"Why?" Jameson typed. "Why do you need the large planet?"

The two Cygnans hesitated. Their stumpy eye polyps twitched. They were facing a problem about vocabulary.

"Jameson eats the green food made from growing things," Triad skirled. "The small animal from Earth eats the white liquid. Then Jameson has the power to move. Then the animal has the power to move.

Does Jameson understand?"

"Jameson understands," he tapped out on the keys.

"An engine must eat. Then this place may move."

Fuel. They were talking about fuel. A couple of quick exchanges and he had the Cygnan word for it.

Tetrachord spoke up. "The fuel which our engines eat is the mother-of-matter."

He got it right away. Hydrogen!

They were using Jupiter for fuel. To the Cygnans, that was all Jupiter was good for. They'd simply dropped in on a handy solar system to tank up.

Gas giants were common throughout the universe. They were a necessary consequence of planetary formation. Most of those that had been detected, like the superjovian companions of 61 Cygni and CIN 2347, were considerably more ma.s.sive than Jupiter itself. Using them for refueling stops must be a convenient way to star-hop.

Mother-of-matter-that was as good a description as any for the most basic of the elements. Rather poetic, in fact.

Jupiter was composed almost entirely of hydrogen. The giant planets, with their tremendous gravitational strength, were able to hang on to the light gases that had given them birth. Oh, the atmosphere was placed with helium and with such impurities as water, methane, and ammonia. And somewhere at the center of that vast slush ball, like a cherry pit, was a small rocky core about the size of the planet Earth.

But mostly it was hydrogen-an atmosphere hundreds of miles deep, squeezed gradually by its own unsupportable weight until it began to behave like a liquid. There was no clear boundary. It became an ocean without a surface, some twelve thousand miles deep, an ocean in which the planet Earth could have sunk without a trace. It was also an ocean without a bottom. At that depth, under a pressure of three million Earth atmospheres, the thickening syrup of molecular hydrogen underwent another transformation. Its molecules dissociated into atoms. It turned into a metallic form never seen on Earth: a dense fluid that could conduct electricity.

It would make a bottomless reservoir of hydrogen fuel. Siphon off the upper atmosphere and the lower layers, released from pressure, would boil up into a gas again. Drain the oceans that weighed on that strange ball of metallic hydrogen and the viscid stuff would turn into the molecular form, in turn boiling off as a gas at its surface.

Jupiter would make a great fuel tank. How close to the speed of light could you get before you used it up?

Jameson looked over at the triple image of Jupiter on the circular screens. How much of Jupiter was gone already? It was impossible to guess. The Cygnans could have stolen a ma.s.s equal to several Earths without making an appreciable dent in it. And as the surface pressure let up, what was left of Jupiter would expand. To Jameson's naked eye there was no difference in Jupiter's size.

Jameson's fingers rippled over the keys. The Moog cleared its electronic throats and said, "How? How is it possible for you to do this thing?"

"The planet you call Jupiter will fall," Tetrachord repeated for the fourth time. "And we will fall after it."

"Yes, but what will it falltoward? " Jameson asked, getting desperate. It wasn't easy discussing orbital mechanics in babytalk.

"It will fall to that-which-pulls," the Cygnan tweetled, "when that-which-pulls grows heavy enough."

Jameson clenched his big fists in frustration. They were going around in circles. Any more of this and the Cygnans would give up on him, as they always did when the going got tough. He might never have another chance to reopen the subject.

He composed his thoughts and ran his hands over the keys again.

"Where is that-which-pulls?"

Tetrachord gestured vaguely toward the bulk of Jupiter on the screens. "You see it," he said.

What did that mean? Was Jupiter supposed to fall toward itself? "Give me a closer look," Jameson said.

Tetrachord twisted a serpentine neck toward the triple screens and warbled at them. The console evidently could be voice-activated as well as manually controlled by the little tuned wires.

The view of Jupiter enlarged. The screens zoomed in on a segment of the hoop of glowing wire that circled the giant planet above the cloudtops. A rim of clouds stretched from Jupiter to the hoop like a hat brim.

The zoom stopped. Jameson was looking at a sash of light stretching in triplicate across the screens.

The close-up of a piece of halo told him nothing. "I see that the light draws hydrogen ('mother-of-matter') from Jupiter. But where does the hydrogengo? How is it stored?"

That earned him a lot of disconcerted whistling. "To store hydrogen has not-meaning. It is to be eaten by that-which-pulls."

Back to square one. Exasperated, Jameson said, "I don't see anything there except light."

They seemed puzzled that Jameson couldn't see what they were showing him. They held a bagpipy conference about it. Then Tetrachord made some adjustments at the keyboardlike row of fretted miniature guitar necks.

As Jameson watched, the glowing bracelet that circled Jupiter began to flicker. It became a series of fireb.a.l.l.s chasing one another's tails. The fireb.a.l.l.s finally slowed so that he could see them. What he saw was a herringbone frieze of overlapping shapes. Through the ghost images he could make out the basic form of the thing.

It was a strobe effect. Jameson was looking at a frozen frame of the object that circled Jupiter at a blurring speed that by now must be an appreciable fraction of the speed of light.

The Cygnans needed no such visual coddling, he suddenly realized. The neurons in their visual pathways must be able to fire selectively, at millions of times per second, as naturally as a human being might squint for better focus. They carried their own built-in strobe flash.

"That-which-pulls," Triad hummed.

It was one of their probes-an elongated pyramid like a flat-sided spike. It was doing nothing but circle.

Jupiter, again and again, picking up speed with each circuit.

"Does Jameson know that as a thing goes faster, it grows heavier?" Tetrachord asked.

By G.o.d, the creature was quoting the theory of relativity at him! What Cygnan Einstein, thousands of years in the past, had arrived at the great keystone equation governing the increase of ma.s.s with velocity?

Jameson dug into his memory for what he had learned at his Academy cla.s.ses long ago.

"Jameson knows that if a thing would go as fast as light is fast, its weight would be..."

d.a.m.n! What was the Cygnan word for "infinite"? He'd never learned it. Perhaps there wasn't one. And did they understand "weight" to mean "ma.s.s"? And how did you express the concept of a square root in Cygnanese? The square root of one minus the square of velocity divided by the square of the speed of light-how did you go about saying a thing likethat in pidgin? Maybe the Cygnans didn't use square roots, either. Perhaps they arrived at their results in an entirely different fashion-like Russian multiplication.

He stared at the little triangular probe on the screens. Could a thing that small really move the biggest planet in the solar system? His instincts said no. Einstein said yes.

What was its ma.s.s by now? Enough to make the outer fringes of the Jovian atmosphere fall into it.

Jameson could see the threadlike stem of a whirling tornado sucked into the needle craft-a tornado that was whipped round the circ.u.mference of Jupiter at thousands of miles per second, unwinding the giant planet like a ball of twine.

The probe couldn't be more than a few dozen meters in length. But its speed made it legion; it zipped around the planet like a horde of hydrogen-sucking vampires, bleeding Jupiter's substance from the great continuous wound at the equator.

Faster and faster, squandering that bottomless reservoir of hydrogen to push itself fractionally close to the unattainable speed of light, this one tiny gnat could unpeel Jupiter layer by layer. Then, pregnant with stolen ma.s.s, it would reach a point on the curve where it outweighed Jupiter itself-or what was left of Jupiter.

Long before that point, Jupiter would begin to respond to its gravitational tug. The two bodies would be revolving around a common center of gravity, rising through Jupiter, then outside it. Some of the energy could be diverted to form a vector.

The giant planet would then follow the little robot ship like an elephant on a tether. The gigantic alien ships, in turn, would be drawn along in Jupiter's wake, using its bulk as a convenient shield against the inferno of X-rays and gamma radiation sparking off the planet's forward face.

Jameson scratched the stubble on his chin. The mysterious Earth-sized planet that the Cygnans had deposited in orbit around Jupiter was-he guessed-the discarded core of another gas giant. The sheer extravagance of it took his breath away. The Cygnans had to be the wastrels of the universe.

Maybe it was the only way to travel between the stars at relativistic speeds. Take along a gas giant for a fuel tank. Or, more accurately, have it takeyou along. A fuel supply-and protection against the h.e.l.lish storm of radiation that happened when you collided with interstellar hydrogen at a speed approaching that of light.

The scale of it was staggering. But after all, he told himself reasonably, the principle wasn't too different from that of an early chemical rocket burning tons of fuel to put a few pounds into orbit. As the fuel was burned up it imparted its energy to what was left of the payload-a point that had been lost on some of the early critics of s.p.a.ce travel, who insisted that not even the most powerful known fuels contained enough energy to boost themselves to escape velocity.

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 ma.s.s to move Jupiter and Jupiter lost enough ma.s.s to be moved didn't really matter; it could be expressed as a differential equation. Half a Jovian ma.s.s-or a tenth of it-was still plenty of ma.s.s 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 ma.s.ses to brake with. Burning 90 percent ofthat would still leave them with three Earth ma.s.ses at the end of their journey.

Be generous. a.s.sume some inefficiency in their system. Surely they couldn'ttotally convert matter to energy. Throw away a couple of Earth ma.s.ses. 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. a.s.sume evenmore 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 s.p.a.ceship, 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 a.s.sumptions. Itfelt right!

h.e.l.l, 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 ma.s.s. 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 ma.s.s to move Jupiter? The probe didn't have to accelerate any ma.s.s except itself; its fuel tank was external-Jupiter's atmosphere. He frowned, trying to recall some of the theoretical scuttleb.u.t.t 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 s.p.a.cecraft could attain 99.999,999,999,999,999,996 percent of the speed of light by annihilating a ma.s.s 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 h.e.l.l did the Cygnans keep the probe from flying off into s.p.a.ce? At that speed, with its abnormal ma.s.s, 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 matterhow much energy was diverted to tie the probe down in orbit? Just takewhatever was needed out of all that kinetic energy stolen from Jupiter's ma.s.s, no matter how mind-boggling the sum. Blow it out in a direction perpendicular to the orbit. Or change the att.i.tude 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 letthat 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 s.p.a.ce! 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 system 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 s.p.a.ce, 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 ma.s.s 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...o...b..ting 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...o...b..t 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.