Wheelers

by Ian Stewart

  "First we dive, then swing and accelerate. But that just the begin, okay? After that, things get hairy. Then get very hairy, right? So you just stay calm . . . trust me . . ." Despite the Cuckoo's remembered admonition, Nagarjuna had to say it: "I the best!"

  The preparations took a while. Moses found it easy to remain calm, though few others would have done so in his place. But few others had had his upbringing, where it was fatal not only to show fear, but to feel it. He sat in the passenger seat and watched every move that the pilot made. Maybe one day he would become a pilot, like Auntie Pru.

  Nagarjuna spoke into his mike: "Check— vajra-1 in slot?" In Pali, vajra was a thunderbolt. Moses didn't hear the reply, but the pilot seemed satisfied.

  "Go for first burn," said Nagarjuna. He fiddled with various controls, spoke into his throat mike. The tiny ship complied; Moses felt a growing weight on his chest, as if a large animal were sitting there. The animal grew heavier, until he wondered if his ribs would crack under the strain . . . then it vanished.

  He felt sick. Until now, weightlessness hadn't bothered him, but this time it was different—the sudden cessation, perhaps?

  Nagarjuna grabbed two sick-bags, tossed him one, and Moses threw up. To his surprise, the pilot did the same in the other. "I fine," said Nagarjuna shakily, still looking green around the gills, "on own. But if anyone else get sick . . . me, too. 'Sympathetic reaction,' my teacher say. You okay?"

  "I'm fine."

  "Good. Hang on to feeling. Because very soon you not be!"

  The Moon in the cruiser's flatfilm screens grew, fast. It changed from a small ball to a huge curved dome, then to a straight line, black on one side, shining gray and silver on the other. The sharp division bisected the screen.

  "Course correction," said Nagarjuna. "Small, so I think you get no sick, okay? Good." Imperceptibly, the vessel's course zoomed in on a crucial, tiny window in a twenty-four-dimensional space of mathematical variables—its position and velocity had to be controlled into very small confidence intervals, with virtually no margin for error. This was one reason why no sane pilot would consider using their current strategy to shorten the normal journey time to Jupiter—or anywhere else. Of course many pilots were not entirely sane, but there were other reasons, too—reasons that did not apply when the Earth was threatened with annihilation, but excellent reasons on any other occasion. Reasons that Nagarjuna was trying not to think about.

  The Moon's cratered surface was no longer in front of them, but below, and they skimmed across it at a blistering pace. Moses had been told the theory: they were trying to gain velocity by making a partial orbit of the Moon, swinging across its trailing hemisphere as it sailed its unruffled orbit, borrowing momentum that would accelerate the tiny craft to unprecedented speeds while slowing the ponderous satellite by an imperceptible amount. In a million years' time publishers would have to rewrite their diaries; for now, nothing seemed to have changed. The unerring precision of Newtonian conservation laws held sway, an exchange too fine for human senses to perceive.

  Nagarjuna knew that the most critical phase of the entire mission was rushing toward him, relentless, obeying its own inhuman imperatives. His eyes roamed across the readouts as the computer gentled them into the required trajectory. If it made a mistake, there was little he could do—they were provisioned for eight days, no more, to avoid excess mass. At their present speed, it would take four months to reach Jupiter . . . much too slow.

  Nagarjuna broke out in a cold sweat at the thought of just what they had to accomplish—and how. Pre-Pause technology would have made the task easier—at least until the vessel outsmarted itself and did something really stupid.

  The trajectory uncurled, the Moon receded. Again Nagarjuna asked the computer to verify their course; again he breathed a sigh of relief.

  Now it was up to the mass-drivers and their operators. And to the jury-rigged gadgetry in the large metal box, the one that bore an uncanny likeness to a coffin.

  As a fortune in palladium converged on Nagarjunas cruiser, the pilot readied himself for the big moment. He was going fishing in the celestial ocean, with a net on the end of an elastic rope.

  It was an old idea, but nobody had ever been desperate enough to try it before. The astronautics journals of the early 2100s called it PAMT—Propulsion by Accumulated Momentum Transfer—but nobody used the clumsy acronym. Instead, the method was generally known to spacers as mass-sailing.

  The name came from an older idea still, which in a sense was really the same one. The principle of the light-sail had been around for as long as space travel itself. Attach a life-support module to a huge, gossamer-thin parachute; hitch a lift from the Sun's inexhaustible outpouring of photons. As the physics of the solar wind became better understood, the theorists repeatedly redesigned their hypothetical spinnakers to surf the plasma wind of electrons, protons, alpha particles, and whatever else the Sun was prepared to eject.

  Mass-sailing was the same idea, but the particles were bigger and the sail was correspondingly smaller and more robust. "Rustic" might be a better description.

  Nagarjuna opened the external welded-on coffin and readied his celestial fishnet. This would not be his only chance—other equally valuable rocks were already trundling along alternative trajectories, in case he missed and had to try again—but it was the best chance, the one that would give them the best start on the fast track to Jupiter.

  He had no intention of missing.

  Moses watched, his face devoid of emotion, as the cruiser's radar sensed the rock's approach and passed the information on to the computers, which readied Nagarjuna's net. The maneuver would be semiautomatic, but there were enough imponderables that the young monk's human touch would be essential in laying the trap. The almost immediate release would have to be done on automatics.

  Then he would have to do it all again, and again, and again . . . With faster and faster rocks, hurled from farther and farther away . . . Not to mention what would happen when they neared their distant goal.

  Don't think about that.

  Hours passed. The radar signals became stronger, clearer, more accurate. Nagarjuna decided that the moment was ripe and set his net. It slid out of the coffin at the end of its twenty-mile tether, and his nimble fingers manipulated a web of electrostatic forces to spread it apart into a welcoming string basket. . . only the string was multistranded nanofiber cable and the tether was capable of stretching to ten times its length before there was any danger of it snapping. The trick was to spread the net without letting instabilities collapse any part of it. The computers helped, but the monk had to make occasional manual corrections.

  His net safely and accurately spread, Nagarjuna slipped out of his seat and satisfied himself yet again that Moses was strapped down. The acceleration would be fierce, but only for an instant.

  They never saw vajra-l.

  They felt the gut-wrenching jerk as it slammed into the net, though. The tether extended, heating rapidly as it absorbed a tiny part of the rock's momentum. The tiny cruiser was pulled forward like a toy on the end of a length of string.

  The tether stretched farther. Within a few seconds its front end was two hundred miles ahead of the cruiser, moving at twice the velocity. The rear end was already dragging the tiny craft forward.

  There was more to come.

  Stresses soared in the carefully engineered layers of the tether. Stretched to its limit, it now began to contract, whipping the cruiser toward the far larger mass of the captured rock. Now the tether began to shrink as rapidly as it had lengthened; Moses and Nagarjuna were pressed deep into their seats by invisible, but terribly real, forces. If the tether hadn't been stretchable, the two of them would have been splattered against the bulkhead, mixed up with the wreckage of their seats. Thanks to elasticity, the acceleration remained bearable. Just.

  The most dangerous moments were still to come, though, because now Nagarjuna had to make sure that they were not dragged into a collision with the rock. One
squirt with the attitude jets ought to take care of that. If not, they would never know.

  As the cruiser slid past the speeding rock, and at the instant that its velocity attained a precomputed vector, the center of the net opened up, releasing the rock. The acceleration stopped, and all was as before . . . except that now the cruiser had received a permanent boost to its velocity.

  Nagarjuna ignored his aching joints, consulted the schedule on his screen, and began to redeploy the net. Vajra-2 was due in fifteen minutes.

  While the cruiser's speed was beginning to build, while the closest bodies were the Moon and the Earth, it would be fueled by rocks from the lunar mass-driver. Each successive rock would be tossed along a subtly different trajectory, at a higher speed; step by step the craft would be accelerated to unprecedented velocities. The plan only stood some chance of succeeding because the monks had plenty of experience shuttling far larger rocks around the inner solar system—the record was a potato-shaped lump of impure nickel-iron alloy, eight miles by five by three. The difference, on this occasion, was the speed of the rocks and the short intervals between them.

  Initially, the cruisers position could be tracked with enough time left to change the alignment of the mass-driver, reducing errors that would otherwise accumulate until Nagarjuna's net missed an incoming rock and his cruiser floundered, its mass-sail becalmed in momentumless vacuum. As the cruiser's velocity increased, however, two new problems would arise. It would be moving faster, and it would be farther away.

  Nagarjuna had to be given breaks when he could sleep. This increased the journey time, but it also worked in their favor, because during those periods of downtime the Buddhist engineers could recompute the rocks' trajectories. As the craft's path took it out toward Martian orbit, the mass-drivers near Mars would take over from the lunar one. But even then, a stage would quickly be reached when a passively thrown rock would not be accurate enough.

  The answer: active rock.

  Floating in space near Euphrosyne, the Pitching Machine cranked into life and began to live up to its name. In its capacious slings were mounted five gigantic rocks. Others floated nearby, ready to be tugged into position as used slings were vacated. To each the neo-Zen engineers had secured small but powerful rocket motors, with which they could make remote adjustments to the rock's velocity, attitude, and position as it sped along its trajectory. Nagarjuna would need all the help they could give him.

  In the virtual mind's eye of the Pitching Machine's computer, its target was an empty region of space much like any other— until the ticking of a clock suddenly transformed it into an infinitely desirable slice of celestial real estate.

  The Pitching Machine surged with electricity as its powerful electromagnets came up to peak current in a precisely timed sequence. The first of the active rocks hurtled forward, propelled by a wave of magnetism. Already it was moving much faster than the Buddhist engineers had ever achieved before, and still the rock had covered less than a quarter of the mass-driver's length. But then, it was a much smaller rock than those they usually threw about the solar system.

  The sling approached the end of the rail-gun and decelerated fast. The rock continued under its own momentum, careering out of the solar system fifty times as fast as the speediest spacecraft ever built; soon the Pitching Machine would be hurling rocks even faster. All Nagarjuna had to do was catch them, let them pull him along as the elastic tether absorbed the strain, wait for the twang as the tether began to contract again, and let go. All the neo-Zen engineers had to do was calculate the path of the stream of rocks, align their mass-drivers, jockey the active rocks along whatever path was required, and hope that the entire ridiculous lash-up conformed to their simulations.

  All.

  The neo-Zen pilot fingered the knee flap of his sky-blue suit, wondering what was going on in the child's mind. Moses looked calm, but he nearly always did. It was hard to read his emotions. The mistake would be to assume that therefore he didn't have any Nagarjuna wasn't that stupid. He smiled at the boy and said a few reassuring words.

  Moses didn't need reassurance. He was bored, excited, and— despite his years as a street child—terrified all at the same time, but he was going to Jupiter] To meet the aliens] For the first time, his life had a goal, and it thrilled him to the core. It just hadn't occurred to him to express any of this, for to Moses emotions were things you mostly kept to yourself—to keep people from using them against you. There were many things that he did need, but reassurance wasn't one of them. Reassurance was when people tried to tell you everything was fine, when palpably it wasn't. He hadn't grasped that reassuring others wasn't what it was about. It was reassuring yourself by believing your own lies . . .

  In their own distinctive ways, Nagarjuna and Moses awaited their respective destinies with the customary mixture of awe, hope, despair, fear, and implacable obstinacy that is the lot of those that the rest of us call heroes.

  If it had been a vidifilm, you would have heard space sizzle as the rock from Sling 4 hurtled toward its goal, but no sound traversed the silent vacuum, for there was no air to carry it. As Nagarjuna watched its astonishing progress on his screen, though, he wouldn't have been surprised if space had started to sizzle.

  The stream of rocks that had propelled them toward Jupiter faster than any human had ever traveled before had temporarily dried up, but Nagarjuna was drenched in sweat. The net had proved a bitch to control, and he would shortly say as much to his mission controllers in the Cuckoos Nest, though in more professional phraseology. It was unlikely that the Way of the Wholesome would ever repeat this particular exercise, but even so the neo-Zen engineers ought to be told to increase the damping on the nets elastic oscillations. At least twice he had managed to reset it with less than a minute to spare. Once he had been within seconds of missing a rock entirely, which would have destroyed the whole sequence, lost them a week or more, and very probably killed them both by making it impossible to get to Jupiter before their supplies ran out—assuming they could reach their goal at all after such a mistake.

  The cruiser was now two-thirds of the way to Jupiter and had attained its maximum speed. The next problem would be to slow down.

  As the mission moved into its final phase, the activity around the Pitching Machine went up several gears. Neo-Zen pilots in tiny mining vessels grappled with rocks that they would normally have considered a piece of cake—but not when they were under severe time pressure and surrounded by dozens of others in exactly the same circumstances.

  There were accidents. Two tugs converged on the same rock at the same instant and collided. The wreckage was shoved unceremoniously aside by a third, whose pilot was well aware of the possible consequences for his two fellows. Others would do their best to rescue them and tend their injuries. All that mattered was to keep the stream of rocks clicking tidily along.

  These rocks were little different from their predecessors, save for two tiny details. The rockets mounted on them were larger, more powerful, and more numerous. And the rocks' trajectory was more ambitious. They were aimed not at Jupiter, but to miss the giant world by a hair's breadth—grazing its upper atmosphere, swinging around the planet in a half-circle, and returning from its opposite limb so that they headed back toward the oncoming cruiser. As far as Nagarjuna was concerned, it would be as if Jupiter were spitting rocks back at him. All he had to do was catch them, transfer some of their momentum to his ship through the elastic tether, and let them go before the tether broke.

  Easy.

  There were many imponderables, and the neo-Zen pilot tried hard not to ponder them. The biggest worry had nothing to do with the technology: it was that the Jovians might interpret the stream of rocks as a threat, and use their gravitic technology to divert it. The mission controllers had convinced themselves that this was unlikely, for a variety of reasons. The rocks were small enough to burn up harmlessly in Jupiter's atmosphere, unlike massive comets . . . The planet must have been encountering such tiny bodies several times
a decade, but Earth's astronomers had recorded no previous anomalous movement of the Jovian moons, implying that the Jovians did not normally divert rocks of that size . . . Moreover, these rocks weren't even on course to hit the planet, just to come close . . .

  Nagarjuna wasn't the only person to be worried by the logic. One popular interpretation of events, among those sane enough to be mildly paranoid about the whole business, was that the Jovians had diverted the comet Earthward as a deliberate act of interplanetary war. How, then, might they react if their enemy were suddenly to start spraying small asteroids into their vicinity? What might they think? Suppose, for example, that those rocks were carrying nuclear weapons! Suppose their apparently harmless trajectories could be controlled when they approached the target world . . .

  Suppose. Suppose there was another way to slow the cruiser down quickly enough to get Moses to Jupiter within the required time interval. But there wasn't. The only alternative was to whip the cruiser around Jupiter so that it was heading back into the rock stream. That would have been more satisfactory, in some ways, but it would take too long—and run the risk of traversing Jupiter's rings. And the aliens might see a spacecraft as a threat when a rock might be ignored. So, on balance, this way was less risky.

  Nagarjuna had spun the tiny cruiser end for end, so that now it faced back toward Earth. The rocks would be coming from behind him, as before. The main difference would be the relative speed, now virtually double his own, for the stream of rocks would whip around Jupiter without losing momentum, but this time he would be plunging backward into them.

  Nagarjuna was expendable, Moses was not. There were precautions to be taken and contingencies to be planned for. The pilot said a few brief words of encouragement to the boy, who kicked himself across the cabin to a waiting emergency pod, a thick rind of plastic foam that had been split open like two halves of some enormous fruit. The pod held enough air and water to keep a person alive for twenty-four hours. There was no food—something more vital would run out before it was needed—and only the most rudimentary of sanitary arrangements, a kind of oversized diaper. When closed, the pod was sealed to vacuum.