Incandescence

11

In the center of the bulge, a billion and a half stars wheeled around in a disk fifteen hundred light years wide. The astronomers of the Amalgam called this the NSD—the Nuclear Stellar Disk—and had long ago resigned themselves to observing it from afar, as just one more example of the kind of structure seen in a billion other galaxies. It was a telescopic object, not a destination for travelers.

Many of the stars in the NSD were infant prodigies: hot, bright, fast-burning giants born a few tens of millions of years ago in the clouds of gas swept inward by the complex dynamics of the galactic core. Others were older, smaller stars that had wound their way in over billions of years, their orbits slowly decaying as they lost energy to chance encounters.

The meteor that the Aloof had captured had managed to climb just beyond the edge of the NSD. Given that the rock had not been melted by the impact that had sent it on its way, there were limits to the speed with which it could have been blasted free of its parent world. If that world had been bound to a star at the time, meteor and star could not have parted company too quickly.

Over fifty million years, the two might have completed as many as ten laps around the galactic center, with their orbits gradually diverging as they came under the sway of different neighbors. However, if the star in question was assumed to be the Interloper that had scrambled the system of the planet that Rakesh had named Touched-by-Steel, then the possibilities became much more tightly constrained. Many of the stars that might have been close enough to the meteor itself certainly hadn’t traveled far enough from the galactic center at any time in the last two hundred million years to have kidnapped the Steelmakers’ world. According to the models Rakesh ran, only forty-six stars could have captured the planet, sunk down into the NSD, and then been in the right place fifty million years ago to make sense of the meteor’s trajectory.

When the remaining siblings of Touched-by-Steel, the three gas giants and their moons, proved to be untouched themselves, Parantham asked the map to take Lahl’s Promise to the first of the forty-six stars.

This time, the Aloof’s hidden travel agent delivered no pleasant twist to their itinerary. The jump-cut in their consciousness filled the sky with hot blue stars that far outshone the sun they now orbited, but as the seconds ticked by and the cabin window completed its three hundred and sixty degree pan, no planet swam into view.

They scoured the region with their instruments, but this star’s sole companion was a sparse disk of rubble, with all the fine dust that might normally have been expected blown away by the wind from the neighboring stars. No gas, no ice; just barren rock. With volatiles so rare, Rakesh thought, it must have been a challenge for the Aloof’s engineering spores to scrape together the raw materials to reconstruct the whole ship, unless they’d developed femtomachines sophisticated enough to make transmuting the elements more efficient than scavenging for them.

The second candidate on their list had managed to hold on to even less detritus than the first. The winds from the new-born giants were not as strong here, but if any planets or asteroids worthy of the name had once accompanied this star, they had long ago been dislodged from their orbits by interfering neighbors. Rakesh had learned as a child that life could only thrive out in the disk, and however far the Steelmakers had progressed it was growing ever harder to see them as much of an exception to that parochial rule. Maybe life had flourished in this region, in some as yet undiscovered niche that had nothing to do with planets sitting in stable orbits around stars for billions of years; maybe the Aloof were descended from such creatures. The fact remained, though, that his cousins seemed to have hitched their fortunes to a way of life that simply couldn’t last here.

The third star possessed a substantial asteroid belt, but still no planets. Rakesh thought, This is how it’s going to be: sometimes a few more rocks, sometimes a few less. Each star’s chaotic history of close encounters would sweep a slightly different range of orbits clean, but there’d always be a smattering of junk clinging on.

Parantham said calmly, “The isotope signature of most of these asteroids matches our rock.”

Rakesh viewed the data. Point after point coincided, error bars overlapping. What’s more, the models he ran rejected the notion that these asteroids had been born from the same gas cloud as the star they orbited. It looked as if they’d found the Interloper, and the shattered remnants of the Steelmakers’ world.

Rakesh was shaken, though he knew he had no right to be surprised that the search had ended badly. The Interloper had dragged this world into ever more dangerous territory; the real miracle was that it had enjoyed such a long era of safety and stability around its birth star. “So this is their graveyard,” he said.

“We don’t know that,” Parantham replied. “We know that the Steelmakers built at least one interplanetary probe. At some point they might have built star ships, or engineering spores. They might have left this world behind long before it was broken up.”

Rakesh had his doubts that the Steelmakers—as a species, let alone a technological culture—could have survived their planet’s capture by the Interloper. Still, it was possible that in the intervening hundred million years a second intelligent species had arisen in their place. In any case, he’d honor his promise and sift through the ruins. He owed it to the Steelmakers and whoever might have followed them to do his best to learn their history and bring it back to the Amalgam.

Dynamical models indicated that the Steelmakers’ world had been tidally disrupted, rather than smashed apart by a head-on collision. A compact stellar remnant—most likely a neutron star—had passed through the system fifty million years before, coming close enough for the difference in its gravitational pull from one side of the planet to the other to tear asteroid-sized rocks right out of the mantle and send them fountaining into the sky. Though common sense made that sound like the work of a monstrously powerful force, the models suggested that the tidal stretching had only exceeded the planet’s gravity by a modest amount, perhaps as little as fifty per cent. If there had been any hapless descendants of the Steelmakers around, the tidal force itself would have left them unscathed, but that would have been the least of their problems. Some might have survived the initial quakes, as the pressure bearing down on the planet’s interior was lessened in places and strengthened elsewhere, fracturing the crust like the skin of a squeezed grape. Some would have felt their own weight growing, but not unbearably, and even where the tidal stretch turned gravity skyward, some might have had the presence of mind to grip something anchored securely to the ground and cling to life for a few more minutes as the air around them grew thinner. In the end, though, the ground itself had had nothing to hold it together against its own reversed weight, and the planet had simply disintegrated.

Rakesh worked with Parantham to design a probe swarm to send into the ruins. Each probe would be about a micrometer wide, and would hop from asteroid to asteroid by riding the currents of the stellar wind—not the Interloper’s feeble exhalation, but the overpowering breath of the neighborhood giants. On each rock they visited, the probes would gather energy from sunlight to feed a small band of exploratory nanomachines.

The wind couldn’t carry the probes all the way from Lahl’s Promise into the asteroid belt, so they had the workshop build half a dozen delivery modules, driven by ion thrusters, each carrying a kilogram or so of probes to scatter as they arced along the edge of the belt. These delivery modules would also act as information relays, with instruments to track the probes closely and elicit stored data from them.

The modules filed out of the workshop, flung away from the ship by centrifugal force before their thrusters lit up. Rakesh watched their blue exhaust trails through the cabin window. “Do you regret coming with me now?” he asked Parantham.

“Not at all!” she said. She seemed shocked by the question. “Why would I?”

“If the Steelmakers are dead, with no descendants. “

“Then that’s sad,” she said, “but history is full of sad stories. If there’s no chance of meeting them face to face, I’ll happily settle for archaeology. Archaeology in the disk is finished: every ruin has been tomographed down to the molecular level, every scrap of ancient language and every artefact has been interpreted to death. I was promised nothing but a rock full of microbes when I signed up for this, remember? And you expect me to be having second thoughts just because the sentient species we’ve discovered might have lasted less than one hundred and fifty million years?”

Rakesh couldn’t argue with anything Parantham had said, but his own sentiments were very different. “Maybe at the back of my mind I thought the worst case scenario would be a thousand-year-long slog that ended with nothing but bacteria, while the best case would take us straight to the Planet of the Long Lost Cousins, who I could invite into the Amalgam to live happily ever after. Now that we’ve caught a glimpse of the real story, it seems that it’s bacteria who would have had the best chance of living happily ever after.”

He could easily picture his own village on Shab-e-Noor with a dark pinprick crossing the sky, the ground rumbling, an ominous lightness. Of course, that couldn’t happen in the Age of the Amalgam; there was no conceivable cosmic threat out in the disk that could not be detected and neutralized. Such vulnerability had been relegated to history. Nevertheless, the image haunted him in a way that went beyond mere empathy for its putative victims. There was a chill in his bones at the recognition that, in the broadest sense, he’d stepped out from the shadow of the same kind of ax. His ancestors had been luckier than the Steelmakers, that was all.

The first wave of results from the probes came in while Rakesh was in the kitchen, cooking breakfast.

Dead microbes had been found in more than sixty per cent of the asteroids sampled so far. That figure was surprisingly high; either the biosphere of the Steelmakers’ world had extended deep into the mantle, or the rubble that originated from the depths of the planet had been cross-contaminated by other debris, from closer to the surface.

The genome fragments and general morphology closely matched those of the microbes they’d found in the Aloof’s meteor. Along with the isotope data, this left Rakesh with no doubt that they’d found their target. Half of the Interloper’s asteroid belt consisted of rocks virtually identical to the one that had triggered their search.

“The Aloof should give us a treat and a scratch behind the ears now,” he told Parantham as he filled her plate.

She stared at him as if he’d lost his mind.

“On my home world,” he explained, “we have domestic animals that can find things by scent. You give them a whiff, then they go searching for something that smells the same.”

“You don’t have machines to do that?”

“Of course we do. But these animals enjoy it, it’s part of their ancestry. If they don’t get the chance to exercise their skills, they get sick with boredom.”

“Like the gang back at the node?” Parantham suggested dryly.

“Well, yes.” Rakesh hadn’t intended the comparison to be taken literally, but he felt a momentary frisson of unease. “I suppose that’s one theory we can’t rule out: the Aloof took pity on us and offered us a chance to chase a strange new scent across their paddock.”

“It doesn’t show much pity if they only do it for a couple of people every million years.” Parantham shook her head. “We’re not their pets. They’ve kept a few secrets from us; good for them. It doesn’t make them our superiors.”

“A few secrets?” Rakesh laughed. “We mapped their gamma ray data routes. They get to read our minds, down to the last byte. And you’re the one who told me that the gamma ray network was probably just a honey pot.”

“I’m not saying that the relationship is symmetrical,” Parantham conceded. “They’ve certainly out-somethinged us. “

“Outwitted?” Rakesh suggested. “Outsmarted? Outmanoeuvred?”

“Out-sphinxed us,” she replied. “We stared into the bulge for a million years, trying to get a reaction, and they just stared back out at us, stony-faced. We did much more than blink; we gave up the game completely. I don’t believe it’s harmed us, though. I don’t believe it’s a loss on our part, or a victory on theirs. It’s just a difference in our natures. We never wanted to keep our nature and our history secret. It’s a game we never wanted to win.”

Rakesh was woken by the next wave of results. He watched the data and images spinning in his skull as he walked down the corridor to the control cabin, where Parantham was already seated.

“It’s alive!” she crowed. “DNA-based, multi-cellular, engineered. but then drifting genetically, running wild for tens of millions of years.”

The probes had found a scum of fungus-like growth clinging to parts of some of the asteroids. These were not just colonies of microbes; the cells showed specialization, and were organized into distinct clusters. Though the anatomy of the clusters included a protective skin, all of the cells were individually tough enough to retain internal liquid water while exposed to vacuum, over a considerable temperature range, with antifreeze compounds and vapor-reducing soluble polymers augmenting the sheer strength of the cell walls. Their genome showed clear evidence of sophisticated engineering, and although they shared a common ancestor with the dead microbes, most of the traits that ensured their survival in their present harsh environment appeared to have been artificially introduced.

The creation of the species couldn’t be dated exactly until mutation rates and generation times had been measured, but on general biochemical grounds it seemed likely that this fungus had been deliberately constructed at about the time the Steelmakers’ world was torn apart.

Rakesh immersed himself in a diagram of metabolic pathways. “It lives on the stellar wind,” he marveled. “That’s its energy source. For raw materials, it’s coping on the asteroids, but there are vestigial enzymes that suggest it might have thrived with a slightly different substrate. So it spread to the asteroids from somewhere else, and adapted to them over time, but the original species was happier in a different environment.”

Parantham said, “You look up into the sky, and a neutron star is coming. There is no transport network to whisk you away to safety, and you can forget about deflecting this planet-killer. What do you do?”

“Build a spaceship.”

“To go where? There are plenty of stars around, but they’re all devoid of companions. A hundred million years ago your ancestors visited another planet, but the space program has grown a little rusty since then.”

Rakesh grimaced. “So I give up on the idea of running, and make a fungus that will outlive me? I know I’ve been spoiled by high-tech immortality, but that doesn’t sound like much consolation to me.”

Parantham said, “Perhaps it’s just the bottom of the food chain. Make a fungus that will outlive you, then a few species that can eat it, and so on. Then give birth to a child that can live on them.”

“Maybe.” Rakesh ran his fingers through his hair. “Live on them where, though? Those old genes I mentioned were for enzymes that relied on elements that most of the asteroids don’t have. If you know that your world is going to be torn apart, and there are no other planets in sight, where exactly do you expect to live, if not on the scrap heap that’s left behind?”

A few hours later they had the answer, from their telescopes rather than from the probes. Near the edge of the belt, an object some six hundred meters across with a highly atypical spectrum had been found orbiting among the rocks. The telescope’s image showed a gray ellipsoid, pitted and corroded, but clearly too regular to be an asteroid itself. Spectroscopy revealed that its surface contained molecular filaments, carbon nanotubes with elaborate chemical modifications that both strengthened them and protected them against the stellar wind. A variety of the vacuum-hardened fungus they’d detected in the asteroids could be seen in the indentations of small impact craters, where the wind couldn’t reach in to scour it off.

“The material is advanced beyond the Steelmakers’ technology,” Rakesh mused, “but it’s not one hundred million years ahead. They must have gone through a long Dark Age before they finally rose up again.” Only to be cut down once more? That wasn’t clear. Their home world was in ruins, but this artefact was in one piece.

Parantham said, “That surface looks as if it hasn’t been repaired in fifty million years.”

“Not everyone cares about surfaces,” Rakesh replied. “There could still be someone home.”

They sent a surveyor probe, which tomographed the artefact with ambient neutrinos. Inside was a maze of tunnels and caves. Apart from these empty spaces, there was an intricate pattern to the density of the structure itself: parts of the walls were solid as basalt, while others seemed as spongy and permeable as limestone.

Parantham beamed a radio signal from the surveyor probe down to the artefact, a simple message of greeting repeated across the frequency spectrum. The faint passive echo that came back suggested some long strips of conducting material, but no resonant circuits: electrical wiring, perhaps, but no obvious low-tech receivers or transmitters.

An analysis of the artefact’s thermal emissions showed no significant amounts of heat being generated within, beyond what might be expected from a small amount of the fungus, and perhaps other species. There was no obvious stream of waste, organic or otherwise, leaving the artefact, though with the stellar wind as its only input any putative ecosystem would have to cling tightly to all of its materials.

Rakesh said, “It’s time to send in the jelly babies.”

“Ha! You were far more cautious with Steel Mountain,” Parantham reminded him.

“If we trigger some elaborate defensive response,” Rakesh said, “then at least I’ll die happy. Knowing that this civilization survived.”

There was no entranceway into the artefact, but the surveyor probe identified a system of narrow cracks in the exterior wall that ultimately led to one of the internal tunnels. If they made their avatars even smaller than before, about a fifth of a millimeter tall, they would be able to squeeze through.

Rakesh glanced up one last time into the sky full of hot blue stars before following Parantham into the chasm.

As the walls twisted around them, they soon reached a point where the stars were hidden and everything was swallowed by the deep shadows of vacuum; by switching to infrared vision, though, it was possible to grope their way down by the thermal glow of their surroundings. Their avatars sported adhesive pads on their hands and feet, tailored to the chemistry of the bare surface, but the infestations of fungus made their grip less secure.

Rakesh sent nanomachines from a stock in his avatar’s arm into a patch of fungus, to sequence it. There were at least nine distinct species present, and they all showed marked differences from the kind found on the asteroids. The vestigial enzymes he’d noted there were being produced in far greater quantities here, and seemed to interact with several components of the wall material. As he pondered the modified diagram of metabolic pathways, he realized what was happening. The walls acted as a reservoir for the raw materials that the fungus needed, but the fungus did more than leech essential nutrients out of its environment. As part of its life cycle, it returned everything it took, with the added bonus that structural flaws in the wall were repaired in the process. The system wasn’t perfect, but a few cracks after fifty million years wasn’t bad.

It was a tortuous business navigating the fissure, but Rakesh wasn’t tempted to disconnect his senses and leave his avatar on autopilot. He didn’t know if he was entering a mouldy tomb or a thriving metropolis, but he had no wish to dilute, or distance himself from, the experience. As painful as he found it to be forced to confront the bleak prospects for life in the bulge, this expedition was exactly what he’d been seeking ever since he’d left his home world. Who else on Shab-e-Noor, who else in the whole disk, would be able to tell their descendants: “We climbed down through a gap in the wall, not knowing what we would find inside the structure after fifty million years?”

When they emerged into the tunnel, Rakesh found himself immersed in a featureless glow. The tunnel wall was so close to being uniform in temperature that its thermal emissions rendered everything in contrast-free monochrome. It was almost impossible to interpret what he saw, let alone navigate by it.

“Is it just me who’s gone blind?” he asked Parantham.

“IR sensitivity isn’t enough. We need to rewrite our whole visual processing system,” she suggested.

Rakesh searched the library. Leaving aside olfactory and tactile modes—sniffing or groping your way through the dark—most underground species employed vibration sensors or sonar. The walls here were excellent sound conductors, but even so that would be of limited use. He found a mode of IR-based perception that some asteroid-mining robots and a few tunnel-dwelling species relied upon. It involved extracting and interpreting very small temperature differences from thermal emissions; it was exactly what he and Parantham needed.

The tunnel snapped into focus, decorated with elaborate patterns where the fungus in all its variety grew. Despite the strangeness of the view, the new system felt right: Rakesh knew where he was now, how to move, and what to expect to see when he did. It was unsettling to be reminded that vision was a highly refined form of knowledge, a set of propositions about the world that needed to be deduced, not some passive stream of data that simply flowed into his skull as effortlessly as light into a camera.

They set out along the tunnel, which loomed over them like some monumental feat of engineering. It was only about two centimeters wide, but Rakesh had no way of knowing whether its builders would have viewed it as a cramped passageway, a great highway, or something in between.

They’d chosen not to use the avatars’ vibration sensors as their primary mode of perception, but that didn’t stop them picking up a faint but rising beat conducted through the tunnel wall. “Should we go and explore that?” Parantham asked.

Rakesh said, “It sounds as if it’s coming toward us already.”

A giant creature came scurrying around a bend in the tunnel. It was moving on twelve legs like a busy arthropod, about a millimeter across. Their mode of vision rendered it translucent, revealing hints of membranes and chambers flexing and contracting within.

When it changed course to charge straight toward them Rakesh suppressed the urge to flee; their avatars were extremely robust, and in any case easily replaced. The creature halted and inclined the axis of its body toward him; it seemed presumptuous to assume that it was lowering its face, when Rakesh could make no immediate sense of the complex mass of bristles, knobs and tendrils that confronted him. A cluster of these organs suddenly sprang forward and made contact with his avatar, wrapping it and holding it firmly; he steeled himself for the shock of being vicariously swallowed, but after a moment the creature unwrapped him and disengaged. It stood motionless for a second or two, as if pondering the need for another taste, then it turned away and continued down the tunnel, as rapidly as it had approached.

Parantham said, “We should follow it.”

“Yes.”

The avatars had small fusion-powered ion thrusters attached like backpacks; with no gravity or air resistance to overcome, catching up with the creature and flying a few centimeters above it was easy. Having dismissed them as inedible once, the creature seemed untroubled by their presence, if it was aware of them at all.

The creature had shed cells on Rakesh’s avatar, and he had the nanomachines sequence them as he flew. They shared the fungus’s vacuum-hardening traits, and a large proportion of its other genes, both natural and introduced.

Parantham said, “I’d like to run a morphogenetic model. What do you think?”

“Coarse-grain it, and I think that would be ethical.” Software could take the genome and use it to simulate a growing embryo. A fine-grained simulation would necessarily experience everything that a real organism would, but a coarse-grained simulation could provide information about the range of generic experiences that were possible, without anyone actually experiencing them.

“All right.”

In a spare corner of Parantham’s mind, a sketch of the virtual creature took shape. While Rakesh watched the adult below him scurrying along, pausing now and then to graze on patches of fungus, a second viewpoint showed him an annotated diagram of the developing embryo in its egg case. As morphogen gradients washed over the dividing cells, eight distinct segments formed, the middle six slowly sprouting a tightly folded pair of legs each. Mouthparts, excretory and reproductive organs were whittled out of the growing mass of cells. The developing nervous system was extremely simple, and by the time the egg hatched it was close to hardwired: a handful of innate drives and reflexes would enable this creature to move, feed and mate, but it had no potential to do anything more complex.

Like all DNA-based life it was Rakesh’s distant cousin, but it was unlikely to be a direct descendant of whoever had built this ark.

“It doesn’t use infrared at all,” Parantham observed. “It listens for sounds conducted through the tunnel wall.”

“So how did it home in on me when I was standing stock still?” Rakesh examined the model’s results more closely. “Aha. Resonances set up by its own footsteps. A kind of sonar, after all.” It was impossible to say exactly how this creature’s natural ancestors had lived, but the engineered traits it possessed were extensive and ingenious. The Arkmakers might not have had the technology to locate, let alone reach, another planet like their own, but they had worked hard to adapt life to their new environment.

“So where are its designers?” he asked Parantham.

“Be patient,” she replied. “We’ve barely scratched the surface.”

When they reached a fork in the tunnel, they launched a small probe to keep following the twelve-legged creature, and they took the other turn, in toward the center of the ark.

Rakesh kept waiting for a riot of new lifeforms to appear before his eyes—to cross some threshold that marked the end of the barren outskirts, and witness a sudden explosion of diversity—but all they saw were the same kinds of fungus and the same spider-like creatures eating it. In fact, the further they went the sparser the fungus became.

“The stellar wind powers the whole ecosystem,” Parantham mused. “But it barely penetrates this deep. Parts of the walls are permeable to it, parts are not; it’s as if they designed the ark to have a certain flow, a certain set of currents running through it. But the wind must have been much stronger then. These days, it’s too feeble to do the job. And there doesn’t seem to be any other mechanism for transporting energy in toward the center.”

Rakesh didn’t reply, but he couldn’t help following her argument to its logical conclusion. The Arkmakers had invented a whole new ecosystem to live in after the death of their planet, but the vagaries of the bulge had defeated them yet again. They had relied on the hot winds from nearby giant stars as their new primary energy source, turning their backs on the relatively weak radiation from their small but stable foster-sun. Giant stars had short lives, and while new ones were always being born, in any particular place the stellar wind could ebb and flow dramatically on a time scale of just a few million years. The Interloper might have finished off the Steelmakers, but it could have provided whoever came after them with more or less constant light for another three billion years.

As they flew deeper into the ark, the fungus disappeared completely. The interior was barren. With nothing to repair them, the walls became increasingly cracked; small thermal stresses over the millennia had torn at the structure, in places reducing it to loose piles of rubble. Electromagnetic probes revealed what might once have been a network of copper wires running through the walls—distributing power, perhaps, or information—but they were just fragmented segments now, worn and snapped by minuscule but relentlessly patient forces.

About halfway to the center, rubble blocked their way. They despatched a swarm of small probes that could squeeze into the interstices, then backtracked and took a turn sideways, to see if there was anything that they could explore “for themselves”. Rakesh had grown used to his new body, and he was reluctant to give it up and return his senses to Lahl’s Promise.

He said, “Fifty million years is a long time to expect anyone to stay cooped up in a place like this. Maybe the Arkmakers finally developed interstellar travel, mined the asteroids for some raw materials, and then headed right out of the NSD to search for a safer home.”

“That’s possible,” Parantham replied. “And if we can’t see the hole where they burrowed out of this cocoon, maybe the fungus sealed it off.” She hesitated. “The design still doesn’t make a lot of sense to me, though. Even if the stellar winds were much stronger when this place was built, I can’t see what the flow was optimized for. There’s a very precise gradation in the density of the wall material; it’s far too regular to be accidental, so it’s either an engineering feature that I just don’t understand, or a reflection of a very weird esthetic. If you chop this place in half along the center of the long axis, there’s more flow-through in one half than the other. What’s that all about? And I’d swear the local variations are designed to induce turbulence that would scatter the fungus as widely as possible, but the fluid dynamics is all wrong for any plausible stellar wind that might have blown into this system.”

The way ahead was blocked by a cloud of rubble again. Rakesh braked and let himself float in the middle of the tunnel.

He said, “This thing has no engine. How do you think they got it clear of the planet when the neutron star came through?”

Parantham shrugged. “They might have taken it up in small pieces, and used the fungus to weld them together.”

“That’s assuming they had rockets at all,” Rakesh said.

“Well, yes. If they didn’t, they could have simply built it on the ground and let the tidal force lift it. That would have been a very risky strategy, though.”

“Where would the safest launching site be?” Rakesh glanced at a model and answered his own question, “The point furthest from the neutron star at the moment when its tidal force canceled the planet’s gravity. Assuming that it survived the quakes, the ark would simply drift up into space.”

Parantham said, “It wouldn’t have had much of a head start, though, before all the rocks came tumbling after it. Collisions between the debris would redistribute its momentum, creating some fragments that would outrace the pack. You couldn’t avoid a serious peppering, at the very least.”

“I suppose they could have made more than one ark, to improve the odds,” Rakesh suggested. “The others might have been destroyed by debris, or captured by the neutron star.”

Parantham let out a long, reproachful moan. “Captured by the neutron star?”

Rakesh was bemused. “You don’t think that’s possible?”

“Of course it’s possible. And that’s exactly what they wanted! This one’s the failure, the one that was left behind!”

“How is it a failure to get left behind?”

“The giants’ stellar wind,” she said, “has a greater energy density than middle-aged starlight, but there’s something that would give it even more oomph: the gravitational field of a neutron star. The neutron star would have drawn the wind into an accretion disk around it, far richer in energy than anything else in sight. The Arkmakers saw this monster coming, and thought: if it’s going to pulverize our home, better to learn to drink from that whirlpool than skulk around in the ruins waiting for the next disaster.

“This ark, and everything in it, was designed to survive in an accretion disk. The asymmetrical flow-through would have given it a kind of buoyancy, pushing it back out into larger orbits if it ever sank in too deep.” Parantham ran a model, and piped the output to Rakesh. “The wind in the disk would have been strong enough to keep the fungus alive almost everywhere, to support the food chain throughout the ark.”

Rakesh absorbed the model’s results. Parantham’s conclusions were hard to dispute.

“So this place was starved from the beginning?” he said. “When they missed the neutron star, they had no hope?” The children of the Arkmakers, designed to escape the fate of their planet-bound parents, had found themselves stranded with the wrong biology, trapped inside an ingenious machine for extracting energy from an exotic new source that was receding into the distance at a few hundred kilometers a second.

Parantham said, “No hope for themselves. But I can’t believe this was the only ark. There could have been a dozen, there could have been a thousand. If they really saw no prospect of fleeing from the neutron star, every resource on the planet would have been used to maximize the chances of hitching a ride.”

Rakesh looked around at the ruins of this desperate strategy, and tried to picture the same tunnels teeming with life while the hot wind from a neutron star’s accretion disk whistled through the walls. Perhaps the extraordinary gamble could have paid off, if they’d repeated it a sufficient number of times.

“If they hitched a ride, where did it take them?” Rakesh asked. When he and Parantham had first realized what it was that had created the asteroid belt, they had run dynamical models and checked the maps, but they’d been unable to locate the neutron star that had done the deed. The only thing that had been clear was the general direction of its motion.

“Toward the center,” Parantham replied. “Deeper into the core.”