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.