The Clockwork Rocket

by Greg Egan

  Yalda was upside-down and two strides above the ground before she felt the tightness around her lower wrist; she dropped the mallet and reached down frantically toward the catapult, but it was already too late to grab any part of it. With her rear gaze she stared up at the sack’s drawstring, twisted around her hand. She must have left the drawstring protruding from the side of the launching plate, and then slipped her arm through its loop.

  Her first, idiotic, impulse was simply to disentangle herself from the sack—as if it alone were the cause of all her problems, and if only she were free of it she’d drift gently back to the ground. Her next thought was to pull it closer to her body, which she did. Then she freed her wrist from the string and clutched the coarse fabric of the sack against her chest, but she managed to stop herself from completing the plan: tossing the sack upward in order to propel herself back toward the mountain. Her instinctive sense that this tactic ought to work was almost overpowering; had she been stranded in the middle of a chamber inside the Peerless, it would certainly have done the trick. But even if she’d pushed the sack away with all the strength in her four arms—even if she’d burst the seams of her cooling bag and extruded two more—it would not have been enough. She knew how long she’d labored to turn the crank on the catapult, how much energy she’d put into the springs. A single burst of effort couldn’t match that. And a partial victory that merely slowed her ascent would be completely useless, if in the process she lost the means to do anything more.

  Yalda glanced back at the receding light from the tunnel mouth. If she panicked and acted without thinking, she was dead. Her rapidly increasing distance from the ground was terrifying, but it was not her real enemy. It didn’t matter how long it took her to reverse direction; once she was headed back to safety, the length of her trip would be irrelevant. Or very nearly so: the sole criterion was that she needed to return before her air canister ran out—and it held enough for an entire six-bell shift.

  Could the canister itself help? She ran a hand over its cool surface, imagining a swift burst of air that would send her hurtling down to safety. But without any tools, she doubted she could break open the valve that limited the outflow—and even if she could, the momentum of the entire contents might not be sufficient for the task. What’s more, a marginal success would make a lie of her indifference to distance; if she ended up drifting slowly back toward the ground, without cooling she could easily die of hyperthermia on the way. There were a dozen replacement canisters in the catapult’s tool hold, but did she really want to smash this one open and gamble on reaching the others in time?

  No air rocket, then. All she had for exhaust was the rubble in the sack, and all she had to propel it was her own strength. But the catapult had put her in this plight using nothing but stored muscle power; if she parceled out the rubble in a manner that allowed her to expend more energy on this task than she had on turning the crank, she should be able to reverse the consequences.

  Her body’s slow spin had her facing back down toward the light from the tunnel again. Her fellow workers would certainly miss her as the rubble sacks began filling up the pulley line, but they would not be in a rush to come looking for her; she could easily have spent all this time on minor repairs to the catapult. The rock face was where serious accidents happened; what kind of fool managed to shoot herself into the void? But whenever they did start to worry about her absence, she could forget about anyone throwing her a rope; she was already too far away for that.

  No matter; if she stayed calm, she could fix this. She identified the point on the star-trail horizon that, as near as she could tell, marked the direction in which she was traveling: precisely opposite the shrinking patch of light on the ground. She loosened the drawstring and opened the sack a little, fearful of spilling the contents with her jostling, then reached in and took out a handful of rubble. She waited for her rotation to bring the target in front of her again; she wasn’t going to try to reconfigure her flesh to give equal strength to a backward throw. Then she brought back her arm and flung the handful of rock away with all her strength.

  The effort felt puny and ineffectual—and she suddenly realized that in her haste and agitation she’d been laboring under yet another delusion. If she threw a heavy object, like the whole sack of rubble, the energy she put into it would be limited by the maximum force her muscles could apply. If she divided the sack in two and threw each half separately, the same force would let her throw a half-sack faster than a whole one, transferring as much energy to each half as she would have used on the whole.

  Two throws, twice as much energy—hooray! But two throws would still not be enough, so why not four throws, a dozen, a gross, taking her time but increasing the total energy as much as she needed? That was what she’d been thinking: matching the energy she’d put into the catapult would simply be a matter of eking out the rubble with sufficient care.

  But the pattern of throwing ever smaller weights ever faster only held true up to a point—for a half-sack, yes, but not for a handful of pebbles. By that point, the limiting factor would be the speed at which her muscles could contract, not the force they could apply. And when the speed was fixed, the energy she could put into a given quantity of rubble became proportional to its mass—which meant that it would add up to the same total, regardless of how many separate throws she made.

  It didn’t matter how much strength she still had in her body; it didn’t matter that she could have cranked that catapult ten dozen more times without tiring. Her fate was completely determined by the total mass of rock in the sack, and the speed at which she could throw, not the greatest load, but the smallest.

  Yalda looked back at the mountain. She could see three other worksites now, the bright mouths of tunnels further down the slope. But her trajectory was carrying her off to the side, and an expanse of dark rock was now spread out below her. There was an entire second line of worksites a half-turn around the mountain; the full set of engines would consist of a dozen diametrically opposite pairs. But if any of those sites came into view she’d know that something was wrong—that she’d been misdirecting her throws and inadvertently bending her trajectory.

  She took another handful of rubble from the sack, waited for the target, then threw it. Her spin provided a rhythm for the process, giving her a chance to rest her arm without delaying the next throw too long. After a dozen cycles, she switched arms. She couldn’t extrude any new limbs without damaging her cooling bag, but although she felt some jarring at the end of each throw it didn’t build up to enough pain or damage to slow her down.

  What she could have done with, though, was a good slingshot.

  Yalda could see ten of the worksites now, with the remaining two from her side of the mountain probably just hidden behind small outcrops. All these engines would be completed, with or without her. The Peerless would get its spin, the crops would thrive once more. The real purpose of their journey would soon come to the fore again. Sabino had opened up a path that the brightest young students—Fatima, Ausilia and Prospera—would follow. Her death would not mean the end of anything.

  And Nino? She cut off the morbid train of thought. The rubble sack was still more than half-full, her situation had not yet been proved unsalvageable.

  As she threw another handful of rubble, she saw a flash of light in her rear gaze. She tried to place it exactly, to backtrack from the afterimage, but her spin confused her. Had she glimpsed one of the other worksites, the lights from its tunnel peeking briefly over the edge of the mountain? It had been too bright for that, hadn’t it? The tunnel mouths all faced the same way around the mountain—so those at the other worksites would be pointing away from her. The most she could have seen was the spill of light from the ground near the pit, and the scatter in the dust haze. How could that have outshone the sites where she was facing straight into the tunnels?

  A few turns later, she was facing the mountain when she saw a second flash: far from any of the worksites, surrounded by blackness. She
wondered if someone might have lit a sunstone lamp inside one of the observation chambers—but why would they do such a thing, let alone light it only for an instant?

  The third flash was at a different location, still nowhere near any worksite—and too brief and too bright, Yalda concluded, to be an artificial source at all. Something must have collided with the Peerless—something small that nevertheless carried enough energy to turn the rock white-hot.

  The telescopes had shown a corridor devoid of matter, but there’d been a limit to the sensitivity of those observations. Any speck of dust here, drifting along at a leisurely rate relative to the ordinary stars, would now be like a Hurtler to the Peerless. That was the price of taming the Hurtlers by matching their pace: ordinary dust could now do as much damage to the mountain as a Hurtler could do to an ordinary world.

  So much for the city of carefree scholars, working in safety and tranquility until the secrets of the cosmos were laid bare to them. Just like the people they’d left behind, they would be living with the constant threat of conflagration. And not for four years: for generations.

  Worst of all, Yalda realized, she was probably the sole witness to these events. The dust could have been striking the mountain for days, but most of the surface was invisible from the worksites and observation chambers. She had to get back and organize a fire watch for the Peerless; they had to prepare themselves to reach and douse a wildfire anywhere on the slopes, or risk going the way of Gemma.

  Yalda cast another few stones—imagining them heavier in the hope of tricking her body into dispensing a little more force. The sack was a quarter full. She believed she was still heading away from the mountain, but judging tiny changes in the view at this distance was almost impossible.

  How could they keep a lookout for fires? From a cage tethered on a rope, high above the surface, stabilized… somehow. Once the mountain was spinning, though, the problem wouldn’t be stability, but the strength of the rope.

  And once the mountain was spinning, it would be far harder to move around on the surface. Weightlessness had made it difficult enough, but every part of the slope would be transformed into a ceiling. How did you douse a raging fire on a ceiling?

  The sack was empty. Yalda clutched it to her chest, unwilling to presume that she’d have no further use for it. Was she moving toward the mountain, or away from it? For some time now, she hadn’t discerned any change in the angle it occupied in the sky, but she’d been too distracted to give the task much thought. She needed to pick a few distinctive stars close to the edge of the mountain, then wait to see if they crept away from it, or whether its silhouette slowly grew and hid them.

  Yet another flash of light came from the mountain, this one very close to one of the worksites. Perhaps someone there, outside the tunnel on catapult duty, would have seen it? Yalda counted the pit-lights down from the summit, and realized that the site was her own.

  The light winked again, from exactly the same direction. Not an impact, then. By now, she realized, her team would be out scouring the area for her, their sunstone lamps occasionally turning up into the sky. Yalda pictured them inspecting the catapult, feeling how loose it was, wondering if anyone could possibly have been careless enough—

  The same light appeared, brighter than before, crossing her line of sight so slowly that it dazzled her. When she completed a half-turn it struck her rear gaze and stayed—wandering a little, but never fading out completely.

  The lamp wasn’t on the surface of the mountain; it was moving straight toward her through the void. And it couldn’t be aiming itself, searching her out itself.

  Yalda spread the empty sack out in front of her, hoping to make a larger, more reflective target. The approaching light began wavering oddly, as if seen through a heat haze. Through a burst of air, spreading out through the void. Some beautiful idiot had come after her—launched along the same trajectory by the catapult—and now they were using compressed air to brake. Not from a tiny canister like her own, but from one of the giant cylinders that powered the jackhammers.

  The dazzling light overshot her, passing to one side. It rebounded, then overshot in the other direction. It was excruciating, but Yalda could do nothing to meet her rescuer halfway. By trial and error, by eye and airburst alone, the distance and difference in speed that separated them was whittled down to the point where the lamp became superfluous and its owner shut it down. No longer blinded by its glare, by starlight alone Yalda could see the figure before her, clutching an air tank and a coil of rope, wrapped in a familiar cooling bag.

  Fatima took hold of a portion of the coiled rope and tossed it toward Yalda. This sent her gliding backward, but she didn’t bother trying to compensate, she just let the rope uncoil. Yalda reached out and grabbed the end, then brought it around her waist twice and held on tight.

  There was a jolt as the rope went taut, then they were bound together, moving in a broad circle around a common point. Yalda dragged herself along the rope a short way, then gestured to Fatima to use an airburst to get rid of some of their angular momentum. By the time they were within arms’ reach of each other, their spin was almost gone.

  Fatima took hold of Yalda’s helmet and pressed it against her own. “Help me get down. Please.”

  She sounded terrified, and for a moment Yalda couldn’t reply. How could she have come after her at all, if she was so afraid?

  “Let me take the canister from you,” Yalda suggested gently. “Don’t release it until I’m holding it.”

  Fatima had two arms wrapped around the cylinder. Yalda embraced it herself the same way, then eased it out of Fatima’s grip.

  With her other hands, she rearranged the rope, forming two coils and bringing them around their bodies, then securing the connection with a series of knots. Fatima was shivering; she’d already done more than Yalda could have asked of anyone. It was her own job now to get them safely down.

  “I keep thinking about Benedetta,” Fatima said. “Landing is the hardest thing.”

  “This won’t be like that,” Yalda promised. “No fire, no heat, no danger—” She noticed the sunstone lamp still strapped to Fatima’s shoulder. “We won’t need that anymore.” She pulled it loose and swatted it gently away into the void; with all the jarring it had suffered already, it was a miracle it hadn’t exploded.

  Yalda found her target on the horizon and opened the valve on the air cylinder a notch; the effortless kick against her arms was the most beautiful sensation she’d ever felt. She’d never know if she’d already been heading back toward the ground before Fatima reached her; she didn’t want to know.

  A pinprick of light appeared on the dark rock below them. “Did you see that?” Yalda asked Fatima. She’d been hoping that she might have been delirious before—or that Fatima’s ascent might have included enough unlikely swerves for her search lamp to account for everything.

  “Yes. What was it?”

  “I have no idea,” Yalda lied. “Don’t worry; we’ll work it out later.”

  As the mountain loomed closer, the line of worksites spread out beneath them, the most distant fading to black. Yalda made a sideways correction, steering them toward the mouth of their own tunnel. When that patch of bright rock began to grow alarmingly, she squirted air down, slowing their descent. For a pause or two she thought she might have overdone it and launched them away from the mountain again, but they were close enough now that the cues did not remain ambiguous for long. She used another quick burst to slow their horizontal motion, lest they scrape all their skin off on the rock.

  As the guide rails running past the tunnel mouth rushed into view, Yalda discerned a new feature: the team had tied dozens of lengths of rope to the rails, spread out along a couple of stretches, pointing away from the rock with their free ends high above the ground. If she could steer into this soft, forgiving fence—

  “Try and grab the ropes!” she urged Fatima, as they swooped toward them. “The more arms to share the jolt, the better.”

 
A flicker before the glorious fool-catcher came within reach, Yalda used a tiny kick from the cylinder to give them a slight upward velocity. Then she dropped the cylinder and flailed around, managing to seize one of the ropes. Fatima had gripped another one, in two places. Yalda brought all her own hands onto her rope before it went taut; the shock to her joints made her cry out in pain, but she didn’t lose hold of it.

  They were a few strides above the guide rails. Yalda had been expecting to have to drag herself hand-over-hand down to the ground, but the ropes’ elastic tug had delivered a little more force than was needed to stop them, and they were actually drifting slowly toward the surface.

  Fatima began humming from the shock. Yalda almost joined her, but she was afraid that if she started she’d never stop.

  She said, “We’re safe. You did it, my friend, and now we’re both safe.”

  18

  Lavinio said, “Without gravity, I think this is the best we can do.”

  Yalda bent down from the ropes that crossed the test field and examined the plants. The wheat stalks were barely two spans high.

  “They’re… mature? They’re making seeds?” The tiny structures protruding from the stalks certainly resembled seed cases, but they were so small that it was hard to be sure.

  “Yes, they’re mature,” Lavinio confirmed.

  “But they’re a twelfth the size of normal wheat!”

  “However long we keep the seedlings in the centrifuge,” Lavinio explained, “they always stop growing when we take them out—but if we raise them to this height first, they don’t die when we replant them in the fields. They don’t get any bigger, but they do form seeds of their own.”