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.