Seveneves

by Neal Stephenson

  The payload in this case was a single arklet hull that had been augmented with a “side” door: an airlock that could accommodate one person in a space suit. Other than that, it had been stripped to the minimum complement of equipment needed to keep a crew of four alive for a few days. To its mass, of course, needed to be added that of the actual humans and their food and other essentials. The lightness of a bare arklet hull was startling; the newer hulls, made of overwrapped composites, weighed in at eighty kilograms. Stripped of everything that made it comfortable and inhabitable over the long term, and including the “side door,” the maneuvering thrusters, and a reasonable supply of thruster propellant, the mass of New Caird was about ten times that. The humans weighed three hundred kilograms. The rocket motor that would be doing all the important burns weighed another two thousand. So, in round numbers, the payload mass—the stuff that actually had to get delivered to the docking port of Ymir—was some thirty-five hundred kilograms. The mass ratio of seven meant that its propellant load, at the beginning, was going to be some twenty-one thousand kilograms of liquid hydrogen and liquid oxygen.

  The Shipyard had been stocked with several cryogenic propellant tanks of various sizes, some designed to hold LH2 (liquid hydrogen) and others built to the somewhat different specifications needed in the case of LOX (liquid oxygen). The chosen tanks were bolted together in a stack with the rocket engine mounted “below” and thermal protection wrapped all about. New Caird proper—the arklet with the humans in it—projected forward on a scrap of scaffolding just long enough that her maneuvering thrusters wouldn’t damage any of the other parts when they came on.

  While the MIV was being constructed, twenty-one thousand kilograms of water had to be split into hydrogen and oxygen, chilled to cryogenic temperatures, and stored. The Shipyard’s port side already had some LH2 and LOX premade. In general, though, they tended not to keep a lot of them on hand, because they were tiresome substances to work with. The demand was supplied by the naval reactor on the Shipyard’s long arm, which was brought up to full power for the first time since it had been launched, piece by heavy piece, from Cape Canaveral on a series of heavy-lift rockets. Pumping juice down heavy cables to the splitters, it was able to turn twenty-one tons of water into gases and chill the gases to cryogenic temperatures while the other preparations were being made.

  This was a lot of water—roughly fourteen liters of it for every surviving human. The Cloud Ark recycled water, of course, and was far from running out of the stuff. Nonetheless, the idea of taking that much of it and spewing it into outer space, never to be recovered, gave many people pause: especially the Dump and Run partisans.

  There was a strong counterargument, which was that New Caird’s objective was to take possession and control over a piece of frozen water that weighed as much as Izzy herself, including the giant piece of iron to which Izzy was attached (and would continue to be, if the Big Ride advocates had their way).

  Once New Caird had reached her, Ymir could presumably be slowed down, and brought to a rendezvous with Izzy, by firing her engine. And that was a primitive beast, but it had a basically infinite supply of energy in the nuclear reactor, and a vast stock of propellant in the form of ice. The “steampunk” propulsion system had much lower efficiency, however, than a properly engineered rocket motor. Consequently, the mass ratio that would be needed to slow Ymir down from the high-speed elliptical orbit with which it was falling into Earth’s gravity well, to match the much slower, circular orbit of Izzy, was about thirty-four, which meant that 97 percent of the ice currently attached to Ymir was going to be melted, turned into steam, and jetted out its makeshift nozzle just to slow it down. The remaining 3 percent, however, would still weigh as much as Izzy and Amalthea put together. Split into hydrogen and oxygen, it would supply the rocket fuel needed to power the Big Ride, all the way up to Cleft.

  “I DIDN’T EXPECT IT TO BE BLACK,” DINAH SAID. SHE WAS HEARING HER own voice as if down a mile-long sewer pipe. She was pretty sure she had lost consciousness a minute ago. Maybe she wasn’t all the way back yet.

  Markus was slow in responding. Maybe he had blacked out too. Maybe he was just distracted. “Comet cores are covered in—”

  “Stinky black stuff, yeah, I know that, Markus. Remember who I am?”

  “Sorry. Not enough blood in brain.”

  “But this is just a shard that Sean broke off of Grigg-Skjellerup. Why’s it all covered?”

  “I don’t know,” Markus said.

  They were looking at Ymir from a distance of ten kilometers and closing. They were viewing her on their tablets, through a zoomed-in video camera. Vyacheslav Dubsky, floating closest to New Caird’s forward end, put his face to the vessel’s tiny window and searched the black sky for the black ship, but the squint on his face suggested it was still too far away for naked eyes to be of much use.

  “Maybe he was doing us a favor,” Dinah said. “The black stuff has all kinds of goodies on it. Carbon, obviously. But also nitrogen, potassium—”

  “Micronutrients,” Markus said, “that the Cloud Ark will be needing.”

  “So maybe he used the robots to scrape some of it off Greg’s Skeleton, and loaded up on the gunk,” Dinah speculated.

  “We will know soon,” Vyacheslav said. “Presumably he left a document.”

  “Which we will not be alive to read, unless we stick the landing,” Markus pointed out, “so no more chatter from now on, please. Slava—” and he broke into a string of bad Russian meaning something like I trade places with you now. Vyacheslav responded in equally bad German. Both men were perfectly fluent in English. But they made a private joke of butchering each other’s languages, ostensibly as part of a project to preserve Old Earth’s linguistic heritage. Markus then added, “The rest of you, buckle up.”

  With the deft movements of one who had been in space for two years, Vyacheslav glided aft. He was one of the veteran Russian spacewalkers who had come up to Izzy way back on A+0.17, in the first launch after the moon had blown up. He had been a mainstay of the Scout and Pioneer eras, racking up more time in a suit than anyone, wearing out three Orlans. He was a little worn out too, being sallow and gaunt compared to the strapping hero who had emerged two years ago from the same Soyuz that had carried Rhys and Bolor-Erdene. Markus replaced him at the forward window and buckled himself into the pilot’s seat.

  Behind that was a row of three acceleration couches, mounted on a frame that spanned a diameter of the arklet’s hull. Dinah was loosely belted in on the port side. A few minutes ago she had been tightly belted in. She had not adjusted the straps. The entire couch, and its supporting truss, had been deformed by the same burst of gee forces that had left her so woozy. To starboard was Jiro Suzuki, a nuclear engineer who had been involved with the design of Ymir’s nuclear reactor core. It wasn’t clear whether he was conscious; but then it never was with Jiro. Vyacheslav, the fourth member of New Caird’s complement, settled into the middle position and pulled the top straps of the five-point harness over his shoulders.

  A staccato burst sounded from the gamma spectrometer—the modern equivalent of a Geiger counter—floating in front of Jiro’s face. Then the Eenspektor, as they called this device in butchered Russian, dropped back into the normal sporadic patter.

  Radiation was striking the Eenspektor—and their bodies—all the time, at random moments and in no particular pattern. Sometimes there would be a little burst, and that part of the mind that liked to see meaning in everything would identify it as an event. But then it would die away and be forgotten. That was just the way of the universe, and of the human psyche. There was a lot more radiation in space than there had been down on the ground, but all the survivors had long since come to terms with that, and Jiro had dialed down the sensitivity on his Eenspektor so that it wasn’t screaming at them all the time.

  If it started screaming in the next few minutes, it wouldn’t be because of some faraway cosmic event. It would be because of a radiation leak from Ymir. />
  “Starting to see the exhaust trail,” Markus commented. “Can you see it on video? It’s faint. The sunlight hits it perfectly a few hundred meters aft of the nozzle bell.”

  He was referring to a thread of steam that emerged from Ymir all the time, even when her engine wasn’t powered up. This was how Konrad and Doob and the other astronomers aboard Izzy had been able to track the ship’s course using their optical telescopes, and to verify that the params encoded in Sean’s last transmission had been accurate. Wispy as it was, the steam trail reflected more light than did the ship itself.

  It was created by the slow, steady boiling of ice caused by the latent radioactivity in the ship’s fuel rods. When the control blades were pulled out and the reactor operated full blast—which was almost never—it produced four gigawatts of thermal power by splitting uranium and plutonium into smaller nuclei, many of which were themselves unstable isotopes. As these fission fragments decayed into “daughters” and “granddaughters,” heat continued to be generated even when the reactor had been shut down. There was nothing that could stop it, so some loss of ice, in the form of this tenuous trail of steam, was unavoidable. It was okay. Ymir had plenty more where that came from, and Sean would have allowed for it in his calculations.

  Sean, never the most emotionally sharing kind of guy, further throttled by his makeshift radio, had not supplied details as to what had killed him and his crew. Had it been some kind of disastrous problem with the reactor core, he probably would have given them a heads-up. For that matter, Ymir wouldn’t have made it this far if the system hadn’t been basically working. So Jiro was not coming into this thing expecting a total nightmare. But there was no telling.

  No one spoke for several minutes as Markus monitored their approach and occasionally touched the controls, spanking New Caird into a slightly different course.

  They had gotten here by means of two large burns. The first, and smaller, had placed them into an ellipse that had shot beyond the orbit of the former moon. After several days of weightless coasting away from the Earth, they had succumbed to the force of gravity, looped lazily around, and begun to fall back again toward the burning planet. This had been timed in such a way that, about a day later, they would be overtaken by Ymir, coming in on a roughly parallel track. But Ymir was traveling much faster—coming in hot, as Sean had told them—basically because it had been falling in toward the Earth from an extremely high starting place, gathering speed relentlessly for weeks. Left unmolested, Ymir would come screaming in toward Earth with a relative velocity of some twelve thousand meters per second, make a hairpin turn just a few kilometers shy of a catastrophic encounter with the glowing atmosphere, then go hurtling back outward again, not to return for a couple of months. Eventually her orbit would decay to the point where she would get dragged down by the atmosphere and destroyed.

  At any rate she would have flashed by New Caird too quickly even to be seen, her relative velocity faster than that of a rifle bullet, had New Caird not just matched her velocity by making a long, precisely timed burn of her main engine. The four members of the crew were still recovering from this. The vehicle’s main engine was oversized—the kit of MIV parts from which she’d been built only had so many options—and so the gee forces had been impressive at the beginning of the burn and brutal toward the end, as the lavish expenditure of propellant had made her lighter and lighter compared to the engine’s formidable thrust. If Dinah had blacked out for a few seconds, why, maybe that was just as well, given that they had aimed themselves almost squarely at the Earth and then launched themselves at it as if on a suicide run. This was necessary in order to go where Ymir was going, but added up to maybe a little more excitement than she really was in the mood for at this moment in her life.

  Earth was, of course, completely unrecognizable. From this distance it was about the size of a tangerine held at arm’s length, and about the same color. Formerly a cool blue-and-white lake in the cosmos, it now hung there like a blob of molten steel thrown out by a welder’s torch. In the belt between the tropics, where most of the Hard Rain was falling, it glowed orange. The color faded and reddened to a kind of sullen brown around the poles, and the whole planet continually sparkled with the bluish light of vaporizing and exploding bolides. In a few days it would blot out half of the sky for a hectic few minutes while they slingshotted around it. By that time, they needed to have Ymir’s main propulsion up and running so that they could execute the huge braking burn that would slow her down to the same velocity as Izzy.

  It was crazy. It was a crazy plan. The crushingly high acceleration that they had survived at the end of the big burn a few minutes ago was a physical reminder that they had only taken enough propellant to sync New Caird with Ymir. If they failed in their basic mission—if they couldn’t dock with Ymir and get her engine working—they had no way of getting back to Izzy, save perhaps by the utterly insane measure of diving into Earth’s atmosphere on their next pass and using the air to slow them down.

  Dinah had been a little slow to absorb the full meaning of the little ship’s name. The James Caird was a small boat that Shackleton had used to make a desperate run for help to save the remnant of his failed South Pole expedition. They had aimed it at South Georgia Island, a speck on the map, in the knowledge that if they didn’t hit it spot on, the prevailing winds would never allow them to turn around and make another try.

  She wondered if that very craziness wasn’t Markus attempting to make a point. The overall situation of the human race was, of course, ludicrously desperate. Doob had been the first to point this fact out in public, two years ago. Planning and preparation had consumed the time since. The work had been hasty, improvised, and politically inflected, but fundamentally it had been a well-ordered and methodical engineering project. As it had to be. But its plodding bureaucratic nature had a kind of lulling effect. How many times in the last two years had Dinah leaned back from a screen full of code and forcibly reminded herself of what was going on, and how bad it was? Unable to keep that squarely in their minds, the fifteen hundred or so survivors tended to live from one day to the next and keep doing what they had done the day before. Of all people, Sean Probst had been the least susceptible to that; he had seen what needed to be done almost immediately, and he had made efforts to do it that had been fantastically strenuous and, in the end, fatal. With his final transmission he had passed that responsibility on to Markus. Dinah suspected that Markus had stepped away from his position at the top of the org chart, and set out on this mission, partly to set an example for everyone else.

  And if that was true, bringing Dinah along was to make a point as well. He would spare no one, play no favorites.

  Markus broke the silence once during the approach: “Definitely a shard. As you said. Not snowball. Not candle.”

  “I agree,” Dinah said. She could see its shape clearly enough, now, on the screen of her tablet.

  Unlike normal ships, which carried their propellant in tanks, Ymir was a big chunk of solid propellant—ice—inhabited by a sort of parasitical infestation of equipment whose purpose was to convert that propellant into thrust. Not knowing exactly what he would find on Comet Grigg-Skjellerup, Sean had come equipped with more than one alternative architecture for putting Ymir together. If the comet core had turned out to be a loose ball of ice-dust, then he’d have had to scoop out what he needed and pack it into something like a snowball, giving Ymir a spherical shape with the reactor embedded in its center. Another option would have been to fashion a long cylinder of ice and plant the reactor in one end of it, then “burn” it forward, consuming the ice en route, like a candle. What they were seeing now looked more like the third architecture, which was the shard. It suggested that, upon his rendezvous with Grigg-Skjellerup, Sean had found it made up of at least one fairly hard and solid crystal that could be relied on to hold itself together structurally during the maneuvers to come. He had split the shard off from the main body of the comet and planted the reactor system somewhe
re near its middle, then embedded the rest of his ship—the part where the humans lived—in what would become its nose. If the equipment had worked as planned, then executing the “burns”—i.e., pulling out the control blades to place the reactor into operation and make steam—had been a matter of sending signals to actuators embedded in the core: motors that would move the rods, valves that would control the flow of steam and water, and so forth.

  Implicit in all of this was a hell of a lot of robot activity, which was why Sean had taken the extraordinary step of traveling personally to Izzy to clean out Dinah’s supply of them before proceeding to his rendezvous with Ymir. The reactor had to be fed with ice. Because ice was a solid, it couldn’t flow through tubes. Robots had to mine ice from the shard and transfer it to a feed system: a set of augers that would move it into the reactor chamber to be melted and vaporized. A Siwi robot could move a lot of material in a hurry by embedding its “tail” in the ice and then using a whirring mill on its “head” to throw off a fountain of fine shavings that could be collected and carried off by Nats. The long intervals of time between burns could be used to store up a supply of shredded ice in hoppers that would feed the augers.

  Downstream of the engine, robots were also needed to maintain the shape of the rocket nozzle. This was a long duct with a wide mouth on the aft face of the shard, tapering to a narrow throat near the reactor. The throat had been constructed on Earth and launched up with the reactor. It was made of a corrosion-resistant alloy called Inconel. Any other material would rapidly wear out from the hot steam blasting through it. Conditions in the long spreading bell of the nozzle, however, were more benign, and so it worked fine for that to be sculpted from ice. Nonetheless, it changed its shape as it was used. Deeper in, where the exhaust was hot, it grew wider as its walls were melted by the torrent of steam. Closer to the exit, where the exhaust had cooled to below freezing, it accumulated on the walls and narrowed the passage. So robots had to scuttle around reshaping the nozzle. This was a fine task for the Nats that Larz had experimented with in Seattle.