Project Solar Sail

by Arther C. Clarke

  To Merton, floating weightless at the periscope, it seemed to fill the sky. As it well 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 them free of Earth, merely by the power of the sunlight it would trap.

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

  Seven knife blades sliced through the 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 would feel no thrust, the instrument board told him 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 attained it. Diana’s design was sound; the vast sail was living up to his calculations. At this rate, two circuits of Earth would build up his speed to escape velocity—then he would 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 only comes to 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 sailing boat 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 fuel!”

  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 television 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 resemblance to a boy’s kite—but a kite more than a mile on its 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 spiderweb—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 Mar’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. 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 round to the other side of Earth and then started to head back into the sun.

  Time for the first check, Merton told himself, 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. It was undulating in slow, stately oscillations, sending multiple images of the sun marching across the heavens, until they faded away at the edges of the sail. 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, now that the race had just started, but 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 physical resources—like that other solitary seaman Joshua Slocum, in his tiny Spray. The American skipper had sailed Spray single-handed round the world; he could never have dreamed that, two centuries later, a man would be sailing single-handed 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 ho
urs, 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 anti-spin devices had failed and the rigging had become twisted. Swiftly, he checked the meters that showed the tension in 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 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 disk 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 sailing boat. It would be at least twenty minutes before Gossamer could slide completely across the face of the sun, and leave him in darkness.

  Diana’s tiny computer—the size of a matchbox, but the equivalent of a thousand human mathematicians—considered the problem for a full second and then flashed the answer. He’d have to open control panels three and four, until the sail had developed an extra twenty degrees of tilt; then the radiation pressure would blow him out of Gossamer’s dangerous shadow, back into the full blast of the sun. It was a pity to interfere with the autopilot, which had been carefully programmed to give the fastest possible run—but that, after all, was why he was here. This was what made solar yachting a sport, rather than a battle between computers.

  Out went control lines one to six, slowly undulating like sleepy snakes as they momentarily lost their tension. Two miles away, the triangular panels began to open lazily, spilling sunlight through the sail. Yet, for a long time, nothing seemed to happen. It was hard to grow accustomed to this slow-motion world, where it took minutes for the effects of any action to become visible to the eye. Then Merton saw that the sail was indeed tipping toward the sun—and that Gossamer’s shadow was sliding harmlessly away, its cone of darkness lost in the deeper night of space.

  Long before the shadow had vanished and the disk of the sun had cleared again, he reversed the tilt and brought Diana back on course. Her new momentum would carry her clear of the danger; no need to overdo it, and upset his calculations by side-stepping too far. That was another rule that was hard to learn. The very moment you had started something happening in space, it was already time to think about stopping it.

  He reset the alarm, ready for the next natural or man-made emergency; perhaps Gossamer, or one of the other contestants, would try the same trick again. Meanwhile, it was time to eat, though he did not feel particularly hungry. One used little physical energy in space, and it was easy to forget about food. Easy—and dangerous; for when an emergency arose, you might not have the reserves needed to deal with it.

  He broke open the first of the meal packets, and inspected it without enthusiasm. The name on the label—spacetasties—was enough to put him off. And he had grave doubts about the promise printed underneath. Guaranteed Crumbless. It had been said that crumbs were a greater danger to space vehicles than meteorites. They could drift into the most unlikely places, causing short circuits, blocking vital jets that were supposed to be hermetically sealed.

  Still, the liverwurst went down pleasantly enough; so did the chocolate and the pineapple puree. The plastic coffee bulb was warming on the electric heater when the outside world broke in on his solitude. The radio operator on the Commodore’s launch routed a call to him.

  “Dr. Merton? If you can spare the time, Jeremy Blair would like a few words with you.” Blair was one of the more responsible news commentators, and Merton had been on his program many times. He could refuse to be interviewed, of course, but he liked Blair, and at the moment he could certainly not claim to be too busy. “I’ll take it,” he answered.

  “Hello, Dr. Merton,” said the commentator immediately. “Glad you can spare a few minutes. And congratulations—you seem to be ahead of the field.”

  “Too early in the game to be sure of that,” Merton answered cautiously.

  “Tell me, doctor—why did you decide to sail Diana yourself? Just because it’s never been done before?”

  “Well, isn’t that a very good reason? But it wasn’t the only one, of course.” He paused, choosing his words carefully. “You know how critically the performance of a sun yacht depends on its mass. A second man, with all his supplies, would mean another five hundred pounds. That could easily be the difference between winning and losing.”

  “And you’re quite certain that you can handle Diana alone?”

  “Reasonably sure, thanks to the automatic controls I’ve designed. My main job is to supervise and make decisions.”

  “But—two square miles of sail! It just doesn’t seem possible for one man to cope with all that!”

  Merton laughed. “Why not? Those two square miles produce a maximum pull of just ten pounds. I can exert more force with my little finger.”

  “Well, thank you, doctor. And good luck.”

  As the commentator signed off, Merton felt a little ashamed of himself. For his answer had been only part of the truth; and he was sure that Blair was shrewd enough to know it.

  There was just one reason why he was here, alone in space. For almost forty years he had worked with teams of hundreds or even thousands of men, helping to design the most complex vehicles that the world had ever seen. For the last twenty years he had led one of those teams, and watched his creations go soaring to the stars. (But there were failures that he could never forget, even though the fault had not been his.) He was famous, with a successful career behind him. Yet he had never done anything by himself; always he had been one of an army.

  This was his very last chance of individual achievement, and he would share it with no one. There would be no more solar yachting for at least five years, as the period of the quiet sun ended and the cycle of bad weather began, with radiation storms bursting through the solar system. When it was safe again for these frail, unshielded craft to venture aloft, he would be too old. If, indeed, he was not too old already . . .

  He dropped the empty food containers into the waste disposal, and turned once more to the periscope. At first, he could find only five of the other yachts; there was no sign of Woomera. It took him several minutes to locate her—a dim, star-eclipsing phantom, neatly caught in the shadow of Lebedev. He could imagine the frantic efforts the Australasians were making to extricate themselves, and wondere
d how they had fallen into the trap. It suggested that Lebedev was unusually maneuverable; she would bear watching, though she was too far away to menace Diana at the moment.

  * * *

  Now Earth had almost vanished. It had waned to a narrow, brilliant bow of light that was moving steadily toward the sun. Dimly outlined within that burning bow was the night side of the planet, with the phosphorescent gleams of great cities showing here and there through gaps in the clouds. The disk of darkness had already blanked out a huge section of the Milky Way; in a few minutes, it would start to encroach upon the sun.

  The light was fading. A purple, twilight hue—the glow of many sunsets, thousands of miles below—was falling across the sail, as Diana slipped silently into the shadow of Earth. The sun plummeted below that invisible horizon. Within minutes, it was night.

  Merton looked back along the orbit he had traced now a quarter of the way around the world. One by one he saw the brilliant stars of the other yachts wink out, as they joined him in the brief night. It would be an hour before the sun emerged from that enormous black shield, and through all that time they would be completely helpless, coasting without power.

  He switched on the external spotlight and started to search the now darkened sail with its beam. Already, the thousands of acres of film were beginning to wrinkle and become flaccid; the shroud lines were slackening, and must be wound in lest they become entangled. But all this was expected; everything was going as planned.

  Forty miles astern, Arachne and Santa Maria were not so lucky. Merton learned of their troubles when the radio burst into life on the emergency circuit.

  “Number Two, Number Six—this is Control. You are on a collision course. Your orbits will intersect in sixty-five minutes! Do you require assistance?”