“Including the plane change?” Doob asked.

A trace of a smile came onto Markus’s face. He knew exactly what Doob was thinking. For, unable to get Cleft out of his head, Doob had shown pictures of his favorite piece of the moon to Markus, to Konrad, to Ulrika and Ivy and some of the others who seemed to make up the informal power structure of the Cloud Ark.

“Let me be clear,” Markus said. “When I speak of the Big Ride, I mean it for real. We take all of Amalthea with us. We raise the orbit to the moon’s. We change the plane. We circularize. And we end up safe and sound in Cleft.”

“And Ymir carries enough water for that mission?”

“Yes,” Markus said, “if we can control her and bring her in.”

“Isn’t that Sean Probst’s job?” Moira asked.

“Not anymore,” Markus said. “The information I just imparted to you was in Sean’s final transmission.”

Moira and Doob looked at him sharply.

“The health situation has been not so good, for a long time,” Markus explained. “Sean was the last member of the expedition to die.”

“Are you saying that Ymir is a ghost ship?!” Doob asked.

“Yes.”

“And there’s no way to remote-control her,” Moira guessed.

“Unfortunately Dinah’s Morse code cannot help us in that regard,” Markus agreed.

“So someone has to go and—”

“Someone has to go and land on that fucking big piece of ice,” Markus said, “and get inside of Ymir and restart the nuclear reactor and commit the final burns that will bring her into sync with Izzy.”

“Who the hell—” Doob began, but Markus cut him off by pointing to himself. He did this in a somewhat awkward fashion that, deliberately or not, looked like a pantomime of suicide by handgun. He said, “I am placing Ivy in command of Izzy and the Cloud Ark tomorrow. I am assembling a crew that will depart in a MIV and make a rendezvous with Ymir. We will board her and manually execute the procedures needed to bring her under control and get her payload to Izzy. We will then use what is left of the ice to raise Izzy’s orbit—and we will bring Amalthea with us on the Big Ride.”

“That’s . . . major,” Moira said. “Who knows? When were you going to announce it?”

“I just decided it now.” Markus sighed. “Listen, it is the only way. In my heart I always considered Dump and Run and Pure Swarm both to be too risky. What happened with the HGA just makes this more obvious. The only wise course is the Big Ride. It will take a long time—two years or something. But during all that time the most important resources can be sheltered within Amalthea. And by that I mean you and your equipment, Moira. You can have whatever resources you need from the Mining Colony to create a safe location for the genetics lab.”

“Okay,” Moira said, “I’ll talk to Dinah.”

“Talk to whomever she delegates,” Markus said. “Dinah is going to have to come with me on the expedition. I need her to deal with all of those verdammt robots.”

“How can I help?” Doob asked. He wondered if Markus might dragoon him as well, and was torn between being afraid of that and tremendously excited.

“Figure out how we are going to do it,” Markus said, after considering it for a few moments. “Lay in a course for Cleft.”

“Yes,” Doob said. “I’ll do that.” The little boy in him was crestfallen that he wasn’t going on the adventure. Then he reminded himself that he was already part of the biggest adventure ever, and that, so far, it had been altogether miserable.

ALL CONVERSATIONS WORTH HAVING ABOUT SPACE VOYAGES WERE couched in terms of “delta vee,” meaning the increase or decrease in velocity that had to be imparted to a vehicle en route. For, in a common bit of mathematical shorthand, the Greek letter delta (Δ) was used to mean “the amount of change in . . .” and V was the obvious abbreviation for velocity. The words “delta vee,” then, were what you heard when engineers read those symbols aloud.

Since velocity was measured in meters per second, so was delta vee. The delta vees bandied about in spaceflight discussions tended to be large by the standards of what Markus was now calling Old Earth. The speed of sound, for example—a.k.a. Mach 1—was three hundred and some meters per second, and most earthbound people would consider it awfully damned fast. But it hardly rose to the notice of most people who talked about space missions.

A common delta vee benchmark had been the amount needed to get something from an Old Earth launch pad to an orbit like Izzy’s. This was some 7,660 meters per second, or more than twenty-two times the speed of sound: an impossible figure for any object that had to fight its way through an atmosphere. Once a vehicle had reached the vacuum of space, though, things became simpler: rocket engines worked more efficiently, drag and aerodynamic buffeting were absent, and the consequences of failure weren’t invariably catastrophic. Getting it from point A to point B was a matter of hitting it with the right delta vee at the right time.

Sean Probst’s delta vee history, from his departure from Earth until his departure from life, had gone something like this. The launch from terra firma to Izzy on Day 68 had required a delta vee of 7,660 m/s according to a naive calculation; but as any old space hand would know, losses due to atmospheric friction and the need to push back against gravity would have elevated the practical number to more like 8,500 or 9,000.

Once he had collected Larz and most of Dinah’s robots, Sean had needed to execute a plane-change maneuver to get from the Izzy orbit—which was angled at about fifty-six degrees to the equator—to the equatorial orbit in which Ymir was being assembled. This was one of those circumstances in which human intuition got it all wrong. The Izzy orbit and the Ymir orbit did not seem all that different in most respects. Both of them were a few hundred kilometers above the atmosphere. Both were essentially circular (as opposed to elliptical). And both went in the same direction around the Earth. The only real difference between them was that they were at different angles. And yet the delta vee required to get from one to the other was large enough that it had been necessary to launch a separate rocket, carrying nothing but extra propellant, just to refuel Sean’s vehicle in preparation for the plane-change burn.

Once Ymir had been assembled, a delta vee of some 3,200 m/s had been needed to place her in a very elongated elliptical orbit that had taken her out to L1. En route, the plane-change problem had once again reared its head. Essentially everything in the solar system, including Comet Grigg-Skjellerup, was confined to a flat disk centered on the sun. The imaginary plane through that disk was called the ecliptic. Conveniently for people who liked seasons, but not so good for interplanetary travelers, Earth’s axis and equator were angled with respect to the ecliptic by 23.5 degrees, and so Ymir’s initial orbit had been off-kilter by that amount. Fortunately, plane-change maneuvers were much less “expensive” (meaning they required a lot less delta vee) when they were performed far away; and Ymir was, of course, going very far away. So, they had done the plane change out at L1 range, as part of the same burn, totaling some 2,000 m/s, that took her out through the L1 gate into heliocentric orbit.

That orbit, more than a year later, had intersected that of Comet Grigg-Skjellerup. As Ymir had drawn near to the comet core, she had used another 2,000 m/s of delta vee to sync her orbit with its.

All of these maneuvers, up to the arrival at Grigg-Skjellerup, had been achieved by using Ymir’s rocket engines, which were altogether conventional: they burned propellants (fuel and oxidizer) in a chamber, making hot gas, which was vented out of a nozzle to produce thrust. The final burn had emptied her propellant tanks, so this was a one-way journey unless the nuclear propulsion system could then be turned on.