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 somewhere 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.