Protector

by Larry Niven

  “The Pak fleet is a hundred and seventy-three years away,” he pointed out now. “We’ll be at Wunderland in five years. What makes you think you need a gunner? What am I doing here, anyway?”

  Brennan took a handhold at the rim of a fusion bomb’s rocket nozzle. “You could say I’ve learned some humility. I thought of looking for a Pak fleet, long ago, but I didn’t. The probability was just too low. Well, I’ve stopped taking chances.”

  “What chances? We know where the Pak fleet is.”

  “I didn’t want to worry you. It’s a long shot.”

  “Worry me! I’m bored!”

  “All right, let’s go back a bit,” said Brennan. “We know where the first fleet is, and how big it is. The second fleet wasn’t launched for another three-hundred-odd years. All I’ve found of it is a patchy source of those same chemical exhausts, off center to the first fleet and moving a bit faster. They wouldn’t follow directly behind the first fleet. It’d be eating up too much of their fuel.”

  “How big?”

  “Smaller. Order of a hundred and fifty ships, assuming they didn’t change the design, which they may have. I can’t tell.”

  “Is there a third fleet?”

  “If there is, I’ll never detect it. They had to go out for new resources to build the second fleet. They may have had to mine worlds in nearby systems and build the ships there. How long would it take them to build a third fleet? If it’s there, it’s too far away for me. But the point is that there had to be a last fleet.”

  “So what?”

  “I’m suggesting that when the last fleet left—the second or the third or the fourth, it doesn’t matter—some protectors stayed behind. We assume they were the ones without breeder descendants. They stayed behind partly to save room on the ships, and partly because they might do some good on Pak.”

  “On an empty world? How?”

  “They could build a scout fleet.”

  It was not the first time Roy had worried about Brennan’s sanity. The changes in his physiology, plus twenty-two decades alone…but if Brennan were insane, he might be too bright to give it away.

  Gently Roy pointed out, “But your scout fleet would be at least five hundred years behind the rest.”

  “Sounds silly, does it? But they’re free to experiment. They don’t have to use a proven design, because they’re only risking themselves. They don’t need a cargo pod. They could take three gravities forever, I think; I know I could. That cuts down on their supply weight, because the trip takes less time. With the breeders gone they can do all kinds of things…like making new metal mines by setting up eruptions in the crust of Pak.”

  “You’ve got quite an imagination.”

  “Thank you. What I’m getting at is that they could plan to pass the first wave of refugee ships about where the Pak telescopes aren’t good enough to scout the territory any further. From there on they lead the fleet. Still bored?”

  “No. You’re daydreaming, though. They might never have built these hypothetical ships. Whatever sent them scurrying out of the galactic core might have caught the scouts.”

  “Hell, it could have caught the third wave and brushed the second. Or the scout ships might have blown up. Or—lest you miss the point of all this—they could be arriving now.”

  “You haven’t found them?”

  “What, with a whole sky to search? They wouldn’t just come down our throats; they’d converge on Sol from random directions. I would, if I were doing it. Remember what they’re expecting to find: a world of Pak protectors running a civilization two hundred years old. That’s enough time to build up a virgin world, starting with a population of…oh, thirty million breeders of all ages would have given Phssthpok about three million newly changed protectors. The scouts wouldn’t want to give away the position of their fleet.”

  “Uh huh.”

  “There is something I can do, but it’ll take a few days of work to make the tools. First I’m going to make sure you can fight this ship. Let’s go back to the lifesystem pod.”

  A directed magnetic field would churn the interstellar plasma as it was guided into a Bussard ramjet. As a weapon it might be made to guide the plasma flow across the ship itself. The gunner would have to vary his shots, or an enemy pilot could compensate for the weapon’s effect. If the local hydrogen density were uneven, that would hurt him. If the plasma were dense enough locally, the enemy could not even turn off his drive without being cremated. Part of the purpose of the ram fields was to shield the ship from the gamma ray particles it was burning for fuel.

  “Hit him near a star, if you get the choice,” said Brennan. “And don’t let him do that to you.”

  The laser was surer death, if it hit a ship. But an enemy ship would be at least light-seconds away at the start of a battle. It would make a small, elusive target, its image delayed seconds or minutes. The thousand mile wings of a ram field would be easier to hit.

  The guided bombs were many and varied. Some were simple fusion bombs. Others would throw bursts of hot plasma through a ram field, or carbon vapor to produce sudden surges in the burn rate, or half a ton of pressurized radon gas in a stasis field. Simple death or complicated. Some were mere decoys, silvered balloons.

  Roy learned.

  The wreck of Kobold was almost three months behind them, and Roy was at war. Lately he had come to enjoy these simulated battles, but he wasn’t enjoying this one. Brennan was throwing everything at him. The Pak scouts had used a three gee drive until they crossed his wake, and then Wham! Six gees and closing. Some of his missiles were going wild; the scouts were doing something to the guidance. The pair dodged his laser with such ease that he’d turned the damn thing off. They’d used lasers on him, firing not only at his ship but at the field constriction behind him where hydrogen atoms met and fused, so that Protector surged unevenly and he had to worry for the generator mountings. They threw bombs at unreasonable velocities, probably through a linear accelerator. He had to dodge in slow random curves. Protector was not what you’d call maneuverable.

  Three days he’d been in the lifesystem module, eating and drinking there and using pep pills instead of sleep. Playing Brennan’s game. He was mad clean through. Within ships he could infer only from instruments, he imagined hard faces like Brennan’s.

  Two scouts closing from behind, and finally he hit one with the directed magnetic field and watched its ram field flare and dissipate.

  That was when he realized that there were two pairs of ships in tandem. Damn Brennan anyway! He’d hit a lead ship, but the trailing ship was still there…and slowing. Somehow the loss of the lead ship had slowed it. Roy concentrated on the second team, which was still closing.

  He tried a turn. Two ships linked should be less maneuverable than one…and an hour later he knew that they were. He’d turned only a fraction of a minute of arc, but they had turned less. He could keep up his dodging and still turn inside them.

  He tried some of his weaponry on the lone ship behind him.

  Then half his weapons board was red, and he had to guess what had exploded in the trailing pod. Probably that idiot projector: he’d been trying to punch a hole in the lone ship’s ram field. He bet his ship he was right, and gambled further that the explosion had wrecked his laser, which might otherwise have been of some use. He fired a flurry of bombs from the side of the cargo pod opposite the explosion. The lead ship of the remaining pair flared and died.

  That left two, each the trailing ship of a pair, making less than his own acceleration. He dithered a bit, then ran for it. He continued to dodge missiles and laser beams.

  The scouts fell away. He watched them dwindling…and then one wasn’t dwindling…and it finally dawned on him that that one had picked up acceleration somehow and was coming up from behind at something like eight gees.

  Roy’s first impulse was to scream, “Brennan! What are you trying to pull?”

  He’d done that before. This time he restrained it. Because he’d guessed the an
swer: the second ship was burning Protector’s own exhaust! Never mind how: that was it, that was why they moved in tandem.

  He dropped two half-tons of radon with the drives disconnected.

  Radon has a short half-life: it has to be kept in stasis. The generator was outside the bomb shell, and was partly soft iron. The enemy’s ram field tore it apart. A minute later the radon was in the constriction, and incredible things were happening: radon fusing to transuranian elements, then fissioning immediately. The constriction exploded. The ram field sparkled like a department store Xmas tree gone manic. The Pak ship flared into a small white point, fading.

  The last Pak ship was far behind.

  Coming out of it was a slow process. Roy had to keep telling himself: this isn’t real, this is only pretend. He jumped violently when Brennan’s alien head poked through the twing.

  Then he shouted, “What the hell was that about, him burning my exhaust?”

  “I just knew you’d bring that up,” said Brennan. “I’ll tell you in detail, but first let’s talk about the battle.”

  “Screw the battle!”

  “You did well,” said Brennan. “There isn’t much left of your weapons pod, but that’s okay if you don’t meet any more scouts. You don’t have reserve fuel to get into orbit around Wunderland; you used too much. But you can abandon Protector and land with the cargo ship.”

  “That’s nice. That’s very reassuring. Now tell me how a Pak scout can burn my own exhaust and come tearing up my tailpipe!”

  “It’s one possible configuration. In fact, it’s the one I’m about to start looking for, because it’d be easy to find. I can show you better with diagrams.”

  Roy had calmed down a bit when they reached the Flying Dutchman’s control room. He had also started to shake. Three days in Protector’s control chair had left him exhausted.

  Brennan looked at him thoughtfully. “Want to put this off?”

  “No.”

  “Okay, I’ll make it quick. Let’s look at what your ram field does. It picks up interstellar hydrogen in a path three thousand miles across. It sweeps it in via magnetic fields, pinches it together hard enough and long enough to produce some fusion. What comes out is helium and some leftover hydrogen and some higher-order fusion products.”

  “Right.”

  “It’s also a hot, fairly tight stream. Eventually it’ll spread out into nothing, like any rocket exhaust. But suppose a ship were following you, here.” Brennan made pictures on the screen: two tiny ships, the second following a hundred miles behind the first. He spread a wide cone before the lead ship, converging it almost to a point behind the ship. A needle shape with the ship in its point—the ship’s protective shield—brought the incoming hydrogen into a ring shaped constriction.

  “You’re collecting the fuel for him. His ram field is only a hundred miles across—” Brennan drew a much narrower cone. “—and it gives him finer control over his fuel flow. It’s already hot and dense. It burns better, in higher-order fusion. The exhaust would be rich in beryllium.

  “It’s just one of the things those last remaining Pak might have tried. The lead ship would be nothing but a ram: no onboard fuel, no insystem motor, no cargo. It would have to be towed up to ramscoop speed. The following ship is heavier, but it gets more thrust.”

  “You think that’s what’s coming at us?”

  “Maybe. There are other ways to work it. Two ships, independent, held together by a gravity generator. In a pinch they could split up. Or the lead ship might be the ship proper, with the hind ship only an afterburner. Either way, I can find them. They’ll produce beryllium frequencies like a neon sign on the sky. All I’ve got to do is build the detector.”

  “Need help?”

  “Eventually. Go to sleep. We’ll try another dry run in a month or so.”

  Roy stopped in the doorway. “That long?”

  “Just to keep you on your toes. You’re as ready as you’ll ever be. Only, be more careful with that electromagnetic projector. When you wake up I’ll show you what the Pak scouts did to it.”

  “What you did to it.”

  “What they would have done. Go to sleep.”

  Brennan was in the machine shop for three days. If he slept he slept there. He skipped meals there. Whatever he was doing filled the machine shop with constant racket and sent a humming vibration through the rock of the Flying Dutchman.

  Roy read a couple of old novels stored in the computer. He floated through bare rock caverns and corridors, and was oppressed by the sensation of being underground. He worked himself to exhaustion in the exercise room. Free fall had cost him some muscle tone. Have to do something about that.

  He researched Wunderland and found about what he expected. Gee: 61%. Population: 1,024,000. Colonized area: 3,000,000 square miles. Largest town: München, population: 800. Farewell, city life. Come to that, München would probably look like New York to him by the time he got there.

  There was a time on the fourth day when he found the machine shop quiet and Brennan apparently asleep. He was about to leave when Brennan opened his eyes and started talking.

  “You depend too much on those long, slow turns,” he said. “The way to dodge Pak weaponry is to vary your thrust. Keep opening and closing the constriction in the ram field. When they throw something like a laser pulse into the constriction, open it. Nothing’s going to fuse if you don’t squeeze the plasma tight enough.”

  Roy wasn’t flustered. He was getting used to Brennan’s habit of resuming a subject that may have been broken off days ago. He said, “That last ship could have done that when I threw radon at him.”

  “Sure, if he did it fast enough. At good ramscoop velocities the shit should be in the constriction before he knows it’s reached the ram field, especially as you didn’t put any rocket thrust on it. That was good thinking, Roy. Memo for you: don’t ever follow a ship that’s running. There are too many things he can throw into your ram field. Hopefully we’ll be doing the running in any battle.”

  Roy remembered what he had come for. “You’re two days past dinnertime. I thought I’d—”

  “Not hungry. My prism’s in the oven, and I’ve got to wait for it to cool.”

  “I could bring—”

  “No thanks.”

  “Any significance?”

  “Didn’t I tell you I was predictable? If there aren’t any Pak scouts in the vicinity, you could just as well go on to Wunderland alone. Most of what I know about the Pak is stored in the computer. When a protector feels not needed, he doesn’t eat.”

  “So you’re kind of hoping we find Pak scouts.”

  Brennan laughed: a credible chuckle, though his mouth didn’t move. His face wasn’t hard, exactly; it was like wrinkled leather. It was his mouth that was like hard shell. Too much of human expression is in the mouth.

  On the evening of the same day he came out towing three hundred pounds of machinery, of which a big, solid crystal prism was a prominent part. He wouldn’t let Roy help tow it, but they set it up together at the focus of the Flying Dutchman’s telescope. Roy brought him a sandwich then, and made him eat it. The Jewish mother role irritated him, but so did the thought of going on to Wunderland alone.

  Brennan was gone when Roy came looking for him, around mid-afternoon of the fifth day. Roy found him in the one room from which he was forbidden, the hydroponics garden. Brennan was moving down the side of an open tank, consuming sweet potatoes one after another.

  The prism threw a rainbow spectrum across a white surface. Brennan pointed to a bright green line. “Beryllium light, blue-shifted,” he said. “And the helium lines are up in the violet. Ordinarily beryllium is in the infrared.”

  “Blue-shifted.” Any school child knew what that meant. “He’s coming down our throats.”

  “Maybe not. He’s coming toward us, but maybe not dead on. We’re only a couple of light-weeks out from Sol, and he’s a light-year away, and I think he’s decelerating. I’ll have to check to see if were ge
tting his exhaust. But I think he’s headed for Sol.”

  “Brennan, that’s worse.”

  “It’s just as bad as it can get. We’ll know in a month. He’ll have moved by then. We’ll have some parallax on him.”

  “A month! But—”

  “Just a minute. Calm down. How far can be go in a month? He’s way below lightspeed; we’re probably going faster than he is. A month won’t cost us much—and I’ve got to know how many there are, and where they are, and where they’re going. And I’ve got to build something.”

  “What?”

  “A widget. Something I dreamed up after we found the Pak fleet, when I saw that there might be Pak scouts around. The designs are in the computer.”

  Roy did not fear loneliness. He feared its opposite. Brennan was an odd companion, and Protector was going to be cramped when they finally left the Flying Dutchman. For a week or so Roy stayed away from the observatory, consciously savoring his alone-ness. In the empty exercise room he hovered in midair, swinging his arms and legs in wide circles. Later he would want to remember the room. Even this half-hollowed ball of rock was too small for a man who would rather be climbing a mountain.

  Once he suggested another dry run. Brennan’s models of the Pak scouts would be more accurate now. But Brennan wasn’t having any. “You know as much as you’re ever going to about fighting Pak. Does that scare you?”

  “Hell, yes.”

  “Glad to bear it.”

  One day Brennan wasn’t in the laboratory. Roy went looking for him. The longer it took the more stubborn he got; but Brennan didn’t seem to be anywhere aboard.

  He finally asked himself, “How would Brennan handle this? Logic. If he’s not inside, then he’s outside. What’s outside that he might need?”

  Right. Vacuum, and access to the surface.

  The tree, the grass, the mud of the pond bottom were all freeze-dried and dead. The stars were bright and eerie, and more real than they had seemed on a vision screen. Roy could see them as a battlefield: the unseen worlds as territories to be fought over, the gas shells around stars as death traps for an unwary warrior.