The Sentinel

by Arthur C. Clarke

  He was still trying to think of something to say when the PRIORITY signal started flashing on the radio board. The stowaway looked at his watch.

  “I was expecting that,” he said. “It’s probably the Prime Minister. I think I’d better speak to the poor man.”

  Saunders thought so too.

  “Very well, Your Royal Highness,” he said sulkily, and with such emphasis that the title sounded almost like an insult. Then, feeling much put upon, he retired into a corner.

  It was the Prime Minister all right, and he sounded very upset. Several times he used the phrase “your duty to your people” and once there was a distinct catch in his throat as he said something about “devotion of your subjects to the Crown.” Saunders realized, with some surprise, that he really meant it.

  While this emotional harangue was in progress, Mitchell leaned over to Saunders and whispered in his ear:

  “The old boy’s on a sticky wicket, and he knows it. The people will be behind the prince when they hear what’s happened. Everybody knows he’s been trying to get into space for years.”

  “I wish he hadn’t chosen my ship,” said Saunders. “And I’m not sure that this doesn’t count as mutiny.”

  “The heck it does. Mark my words—when this is all over you’ll be the only Texan to have the Order of the Garter. Won’t that be nice for you?”

  “Shush!” said Chambers. The prince was speaking, his words winging back across the abyss that now sundered him from the island he would one day rule.

  “I am sorry, Mr. Prime Minister,” he said, “if I’ve caused you any alarm. I will return as soon as it is convenient. Someone has to do everything for the first time, and I felt the moment had come for a member of my family to leave Earth. It will be a valuable part of my education, and will make me more fitted to carry out my duty. Goodbye.”

  He dropped the microphone and walked over to the observation window—the only spaceward-looking port on the entire ship. Saunders watched him standing there, proud and lonely—but contented now. And as he saw the prince staring out at the stars which he had at last attained, all his annoyance and indignation slowly evaporated.

  No one spoke for a long time. Then Prince Henry tore his gaze away from the blinding splendor beyond the port, looked at Captain Saunders, and smiled.

  “Where’s the galley, Captain?” he asked. “I may be out of practice, but when I used to go scouting I was the best cook in my patrol.”

  Saunders slowly relaxed, then smiled back. The tension seemed to lift from the control room. Mars was still a long way off, but he knew now that this wasn’t going to be such a bad trip after all . . .

  THE WIND

  FROM THE SUN

  “The Wind From the Sun” was written just twenty years ago, but is far more topical now than it was in 1963. I have in front of me at the moment a folder full of technical papers assembled by the World Space Foundation in support of its Solar Sail Project—conducted in cooperation with the University of Utah, the Jet Propulsion Laboratory, and the Radio Amateur Satellite Corporation, with the assistance of the Charles A. Lindbergh Fund.

  Let me quote from the Foundation’s leaflet, so that you will better appreciate the background of the story that follows:

  “In 1924, Fridrikh Tsander, perhaps as a result of suggestion by Konstantin Tsiolkovsky, noted that in the vacuum of space, a large thin sheet of reflective material illuminated by the Sun and controlled in orientation could be used as a propulsion device requiring no propellant. This propulsion device is now called a solar sail. In 1973, NASA sponsored a design study which led to a full-scale evaluation of solar sailing for the proposed Halley’s Comet rendezvous mission. Plans for this mission were suspended in 1977, but not before solar sailing had received a thorough technical review confirming its feasibility and unique advantages.”

  The World Space Foundation hopes to launch a small solar-sailer, either from the U.S. Shuttle or the European Space Association “Ariane” rocket, in connection with Vancouver’s EXPO ’86. Anyone wishing to support this project can contact the WSF at P.O. Box Y, South Pasadena, Calif. 91030.

  There is also an enthusiastic French group (U3P—Union pour la Promotion de la Promotion Photonique, 6 rue des Ramparts Coligny, Venerque 31120, Portet-sur-Garonne) planning a solar race around the Moon, hopefully by 1985–6. (Unmanned, of course—again the ESA Ariane would be used as a launcher.)

  And a few months ago I received a fascinating letter from Dr. V. Beletsky, of the Keldysh Institute of Applied Mathematics, Moscow, enclosing his book Essays on the Motions of Space Bodies. One whole chapter is devoted to an analysis of “The Wind From the Sun,” with a detailed integration of the trajectories of “Diana” and “Sunbeam.” To my pleased surprise, Dr. Beletsky wrote: “The data mentioned in your story has proved to be quite sufficient to integrate the differential equations of yacht motions. Integration results almost completely agree with situation in your story!! Have you also integrated the equations of yachts’ motions? If not, why such close agreement of such unobvious details. If yes, why is such important characteristic as the total flight time not in agreement?—2 days in your story and 5 in mine. . . .”

  I had to confess that any agreement must have been more luck than integration. Though I had done some back-of-the-envelope calculations to make sure that the velocities and accelerations were not ridiculous, I had certainly not computed the orbit in any detail.

  Incidentally, the story’s original title, under which it first appeared in Boy’s Life (March 1964) was the rather obvious “Sunjammer.” However, Poul Anderson had the same idea almost simultaneously, so I had to make a quick change of name. . . .

  THE ENORMOUS DISC OF SAIL strained at its rigging, already filled with the wind that blew between the worlds. In three minutes the race would begin, yet now John Merton felt more relaxed, more at peace, than at any time for the past year. Whatever happened when the Commodore gave the starting signal, whether Diana carried him to victory or defeat, he had achieved his ambition. After a lifetime spent designing ships for others, now he would sail his own.

  “T minus two minutes,” said the cabin radio. “Please confirm your readiness.”

  One by one, the other skippers answered. Merton recognized all the voices—some tense, some calm—for they were the voices of his friends and rivals. On the four inhabited worlds, there were scarcely twenty men who could sail a sun yacht; and they were all there, on the starting line or aboard the escort vessels, orbiting twenty-two thousand miles above the equator.

  “Number One—Gossamer—ready to go.”

  “Number Two—Santa Maria—all O.K.”

  “Number Three—Sunbeam—O.K.”

  “Number Four—Woomera—all systems GO.”

  Merton smiled at that last echo from the early, primitive days of astronautics. But it had become part of the tradition of space; and there were times when a man needed to evoke the shades of those who had gone before him to the stars.

  “Number Five—Lebedev—we’re ready.”

  “Number Six—Arachne—O.K.”

  Now it was his turn, at the end of the line; strange to think that the words he was speaking in this tiny cabin were being heard by at least five billion people.

  “Number Seven—Diana—ready to start.”

  “One through Seven acknowledged,” answered that impersonal voice from the judge’s launch. “Now T minus one minute.”

  Merton scarcely heard it. For the last time, he was checking the tension in the rigging. The needles of all the dynamometers were steady; the immense sail was taut, its mirror surface sparkling and glittering gloriously in the sun.

  To Merton, floating weightless at the periscope, it seemed to fill the sky. As well it might—for out there were fifty million square feet of sail, linked to his capsule by almost a hundred miles of rigging. All the canvas of all the tea clippers that had once raced like clouds across the China seas, sewn into one gigantic sheet, could not match the single sail that Diana
had spread beneath the sun. Yet it was little more substantial than a soap bubble; that two square miles of aluminized plastic was only a few millionths of an inch thick.

  “T minus ten seconds. All recording cameras ON.”

  Something so huge, yet so frail, was hard for the mind to grasp. And it was harder still to realize that this fragile mirror could tow him free of Earth merely by the power of the sunlight it would trap.

  “ . . . five, four, three, two, one, CUT!”

  Seven knife blades sliced through seven thin lines tethering the yachts to the mother ships that had assembled and serviced them. Until this moment, all had been circling Earth together in a rigidly held formation, but now the yachts would begin to disperse, like dandelion seeds drifting before the breeze. And the winner would be the one that first drifted past the Moon.

  Aboard Diana, nothing seemed to be happening. But Merton knew better. Though his body could feel no thrust, the instrument board told him that he was now accelerating at almost one thousandth of a gravity. For a rocket, that figure would have been ludicrous—but this was the first time any solar yacht had ever attained it. Diana’s design was sound; the vast sail was living up to his calculations. At this rate, two circuits of the Earth would build up his speed to escape velocity, and then he could head out for the Moon, with the full force of the Sun behind him.

  The full force of the Sun . . . He smiled wryly, remembering all his attempts to explain solar sailing to those lecture audiences back on Earth. That had been the only way he could raise money, in those early days. He might be Chief Designer of Cosmodyne Corporation, with a whole string of successful spaceships to his credit, but his firm had not been exactly enthusiastic about his hobby.

  “Hold your hands out to the Sun,” he’d said. “What do you feel? Heat, of course. But there’s pressure as well—though you’ve never noticed it, because it’s so tiny. Over the area of your hands, it comes to only about a millionth of an ounce.

  “But out in space, even a pressure as small as that can be important, for it’s acting all the time, hour after hour, day after day. Unlike rocket fuel, it’s free and unlimited. If we want to, we can use it. We can build sails to catch the radiation blowing from the Sun.”

  At that point, he would pull out a few square yards of sail material and toss it toward the audience. The silvery film would coil and twist like smoke, then drift slowly to the ceiling in the hot-air currents.

  “You can see how light it is,” he’d continue. “A square mile weighs only a ton, and can collect five pounds of radiation pressure. So it will start moving—and we can let it tow us along, if we attach rigging to it.

  “Of course, its acceleration will be tiny—about a thousandth of a g. That doesn’t seem much, but let’s see what it means.

  “It means that in the first second, we’ll move about a fifth of an inch. I suppose a healthy snail could do better than that. But after a minute, we’ve covered sixty feet, and will be doing just over a mile an hour. That’s not bad, for something driven by pure sunlight! After an hour, we’re forty miles from our starting point, and will be moving at eighty miles an hour. Please remember that in space there’s no friction; so once you start anything moving, it will keep going forever. You’ll be surprised when I tell you what our thousandth-of-a-g sailboat will be doing at the end of a day’s run: almost two thousand miles an hour! If it starts from orbit—as it has to, of course—it can reach escape velocity in a couple of days. And all without burning a single drop of fuell”

  Well, he’d convinced them, and in the end he’d even convinced Cosmodyne. Over the last twenty years, a new sport had come into being. It had been called the sport of billionaires, and that was true. But it was beginning to pay for itself in terms of publicity and TV coverage. The prestige of four continents and two worlds was riding on this race, and it had the biggest audience in history.

  Diana had made a good start; time to take a look at the opposition. Moving very gently—though there were shock absorbers between the control capsule and the delicate rigging, he was determined to run no risks—Merton stationed himself at the periscope.

  There they were, looking like strange silver flowers planted in the dark fields of space. The nearest, South America’s Santa Maria, was only fifty miles away; it bore a close resemblance to a boy’s kite, but a kite more than a mile on a side. Farther away, the University of Astrograd’s Lebedev looked like a Maltese cross; the sails that formed the four arms could apparently be tilted for steering purposes. In contrast, the Federation of Australasia’s Woomera was a simple parachute, four miles in circumference. General Spacecraft’s Arachne, as its name suggested, looked like a spider web, and had been built on the same principles, by robot shuttles spiraling out from a central point. Eurospace Corporation’s Gossamer was an identical design, on a slightly smaller scale. And the Republic of Mars’s Sunbeam was a flat ring, with a half-mile-wide hole in the center, spinning slowly, so that centrifugal force gave it stiffness. That was an old idea, but no one had ever made it work; and Merton was fairly sure that the colonials would be in trouble when they started to turn.

  That would not be for another six hours, when the yachts had moved along the first quarter of their slow and stately twenty-four-hour orbit. Here at the beginning of the race, they were all heading directly away from the Sun—running, as it were, before the solar wind. One had to make the most of this lap, before the boats swung around to the other side of Earth and then started to head back into the Sun.

  Time, Merton told himself, for the first check, while he had no navigational worries. With the periscope, he made a careful examination of the sail, concentrating on the points where the rigging was attached to it. The shroud lines—narrow bands of unsilvered plastic film—would have been completely invisible had they not been coated with fluorescent paint. Now they were taut lines of colored light, dwindling away for hundreds of yards toward that gigantic sail. Each had its own electric windlass, not much bigger than a game fisherman’s reel. The little windlasses were continually turning, playing lines in or out as the autopilot kept the sail trimmed at the correct angle to the Sun.

  The play of sunlight on the great flexible mirror was beautiful to watch. The sail was undulating in slow, stately oscillations, sending multiple images of the Sun marching across it, until they faded away at its edges. Such leisurely vibrations were to be expected in this vast and flimsy structure. They were usually quite harmless, but Merton watched them carefully. Sometimes they could build up to the catastrophic undulations known as the “wriggles,” which could tear a sail to pieces.

  When he was satisfied that everything was shipshape, he swept the periscope around the sky, rechecking the positions of his rivals. It was as he had hoped: the weeding-out process had begun, as the less efficient boats fell astern. But the real test would come when they passed into the shadow of Earth. Then, maneuverability would count as much as speed.

  It seemed a strange thing to do, what with the race having just started, but he thought it might be a good idea to get some sleep. The two-man crews on the other boats could take it in turns, but Merton had no one to relieve him. He must rely on his own physical resources, like that other solitary seaman, Joshua Slocum, in his tiny Spray. The American skipper had sailed Spray singlehanded around the world; he could never have dreamed that, two centuries later, a man would be sailing singlehanded from Earth to Moon—inspired, at least partly, by his example.

  Merton snapped the elastic bands of the cabin seat around his waist and legs, then placed the electrodes of the sleep-inducer on his forehead. He set the timer for three hours, and relaxed. Very gently, hypnotically, the electronic pulses throbbed in the frontal lobes of his brain. Colored spirals of light expanded beneath his closed eyelids, widening outward to infinity. Then nothing . . .

  The brazen clamor of the alarm dragged him back from his dreamless sleep. He was instantly awake, his eyes scanning the instrument panel. Only two hours had passed—but above the accelerometer,
a red light was flashing. Thrust was falling; Diana was losing power.

  Merton’s first thought was that something had happened to the sail; perhaps the antispin devices had failed, and the rigging had become twisted. Swiftly, he checked the meters that showed the tension of the shroud lines. Strange—on one side of the sail they were reading normally, but on the other the pull was dropping slowly, even as he watched.

  In sudden understanding, Merton grabbed the periscope, switched to wide-angle vision, and started to scan the edge of the sail. Yes—there was the trouble, and it could have only one cause.

  A huge, sharp-edged shadow had begun to slide across the gleaming silver of the sail. Darkness was falling upon Diana, as if a cloud had passed between her and the Sun. And in the dark, robbed of the rays that drove her, she would lose all thrust and drift helplessly through space.

  But, of course, there were no clouds here, more than twenty thousand miles above the Earth. If there was a shadow, it must be made by man.

  Merton grinned as he swung the periscope toward the Sun, switching in the filters that would allow him to look full into its blazing face without being blinded.

  “Maneuver 4a,” he muttered to himself. “We’ll see who can play best at that game.”

  It looked as if a giant planet was crossing the face of the Sun; a great black disc had bitten deep into its edge. Twenty miles astern, Gossamer was trying to arrange an artificial eclipse, specially for Diana’s benefit.

  The maneuver was a perfectly legitimate one. Back in the days of ocean racing, skippers had often tried to rob each other of the wind. With any luck, you could leave your rival becalmed, with his sails collapsing around him—and be well ahead before he could undo the damage.

  Merton had no intention of being caught so easily. There was plenty of time to take evasive action; things happened very slowly when you were running a solar sailboat. It would be at least twenty minutes before Gossamer could slide completely across the face of the Sun, and leave him in darkness.