Oh, Enry knows his subject, I'll give him that. But his subject's the Father World. Those disks out there are something nobody's an expert on. But my group's been doing theoretical studies for twenty years now—you'd be surprised at some of the computer simulations we've come up with!” She wrinkled her nose. “We really ought to have an archeologist with us, too."

“An archaeologist, is it? Spare me! A theoretical paleontologist's farfetched enough on a preliminary scouting expedition like this."

“Does that mean you'll—"

A monitor came puffing up, a man with a broad, law-abiding face and a long-suffering expression.

“Sorry, Captain. She slipped past us. I'll get her out of here."

Ame turned on him. “I'm not going with you. I'm going aboard, isn't that right, Bram-tsu Captain?"

The monitor looked doubtfully at the lightweight knee-lengths and slipover she was wearing. “If you want, I can get one of the other safety marshals and we can escort her forcibly."

“No, it's all right. I'll talk to her,” Bram said. “Go on, Marshal, I'll take responsibility for sending her back."

The monitor raised a quizzical eyebrow and withdrew. Bram turned to Ame. “You haven't rehearsed with us. There's the question of equipment—"

“Oh, that! There are spare space suits in the ship, and one of them is bound to fit me—it isn't as if they had to be custom fitted. I've ridden with Lydis lots of times before—on trips to other branches and even to the probe. And when it comes to that, I've spent as much time in vacuum as anybody. If I can climb around the branches under spin, I ought to be able to manage on one of those nice flat things out there."

Bram refrained from bringing up Jao's speculations on the nature of disk gravity gradients. “Are you sure you want to leave the twins alone for that long?"

“Smeth will take care of them. They adore him. He spoils them like mad. They're two years old now—they don't need to have me around constantly."

Bram sighed. “Everybody and their gene sibling wanted to be included on this trip. I almost had a riot. I had to promise that if there are no problems, everyone who wants to will get a turn while we're parked in this orbit. And here I am, giving preference to a descendant. They'll have my hide for nepotism."

“I can go, then?"

He gave in. “Your great-great-grandmother is the pilot. It's her decision. If she says you can go, then you're on. Otherwise you promise to leave quietly, all right?"

“I promise."

Above, Jao stuck his head out of the hatch. “What's holding you up? Lydis's already lost a turn while you've been palavering."

Ame scrambled up the landing leg ladder, with Bram behind her carrying the sack of equipment. It was bulky; he felt the handle of a digging tool through the fabric as it swung against his hip.

At the top, Bram twisted around and caught sight of Mim waving to him from the other side of the barrier. He waved back and squeezed through the hatch after Ame.

Jao filled the air lock, huge and grinning. “Stay here a minute with her,” Bram said, “while I—"

“Lydis says it's okay,” Jao said. “Let's get going."

A great rumbling sound filled the bay as the curtain rolled around its track and sealed off the cylindrical launch chamber. The crowd on the other side would be streaming toward holo monitors to watch the drop as relayed by the exterior pickups.

Bram turned sternly to Ame. “You had it all arranged with her in advance, didn't you?"

She wrinkled her nose at him. “She said you'd only be stuffy about it, and she was right."

Bram shrugged and sealed the outside hatch, then, after herding Jao and Ame through the air lock, screwed the inner lid into place.

He looked around the dome-shaped cabin. The landing vehicle was basically a squat hemisphere supported on five arched legs. It had started out as a Nar design with the pilot's seat in the middle, but like the rest of the considerable fleet the tree had been stocked with, its interior had been rearranged during the intergalactic crossing to give it something resembling front-back orientation, and the controls had been shifted to conform to human morphology. Lydis and her copilot sat facing one of the five bulging ports—the one that had been designated “forward.” The rear of the cabin contained passenger couches—more than the current mission profile called for—storage, equipment, and minimum amenities.

“You have exactly ten minutes to tie yourselves down,” Lydis said. “I don't intend to sit here for another go-round."

“Sorry,” Bram said.

He nodded to his daughter's co-pilot, a wiry, nonchalant fellow named Zef, then helped Enry and Jao stow their gear. Ame went to a locker and helped herself to a spare vacuum suit. While she struggled to get into it, Bram hefted the clanking sack she had given him and, after a moment's reflection, shoved it into a padded locker. “I hope there's nothing breakable in there,” he said.

“Nothing very breakable,” she said.

They climbed onto their couches and fastened the armpits-to-hips webbing in place. Jao cranked his couch to a sitting position.

“I want all you treelubbers to lie prone for the drop,” Lydis said. “And while you're at it, put your helmets on."

She herself was sitting upright, as was Zef. Jao pointed that out.

“You do everything by the list when you ride with me,” Lydis said. “Otherwise, you can get out and walk."

Grumbling, Jao complied.

“It's surprising, the number of things that can go wrong,” Zef said cheerfully. “Why, I saw a fellow explode once because he forgot to screw his helmet on all the way and nobody'd told him the cabin wasn't going to be pressurized that trip."

“Oh, stow it,” Jao said. “I'm not falling for any more of your stories."

Zef laughed. “It's not that we care about the safety of our passengers. We just don't want a lot of helmets floating around and bumping into things."

Jao started to reply, but his voice was cut off as Lydis watched the passengers off the Talk circuit. Abruptly Zef dropped his smile and became all business. Bram found himself gripping the armrests of his couch. Drop must be imminent.

He was still plugged into the Listen circuit, though. He could hear Lydis talking to Jun Davd back in the observatory.

“I have your readout,” she said. “Please confirm."

“Three minutes more and you'll be in optimum drop position. As tangent as you can get. Do you want the computer to open the trapdoor for you?"

“No, I'd rather do it by feel. The computer doesn't have nerves in its bottom, and it has too much faith in the invariance of mechanical systems. I'm going to have to make a lot of small burn corrections, anyway, once we're out there. Just keep feeding me the figures."

It was a point of pride with Lydis to be in fingertip control. She believed piloting was an art, not a science.

“A computer with nerves in its bottom!” Jun Davd chuckled. “My goodness. We'll work on it."

By craning his neck, Bram could see one of the duplicate screens left over from the original Nar installation, next to the observation blister closest to him. In a simplified computer cartoon it showed a great dull-red disk, slightly angled to give a sense of perspective, and a jolly little green representation of Yggdrasil, much out of scale, floating above and to one side of it. Discreetly flashing and dotted lines showed the direction of rotation of both bodies and their intersecting orbits around the rice-grain sun shining through a cluster of red lobes at the center of the system.

It obviously hadn't been very practical to put Yggdrasil into orbit around the rim of a disk-shaped body with a circumference of two hundred seventy million miles. And parking Yggdrasil sixty degrees ahead of the disk—at the stable point which in this crazy system neatly coincided with the point of equilibrium with the disk ahead of it in orbit—would still have placed them an inconvenient forty-five million miles away from the forward edge of the disk and all of one hundred million miles away from the present “top” of the disk, which they had chosen as their likeliest base of operations.

So instead, with Jun Davd's help, Bram had put Yggdrasil into a solar orbit that intersected the disk's orbit at a tilted angle. It started above and behind the disk at a distance of only a few million miles, slanted down at a tangent that almost grazed their target point on the rim, and continued on past to a point ahead of the disk in orbit that would place Yggdrasil directly “above” the spot where the disk's own slow rotation would have brought the explorers’ base of operations by that time.

Thus, for at least the first half year, travel time between Yggdrasil and the main landing site would be measured in days rather than months. At that point, Yggdrasil's solar orbit could be converted into a powered orbit around the rim, which would take it back to its starting point for another such orbital stern chase.

Bram kept his eye on the pulsing orange line that emanated from the tiny cartoon Yggdrasil on the screen and ended tangent to the disk. It represented a vector of the momentum that would be imparted by Yggdrasil's own orbital motion plus the added kick from Yggdrasil's rotation at the moment of release.

Lydis would add her own increment of momentum by firing the spacecraft's engines once she was in a position to judge how well lined up she was. Then she would have to cut it fine at the other end, killing all her pseudoorbital velocity and matching the speed of her target on the rotating edge of the disk—so that the net cancellation of both would come out even at the precise moment of touchdown.

No wonder she didn't trust the computer.

Once launched, the complex orbital mechanics boiled down to an eyeball-and-seat-of-the-pants job, and Bram himself trusted Lydis's instincts more than he trusted the unreeling chains of glowing figures superimposed on the computer cartoon that kept changing their final decimal places.

“Hold on to your valuables,” Zef warned through the helmet circuit.

The trapdoor beneath the spacecraft sprang open, and they fell through. Sudden weightlessness was a faint thrill along Bram's spine till his body adjusted. He made an incautious movement and floated an inch off the couch, held down by the webbing pressing against his chest.

He lost interest immediately in the computer display and applied himself to the view outside the blister. Yggdrasil's great gnarled branches floated by, pierced by random points of light from people's living quarters.

The tree rose until it was a green cloud above them. It began to dwindle and in minutes was far enough away that its shape could be seen against a sprinkling of stars: a double-ended mushroom divided by darkness.

He turned his head to see how Enry was taking it and saw that the man was sweating inside his helmet. Of the four passengers, Enry was the only one who had never been away from the tree; Ame had gone on jaunts with Lydis, and Jao had gone with Bram on comet-chasing expeditions. Bram could understand how Enry felt. It was a wrenching experience to part from the entity that nutured you in blind universe.

From this angle, line of sight was out of the system, and nothing could be seen except the stars. Now, with Yggdrasil shrinking overhead, Lydis rotated the ship to point toward their destination.

An uncanny collection of glowing circles rose to fill the viewport. Here, above the equatorial plane of the system, one could look down past their scalloped fence into the inner heart where the polar disks orbited. Their tininess was an illusion of distance; they still dwarfed the enclosed sun. One of them was skewed; Jao had been right about that. A collision with a leftover planetoid or a solar flare some time in the past had altered its carefully timed spin. The sun spilled its light through like a glowing egg in a nest.

The turning of the craft continued, and now an enormous knife edge cleaved the sky: the disk that was their destination.

Lydis applied a touch of her lateral jets once more, and the turning stopped until the knife edge was suspended directly overhead, Bram studied it through the bubble dome. At the tip where the line ended was an illuminated dot, like a tiny flower on a stem.

“You can see the moon from here,” Lydis's voice came through the suit radio. “It looks as if it's resting on the rim from here, but of course it's not. The structures we sighted through the big telescope are beneath it. We should begin to make them out at about a quarter million miles. They're huge."

The diskworld had proved to have moons—eleven of them, equally spaced, in synchronous orbit around the rim. Where the twelfth should have been, the narrow ribbon of landscape slumped suggestively across a span of twenty million miles.

The orbits of the moons were impossible—too close and too slow. “They have no right to hover like that,” Jun Davd had said.

For once, Jao had had no theories, except for a halfhearted, “Antigrav, maybe?"

“What I'm interested in is, what are they hovering over?” Jun Davd had mused. He had kept his instruments trained on the moons during Yggdrasil's long inward sweep from the outer limits of the plundered system, and had done much juggling with computer enhancement and other techniques. Some two billion miles out, he had been rewarded. “It's some kind of support complex,” he had announced, showing dubious pictures of a patchy grid, which might have been nothing more than the computer's desire to please. “Roadways, maybe. Ditches or canals or the remnants of a buried transport system. Street layouts with the rubble showing differently in the infrared ... casting low shadows..."

He had set a course for the largest of the complexes on the disk whose orbit they could most conveniently intercept. It struck Bram as finicky and bizarre, and Jun Davd agreed with him. But it was a planetary body with interplanetary distances; the next largest complex was a third of the way around the rim—ninety million miles away. Too far to walk. It was definitely a problem in space navigation.

“Hold on,” Lydis's voice said. “I'm going to give you some weight now."

There was a gentle shove on Bram's chest, pressing him into the couch. A rain of small objects came from above; someone had forgotten to secure some minor gear. Zef turned to glare at the culprit, and Jao grinned sheepishly within his helmet.

The burn was a leisurely one, lasting a half hour at what Bram estimated to be about a quarter of a gravity. There was plenty of hydrogen and oxygen to be profligate with since Yggdrasil had drunk its fill of comets.

Lydis saw her passengers fidgeting. “I know it's hard to lie still when there doesn't seem to be any reason for it,” she said, “but I don't want any mass moving around while I'm doing this."

At last they went weightless again. “All right,” Lydis said. “You can get up now. Take off your vacuum suits if you like. I'm not going to fire the jets again for about two days."

Everybody gratefully desuited. Jao scratched mightily. “I don't think you were worried about leaks at all,” he said. “I think you just wanted to keep us quiet."

“Where'd you ever get an idea like that?” Zef said.

They all crowded to an observation blister to have a look at their destination; Lydis had rolled the ship over after the main burn so that people wouldn't have to crane their necks to look through the overhead dome.

Enry, pale, said, “How wide is the rim? It still looks like a one-dimensional line from here."

“About fifty miles,” Jao said. “Talk about thin! We wouldn't see it at all from this angle if it wasn't for scattered light from over the edge."

Ame said, with a trace of awe, “The former human race was efficient. “Just about all the working surface is on the fiat sides."

Jao nodded. “But don't forget, even a fifty-mile width gives a surface area on the rim alone of thirteen and a half billion square miles. That's equivalent to the surface areas of seventy of your normal, terrestroid-style planets like the Father World. That's a lot of elbow room, even if it is all east and west."

“I'm glad we don't have to dig it all up,” Ame said, with a glance at Enry to see how he was taking her presence.

Enry rose to the occasion. He was stuffy but nice. “I could use a little help,” he said.

A perfunctory laugh went around. Bram asked Lydis for a magnified view through the ship's telescope and got nothing more than a fuzzier line topped by a blurred speck that might have been construed as a crescent.

“It's going to be a very strange place,” he said.