His mind raced. Think of Jupiter, then, as being a series of fuel tanks which are progressively discarded as they are used up. The point at which the little probe gained enough mass to move Jupiter and Jupiter lost enough mass to be moved didn't really matter; it could be expressed as a differential equation. Half a Jovian mass—or a tenth of it—was still plenty of mass left to play around with.
The Cygnans could afford to be profligate with their stolen planets. Suppose, Jameson thought feverishly, they burned 90 percent of Jupiter to attain a velocity of, say, 98 percent of the speed of light for their strange caravan. Ninety-eight percent, he guessed, would be about the point where a law of diminishing returns set in and the implacable equations of relativity demanded the expenditure of impossible amounts of energy to attain infinitesimal increments of velocity. So what? They'd coast at .98c. That would still leave them more than thirty Earth masses to brake with. Burning 90 percent of that would still leave them with three Earth masses at the end of their journey.
Be generous. Assume some inefficiency in their system. Surely they couldn't totally convert matter to energy. Throw away a couple of Earth masses. That would still leave them with an Earth-sized rocky core.
Like the one that was now in orbit around Jupiter.
But wait a minute. Assume even more inefficiency!
Once they got going, they might pick up enough interstellar hydrogen to make up the difference. After all, why waste that manna of hydrogen infall as they plowed through interstellar gas clouds?
A twentieth-century scientist named Bussard once had calculated that an intake area of about 80 miles in diameter would be a sufficient-sized scoop for an interstellar ramjet feeding on clouds of ionized hydrogen. True, he was talking about a 1,000-ton spaceship, but he was also talking about ordinary hydrogen fusion.
Even at the final stages of their journey, with their Jovian giant shrunk to Earth size, they'd have a scoop with a diameter of 8,000 miles. And that was just the small end of a truncated cone—the impact area. The size of the large end would be anybody's guess. It would depend on such things as gravitational attraction and the rate of ionization induced by the planet's magnetic field. It could sweep an area hundreds of thousands of miles in diameter.
Jameson felt his cheeks burning with excitement. He wished he had Ruiz to talk this over with. Ruiz would be able to work out the math. But he was sure that he was right in his assumptions. It felt right! Hell, he could be wrong by a factor of ten and the principle would be the same. Burn 99 percent of a gas giant to get the remaining 1 percent—plus your piddling fifteen-mile ships—up to velocity. Then brake by burning 99 percent of the remaining 1 percent. Maybe they'd ridden here on a superjovian, ten or twenty times Jupiter's mass. Maybe they'd have to limp out of the solar system at a mere 90 or 95 percent of the speed of light.
“The word is infinite,” Tetrachord said. He sounded like a cageful of twittering birds, all except the sound for “infinite,” which was a single sweeping glissando spanning two octaves, with a little turn at the top. Was it Jameson's imagination, or was there an overtone of approval in the Cygnan's voice, like a person patting a smart puppy on the head for doing a trick?
“But an infinite weight can never be reached,” Jameson responded quickly.
“No.” Again Jameson thought he heard approval. “But that-which-pulls will be heavy enough in...” There was a phrase for a unit of time.
That was the sticky point. The Cygnans, in their arrogance or self-sufficiency, had never bothered to give Jameson a scale involving Earth's year or period of rotation. Perhaps they simply didn't care to study the planets encountered during their transient pit stops. They used a Cygnan time scale, and to understand the timetable for this cosmic theft he was going to have to find out something about the Cygnan home planet. Perhaps it didn't even exist any more, but it was still part of their cultural baggage.
Just how close to the speed of light could the little probe itself get in its flashing circuit, and how long would it take it to gain enough mass to move Jupiter? The probe didn't have to accelerate any mass except itself; its fuel tank was external—Jupiter's atmosphere. He frowned, trying to recall some of the theoretical scuttlebutt that had come out of the abandoned studies for a Centaurus probe. A twentieth-century rocket expert named Sanger had estimated that a respectable-sized spacecraft could attain 99.999,999,999,999,999,996 percent of the speed of light by annihilating a mass the size of the Moon. You could get there in a year at one g, in less than four days at 100 g's.
The probe the Cygnans called “that-which-pulls” had been at it for more than six months. It must have brushed the speed of light within a few days. Now it was nibbling away at the remaining fraction, fighting an uphill battle against relativistic imperatives. The efficiency of its engines would be diminishing, from an outside observer's viewpoint, by the penalty of an enormous time-dilation effect. But it was certainly getting there.
Now how the hell did the Cygnans keep the probe from flying off into space? At that speed, with its abnormal mass, and with its tight turns around Jupiter, the centrifugal force must be ... unthinkable!
He was about to ask Tetrachord when he realized that the question was without meaning. What did it matter how much energy was diverted to tie the probe down in orbit? Just take whatever was needed out of all that kinetic energy stolen from Jupiter's mass, no matter how mind-boggling the sum. Blow it out in a direction perpendicular to the orbit. Or change the attitude of the robot craft to provide a vector that would balance the enormous forces. The method didn't even have to be particularly complicated. A simple feedback mechanism would do.
He took a closer squint at the cascade of repeated shapes frozen on the screens. There! He could make it out! A plume of light sprouting from the waist of the ship, driving it inward toward its primary.
He shuddered. What was that spray of light? A waste product of the drive, as heat is the byproduct of the work done by a mechanical engine? He suspected that the Cygnan exhaust was something akin to pure gamma rays. It wouldn't do to let that stuff get too close to an inhabited planet, to say nothing of that terrible bow wave!
“How will you remain in the shadow of the light-that-kills?” he asked impulsively, improvising words and hoping his meaning was getting across.
The two of them twittered happily. They were proud of their puppy. It had done another trick.
Tetrachord plucked at the console, wiping the viewscreens. The picture was replaced by a triptych of bright toylike geometric shapes rotating and counter-rotating in intricate patterns like the mechanism of a transparent clock. The moving parts ticked away in tiny jumps in sequence around the three identical screens.
A model of their system! Here was a planetarium for Cygnan schoolchildren, showing them how their convoy of artificial worldlets traveled through space! He'd hit the jackpot again. He didn't need any math to grasp the basics of what was being shown to him.
At the center was a glowing red ball, moving against an abstract background of stars. That was the gas giant. The one they had already used up, soon to be replaced by Jupiter. Around its waist was a string of tiny chips that pulsed in sequence to indicate motion. An abstraction of the robot probe, tailored to Cygnan perceptions. He squinted more closely. On one of the three screens he could see a fan of light directed outward—the lethal spray of radiation.
Enclosing the two-body system in a wider, polar orbit was a smaller sphere of opalescent gray. One of the gas giant's moons, dragged along by its kidnapped parent. It rotated in a plane that was almost vertical to the loop described by the probe. It put Jameson in mind of a gyroscope configuration, or perhaps one of the simplified representations of an atom that used to be popular in his schoolboy days. Then, in orbit around the moon—again in another plane—he saw five glittering mites, three-armed asterisks spinning on stems. The Cygnan ships.
The ships, in turn, were rotating around a common center of gravity, chasing one another around in a circle. It was the sys
tem of five ships, considered as a single body, that rotated around the moon.
It was beautiful.
He sat watching the show, transfixed, like a boy mesmerized by a complicated set of electric trains. Everything had its own motion. Nothing collided.
He watched it for a long time to be sure how it meshed.
To keep from being flung into space, the little probe, that-which-pulls, had to keep blowing off energy all the time the intricate procession was traveling, even when it had stopped accelerating. It was a worm, draining away the substance of the planet. There could be no such thing as coasting without consuming fuel.
The Cygnans would have to drop in periodically on star systems close to their line of flight to refuel. Otherwise they would find themselves without enough mass to brake and be doomed to go flying forever through the universe.
How many stars had they plundered of gas giants during their long hegira?
Each shanghaied planet, of course, would have cost the Cygnans at least a year's braking time, plus another year to boost up to relativistic speeds again. For the rest of it, how long would it have taken, ship's time, to travel the 10,000 light-years that Ruiz had postulated? At 98 percent of the speed of light, Jameson knew, the time-dilation effect would be approximately fivefold. The Cygnans had been cooped up in their triangular cans for more than two thousand years of subjective time. They had managed to maintain a technological civilization, but he guessed things were getting a little stale.
Triad was tootling at him. “Does Jameson see how we are always in the shadow of the moon?”
Jameson returned his attention to the screens. He tried to keep track of the separate motions.
The pilfered moon, despite appearances, wasn't revolving around its parent. It was orbiting around the common center of gravity shared by the gas giant and the probe that was in tight orbit around it. But since the center of gravity was so close to the planet's surface, of course, it made no practical difference to what he was seeing.
The moon, from his point of view above the system, was rotating counterclockwise around the gas planet. The direction of motion of the whole system was upward, toward twelve o'clock. The mitelike ships, in orbit around the moon, were traveling clockwise.
Their orbit had the same period as the moon's orbit around its primary. They were always in a trailing position, shielded from impact with interstellar hydrogen by three thousand miles of rock. At every point of the opposing orbits, the ships were in the lee of the radiation.
He watched the clockwork simplicity of it.
With the moon in six-o'clock position, the rosette of ships was also in six o'clock in its elliptical lunar orbit. It was shielded by both the moon and the bulk of the giant planet itself.
When the moon emerged from that cone of safety, to three-o'clock position, the ships were at the moon's nine o'clock. But since the moon was by that time tilting its twelve inward toward the giant, the ships were still in a trailing position. With the moon at twelve o'clock, leading the whole procession, the cluster of ships was safely behind it, at the moon's twelve o'clock. Another quarter turn for both orbiting systems put the moon at nine and the ships at three—still in the moon's radiation shadow.
The ships’ orbits, he suspected, would have to be adjusted continually to match their period to the moon's rotation—especially as the mass of the primary shrank. But surely, maneuvering the five ships would require only a fraction of the total energy expenditure eaten up by moving a Jovian or superjovian!
It was beautifully simple and elegant! Jameson watched in admiration for long moments.
Even the deadly probe, with its radiation backlash, was never at the crossroads of the moon's orbit at the two points where their paths intersected. Everything ticked along beautifully.
“I see,” Jameson said. “Your ships are safe.”
The two Cygnans whistled their approval. Tetrachord wiped the screens and dropped down on four legs. One of his upper limbs twined around Triad in an almost-human gesture of affection.
Jameson blared the sharp fanfare for attention. Startled, the Cygnans jerked their heads in his direction.
“What about Earth? My planet. Will it be safe when you leave this system?”
Consternation. Much twittering back and forth. Jameson had the impression that they had never thought about it, that it hadn't occurred to them to care.
Finally Tetrachord punched in an inquiry to the ship's computer, or whatever passed for one aboard the Cygnan vessel. There were flashing images that made no sense to Jameson. They hadn't bothered to adjust the screen for human vision this time.
Tetrachord twisted around. His eyestalks stretched like taffy in Jameson's direction.
“Jameson,” the creature said. “We will cross the orbit of your planet when we leave. We will pass close to your sun and swing around it to change direction.”
Jameson got a crawly sensation down his spine. The Cygnan caravan would cross the Earth's orbit twice.
“Just how close to Earth will you pass?” he asked.
There was no answer for a while. Jameson found he was holding his breath.
The Cygnans wouldn't have reached anything near light-speed by the time they crossed Earth's path, of course, so the deadly shower of X-rays that had announced their approach to the solar system would be no danger. But the probe's deadly drive would be on. That in itself might be enough to sterilize a hemisphere if it got too close and was pointed in the wrong direction. Then, too, there was Jupiter's own radiation belt, extending millions of miles into space. The Cygnans themselves would be safe from charged particles in the zone swept clean by their moon, but Earth might not be so fortunate.
And there certainly would be tidal effects.
Jameson trembled at the thought of what might happen if a Jupiter-sized mass passed too close to Earth. Earthquakes, floods, perhaps even the breakup of the Earth's crust.
What if the Earth's orbit were changed, moved a couple of million miles closer to the sun? Or pulled farther away? Or changed, like Pluto's, to a more elliptical orbit? Earth's climate could be permanently altered—an eternal ice age, with much of terrestrial life obliterated, or a water world, steaming under the melted polar caps!
Earth might even be plucked out of orbit to fall into the Sun.
“How close?” he repeated urgently.
“Jameson will be safe,” Triad hummed soothingly. “We will take Jameson with us.”
“Dammit!” he exploded. “That's not what I asked! What about the Earth?”
He stopped. He'd unthinkingly used human speech.
They didn't understand the words, but the violence of his outburst had startled them.
Triad pressed herself against her larger companion. The soft, rat-sized thing plastered to her abdomen reacted to her distress by digging in more firmly with its insectlike legs.
Tetrachord hissed reflexively at Jameson. His upper body stretched to become a foot taller.
Jameson stood facing the alien pair, fists clenched. The kitten had dropped off his lap and scurried away. After a moment, Jameson's fists fell to his sides. The tension in the bodies of the two Cygnans gradually relaxed.
Jameson stooped over the keyboard of the Moog again and played out his question. “Where will the Earth be when you pass?”
There was a pause while they digested his query. Finally Tetrachord said, “We do not know.”
“Find out,” Jameson said. There was no Cygnan word for “please.”
They exchanged some running cadenzas, too fast for Jameson to follow. Then Tetrachord, still with a couple of arms around Triad, turned to his electronic zither and twisted some frets. A rapid chirping came out of the console. Tetrachord chirped back at it. For some reason the Cygnan had not encoded the question to the ship's computer. He'd asked someone.
Jameson couldn't understand the reply. Colloquial Cygnan would always be beyond him.
After a delay, a picture formed on the tripartite screen—another nursery diag
ram, like the one he'd been shown of the Cygnan travel arrangements. This one showed a series of concentric triangles with a glowing yellow triangle in the center. The Sun and the orbits of the planets! Jameson gulped. Was that how the Cygnans saw circles? It hadn't been so in the previous projection, but perhaps this was someone's shorthand sketch or working diagram.
An irritated shrilling came from the console. A small green triangle appeared at the third place from the sun and moved back and forth along its track until it found a place and settled down. The orange triangle representing Jupiter jumped out of its orbit and moved jerkily Sunward. It dragged the yellow line of its orbit with it, opening the triangle into a four-sided evolute of ellipse. The evolute stretched as Jupiter intersected the inner solar system, traced a sharp V around the Sun, and headed out into the depths of space again.
Mercury and Venus jumped in their orbits. The white triangle representing Venus had been set spinning. But Earth had been spared.
“Your planet will be on the other side of the Sun,” Tetrachord said, unnecessarily.
Jameson eased himself down on the Moog's stool. His knees were trembling. He became aware that he was drenched with sweat.
If he could believe the Cygnans—and if they didn't decide, from some incomprehensible alien motive, to recompute their line of flight—Earth would be allowed to live.
As long as nothing delayed the Cygnan's departure.
Chapter 20
The attendant was old. If it had been a human being, Jameson would have said it shuffled. By this time he was familiar enough with Cygnans to know that this one's characteristic darting body movements were stiffer and slower than Tetrachord's or Triad's. Its mottled hide was duller, drier, less glossy. Did older Cygnans outlive their parasites, as terrestrial animals sometimes did? At any rate, there was no sluglike pest hanging from its belly, though Jameson thought he detected an old cicatrix where a tiny bloodsucking head might once have been embedded.
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