Greg Egan - The Clockwork Rocket

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.