by Greg Egan
The Stone’s rocky core was visible now, its outline mutating as it spun, like a pallid gray half-moon reflected in a trembling puddle. The asteroid was approaching at an absurdly slow rate by astronomical standards, outpacing them by a mere fifty meters a second; they might have been back on Earth, hurtling along a railway line – albeit in some surreal ghost train ride where a turnstile wrapped around a giant boulder threatened to block their way. But as the boulder drew closer and the Baza passed between the two layers of cables that stretched out from the rock, any sense of a horizontal passage skirting the obstacle vanished. They were swerving around it, but they were swerving upward.
Darpana’s gleeful cry was barely audible, but to Heng it sounded subdued more out of consideration for her fellow passengers than from any lack of energy or enthusiasm. As their weight shot up from nothing toward a gee and a half he could see her grimace with delight, as if the visceral thrill that had gripped her the first time remained as intense as ever. The portions of the cables nearest to them were traveling backward with the asteroid’s spin, putting them almost at rest with respect to the Baza, and the eddy currents induced in them by the ship’s magnets served as a brake, quickly dragging the relative velocity down to zero. Within seconds the Baza was firmly locked to the cables at two points, swinging along with them but still free to pivot around its center of mass and stay true to its original alignment. While the stars beyond the window remained serene and motionless against the frame of the cabin’s interior, the nine bunks turned like roasting spits, swiveling to remain horizontal under the shifting centrifugal gravity. The view that had lain to Heng’s left was dropping below him; the whole ship, and the whole cosmos beyond, seemed to be rotating around the fixed axis of his spine.
In ten seconds they’d completed half a circle, and the stars were rising on his right. He tensed himself for the fall of release, but the navigator opted for another full turn as it worked to refine their course, ensuring that the next encounter would be as perfectly aligned as this one. As Heng gazed up at the stars they were replaced by a blur of rock, sunlit for a moment then fading to black and slipping away.
When the stars rose for a second time the navigator finally broke its grip on the cables, and Heng’s surroundings stopped tumbling. The Stone came into view on his right, retreating, the stars behind it unchanged.
The Baza had performed a U-turn around the Stone, but with respect to the Earth, rather than reversing its motion it had just gained an extra hundred meters a second. Over the next hour the Stone would give a similar boost to every ship in the convoy – and then it would be free to spend a couple of years harvesting sunlight, replenishing its spin and tweaking its orbit until it was back in position to reprise its role for another group of travelers. It had taken three decades to nudge this rock and its companions out of the Amor group and into their tailored orbits, but the foresight of the pioneers who’d begun the process had paid off for the generation that followed. The Baza was not so much a spacecraft in its own right as a life support capsule being tossed from Stone to Stone, but this choreographed relay race would deliver it to Mars in just four and a half months.
Rohini addressed her granddaughter calmly. “You should sleep now, darling, if you can.”
“But it’s exciting!” Darpana protested.
“It is,” Rohini agreed. “But we have thirty more days just like this ahead of us, and if you make yourself sick you won’t enjoy them at all.”
Darpana was silent, but then she seemed to accept the argument. “I’ll try to sleep before the next one.”
“Good girl.” Rohini relaxed back onto her couch.
Darpana looked past her and caught Heng’s gaze. He smiled, then let his eyelids grow heavy, hoping the action would be as contagious as a yawn but less obviously manipulative. By the time he thought it might be safe to check whether he’d had any success, he’d fallen into a warm half-sleep himself, ready to wake in an instant if the Baza required it but unwilling to surface for anything less.
#
It only took a small change to the sun-side window’s tint to brighten the cabin and bring on a notional dawn. Heng rose before any of the passengers to use the toilet, sponge his body and change his clothes. When the Baza finally reached the hundredth Stepping Stone and they climbed aboard for the middle stage of the journey it would feel as if they’d gone from a shanty boat to a luxury ocean liner, but until then these few minutes each day would be the pinnacle of privacy.
As he swung out of the ablutions room he was hit by the glorious aroma of someone’s breakfast sizzling in the microwave. Only Iqbal and Noor were in the kitchen nook, but the other passengers were stirring, woken by the smell.
“Do you feel like sharing?” Heng inquired. The couple’s meal looked like some kind of spiced omelette rotating in its sealed bag under the lights, and though Heng had dozens of cherished recipes of his own, the sensory appeal of this visible, olfactible reality was far stronger than any remembered culinary delight.
“Of course.” Iqbal turned toward the couches, swiveling on his hand-hold. “Anyone else?”
There was a deafening chorus of requests, and Iqbal waved the count up from two to nine. Heng was pleased; so far as he knew there was no acrimony between any of the families traveling on the convoy, but the smallest sign that they weren’t going to turn cliquey on him was welcome. Company policy was to allow no more than three related people on the same ship – to ensure that a single ruptured hull could take only a limited toll from each family – but the resulting assortment of traveling companions had made his last outward journey an ordeal, as he’d struggled to keep the members of two rival clans from goading each other into violence. Whether Mars itself would prove big enough for both of them was, mercifully, not his problem.
Everyone managed to get through breakfast before the klaxons warned that Stone nineteen was on its way. Heng looked on solicitously as Rohini helped Darpana into her harness, but they didn’t need his assistance; they’d both been rated as “diligent” by their trainer in the course back in Shanghai. Dozens of children had made the trip over the years, and Heng had flown in convoys with a few of them, but he’d never had anyone so young on his own ship.
The log showed that Darpana had slept deeply, and as they tumbled around the first Stone of the new day she whooped with unabashed pleasure. Once they were weightless again Heng climbed free and set about the first round of system inspections, starting with the laundry press and moving through all the water-recycling components. It was tedious work, and this early in the trip it was hard not to assume complacently that nothing could yet have grown clogged, infested or leaky, but he had his cans improvise some rousing percussive music, invigorating but not catchy enough to be distracting, and he managed to get through the tasks without a single nagging beep from the overseer.
When he was done, Heng cast his gaze around the cabin, reassuring himself that everything was in order. Akhila was using the spring set, grunting softly as she forced her legs straight against the machine’s tug, then fighting just as hard to bend them again. Heng’s exercise of choice was running – in the middle stage, when he had the freedom of the Stone’s corridors – but the Baza’s zero-gee treadmill was a poor substitute, and in a space as small as this it just left him feeling more hemmed in. The drugs that lied to his muscle fibers and osteocytes, assuring them that they were still bearing their usual loads, seemed to be enough to keep him from any drastic decline in his weightless months.
Everyone else remained on their couches. Iqbal and Noor were facing each other, smiling slightly, conversing privately or sharing an overlay. Rohini had her eyes closed, but Heng had no reason to snoop on her vitals to check if she’d dozed off or was merely engrossed in some study or entertainment. Punita, Aabid and Chandrakant were all clearly in that state, staring attentively into the middle distance. Only Darpana was looking out the window.
She saw Heng’s reflection in the glass and turned toward him. “Are we in front of
the Tragopan, or behind now? I’ve lost track.”
“In front.”
“My cousin said he’d aim his laser pointer out the window.”
Heng doubted that she’d be able to spot it, but if hunting for a faint red speck against the stars helped her pass the time he wasn’t going to disillusion her.
Darpana had another question for him. “Why do the orbits for the Stepping Stones stick out so much?”
“Ah.” Heng summoned an overlay of the asteroids’ trajectories. Darpana joined the view, and gestured at the largest of the ellipses traced out on the illusory pane between them. “We only want to get from Earth to Mars!” she said. “So why do half the orbits go further?”
“What do you think would be more sensible?” he challenged her.
Darpana replied boldly, “Just start with Earth’s orbit and make it bigger, step by step, until you’ve reached Mars.”
“Show me,” Heng suggested. He cleared the Stones from the pane, leaving only the two planets.
Darpana drew a sequence of a dozen concentric circles, bridging the gap in equal increments. “If I draw hundreds it will be hard to see what’s going on,” she explained.
“No, that’s fine, I get the idea.” Heng waited to see if she’d spot any problems for herself, then he pointed out gently, “None of these orbits cross each other, do they?”
“No.” Darpana didn’t understand the complaint. “Why should they? The Stepping Stones don’t need to meet up! If the orbits are close enough, they could still throw the ships to each other.”
“So the Stepping Stones have one kind of orbit – these circles – but the ships move between them on different kinds of orbits?”
Darpana hesitated. “Yes.”
“How different?”
Darpana sketched a short line from one circle to another. “That’s the kind of path we should take. Straight out to the next Stone – like throwing a ball up to your friend on a balcony. Then she throws it to someone on the balcony above … and on it goes, all the way to the roof.”
Heng could see the appeal of this metaphor, but the reality wasn’t much harder to grasp. “One small problem,” he said, “is that you’d need a phenomenal velocity to go straight up like that. Remember, these Stones are in orbit, so your first one will be moving sideways at about thirty kilometers a second.”
“Right.” Darpana took his point and erased her original line, replacing it with a curve that spiraled around at a modest pitch on its way out from the sun. “How’s that? The Stone above is moving sideways too, so it still ought to be able to catch the ship.”
“How far apart are these orbits?” Heng asked.
“I’m not sure,” Darpana confessed. “How far can a Stone throw us?”
“If you throw something at fifty meters a second from the Earth’s orbit, it will travel a quarter of a million kilometers outward before it starts falling back toward the sun.”
“That’s plenty!” Darpana replied. “A few hundred steps like that would get you to Mars.”
“The catch,” Heng said, “is that each step would take about three months.” Darpana’s spiral had an implausibly steep ascent; he sketched in an elliptical arc that hewed closer to the initial circle, wrapping a quarter of the way around the sun before reaching aphelion. “Of course you could always space your Stones closer than this and catch the ship while it’s still moving outward … but so long as the Stones themselves aren’t moving outward, the ships can’t build up speed in that direction. They’ll be forced to cross the whole gap between the orbits at fifty meters a second, or less.”
Darpana gazed at her concentric circles: an endless set of speed bumps if you tried to cut across them. “So the trip would take decades this way?”
“Yes.” Heng brought back the real Stones’ orbits. “We want to head out as fast as we can – which in the middle stage is so fast that if we kept it up we’d overshoot Mars completely. The Stones need to move on similar orbits to us, so some of them do need to overshoot Mars. You can’t just take Earth’s orbit and enlarge it step by step; you need to squeeze it, making it longer and skinnier so it carries you away from the sun.”
“I think I understand now.” Darpana smiled. “I’ll try explaining it to Lomash, and if I can make him believe me then I’ll know I’ve got the hang of it.”
Heng closed the overlay and left her to commune with the Tragopan, her lips moving silently. It was a shame that her cousin couldn’t have been on the same ship, but the two could still chat endlessly and compare their different views of this leg of the voyage. Heng couldn’t understand how the girl’s parents could have left her behind in the first place, but they’d all be reunited soon enough. The ever-growing warrens of Cydonia Station would be a rich playground for an imaginative child, and if in adulthood she wanted to return to Earth’s wider horizons there’d be nothing stopping her.
His overseer buzzed a reminder: it was time to start checking the air scrubbers.
#
“The weather’s turning,” Liana announced grimly. “There’s still a chance that you’ll be able to ride it out to the mid-stage, but you’d better start getting the passengers accustomed to the possibility of taking shelter early.”
Heng stared at the delicate yellow lacework of coronal loops beside her on the overlay. Before they’d set out, the models had promised them a quiet journey – with just enough solar activity to limit the incursion of cosmic rays. A healthy solar wind repelled a fair proportion of the high-velocity particles from interstellar sources, and the trade-off was usually worth it, with the sun’s own slower protons not too hard to block. But a coronal mass ejection could form shockwaves that accelerated the normally tolerable wind up to energies that would penetrate the ships’ hulls. If that happened, only meters of solid rock could protect them.
“What’s the worst case?” Le asked. His worried face faded into view beside Liana’s as he spoke.
“Two days’ warning,” she replied. “The models aren’t perfect, but they’ve never failed to spot an impending CME once it was that close.”
Heng listened as the other captains questioned her, seeking reassurances that she could not provide. Nobody’s life was in danger; the next Stepping Stone would never be more than twenty hours away. But if the travelers were forced to accept the nearest sanctuary, they’d have no hope of reaching their scheduled mid-stage ride. At best they’d face a massively expensive extraction mission – a fully powered ship launched from Mars orbit, if the families could afford it. If not, all the company itself could offer them was two years or more hunkering down in their shelter, until enough Stones could be brought into position to allow them to resume their journey, or to form an off-ramp taking them back to Earth.
When the conference ended, Heng was left staring at the grid of couches in front of him. There weren’t enough cubic meters of air in the Baza to dissipate the stench of his sweat. “I have an announcement,” he said, more loudly than he’d intended, but no one showed any sign of having heard him. He gestured to shut off their overlays and cans, and the passengers shifted in surprise on their bunks.
Heng explained the forecast, and the possibilities ahead. “The one sure thing is that we won’t be taken by surprise,” he stressed. “The conditions on the sun are being monitored in real time by a dozen satellites, and the models can predict these mass ejections very reliably, days in advance. Whatever’s coming our way, we’ll know about it in plenty of time.”
Chandrakant was indignant. “In time for what? To imprison ourselves! Why didn’t your astronomers see this coming before we left?”
“I’m very sorry,” Heng said. He doubted that it would help to start debating the reasons why the long-range forecasts couldn’t be perfect. “We still have a chance of reaching the mid-stage, but we need to be ready either way.”
“What kind of facilities do the other Stones have?” Punita asked anxiously. “What kind of food, what kind of space for us?”
“The interiors
are all identical,” Heng assured her. “And they’re all stocked with supplies …” He almost said “to last for decades” but thought better of it. “For as long as we could possibly need.”
“But this is nothing for you,” Chandrakant declared bitterly. “Where else would you be? It’s just life as normal.” His brother Aabid muttered something to him in Gujarati; Heng only knew Hindi but the tone sounded reproving. Aabid addressed Heng in English. “This is nobody’s fault. Every traveler faces these risks.”
“It’s a shock,” Heng said. “But what can we do?”
“We’ll make the best of it,” Rohini replied firmly. “Hope for a reprieve, and make the best of what comes.”
Heng finally dared to glance toward Darpana. She was ten years old; two years cooped up inside an asteroid would feel like a lifetime to her.
“Would the whole convoy end up together?” she asked. “Or would it be some of us in one Stone, some in another?”
Heng said, “There’s a good chance we’d all be together.” If the warning came as early as expected they wouldn’t have to settle for the very next Stone on their itinerary, regardless of whether or not the rest of the convoy had already left it behind.
“That’s all right then,” Darpana ruled amiably. “On Mars we’d be underground most of the time anyway.” She gripped the sides of her couch and turned her body around to face the window.
Heng watched the other passengers dialing down their angst. If this child wasn’t going to shed a tear or throw a tantrum, it would be shameful for them to make a greater fuss themselves.
#
“Do you have a big family?” Akhila asked Heng as she moved a piece across the backgammon board overlaid between them.
“Just my parents,” he replied. “I can’t afford to marry yet. Maybe after a couple more trips.”
Akhila looked surprised. Perhaps she’d overestimated the portion of her fare that was ending up in his pocket. “And then you’ll settle back on Earth?”
“Yes. I’m not the pioneering type.”