Seveneves - Страница 70

“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.

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 necessary parts and modules were shuttled about the Shipyard until the new vehicle was ready.