The Jupiter Theft

by Donald Moffitt

  “That's about it,” he said.

  “Run through it again, Doc,” somebody said.

  “All right,” Ruiz said good-naturedly. “Hold that up, Commander, so everybody can see. First we have the robot in a powered orbit around the gas giant. It's traveling at very nearly the speed of light, using up the planet's mass without having to accelerate it, until its own Einsteinian mass outweighs what's left of the planet.”

  “Wait a minute, Doc!” a boisterous voice said. “The centrifugal force would be tremendous!”

  “You weren't listening, Gifford. I said a powered orbit.”

  “Yeah, but did you figure out the centrifugal force?”

  “It doesn't matter,” Ruiz said with rare patience. “Whatever percentage of the total kinetic energy is needed to maintain a tight circle is progressively diverted to a thrust perpendicular to the orbit. The Cygnans aren't concerned with efficiency. They've got a whole planet to use up. By the same token, a percentage of the total kinetic energy is used to provide a vector, so the Cygnans can get the whole system moving in whatever direction they want.”

  Mike Berry had stopped playing the guitar and had moved into the circle. “That's right,” he said. “All it means is that the Cygnans strip their Jovian gas tank of mass faster—or longer—before they can move what's left.”

  “Yeah,” somebody gibed. “Shut up, Giff!”

  Ruiz straightened creakily. Like most of the others, he was wearing one of the crisp new uniforms Jameson had retrieved that morning, but it hung loosely on his emaciated frame.

  “If we've got that settled,” Ruiz said, “would you flip the store-and-recall button for us, Commander? Keep doing it. Thank you.”

  Jameson watched the pictures change on the black surface. Ruiz had painstakingly drawn a four-step animated diagram showing the multiple orbits. The moon jumped in its circle around the Jovian planet, moving counterclockwise. The rosette of dots jumped in its egg-shaped orbit around the moon, moving clockwise.

  “The orbit of the ships around the moon has the same period as the moon's orbit around the planet,” Ruiz went on. “Doubtless the ships’ orbits are continuously adjusted to keep them synchronized as the planet shrinks and the moon's orbit changes. But as you can see, the ships are always sheltered from the radiation caused by collision with interstellar hydrogen.”

  “Beautiful, beautiful,” Mike murmured.

  “Commander Jameson thinks—and I agree with him—that a significant portion of that impacting hydrogen also becomes grist for the Cygnans’ mill. After all, it's already been ionized. It becomes an energy bonus to partially offset the inefficiency inherent in the Cygnan method of travel.”

  “Can I quit?” Jameson said. “My thumb's getting tired.” He passed the lightpad back to Ruiz.

  “It seems kinda complicated,” the Giff said, being a bad boy again.

  “It wasn't complicated for the Cygnans,” Ruiz said. “Don't forget, they started with the components of the system already in place. That moon they use for a shield was the world they evolved on ... I wonder how they feel about traveling with its corpse. The gas giant they used for fuel was the primary that their world revolved around. It would have been natural for them to start building their fleet in orbit around their world. They had a whole population to transfer, a shipboard ecology to establish, a technology to develop. It might have taken millennia. They had the time. Perhaps they didn't develop the technology for that Einsteinian siphon of theirs until the work was well under way. They might not have traveled at relativistic speeds for the first few centuries, while they completed the shakedown for those synchronized orbits. It wouldn't have happened all at once.”

  Jameson was the first to pick up on it. “You said ... they had the time. What do you mean?”

  Ruiz was fiddling with his lightpad. He had more of his day's work stored there. He looked up, almost absently.

  “One of their suns was going to go supernova. And they had half a million years to get ready for it.”

  The crowd around Ruiz had grown. Word had gotten about that something lively was going on. Jameson could see Liz Becque nestled against Omar, her arm around his thick waist, her post at the punchbowl deserted. Even Klein had joined the group. He was at the fringes of the crowd, his knotted arms folded, standing next to Yeh and Chia, so intent on the discussion that he'd forgotten his aversion to his Chinese shipmates.

  “The Cygnans are the children of a binary system that now consists of a black hole and a blue supergiant,” Ruiz said. “They spent twenty thousand years watching one of their suns swallow the other.”

  Beside Ruiz, Captain Boyle nodded gravely. “We can begin to understand something about the Cygnans’ motives now. We knew that they came from the direction of the X-ray source known as Cygnus X-l, about ten thousand light-years away. What Commander Jameson managed to see on the Cygnan computer display confirmed that it was their origin, incredible as that seems.”

  Mike Berry was itchy with questions. “Twenty thousand years. That's longer than all of human history. You sure of that figure?”

  Ruiz swung his narrow beak toward him. “We can calculate the timetable for the evolution of an X-ray binary system from the mass of its components. And we get that from their dynamical behavior.”

  Jameson spoke up. “The Cygnan commentary got eloquent about a ‘Great Mother’ that swallows her ... not ‘children'... maybe something like ‘little brother.’ I assumed it had something to do with eclipses in a double-star system. I saw some spectacular ones.”

  “It was more literal than that,” Ruiz said. “One of their suns was swallowing the other. And the mass exchange took place fast enough to be noticeable over the generations. It must have been a key part of the Cygnan race consciousness since they first crawled out of the ooze and became civilized beings.”

  Liz Becque, shuddered and drew closer to Omar. “You mean they actually would have seen one of their suns getting bigger and the other getting smaller?”

  “As their suns evolved, yes.”

  “Think of it!” Dmitri blurted. “To live under such a sky! When you think of the burden of myth and theology we humans have invented from our own simple sunrises and sunsets and seasons...”

  “The myths would have turned into scientific knowledge,” Ruiz said dryly. “When they got to the point where they knew what we know about close binaries and mass exchange, they would have known exactly what the fate of their suns would be. And exactly when it would happen.”

  Dmitri sucked in his breath. “A race spending its entire history with the foreknowledge of doom—first as myth, then as certain knowledge.”

  Jameson squeezed Maggie's hand for comfort. It was no wonder that the Cygnans had no empathy left over for other life forms.

  Out of a babble of voices, someone asked: “The black hole swallows the supergiant, is that it?”

  “No,” Ruiz said. “It's the other way around—at first. And they hadn't got to be a black hole and a blue supergiant yet. You start with two ordinary, healthy stars. But they're big. That's important. One is about twenty solar masses, the other about six.”

  He sketched two circles in the air. Such was the force of Ruiz's personality that Jameson could imagine that he saw them hanging there.

  “They're in a close binary system. Very close. They circle each other in only four and a half days. There's a gas giant farther out, in their common ecosphere.” He sketched another imaginary circle. “It's many times bigger than Jupiter, but not quite big enough to have been kindled into a star. It's got its own family of satellites. One of them's the Cygnan home world. It's about the size of Mercury.”

  “The smaller of the two bodies now in orbit around Jupiter!” Maggie said brightly.

  Ruiz gave her a hooded look. “It's very likely,” he said. “They had to kill their world to escape a murderous sun. I wouldn't be surprised if they had a sentimental attachment to the coffin.”

  “Wait a minute,” Mike Berry objected. “The Cygn
ans boosted at one gravity. Doesn't that imply that they evolved on an Earth-size planet?”

  Unexpectedly, it was Dmitri who came to Ruiz's rescue. “Takes only a year to boost up or down from light speed at one g,” he pointed out. “The Cygnans could take that easily, with their body structure—six legs, low profile, no rigid bones. Besides, they're spinning this ship at one third g now.”

  Ruiz inclined his head in acknowledgment.

  “You were telling us about the timetable for the evolution of an X-ray binary system,” Jameson prompted.

  “Just so,” Ruiz said. “All right. We start with two healthy stars. Big stars burn out quicker. Right? So the twenty-sol star uses up its hydrogen first. It becomes a supergiant, hundreds of times larger than our own Sun. It's a helium star now...”

  “Without wiping out all life in the system, Cygnans included?” Gifford said incredulously.

  “It's part of a close binary system,” Ruiz said. “It can only expand to the limits of its Roche lobe.”

  “That's why the big star looked pear-shaped!” Jameson said.

  “Correct, Commander. When the star overflows its Roche lobe, the surplus mass is transferred to the companion star. The mass exchange takes only about twenty thousand years. Perhaps less. When it's finished, the twenty-sol star is down to less than six solar masses. The six-sol star has grown to somewhat more than twenty. It adds up to about the same.”

  “So their joint ecosphere isn't affected,” Dmitri put in. “The Cygnans survive.”

  “They survive, yes,” Ruiz said. “But they'd have had a difficult twenty thousand years. Helium stars burn hot. The Cygnans must already have started thinking about moving from the neighborhood. They knew that worse was to come.”

  “The supernova?”

  “Yes. The star that ate its companion's mass is in good shape. It has a new lease on life. It can burn its stolen hydrogen for another six million years. But the other star is doomed. It can burn its helium only for about half a million years. Then the carbon core explodes.”

  “So the Cygnans had half a million years’ warning?” Mike Berry said.

  “That's right. But evidently they didn't wait around that long. It's not very healthy to be in the vicinity of a supernova. From what Commander Jameson observed, it looks as if the Cygnans mobilized their society and got out within the first few thousand years.”

  Dmitri was trembling with excitement. “But Jameson saw the supernova explosion! The Cygnans left a few thousand years after the mass exchange, then kept a watch on the stars they'd left.”

  “Correct, Mr. Galkin.” Ruiz looked as if he was enjoying himself.

  “But—but that means that the Cygnans have been evolving in an artificial environment for half a million years!”

  There was a rush of excited voices. Ruiz waited it out. He held up a hand, and the noise died down.

  “It took longer than that,” Ruiz said. “The Cygnans have been traveling for six million years.”

  Chapter 24

  There was a commotion within the crowd, and Tu Jue-chen came pushing her way through. They parted to make way for her, the Americans with somewhat more alacrity than the Chinese. She stood panting on the step below Ruiz and the ship's officers, shaking a liver-spotted fist at him.

  “You are lie!” she shrilled. “You are lie!”

  Beside Jameson, Maggie whispered, “What's this all about?”

  “I don't know,” Jameson said. “She's got some ideological bee in her bonnet.”

  Ruiz's expression was absolutely correct. “Why do you say that, Comrade Tu?” he said.

  She showed her horse's teeth. “Because,” she said triumphantly, “after six million years they would be socialists!”

  Ruiz appeared to ponder the matter. “How do you know they're not?” he said finally.

  “No!” she spat. “They are degenerate society!”

  “I should think so,” Ruiz said, “after six million years in their circumstances.”

  “Six million years is impossible!” Tu insisted.

  Maggie nudged Jameson. “I don't follow her logic. Either they've been around long enough to become degenerate, or they've been around long enough to be socialists.”

  “Don't bother about the logic, Maggie,” Jameson said. “It's just her way of getting across to the troops that the Cygnans are no longer their socialist buddies from the stars. We're having a change of line.

  “But why?”

  “Comrades don't lock the chosen up in a cage. They can't communicate with the Cygnans, and they know it now.” He frowned. “I wonder if they're planning some kind of action.”

  “They are not travel for six million years,” Tu was screaming. “You tell them, Comrade Chu!”

  The Chinese astronomer had been standing off to one side, going over figures on the lightpad with Maybury. He stirred uneasily and said: “You see, Comrade Tu, the interval of time—”

  “Tell them, tell them!”

  Dr. Chu looked unhappy. “Of course there is always the possibility of error in Dr. Ruiz's computations, bu—”

  Unexpectedly, Captain Hsieh broke in. His round face was stern, his short stocky body stretched to full height. “We must listen to Dr. Ruiz,” he said. “He has learned an important thing about the hsing-ch'ung.” The word he'd chosen meant, literally, “star-worms.” “We must try to understand, so that we may act correctly.”

  “What's happening?” Maggie whispered.

  Jameson said, “There's some kind of shift in power going on. We're a long way from New Peking. I think Captain Hsieh and his supporters have decided they're never going to see it again.”

  People's Deputy Commander Yao Hu-fang spoke out from the crowd. “Go ahead, Dr. Ruiz. We would like to hear you.”

  Ruiz nodded at him. He waited until the tumult died down. Tu Jue-chen wormed her way through the milling crowd and went off to sulk.

  “Comrade Tu is right, in a sense,” Ruiz said, watching her go. “The Cygnans haven't been traveling six million years. They left six million years ago.”

  Dr. Chu put down the lightpad, took a nervous glance at Captain Hsieh for reassurance, and said: “We can be sure about the figures. You see, after the supernova explosion it would have taken the system another six million years to become an X-ray source. The X-ray stage is brief. It could not last more than fifty thousand years. But we know that Cygnus X-1 is an X-ray source now!”

  “Or has been within the last ten thousand years,” Ruiz said.

  “Of course,” Chu said apologetically. “It is ten thousand light-years away.”

  “Go on, Dr. Chu,” Ruiz said. “You're doing fine.”

  “When the helium star exploded, its remnant collapsed. It became a black hole. Cygnus X-1, in fact, was the first black hole to be positively identified, back in the twentieth century. Now, the black hole continued to circle its companion—waiting, as Dr. Ruiz might say, for a chance to take back its stolen mass. “It must wait for six million years, till its companion burns up its ill-gotten hydrogen and becomes a blue supergiant. Blue supergiants are almost always associated with such X-ray sources. A blue supergiant some—oh, twenty times larger than the sun will begin to lose mass in the form of a solar wind, at the rate of about a millionth of a solar mass per year. Some of the mass falls into the black hole. It disappears from the universe forever. But during that fall to infinity, the gas accelerates to tremendous speed, and is heated to a temperature of tens of millions of degrees Kelvin. It is this envelope of falling gas that generates X-rays. For a brief period—not more than fifty thousand years—it will shed X-rays burning with the radiance of ten thousand suns.”

  Chu stopped and mopped his brow. His eyes roved toward Hsieh, then to the People's Deputy Commander.

  “After that,” Ruiz said, “the newly formed blue supergiant overflows its Roche lobe and pours its mass down the black hole at a rate that extinguishes it as an X-ray source.”

  Mike Berry was facing the two astronomers, hands on hip
s. “So you say our six-legged friends left home six million years ago, before all this happened?”

  “There's no doubt of it,” Ruiz said.

  “Then tell me this—if Cygnus X-1 is ten thousand light-years away, and they've been traveling at the speed of light, where have they been all this time?”

  Ruiz looked pleased at the question.

  “That's obvious,” he said. “Making stops.”

  Mike was a very bright person. He chewed it over a few moments, then said: “Dr. Ruiz. You've deduced a hell of a lot about our little buddies from the mass of a star and the mass of a black hole. Now this ought to give us a clue about how long—”

  “It does,” Ruiz said.

  Boyle got to his feet. “Dr. Ruiz, I think—”

  “Let him go on,” Hsieh said. “Our people have a right to know.”

  “You're right,” Boyle said. “Go on, Dr. Ruiz.” He sat down.

  Ruiz looked round the crowd. Almost the whole human colony was there now. In the artificial starlight their faces were a pointillist cobble of silver blobs. Here and there a firefly darted as a glowing joint was passed from hand to hand.

  “The average distance between stars in our part of the galaxy is about six light-years,” he said. “If the Cygnans traveled a more or less straight line getting here, zigzagging from star to star, they encountered sixteen or seventeen hundred of them. If they stopped to refuel at every star, they would have stayed for an average of three thousand six hundred years each time.” He paused. “If they stopped at, let us say, every tenth, star each visit would have lasted some thirty-six thousand years.”

  “But why?” Mike pushed away the joint that someone offered him. “Tod Jameson says the Cygnans aren't interested in the systems they visit! They never make planetfall. Even granting that they'd have to do some robot mining from time to time, refurbish their ships, they ought to be able to travel more than six light-years on a gas giant! Once they got coasting at close to light-speed, they ought to be able to reach another galaxy, for God's sake, without refueling—even with the inefficiency you mentioned before. Just shut off their robot siphon and let it careen away into infinity. Make another one when they want to brake. Why waste two years out of six accelerating and braking if they're going somewhere—”