On one level, Markus’s proposal was perfectly obvious. Of course anything would be safer inside of a huge metal asteroid.
On another level, it had ramifications.
As of a few days ago—pre–White Sky, the last time anyone had been able to think straight—the fate of Amalthea and the Mining Colony had still been subject to debate. Was the asteroid the boulder in the wheelbarrow that had to be dumped? Or was it the aegis that would shelter the entire human race? The argument had come down to statistics. They just didn’t have enough data to make a decision.
By suggesting that Moira’s equipment be moved into the interior of Amalthea, Markus seemed to be committing to a specific course of action.
It was a course that Doob instinctively agreed with. But it was a bit strange for a man like Markus to just decide on a course of action before the numbers were in.
Or did he know something Doob didn’t?
Moira, in any case, went first. “What if we Dump and Run?”
She was referring to a gambit, frequently discussed and war-gamed, in which Amalthea would be cut loose and abandoned, and Izzy, lightweight but unprotected, would boost herself to a higher orbit with fewer bolides flying around in it.
“Then we would simply have to move all of that stuff back to Node X first,” Markus said. “Or wherever we felt was safest.”
This elicited a searching look from Moira. Markus held up his hands. “But I take your point. I am increasingly biased against Dump and Run.”
“You know how I feel about the Swarmamentalists,” Moira said.
She was referring to another of the basic gambits, Pure Swarm, in which everything—presumably including Moira’s lab—would be distributed among arklets, which would then collectively move to higher orbit. People and goods would move among them through a decentralized market-based economy.
“Listen,” Markus said, “now that everyone below is dead, and we don’t have to put up so much with bullshit, you will find that Hu and the others have a more nuanced view than they were letting on before.” He referred to the fact that Zhong Hu, as the foremost swarm theorist and the brains behind Parambulator, was assumed to be a Swarmamentalist.
Doob nodded. It still took some effort to remind himself that the millions of Internet commentators arguing for this or that strategy were all ghosts now.
“You know something,” Doob blurted out. Then, as the thought was coming into his head, he added, “From Dinah. The radio.”
“Yes,” Markus said. “Ymir is coming in hot, high, and heavy.” He surrounded those three words with air quotes.
“What does that mean?” Moira asked. “She’s made of ice, how can she be hot?”
“She is approaching with a high closing velocity. Not unmanageable. But . . . somewhat exciting.”
“And ‘high’?” Doob prompted him.
“Sean also transmitted his params,” Markus said. “It would seem that he did us a large favor. He executed the plane change while it was still easy to do so, way out around L1.”
“So when he says he’s coming in high,” Doob said, “he means that Ymir has a high orbital inclination—close to ours?”
“Very close to ours,” Markus confirmed. “He is dropping this big chunk of ice into our lap.”
“So,” Moira said, “on top of everything else, Sean Probst is now preparing to dive-bomb us with a comet?”
“A piece of one.”
“A big piece,” Doob guessed, “if he specified ‘heavy.’”
“The number was impressive.” As Markus said this, he shifted toward Doob and looked him in the eye.
“Oh wow,” Doob said. “Is it enough for the Big Ride?”
“If we can get Ymir to rendezvous with Izzy, then yes,” Markus said. “It is more than enough.”
The Big Ride was the third of the basic options. It meant to boost Izzy in its entirety—Amalthea and all—to a much higher orbit. It had been considered implausible because of the amount of propellant that would be needed. Not just implausible but—absent the timely return of Ymir—physically impossible. Despairing of Sean’s chances, its supporters had lately tended to suggest scaled-down variants, such as reshaping a small percentage of Amalthea into bolide deflectors and ditching most of its mass.
“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 mat
hematical 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.
No engine had ever been made that was capable of pushing a comet core around the solar system at any appreciable speed. For that, they had needed to embed the nuke-on-a-stick into the heart of the ice payload, construct an ice nozzle behind it, and then pull out the control blades, causing the reactor’s sixteen hundred fuel rods to become very hot. Ice turned to water, then steam, which shot out the nozzle and produced an amount of thrust actually capable of making a difference. So a few months had then been consumed disassembling Ymir and integrating its parts into a chunk of ice carved off the three-kilometer ball.
The question might have been asked: Why just a piece of it? Why not bring the whole comet core back, if water was so desirable? What was the point of sending a large nuclear reactor into space if you weren’t going to use it? And the answer lay in the fact that even a large nuclear reactor did not even come close to having enough power to move such a big piece of ice. The mission would have lasted more than a century, assuming the existence of some kind of a miracle reactor that could operate at full power for that long. In order to get this done in any reasonable amount of time, they could only bring back the bare minimum of ice needed to rendezvous with Izzy and accomplish the Big Ride.
In any case, Sean and his surviving band had used the nuclear engine to impart a delta vee of about 1,000 m/s to the shard they had carved off Greg’s Skeleton, thereby placing it into a somewhat different orbit that had, a few months later, glided into L1. Sean had remained alive just long enough to yank out the control blades one last time and execute a delta vee that had basically reversed the maneuver they’d used to leave the L1 gate almost two years earlier. This had simultaneously brought Ymir into geocentric orbit while executing, as cheaply as possible, the plane change needed to enable a later rendezvous with Izzy. A couple of days later Sean had tapped out the “coming in hot, high, and heavy” message and dropped dead. Of what, they could only conjecture.
The retrieval team that was now being organized by Markus was going to use a MIV, or Modular Improvised Vehicle, assembled from a kit of parts: a sort of Lego set for the construction of spaceships, neatly sorted on a stack of modules, collectively known as the Shipyard, connected to the Caboose.
The Shipyard was a generally T-shaped contraption. One arm of the T’s crossbar, projecting from the port side of the Caboose, was studded with MIV parts. The opposite arm was a cluster of spherical tanks surrounding a collection of splitters. These used electrical power to split water molecules into hydrogen and oxygen, and piped them to chillers, which refrigerated the gases until they became cryogenic liquids that could be stored in the bulging tanks.
So much for the T’s crossbar. Its long vertical stroke was a truss terminated by a nuclear reactor: not a small RTG like the ones on the arklets, but a true reactor, originally designed to power a submarine, considerably souped up for this task.
Markus dubbed the Shipyard’s first product New Caird, after a small boat that had been used in Shackleton’s expedition to Antarctica. She was assembled and made ready for use in ten days: about one-third of the time they estimated it would take for Ymir to arc in from L1 and make her closest pass to Earth.
To design, assemble, and test such a vehicle so quickly would have been unthinkable two years ago. During the interval between Zero and the White Sky, however, the engineering staffs of several earthbound space agencies and private space companies had foreseen the future need to jury-rig space vehicles from standard parts such as arklet hulls and existing rocket engines, and had provided a kit of parts, lists of procedures, and some basic designs that could be adapted to serve particular needs. In effect, New Caird had been designed a year ago by a large team of engineers on the ground, all but three of whom were now dead. Those three had been sent up to join the General Population. Building on their predecessors’ work, they were able to produce a general design—enough to begin pulling the bits together, anyway—within a few hours of Markus’s decision. Details emerged from their CAD systems as they were needed over the following week and a half, and the necess
ary parts and modules were shuttled about the Shipyard until the new vehicle was ready.
New Caird would have to execute one burn to reach an orbit that would intersect Ymir’s and another to match her velocity, so that the crew could board the ghost ship and take the helm. The total “mission delta vee” for that journey, from its departure from the docking port on Izzy to its arrival at a similar docking port on Ymir, was some 8,000 meters per second.
The conversation turned now to mass ratio: a figure second only to delta vee in its importance to space mission planning. It simply meant how much propellant the vehicle needed at the start of the journey in order to effect all the required delta vees.
Laypersons tended to substitute “fuel” or “gas” for “propellant,” making the obvious analogy to the stuff that had been burned by the engines of cars and airplanes. It wasn’t a bad analogy, but it was incomplete. In addition to fuel, most rocket engines needed some kind of oxygen-rich chemical (ideally, just pure oxygen) with which to burn it. Cars and planes had simply used air. Rockets stored the oxidizer in a separate tank from the fuel until the moment of use. The two chemicals were collectively referred to as “propellant,” and their combined weight and volume tended to dominate space vehicle design in a way that hadn’t been true of, say, automobiles, whose gas tanks had been small compared to their overall size.
A convenient figure for characterizing that was the mass ratio, which was how much the vehicle weighed at the beginning (including the propellant) divided by how much it weighed at the end, when all the tanks had been emptied. If you knew how good the engine was, and how much delta vee you needed, then the mass ratio could be calculated using a simple formula named after the Russian scientist Tsiolkovskii, who was credited with having worked it out. It was an exponential: a fact that explained almost everything about the economics and technology of spaceflight. For if you found yourself on the wrong side of that exponential equation, you were completely screwed.
When the relevant numbers for the Ymir retrieval mission were jacked into the Tsiolkovskii equation, the result was a mass ratio of about seven, meaning that for every kilogram of stuff—Markus, Dinah, other personnel, miscellaneous robots, etc.—that they wanted to arrive safely at the docking port of Ymir, they needed to allow for six kilograms of propellant at the moment of departure from Izzy. This wasn’t all that difficult to achieve, especially for a vehicle that would never be exposed to the rigors of passage through the atmosphere.
Seveneves Page 37