Sandy said, "You remember when I was fabbing the burn box and I had to do those measurements of the QSUs? Well, while Joe was busy building the circuits, I printed up a couple copies of the QSUs. I had my Red photos with perfect color-matching, and the precise scales, and when I finished . . . I mean, they were perfect. Then, when I was fitting the QSUs into the burn box, I switched a couple of them."

"Why'd you do that?"

"Because everybody was so worried about what would happen if the Chinese took the ship. It was an obvious possibility, so . . . why not? If everything worked out, I'd just retrieve them and turn them over to you."

"How'd you get them off the ship?"

"In my hand-camera case. Took the camera out, put the QSUs inside, sealed it up . . . and when we evacuated the Chinese from the Odyssey, took a minute to stick it on the far side of the ship with its Post-it pads. I was worried about the battery-that the lack of warmth would kill it. But then I remembered about the radiators. They put enough heat on parts of the hull that the hull actually was warmish, and that's all I needed. With just a little warmth seeping into the camera case, the battery would last for five years. When we got back . . ."

"You used your remote to unstick it. The camera case is in orbit."

"Yup. Saw it pop off the hull myself. It'll take you about a hundred years to find it, with all the other shit that's still floating around up there. What I'll keep to myself, until I get the apology, is exactly what time I let that puppy go. Got it right down to the tenth of a second. With that information, you could find it in an hour."

"Why'd you wait so long to tell me? Why this whole charade?"

"I think we needed it," Sandy said. "I think we needed the whole trial, all the theatrics, all the bullshit about doing research on the readers, all the sincerity, to convince the Chinese that we really didn't have anything, other than the raw science from the I/O. And that's mostly theory-that's gonna get out no matter what we do. Probably printed in Nature & Science. In fact, when I thought about it, publishing the science, even the little bit that we have, would set off a lot of research commotion, which would cover up the fact that we have all of it. For a while, anyway."

Crow nodded and said, "You're right. About all of it." He stood up, climbed out of the van, and said to the marshals, who were waiting on the sidewalk, "Let him out."

As the marshals came around, Sandy said, "You knew I'd been up to something. Why?"

"'Cause you once told me that you'd not only do what we want, you'd do what we need," Crow said. "I believed you. Plus, of course, that shit-eating grin that would pop onto your face, from time to time, during the trial. Santeros actually spotted it."

"Huh. Gotta work on that," Sandy said. "Uh, why are the marshals just . . . letting me go?"

"They're not exactly marshal-type marshals, if you take my meaning."

"Did you ever catch your spy?"

"Can't talk about that."

"Did you ever figure out how he was communicating with the Chinese?"

"No, never did."

"I read that Elroy Gorey died when the GPS went crazy on a twenty-wheeler, swerved across the road and killed him."

"A tragedy," Crow said. "We all felt terrible." Neither his voice nor his face showed the slightest inflection.

The marshals freed him and Sandy climbed out of the van. Crow handed him an envelope. On the outside it said simply: "The White House."

"What's this?"

"The pardon," Crow said. "I'll work on the apology. Listen, my car's right around the corner. You need a lift?"

EPILOGUE.

2179.

DEEP SPACE.

The sun was the most brilliant star in the sky, but that was all that distinguished it from other stars. The white-hot pinprick shed barely as much light as the quarter moon did on Earth, three thousand AU away. It did little to illuminate the ship gliding through the inner Oort cloud.

Earth's first truly deep space mission had already satisfied two of its three mission objectives. The run out to the Oort cloud was the final field test of the technology critical to the interstellar vessel currently under construction in high Earth orbit. Long-duration antimatter containment and propulsion was a proven reality, and deep-space, self-contained life support a proven technology.

The ship's second objective had been to sample several primordial Oort cloud objects, comets yet to be born. It was science's first chance to study truly pristine material from the formation of the solar system and an excellent trial run for the remote-sensing and physical investigation procedures that would be integral to the interstellar ship's research.

The ship closed on its final objective.

Two service modules jetted out from the ship's air lock. Ever so carefully, mindful of four billion pairs of watching eyes back on Earth, they matched velocities with a vaguely egg-shaped module of antique design, human sized, encrusted with insectile appendages, ports, windows, and cameras. The main port was cracked. Crushed storage lockers and canisters surrounded the base of the egg.

There was a small hole in the egg's shell.

The two modern modules linked to the antique's grappling rings. Ever so gently, they shepherded it into the ship's air lock.

The lock closed.

The ship rotated until its nose pointed toward the sun.

Antimatter engines flared, immeasurably brighter than the distant pinpoint sun. In two years, the crew would be back on Earth, accompanied by Dr. Rebecca Johansson, the first voyager and the first casualty of the interstellar age, who was finally returning home.

AUTHORS' NOTE: THE SCIENCE BEHIND THE STORY.

Dear Reader:.

DON'T read this until you've read the novel, because you'll get a whole bunch of spoilers. Some people are fine with that. We know people who read the ends of mysteries first so they can find out whodunit and then enjoy the run-up. We're just warning you.

The science fiction author Greg Benford talks about "wantum mechanics." It's the totally made-up non-science that saves the crew in the last dozen minutes of a bad Star Trek episode. "Captain, if we invert the polarity of the phasers and couple them to the warp drive, we can produce a beam of the never-before-heard-of unbelievablon particles and render the enemy's fleet helpless."

That's one kind of thrill ride, and it's fun. But we wanted to write the kind of high-tech, hard-science thriller where you can't just make up stuff to solve your problem-where you have to deal with the real lemons that life hands you, to make your lemonade.

Such a problem is right where we started. One of us (John) had this idea for a novel. To give the story the right pacing, it needed spaceship technology that wouldn't take decades to build and could get to Saturn in less than six months. Even setting the story five decades from now, he didn't know how to do that without just making stuff up-wantum mechanics. So he reached out to the other of us and said, "Ctein, can you figure out how to make this work, because if you can, we might have ourselves a novel."

Cut to the finale. He did, and we did, and you just read it.

- Here's some of the science behind the story:.

The Big Problem is that space travel is hard. "Rocket science" became synonymous with "really hard" for good reasons. Getting anywhere fast is really, really hard. We couldn't come up with any way to meet the timetable we wanted with present-day technology, so the story is set half a century from now.

It is, in fact (well, in fiction) a fairly boring half century. For the sake of our story we decided that space travel won't make much more progress in the next four or five decades than it has in the last four or five. Science fiction is a game of what-if, not accurately predicting the future.

Still, if you'd told someone back in 1969, at the time of the first moon landing, that nearly half a century later humans wouldn't be doing anything outside of low Earth orbit, not even going back to the moon, they'd have thought you were crazy. It certainly wasn't what your typical science fiction author imagined for the next fifty years. Depressing as the thought is, our scenario may not be as implausible as we'd like to believe.

With fifty years' worth of steady and predictable technological advancements, we could pull off the science. That still doesn't make space travel easy. Space travel's hard because you need high velocities to get anywhere fast, and it's really hard to get high velocities. It takes appalling amounts of energy.

Typical solar system travel times are usually measured in years. The simplest low-velocity, long-duration trip from Earth to Saturn takes about seven years. It's called a "Hohmann transfer" and you can read about it in Wikipedia. That's way too slow for our story. Even then, it takes about as much additional velocity-seven kilometers per second (km/s)-to get yourself from high Earth orbit onto a trajectory that reaches Saturn, as it does to get into Earth orbit in the first place.

Once you get to Saturn, you'll need more delta-vee (rocket scientist shorthand for the change in velocity that you're making) to kill some of your initial velocity, so you'll put yourself in orbit about Saturn instead of flying on past. Then you'll need similar amounts of delta-vee to get you home again, and back into Earth orbit. That's why almost all the robotic probes we've sent out have been one-way missions; returning home means you need a lot more delta-vee at your disposal.

You might be thinking, well what's so tough about that? If it takes a total of twice as much velocity to get you to Saturn as it does to get into Earth orbit, just make the rocket twice as big. Okay, maybe three times as big to account for getting into orbit around Saturn. And the same amount to get you back again. That doesn't seem that hard.

Unfortunately, that's not how it works. Now we're into proper rocket science, something called the "Tsiolkovsky rocket equation." Don't worry, no math here; you can get that from Wikipedia. The rocket equation ties together three things: the amount of delta-vee you want, the exhaust velocity of your rocket, and the mass ratio of your rocket.

What's "mass ratio"? That's just the ratio of what your rocket weighs fully loaded with reaction mass, divided by what it weighs when you've used up all that mass. That empty (or "dry") weight is everything that isn't fuel; it includes the empty tanks that held the fuel.

Exhaust velocity is the magic number. As long as the total delta-vee you want is less than your exhaust velocity, the amount of reaction mass you need isn't too bad. For example, a rocket that burns oxygen and hydrogen, one of the best chemical fuels you can use, has an exhaust velocity around 4 km/s. If you want to get a delta-vee of 2 km/s, the rocket equation says you need a mass ratio of about 1.7. That means you need to carry 0.7 tons of fuel for every ton of dry rocket you're trying to launch. If you want a delta-vee of 4 km/s, the ratio goes up to 2.7-1.7 tons of fuel for every ton of dry rocket. That's not hard to build.

If you want more velocity than that, it starts to get ugly quickly. Suppose you want a delta-vee of 8 km/s, enough to get you into Earth orbit? (In reality, it's a little harder than that, but we're simplifying for the sake of discussion.) You can think of that as being like getting 4 km/s twice. But, for that first 4 km/s, you're trying to push a rocket that is 2.7 times bigger, because it has to be carrying all that fuel to get the second 4 km/s. Your mass ratio winds up about 7.5. Only 13 percent of your ship is actually ship; 87 percent is fuel that you burn up.

It's awfully hard to build a rocket strong enough to survive flight that is 87 percent fuel. Tanks can only be made so lightweight, and there has to be a useful payload, like people or instruments. It's right on the edge of what our engineering is capable of.

Or a little beyond. No one has yet built a successful rocket that just launches from the earth straight into orbit (what's called "single stage to orbit"). Everything we build has stages, so we can throw away the really big and heavy fuel tanks as they get emptied.

In space, where you're not fighting gravity and you can use lower accelerations, you can build a lighter vehicle. Mass ratios of 10 or better are possible. But you've seen how the numbers multiply up. Even starting from Earth orbit, going to Saturn on a Hohmann transfer, entering Saturn orbit, leaving Saturn orbit, and returning to Earth? With our hydrogen-oxygen rocket, it's simply impossible. You're talking about mass ratios way over 100.

Besides, that's not fast enough! We need much, much higher velocities. We've got to stay within the laws of physics, so what can we change? The exhaust velocity. The higher that velocity, the faster our ship can go (for some particular mass ratio).

In our book, the Chinese are using tried-and-true technology, at least for fifty years from now. Nuclear thermal rockets can get much higher exhaust velocities than chemical rockets (see NERVA, Wikipedia). The Celestial Odyssey uses an exotic reactor design called a "lightbulb reactor" that no one's built yet but that engineers have designs for. If we needed them, we could have them in fifteen or twenty years. The Chinese ship pushes that technology to the limits of what engineers think we can do. It heats the exhaust to around 9000C and it uses hydrogen for the reaction mass. It could use any gas as the reaction mass, because it's just heating it up in a reactor, not trying to burn it. Hydrogen is best because lighter atoms move faster than heavier ones at the same temperature, and hydrogen is as light as we can get. That gets the Chinese the highest exhaust velocity, 22 km/s, which is five times better than burning hydrogen and oxygen.

With that kind of exhaust velocity and a mass ratio of 7 or so, the Chinese can get from Earth to Saturn in around a year and a half. (How do we know that? Later.) It doesn't get them back, but Saturn's rings are water ice and the Chinese can break that down to get the hydrogen to refill their tanks.

Could such a spacecraft make it to Saturn in half a year? Not likely. They'd need nearly three times the delta-vee, and the mass ratios would be hundreds to one. The Americans need something better. Enter the VASIMR engines. VASIMR stands for "Variable Specific Impulse Magnetoplasma Rocket." "Specific impulse" is how rocket scientists refer to exhaust velocity. We didn't make the VASIMR up. They're being tested on Earth, fairly small ones. Ours are a lot bigger, and a little better-performing, but it's fifty years from now. Building bigger and better VASIMRs doesn't look hard; powering them does, and we'll get back to that.

Becca's Science Friday interview in Chapter 21 explains why variable exhaust velocity is, in general, a good idea. The Nixon can get more thrust for the same amount of power, when the ship is fully laden, by keeping the exhaust velocity low. That consumes more reaction mass for the same delta-vee, but it gets the ship up to speed faster, shortening the trip time. As the ship gets lighter, it can get by with less thrust and still keep up the acceleration, so the Americans can run the exhaust velocity higher and make more efficient use of the remaining reaction mass.

We built a spreadsheet to let us play around with different velocity profiles. For a trip time of four to five months, we were able to get the ship down to a mass ratio of 10 with an exhaust velocity that varied from 35 up to 300 km/s. That's about half the mass ratio we could come up with for a fixed-specific-impulse ship of any remotely plausible design. Go, VASIMRs!

The reason the Americans' ship uses water instead of straight hydrogen is because it doesn't need to use straight hydrogen. The exhaust velocity that comes out of a VASIMR depends on the charge on the ion and its mass. Hydrogen produces the highest exhaust velocity. Strip off one electron and you're left with a charge of one and an atomic weight of one. Strip one electron off of oxygen and you've got a charge of one but an atomic weight of 16, so the electromagnetic fields in the VASIMR won't push it anywhere as fast. One of the ways the Nixon has to tailor its exhaust velocity is to tailor the mix of oxygen to hydrogen.

So how can the VASIMR keep up with a "lightbulb" ship? The lightbulb gets its initial velocity from one long burn-the rest of the trip it's in free fall, until the very end, when another burn will slow it down so it can go into orbit around Saturn. With a VASIMR, you simply don't turn it off. You're making a much more economical use of your reaction mass, and the accumulating thrust eventually adds up to much more velocity than is possible with a lightbulb.

VASIMRs have a problem, though. They're powered by electricity, lots of it. The only way we know of to generate so much power is a nuclear power plant.

Surprisingly, the reactor isn't the problem. Reactor cores can generate amazing amounts of thermal power. You have to get that heat out or the core melts down, but NASA figured out how to build a liquid-lithium-cooled core the size of a coffee can that would output 2.5 megawatts back in the 1970s. That's already as good, in terms of both watts per kilogram and watts per cubic centimeter, as what the Nixon needs. The two reactors in the Nixon are each four thousand times bigger . . . but they are not better.

The huge problem the Nixon faces is that only a little more than half of that heat can get converted into electricity that goes into thrust and is kicked out the back in the VASIMR exhaust. There are some fundamental thermodynamic principles that make it unlikely we'll ever be able to do much better than that. The rest of the heat, nine gigawatts or so, ends up being waste heat and has to be disposed of before everything melts down. That is the really, really hard problem in space.

There are only four ways to move heat around: convection, conduction, transport, or radiation. The first two can't get the heat off of the ship-in a vacuum, there's nothing to conduct heat away from the ship, nor is there gas circulation to convect it. You could transport it off, use it to heat up reaction mass and send it out a rocket nozzle. But that's just a nuclear thermal rocket like the Chinese have, and it's nowhere near efficient enough for the Nixon. It also requires infeasibly humongous amounts of heat-absorbing reaction mass; nine gigawatts is an awful lot of heat to be dumping off the ship, day in and day out for months and months.

We're left with radiation. Simple physics describes getting rid of heat by radiation (see "blackbody radiation," Wikipedia). At 80F (27C or 300 Kelvins) a two-sided radiator can dump about a kilowatt of heat (1 kW) per square meter into space. That's a lot by human standards, but it means the Nixon would need about nine square kilometers of radiator to get rid of all its waste heat-roughly the area of 1,700 American football fields. That would weigh far too much. Physics works in our favor, though. The amount of power you can radiate goes up as the fourth power of the Kelvin temperature. At 600C (870K) that same square meter of radiator can dispose of 65 kW of heat. That takes the radiator down to a manageable size.

This requires running the whole power plant hotter, because the theoretical efficiency of the power plant is determined by the starting temperature vs. the final temperature (see "Carnot," Wikipedia). We've kicked the final temperature up nearly threefold, so the source temperature has to go up accordingly. That takes us out of the range of boiling water reactors and generators and into the world of pressurized liquid sodium, running at a red heat.

Is this insane? Definitely, by today's standards. We looked at power plant performance and benchmarks for the past century-plus, and extrapolated those trend lines forward fifty years. In half a century it doesn't seem so crazy.

What kind of radiator can run at 600C? The only possible working fluid is molten metal, and that also works in our favor. Melting metals can absorb huge amounts of energy, far more than any other material. Aluminum, for example, melts a little higher than our working temperature, and melting 2.5 kilograms of it per second will suck up a megawatt of heat. Our imagined radiator alloy is a little better than pure aluminum, but not a whole lot. It's all plausible; we're still doing science and engineering, not wild imagining.

Some simple algebra and punching numbers on a four-function calculator tells us the size for the radiator sail, the speed of the sail ribbons, and the thickness of the ribbons. They are interrelated. The speed of the ribbons has to allow enough time for the semi-molten ribbons to completely solidify before they reach the collection beams. The amount of ribbon being extruded each second has to be sufficient to carry all the waste heat from the Nixon.

A minor bit of cleverness. That thin layer of solid cladding that's frozen onto the ribbon as it's extruded does three things for us. It roughens the surface, which improves the radiation characteristics (see emissivity). It keeps the ribbon a ribbon-molten metal has very fierce surface tension, and a thin liquid ribbon would otherwise break up into droplets almost immediately. Finally, it prevents evaporation of the metal. Typical metal vapor pressures are very low at 600C, but you'd be talking about over a quarter kilometer of surface area and a year's time. You'd have to carry along considerable amounts of spare radiator metal to make up for evaporative losses. In space, it's waste not, want not.

So this is all pretty great: we have two ships that will fly more or less the way we need them to. What will those flights look like? That's orbital mechanics, and while the physics of that is high schoollevel Newton's laws stuff, the calculations are lengthy. They would've been impossible without personal computers.

We got lucky on one point. The starting date for the book was an arbitrary choice and the pacing up to the point where the ships launch was dictated by what would keep the story moving properly (and launch windows-the right alignment of the earth, sun, and Saturn). As it happened, we wound up in the easiest time frame for calculating orbits. None of the other planets came anywhere close to possible orbits for our spacecraft. All we had to deal with was the earth, sun, and Saturn.

We did the orbit simulations in a Windows program called Orbiter. That wound up consuming hundreds of hours of computer and personal time. Continuous-boost spacecraft like the Nixon are still exotic enough concepts that very few software packages can deal with them easily. Those programs are expensive and specialized, their normal clientele being major aerospace companies and research institutions.

Orbiter could very accurately calculate the motion of the spacecraft for each step along a trajectory, but those steps had to be carefully and manually specified. When the Celestial Odyssey was falling free, far from the gravitational influence of the earth or Saturn, a step could span months of simulated time (on the computer, that ran in minutes). We just needed the steps to be close enough together that we'd know where the Chinese were at any particular time in case that needed to be mentioned in the story. Entering or leaving orbit involved a lot more steps because the ship's trajectory was changing very quickly.

Simulating the Nixon was much harder. Except when Bad Things happened to it, it was never in free fall. The simulation steps had to be close enough together to accurately model the trajectory of a spacecraft that was always accelerating or decelerating. Spiraling out from Earth orbit took about fifty steps to simulate, similarly for the transit from Earth to Saturn. At each step, the thrust parameters were keyed in, the time interval specified, the step calculated, and the data-velocities and positions relative to the earth, sun, and Saturn-extracted and transferred to a spreadsheet. That spreadsheet also produced a plot for us showing the spacecraft's trajectory against the earth, sun, and Saturn. A different spreadsheet tracked energy budget-reaction-mass consumption, mass ratios, exhaust velocities, and accelerations.

We ran nearly a hundred such simulations as we developed the story. Some of those runs were dictated by plot changes as we developed the novel, but most were experiments designed to answer questions. How does the Nixon's departure date affect the travel time? How long can the Chinese wait before chasing after the Nixon? What's the best way to get back to Saturn? What's the most efficient trajectory for a return to Earth?

Real rocket science. We think we did it pretty well.

It's not what's important, though, not really. You enjoying the story we told you, that's what matters.

Now it's time for a confession. There is one chunk of wantum mechanics in Saturn Run. We don't have the faintest clue how to make antimatter efficiently and in quantity. Antimatter's real, but physicists only know how to make it positron by positron, antiproton by antiproton. The price is currently many trillions of dollars per gram, and all the antimatter ever made amounts to a microscopic fraction of a gram. A way to make tons of it, in solid iron form? We totally faked that out of plausibly appropriate physics jargon. That shouldn't be a big surprise. If we knew how to do this, the Powers That Be wouldn't let us write about it freely. They probably wouldn't even let us walk around freely.

But the rest of it? Real science and real technology, extrapolated as realistically as we could.

(Oh, all right, we made up the aliens, too. But we didn't give them a magic star drive, okay?).

One last thing: Why did we call it the Nixon? Because we thought it was funny.

-JOHN SANDFORD AND CTEIN.

ALSO BY JOHN SANDFORD.

Rules of Prey.