14 – The Listeners

   A hundred million miles beyond Mars, in the cold loneliness where no man had yet traveled, Deep Space Monitor 79 drifted slowly among the tangled orbits of the asteroids. For three years it had fulfilled its mission flawlessly – a tribute to the American scientists who had designed it, the British engineers who had built it, the Russian technicians who had launched it. A delicate spider's-web of antennas sampled the passing waves of radio noise – the ceaseless crackle and hiss of what Pascal, in a far simpler age, had naively called the "silence of infinite space." Radiation detectors noted and analyzed incoming cosmic rays from the galaxy and points beyond; neutron and X-ray telescopes kept watch on strange stars that no human eye would ever see; magnetometers observed the gusts and hurricanes of the solar winds, as the Sun breathed million-mile-an-hour blasts of tenuous plasma into the faces of its circling children. All these things, and many others, were patiently noted by Deep Space Monitor 79, and recorded in its crystalline memory.
   One of its antennas, by now unconsidered miracles of electronics, was always aimed at a point never far from the Sun. Every few months its distant target could have been seen, had there been any eye here to watch, as a bright star with a close, fainter companion; but most of the time it was lost in the solar glaze.
   To that far-off planet Earth, every twenty-four hours, the monitor would send the information it had patiently garnered, packed neatly into one five-minute pulse. About a quarter of an hour late, traveling at the speed of light, that pulse would reach its destination. The machines whose duty it was would be waiting for it; they would amplify and record the signal, and add it to the thousands of miles of magnetic tape now stored in the vaults of the World Space Centers at Washington, Moscow, and Canberra.
   Since the first satellites had orbited, almost fifty years earlier, trillions and quadrillions of pulses of information had been pouring down from space, to be stored against the day when they might contribute to the advance of knowledge. Only a minute fraction of all this raw material would ever be processed; but there was no way of telling what observation some scientist might wish to consult, ten, or fifty, or a hundred years from now. So everything had to be kept on file, stacked in endless air-conditioned galleries, triplicated at the three centers against the possibility of accidental loss. It was part of the real treasure of mankind, more valuable than all the gold locked uselessly away in bank vaults.
   And now Deep Space Monitor 19 had noted something strange – a faint yet unmistakable disturbance rippling across the Solar System, and quite unlike any natural phenomenon it had ever observed in the past. Automatically, it recorded the direction, the time, the intensity; in a few hours it would pass the information to Earth.
   As, also, would Orbiter M 15, circling Mars twice a day; and High Inclination Probe 21, climbing slowly above the plane of the ecliptic; and even Artificial Comet 5, heading out into the cold wastes beyond Pluto, along an orbit whose far point it would not reach for a thousand years. All noted the peculiar burst of energy that had disturbed their instruments; all, in due course, reported back automatically to the memory stores on distant Earth.
   The computers might never have perceived the connection between four peculiar sets of signals from space-probes on independent orbits millions of miles apart. But as soon as he glanced at his morning report, the Radiation Forecaster at Goddard knew that something strange had passed through the Solar System during the last twenty-four hours.
   He had only part of its track, but when the computer projected it on the Planet Situation Board, it was as clear and unmistakable as a vapor trail across a cloudless sky, or a single line of footprints over a field of virgin snow.
   Some immaterial pattern of energy, throwing off a spray of radiation like the wake of a racing speedboat, had leaped from the face of the Moon, and was heading out toward the stars.

III – BETWEEN PLANETS

15 – Discovery

   The ship was still only thirty days from Earth, yet David Bowman sometimes found it hard to believe that be had ever known any other existence than the closed little world of Discovery. All his years of training, all his earlier missions to the Moon and Mars, seemed to belong to another man, in another life.
   Frank Poole admitted to the same feelings, and had sometimes jokingly regretted that the nearest psychiatrist was the better part of a hundred million miles away. But this sense of isolation and estrangement was easy enough to understand, and certainly indicated no abnormality. In the fifty years since men had ventured into space, there had never been a mission quite like this.
   It had begun, five years ago, as Project Jupiter – the first manned round trip to the greatest of the planets. The ship was nearly ready for the two-year voyage when, somewhat abruptly, the mission profile had been changed.
   Discovery would still go to Jupiter; but she would not stop there. She would not even slacken speed as she raced through the far-ranging Jovian satellite system. On the contrary – she would use the gravitational field of the giant world us a sling to cast her even farther from the Sun. Like a comet, she would streak on across the outer reaches of the solar system to her ultimate goal, the ringed glory of Saturn. And she would never return.
   For Discovery, it would be a one-way trip – yet her crew had no intention of committing suicide. If all went well, they would be back on Earth within seven years – five of which would pass like a flash in the dreamless sleep of hibernation, while they awaited rescue by the still unbuilt Discovery II.
   The word "rescue" was carefully avoided in all the Astronautics Agency's statements and documents; it implied some failure of planning, and the approved jargon was "re-acquisition." If anything went really wrong, there would certainly be no hope of rescue, almost a billion miles from Earth.
   It was a calculated risk, like all voyages into the unknown. But half a century of research had proved that artificially induced human hibernation was perfectly safe, and it had opened up new possibilities in space travel. Not until this mission, however, had they been exploited to the utmost.
   The three members of the survey team, who would not be needed until the ship entered her final orbit around Saturn, would sleep through the entire outward flight. Tons of food and other expendables would thus be saved; almost as important, the team would be fresh and alert, and not fatigued by the ten-month voyage, when they went into action.
   Discovery would enter a parking orbit around Saturn, becoming a new moon of the giant planet. She would swing back and forth along a two-million-mile ellipse that took her close to Saturn, and then across the orbits of all its major moons. They would have a hundred days in which to map and study a world with eighty times the area of Earth, and surrounded by a retinue of at least fifteen known satellites – one of them as large as the planet Mercury.
   There must be wonders enough here for centuries of study; the first expedition could only carry out a preliminary reconnaissance. All that it found would be radioed back to Earth; even if the explorers never returned, their discoveries would not be lost.
   At the end of the hundred days, Discovery would close down. All the crew would go into hibernation; only the essential systems would continue to operate, watched over by the ship's tireless electronic brain. She would continue to swing around Saturn, on an orbit now so well determined that men would know exactly where to look for her a thousand years hence. But in only five years, according to present plans, Discovery II would come. Even if six or seven or eight years elapsed, her sleeping passengers would never know the difference. For all of them, the clock would have stopped as it had stopped already for Whitehead, Kaminski, and Hunter.
   Sometimes Bowman, as First Captain of Discovery, envied his three unconscious colleagues in the frozen peace of the Hibernaculum. They were free from all boredom and all responsibility; until they reached Saturn, the external world did not exist.
   But that world was watching them, through their bio-sensor displays. Tucked inconspicuously away among the massed instrumentation of the Control Deck were five small panels marked Hunter, Whitehead, Kaminski, Poole, Bowman. The last two were blank and lifeless; their time would not come until a year from now. The others bore constellations of tiny green lights, announcing that everything was well; and on each was a small display screen across which sets of glowing lines traced the leisurely rhythms that indicated pulse, respiration, and brain activity.
   There were times when Bowman, well aware how unnecessary this was – for the alarm would sound instantly if anything was wrong – would switch over to audio output. He would listen, half hypnotized, to the infinitely slow heartbeats of his sleeping colleagues, keeping his eyes fixed on the sluggish waves that marched in synchronism across the screen.
   Most fascinating of all were the EEG displays – the electronic signatures of three personalities that had once existed, and would one day exist again. They were almost free from the spikes and valleys, the electrical explosions that marked the activity of the waking brain – or even of the brain in normal sleep. If there was any wisp of consciousness remaining, it was beyond the reach of instruments, and of memory.
   This last fact Bowman knew from personal experience. Before he was chosen for this mission, his reactions to hibernation had been tested. He was not sure whether he had lost a week of his life – or whether he had postponed his eventual death by the same amount of time.
   When the electrodes had been attached to his forehead, and the sleep-generator had started to pulse, he had seen a brief display of kaleidoscopic patterns and drifting stars. Then they had faded, and darkness had engulfed him. He had never felt the injections, still less the first touch of cold as his body temperature was reduced to only a few degrees above freezing.

   He awoke, and it seemed that he had scarcely closed his eyes. But he knew that was an illusion; somehow, he was convinced that years had really passed.
   Had the mission been completed? Had they already reached Saturn, carried out their survey, and gone into hibernation? Was Discovery II here, to take them back to Earth?
   He lay in a dreamlike haze, utterly unable to distinguish between real and false memories. He opened his eyes, but there was little to see except a blurred constellation of lights which puzzled him for some minutes.
   Then he realized that he was looking at the indicator lamps on a Ship Situation Board, but it was impossible to focus on them. He soon gave up the attempt.
   Warm air was blowing across him, removing the chill from his limbs. There was quiet, but stimulating, music welling from a speaker behind his head. It was slowly growing louder and louder.
   Then a relaxed, friendly – but he knew computer generated – voice spoke to him.
   "You are becoming operational, Dave. Do not get up or attempt any violent movements. Do not try to speak."
   Do not get up! thought Bowman. That was funny. He doubted if he could wriggle a finger. Rather to his surprise, he found that he could.
   He felt quite contented, in a dazed, stupid kind of way. He knew dimly that the rescue ship must have come, that the automatic revival sequence had been triggered, and that soon he would be seeing other human beings. That was fine, but he did not get excited about it.
   Presently he felt hunger. The computer, of course, had anticipated this need.
   "There is a signal button by your right hand, Dave.
   If you are hungry, please press it."
   Bowman forced his fingers to hunt around, and presently discovered the pear-shaped bulb. He had forgotten all about it, though he must have known it was there. How much else had he forgotten: Did hibernation erase memory?
   He pressed the button, and waited. Several minutes later, a metal arm moved out from the bunk, and a plastic nipple descended toward his lips. He sucked on it eagerly, and a warm, sweet fluid coursed down his throat, brining renewed strength with every drop.
   Presently it went away, and he rested once more. He could move his arms and legs now; the thought of walking was no longer an impossible dream.
   Though he felt his strength swiftly returning, he would have been content to lie here forever, if there had been no further stimulus from outside. But presently another voice spoke to him – and this time it was wholly human, not a construct of electrical pulses assembled by a more-than-human memory. It was also a familiar voice, though it was some time before he could recognize it
   "Hello, Dave. You're coming round fine. You can talk now. Do you know where you are?"
   He worried about this for some time. If he was really orbiting Saturn, what had happened during all the months since he had left Earth? Again he began to wonder if he was suffering from amnesia, Paradoxically, that very thought reassured him, if he could remember the word "amnesia" his brain must be in fairly good shape.
   But he still did not know where he was, and the speaker at the other end of the circuit must have understood his situation completely.
   "Don't worry, Dave. This is Frank Poole. I'm watching your heart and respiration-everything is perfectly normal. Just relax – take it easy. We're going to open the door now and pull you out."
   Soft light flooded into the chamber; he saw moving shapes silhouetted against the widening entrance. And in that moment, all his memories came back to him, and be knew exactly where he was.
   Though he had come back safely from the furthest borders of sleep, and the nearest borders of death, he had been gone only a week. When he left the Hibernaculum, he would not see the cold Saturnian sky; that was more than a year in the future and a billion miles away.
   He was still in the trainer at the Houston Space Flight Center under the hot Texas sun.

16 – Hal

   But now Texas was invisible, and even the United States was hard to see. Though the low-thrust plasma drive had long since been closed down, Discovery was still coasting with her slender arrowlike body pointed away from Earth, and all her high-powered optical gear was oriented toward the outer planets, where her destiny lay.
   There was one telescope, however, that was permanently aimed at Earth. It was mounted like a gunsight on the rim of the ship's long-range antenna, and checked that the great parabolic bowl was rigidly locked upon its distant target. While Earth remained centered in the crosswires, the vital communication link was intact, and messages could come and go along the invisible beam that lengthened more than two million miles with every day that passed.
   At least once in every watch period Bowman would lock homeward through the antenna-alignment telescope. As Earth was now far back toward the sun, its darkened hemisphere faced Discovery, and on the central display screen the planet appeared as a dazzling silver crescent, like another Venus.
   It was rare that any geographical features could be identified in that ever-shrinking arc of light, for cloud and haze concealed them, but even the darkened portion of the disk was endlessly fascinating. It was sprinkled with shining cities; sometimes they burned with a steady light, sometimes they twinkled like fireflies as atmospheric tremors passed over them.
   There were also periods when, as the Moon swung back and forth in its orbit, it shone down like a great lamp upon the darkened seas and continents of Earth.
   Then, with a thrill of recognition, Bowman could often glimpse familiar coastlines, shining in that spectral lunar light. And sometimes, when the Pacific was calm, he could even see the moonglow shimmering across its face; and he would remember nights beneath the palm trees of tropical lagoons.
   Yet he had no regrets for these lost beauties. He had enjoyed them all, in his thirty-five years of life; and he was determined to enjoy them again, when he returned rich and famous. Meanwhile, distance made them all the more precious.
   The sixth member of the crew cared for none of these things, for it was not human. It was the highly advanced HAL 9000 computer, the brain and nervous system of the ship.
   Hal (for Heuristically programmed ALgorithmic computer, no less) was a masterwork of the third computer breakthrough. These seemed to occur at intervals of twenty years, and the thought that another one was now imminent already worried a great many people.
   The first had been in the 1940s, when the long-obsolete vacuum tube had made possible such clumsy, high-speed morons as ENIAC and its successors. Then, in the 1960s, solid-state microelectronics had been perfected. With its advent, it was clear that artificial intelligences at least as powerful as Man's need be no larger than office desks – if one only knew how to construct them.
   Probably no one would ever know this; it did not matter. In the 1980s, Minsky and Good had shown how neural networks could be generated automatically – self replicated – in accordance with any arbitrary learning program. Artificial brains could be grown by a process strikingly analogous to the development of a human brain. In any given case, the precise details would never be known, and even if they were, they would be millions of times too complex for human understanding. Whatever way it worked, the final result was a machine intelligence that could reproduce – some philosophers still preferred to use the word "mimic" – most of the activities of the human brain – and with far greater speed and reliability. It was extremely expensive, and only a few units of the HAL9000 series had yet been built; but the old jest that it would always be easier to make organic brains by unskilled labor was beginning to sound a little hollow.
   Hal had been trained for this mission as thoroughly as his human colleagues – and at many times their rate of input, for in addition to his intrinsic speed, he never slept. His prime task was to monitor the life-support systems, continually checking oxygen pressure, temperature, hull leakage, radiation, and all the other interlocking factors upon which the lives of the fragile human cargo depended. He could carry out the intricate navigational corrections, and execute the necessary flight maneuvers when it was time to change course. And he could watch over the hibernators, making any necessary adjustments to their environment and doling out the minute quantities of intravenous fluids that kept them alive.
   The first generations of computers had received their inputs through glorified typewriter keyboards, and had replied through high-speed printers and visual displays. Hal could do this when necessary, but most of his communication with his shipmates was by means of the spoken word. Poole and Bowman could talk to Hal as if he were a human being and he would reply in the perfect idiomatic English he had learned during the fleeting weeks of his electronic childhood.
   Whether Hal could actually think was a question which had been settled by the British mathematician Alan Turing back in the 1940s. Turing had pointed out that, if one could carry out a prolonged conversation with a machine – whether by typewriter or microphone was immaterial – without being able to distinguish between its replies and those that a man might give, then the machine was thinking, by any sensible definition of the word. Hal could pass the Turing test with ease.
   The time might even come when Hal would take command of the ship. In an emergency, if no one answered his signals, he would attempt to wake the sleeping members of the crew, by electrical and chemical stimulation. If they did not respond, he would radio Earth for further orders.
   And then, if there was no reply from Earth, he would take what measures he deemed necessary to safeguard the ship and to continue the mission – whose real purpose he alone knew, and which his human colleagues could never have guessed.
   Poole and Bowman had often humorously referred to themselves as caretakers or janitors aboard a ship that could really run itself. They would have been astonished, and more than a little indignant, to discover how much truth that jest contained.

17 – Cruise Mode

   The day-by-day running of the ship had been planned with great care, and – theoretically at least – Bowman and Poole knew what they would be doing at every moment of the twenty-four hours. They operated on a twelve-hours-on, twelve-hours-off basis, taking charge alternately, and never being both asleep at the same time. The officer on duty remained on the Control Deck, while his deputy saw to the general housekeeping, inspected the ship, coped with the odd jobs that constantly arose, or relaxed in his cubicle.
   Although Bowman was nominal Captain on this phase of the mission, no outside observer could have deduced the fact. He and Poole switched roles, rank, and responsibilities completely every twelve hours. This kept them both at peak training, minimized the chances of friction, and helped toward the goal of 100 percent redundancy.
   Bowman's day began at 0600, ship's time – the Universal Ephemeris Time of the astronomers. If he was late, Hal had a variety of beeps and chimes to remind him of his duty, but they had never been used. As a test, Poole had once switched off the alarm; Bowman had still risen automatically at the right time.
   His first official act of the day would be to advance the Master Hibernation Timer twelve hours. If this operation was missed twice in a row, Hal would assume that both he and Poole had been incapacitated, and would take the necessary emergency action.
   Bowman would attend to his toilet, and do his isometric exercises, before settling down to breakfast and the morning's radio-fax edition of the World Times. On Earth, he never read the paper as carefully as he did now; even the smallest items of society gossip, the most fleeting political rumors, seemed of absorbing interest as it flashed across the screen.
   At 0700 he would officially relieve Poole on the Control Deck, bringing him a squeeze-tube of coffee from the kitchen. If – as was usually the case – there was nothing to report and no action to be taken, he would settle down to check all the instrument readings, and would run through a series of tests designed to spot possible malfunctions. By 1000 this would be finished, and he would start on a study period.
   Bowman had been a student for more than half his life; he would continue to be one until he retired. Thanks to the twentieth-century revolution in training and information-handling techniques, he already possessed the equivalent of two or three college educations – and, what was more, he could remember 90 percent of what he had learned.
   Fifty years ago, he would have been considered a specialist in applied astronomy, cybernetics, and space propulsion systems – yet he was prone to deny, with genuine indignation, that he was a specialist at all. Bowman had never found it possible to focus his interest exclusively on any subject; despite the dark warnings of his instructors, he had insisted on taking his Master's degree in General Astronautics – a course with a vague and woolly syllabus, designed for those whose IQs were in the low 130s and who would never reach the top ranks of their profession.
   His decision had been right; that very refusal to specialize had made him uniquely qualified for his present task. In much the same way Frank Poole – who sometimes disparagingly called himself "General Practitioner in space biology" – had been an ideal choice as his deputy. The two of them, with, if necessary, help from Hal's vast stores of information, could cope with any problems likely to arise during the voyage – as long as they kept their minds alert and receptive, and continually reengraved old patterns of memory.
   So for two hours, from 1000 to 1200, Bowman would engage in a dialogue with an electronic tutor, checking his general knowledge or absorbing material specific to this mission. He would prowl endlessly over ship's plans, circuit diagrams, and voyage profiles, or would try to assimilate all that was known about Jupiter, Saturn, and their far-ranging families of moons.
   At midday, he would retire to the galley and leave the ship to Hal while he prepared his lunch. Even here, he was still fully in touch with events, for the tiny lounge-cum-dining room contained a duplicate of the Situation Display Panel, and Hal could call him at a moment's notice. Poole would join him for this meal, before retiring for his six-hour sleep period, and usually they would watch one of the regular TV programs beamed to them from Earth.
   Their menus had been planned with as much care as any part of the mission, The food, most of it freeze-dried, was uniformly excellent, and had been chosen for the minimum of trouble; Packets had merely to be opened and popped into the tiny auto-galley, which beeped for attention when the job was done. They could enjoy what tasted like – and, equally important, looked like – orange juice, eggs (any style), steaks, chops, roasts, fresh vegetables, assorted fruits, ice cream, and even freshly baked bread.
   After lunch, from 1300 to 1600 Bowman would make a slow and careful tour of the ship – or such part of it as was accessible. Discovery measured almost four hundred feet from end to end, but the little universe occupied by her crew lay entirely inside the forty-foot sphere of the pressure hull.
   Here were all the life-support systems, and the Control Deck which was the operational heart of the ship. Below this was a small "space-garage" fitted with three airlocks, through which powered capsules, just large enough to hold a man, could sail out into the void if the need arose for extravehicular activity.
   The equatorial region of the pressure sphere – the slice, as it were, from Capricorn to Cancer – enclosed a slowly rotating drum, thirty-five feet in diameter. As it made one revolution every ten seconds, this carrousel or centrifuge produced an artificial gravity equal to that of the Moon. This was enough to prevent the physical atrophy which would result from the complete absence of weight, and it also allowed the routine functions of living to be carried out under normal – or nearly normal – conditions.
   The carrousel therefore contained the kitchen, dining, washing, and toilet facilities. Only here was it safe to prepare and handle hot drinks – quite dangerous in weightless conditions, where one can be badly scalded by floating globules of boiling water. The problem of shaving was also solved; there would be no weightless bristles drifting around to endanger electrical equipment and produce a health hazard.
   Around the rim of the carrousel were five tiny cubicles, fitted out by each astronaut according to taste and containing his personal belongings. Only Bowman's and Poole's were now in use, while the future occupants of the other three cabins reposed in their electronic sarcophagi next door.
   The spin of the carrousel could be stopped if necessary; when this happened, its angular momentum had to be stored in a flywheel, and switched back again when rotation was restarted. But normally it was left running at constant speed, for it was easy enough to enter the big, slowly turning drum by going hand-over-hand along a pole through the zero-gee region at its center. Transferring to the moving section was as easy and automatic, after a little experience, as stepping onto a moving escalator.
   The spherical pressure hull formed the head of a flimsy, arrow-shaped structure more than a hundred yards long. Discovery, like all vehicles intended for deep space penetration, was too fragile and unstreamlined ever to enter an atmosphere, or to defy the full gravitational field of any planet. She had been assembled in orbit around the Earth, tested on a translunar maiden flight, and finally checked out in orbit above the Moon.
   She was a creature of pure space – and she looked it. Immediately behind the pressure hull was grouped a cluster of four large liquid hydrogen tanks – and beyond them, forming a long, slender V, were the radiating fins that dissipated the waste heat of the nuclear reactor. Veined with a delicate tracery of pipes for the cooling fluid, they looked like the wings of some vast dragonfly, and from certain angles gave Discovery a fleeting resemblance to an old-time sailing ship,
   At the very end of the V, three hundred feet from the crew-compartment, was the shielded inferno of the reactor, and the complex of focusing electrodes through which emerged the incandescent star-stuff of the plasma drive. This had done its work weeks ago, forcing Discovery out of her parking orbit round the Moon. Now the reactor was merely ticking over as it generated electrical power for the ship's services, and the great radiating fins, that would glow cherry red when Discovery was accelerating under maximum thrust, were dark and cool.
   Although it would require an excursion out into space to examine this region of the ship, there were instruments and remote TV cameras which gave a full report on conditions here. Bowman now felt that he knew intimately every square foot of radiator, panels, and every piece of plumbing associated with them.
   By 1600, he would have finished his inspection, and would make a detailed verbal report to Mission Control, talking until the acknowledgment started to come in. Then he would switch off his own transmitter, listen to what Earth had to say, and send back his reply to any queries. At 1800 hours, Poole would awaken, and he would hand over command.

   He would then have six off-duty hours, to use as he pleased. Sometimes he would continue his studies, or listen to music, or look at movies. Much of the time he would wander at will through the ship's inexhaustible electronic library. He had become fascinated by the great explorations of the past – understandably enough, in the circumstances. Sometimes he would cruise with Pytheas out through the Pillars of Hercules, along the coast of a Europe barely emerging from the Stone Age, and venture almost to the chill mists of the Arctic. Or, two thousand years later, he would pursue the Manila galleons with Anson, sail with Cook along the unknown hazards of the Great Barrier Reef, achieve with Magellan the first circumnavigation of the world. And he began to read the Odyssey, which of all books spoke to him most vividly across the gulfs of time.
   For relaxation he could always engage Hal in a large number of semi-mathematical games, including checkers, chess, and polyominoes. If Hal went all out, he could win anyone of them; but that would be bad for morale. So he had been programmed to win only fifty percent of the time, and his human partners pretended not to know this.
   The last hours of Bowman's day were devoted to general cleaning up and odd jobs, followed by dinner at 2000 – again with Poole. Then there would be an hour during which he would make or receive any personal call from Earth.
   Like all his colleagues, Bowman was unmarried; it was not fair to send family men on a mission of such duration, though numerous ladies had promised to wait until the expedition returned, no one had really believed this. At first, both Poole and Bowman had been making rather intimate personal calls once a week, though the knowledge that many ears must be listening at the Earth end of the circuit tended to inhibit them. Yet already, though the voyage was scarcely started, the warmth and frequency of the conversations with their girls on Earth had begun to diminish. They had expected this; it was one of the penalties of an astronaut's way of life, as it had once been of a mariner's.
   It was true – indeed, notorious – that seamen had compensations at other ports; unfortunately there were no tropical islands full of dusky maids beyond the orbit of Earth. The space medics, of course, had tackled this problem with their usual enthusiasm; the ship's pharmacopoeia provided adequate, though hardly glamorous, substitutes.
   Just before he signed off Bowman would make his final report, and check that Hal had transmitted all the instrumentation tapes for the day's run. Then, if he felt like it, he would spend a couple of hours either reading or looking at a movie; and at midnight he would go to sleep – usually without any help from electronarcosis. Poole's program was a mirror image of his own, and the two schedules dovetailed together without friction.
   Both men were fully occupied, they were too intelligent and well-adjusted to quarrel, and the voyage had settled down to a comfortable, utterly uneventful routine, the passage of time marked only by the changing numbers on the digital clocks.
   The greatest hope of Discovery's little crew was that nothing would mar this peaceful monotony in the weeks and months that lay ahead.

18 – Through the Asteroids

   Week after week, running like a streetcar along the tracks of her utterly predetermined orbit, Discovery swept past the orbit of Mars and on toward Jupiter. Unlike all the vessels traversing the skies or seas of Earth, she required not even the most minute touch on the controls. Her course was fixed by the laws of gravitation; there were no uncharted shoals, no dangerous reefs on which she would run aground. Nor was there the slightest danger of collision with another ship; for there was no vessel – at least of Man's making – anywhere between her and the infinitely distant stars.
   Yet the space which she was now entering was far from empty. Ahead lay a no-man's land threaded by the paths of more than a million asteroids – less than ten thousand of which had ever had their orbits precisely determined by astronomers. Only four were over a hundred miles in diameter; the vast majority were merely giant boulders, trundling aimlessly through space.
   There was nothing that could be done about them; though even the smallest could completely destroy the ship if it slammed into it at tens of thousands of miles an hour, the chance of this happening was negligible.
   On the average, there was only one asteroid in a volume a million miles on a side; that Discovery should also happen to occupy this same point, and at the same time, was the very least of her crew's worries.
   On Day 86 they were due to make their closest approach to any known asteroid, It had no name – merely the number 7794 – and was a fifty-yard-diameter rock that had been detected by the Lunar Observatory in 1997 and immediately forgotten except by the patient computers of the Minor Planet Bureau.
   When Bowman came on duty, Hal promptly reminded hint of the forthcoming encounter – not that he was likely to have forgotten the only scheduled in-flight event of the entire voyage, The track of the asteroid against the stars, and its coordinates at the moment of closest approach, had already been printed out on the display screens. Listed also were the observations to be made or attempted; they were going to be very busy when 7794 flashed past them only nine hundred miles away, at a relative speed of eighty thousand miles an hour.
   When Bowman asked Hal for the telescopic display, a sparsely sprinkled star field flashed onto the screen. There was nothing that looked like an asteroid; all the images, even under the highest magnification, were dimensionless points of light.
   "Give me the target reticule," asked Bowman. Immediately four faint, narrow lines appeared, bracketing a tiny and undistinguished star. He stared at it for many minutes, wondering if Hal could possibly be mistaken; then he saw that the pinpoint of light was moving, with barely perceptible slowness, against the background of the stars. It might still be half a million miles away – but its movement proved that, as cosmic distances went, it was almost near enough to touch.
   When Poole joined him on the control deck six hours later, 7794 was hundreds of times more brilliant, and was moving so swiftly against its background that there was no question of its identity. And it was no longer a point of light; it had begun to show a clearly visible disk.
   They stared at that passing pebble in the sky with the emotions of sailors on a long sea voyage, skirting a coast on which they cannot land. Though they were perfectly well aware that 7794 was only a lifeless, airless chunk of rock, this knowledge scarcely affected their feelings. It was the only solid matter they would meet this side of Jupiter – still two hundred million miles away.
   Through the high-powered telescope, they could see that the asteroid was very irregular, and turning slowly end over end. Sometimes it looked like a flattened sphere, sometimes it resembled a roughly shaped block; its rotation period was just over two minutes. There were mottled patches of light and shade distributed apparently at random over its surface, and often it sparkled like a distant window as planes or outcroppings of crystalline material flashed in the sun.
   It was racing past them at almost thirty miles a second; they had only a few frantic minutes in which to observe it closely. The automatic cameras took dozens of photographs, the navigation radar's returning echoes were carefully recorded for future analysis – and there was just time for a single impact probe.
   The probe carried no instruments; none could survive a collision at such cosmic speeds. It was merely a small slug of metal, shot out from Discovery on a course which should intersect that of the asteroid.
   As the seconds before impact ticked away, Poole and Bowman waited with mounting tension. The experiment, simple though it was in principle, taxed the accuracy of their equipment to the limits. They were aiming at a hundred-foot-diameter target, from a distance of thousands of miles.
   Against the darkened portion of the asteroid there was a sudden, dazzling explosion of light. The tiny slug had impacted at meteoric speed; in a fraction of a second all its energy had been transformed into heat. A puff of incandescent gas had erupted briefly into space; aboard Discovery, the cameras were recording the rapidly fading spectral lines. Back on Earth, experts would analyze them, looking for the telltale signatures of glowing atoms. And so, for the first time, the composition of an asteroid's crust would be determined.
   Within an hour, 7794 was a dwindling star, showing no trace of a disk. When Bowman next came on watch it had vanished completely.
   They were alone again; they would remain alone, until the outermost of Jupiter's moons came swimming up toward them, three months from now.

19 – Transit of Jupiter

   Even front twenty million miles away, Jupiter was already the most conspicuous object in the sky ahead. The planet was now a pale, salmon-hued disk, about half the size of the Moon as seen from Earth, with the dark, parallel bands of its cloud belts clearly visible.
   Shuttling back and forth in the equatorial plane were the brilliant stars of Io, Europa, Ganymede, and Callisto – worlds that elsewhere would have counted as planets in their own right, but which here were merely satellites of a giant master.
   Through the telescope, Jupiter was a glorious sight – a mottled, multicolored globe that seemed to fill the sky. It was impossible to grasp its real size; Bowman kept reminding himself that it was eleven times the diameter of Earth, but for a long time this was a statistic with no real meaning.
   Then, while he was briefing himself from the tapes in Hal's memory units, he found something that suddenly brought the appalling scale of the planet into focus. It was an illustration that showed the Earth's entire surface peeled off and then pegged, like the skin of an animal, on the disk of Jupiter. Against this background, all the continents and oceans of Earth appeared no larger than India on the terrestrial globe.
   When Bowman used the highest magnification of Discovery's telescopes, he appeared to be hanging above a slightly flattened globe, looking down upon a vista of racing clouds that had been smeared into bands by the giant world's swift rotation. Sometimes those bands congealed into wisps and knots and continent-sized masses of colored vapor; sometimes they were linked by transient bridges thousands of miles in length. Hidden beneath those clouds was enough material to outweigh all the other planets in the Solar System. And what else, Bowman wondered, was also hidden there?
   Over this shifting, turbulent roof of clouds, forever hiding the real surface of the planet, circular patterns of darkness sometimes glided. One of the inner moons was transiting the distant sun, its shadow marching beneath it over the restless Jovian cloudscape.
   There were other, and far smaller, moons even out here – twenty million miles from Jupiter. But they were only flying mountains, a few dozen miles in diameter, and the ship would pass nowhere near any of them. Every few minutes the radar transmitter would gather its strength and send out a silent thunderclap of power; no echoes of new satellites came pulsing back from the emptiness.
   What did come, with ever growing intensity, was the roar of Jupiter's own radio voice. In 1955, just before the dawn of the space age, astronomers had been astonished to find that Jupiter was blasting out millions of horsepower on the ten-meter band. It was merely raw noise, associated with haloes of charged particles circling the planet like the Van Allen belts of Earth, but on a far greater scale.
   Sometimes, during lonely hours on the control deck, Bowman would listen to this radiation. He would turn up the gain until the room filled with a crackling, hissing roar; out of this background, at irregular intervals, emerged brief whistles and peeps like the cries of demented birds. It was an eerie sound, for it had nothing to do with Man; it was as lonely and as meaningless as the murmur of waves on a beach, or the distant crash of thunder beyond the horizon.
   Even at her present speed of over a hundred thousand miles an hour, it would take Discovery almost two weeks to cross the orbits of all the Jovian satellites. More moons circled Jupiter than planets orbited the Sun; the Lunar Observatory was discovering new ones every year, and the tally had now reached thirty-six. The outermost – Jupiter XXVII – moved backwards in an unstable path nineteen million miles from its temporary master. It was the prize in a perpetual tug-of-war between Jupiter and the Sun, for the planet was constantly capturing short-lived moons from the asteroid belt, and losing them again after a few million years. Only the inner satellites were its permanent property; the Sun could never wrest them from its grasp.
   Now there was new prey for the clashing gravitation at fields, Discovery was accelerating toward Jupiter along a complex orbit computed months ago by the astronomers on Earth, and constantly checked by Hal. From time to time there would be minute, automatic nudges from the control jets, scarcely perceptible aboard the ship, as they made fine adjustments to the trajectory.
   Over the radio link with Earth, information was flowing back in a constant stream. They were now so far from home that, even traveling at the speed of light, their signals were taking fifty minutes for the journey. Though the whole world was looking over their shoulder, watching through their eyes and their instruments as Jupiter approached, it would be almost an hour before the news of their discoveries reached home.
   The telescopic cameras were operating constantly as the ship cut across the orbit of the giant inner satellites – every one of them larger than the Moon, every one of them unknown territory. Three hours before transit, Discovery passed only twenty thousand miles from Europa, and all instruments were aimed at the approaching world, as it grew steadily in size, changed from globe to crescent, and swept swiftly sunward.
   Here were fourteen million square miles of land which, until this moment, had never been more than a pinhead in the mightiest telescope. They would race past it in minutes, and must make the most of the encounter, recording all the information they could. There would be months in which they could play it back at leisure.
   From a distance, Europa had seemed like a giant snowball, reflecting the light of the far-off sun with remarkable efficiency. Closer observations confirmed this; unlike the dusty Moon, Europa was a brilliant white, and much of its surface was covered with glittering hunks that looked like stranded icebergs. Almost certainly, these were formed from ammonia and water that Jupiter's gravitational field had somehow failed to capture.
   Only along the equator was bare rock visible; here was an incredibly jagged no-man's-land of canyons and jumbled boulders, forming a darker band that completely surrounded the little world. There were a few impact craters, but no sign of vulcanism; Europa had obviously never possessed any internal sources of heat. There was, as had long been known, a trace of atmosphere. When the dark edge of the satellite passed across a star, it dimmed briefly before the moment of eclipse. And in somr areas there was a hint of cloud – perhaps a mist of ammonia droplets, borne on tenuous methane winds.
   As swiftly as it had rushed out of the sky ahead, Europa dropped astern; and now Jupiter itself was only two hours away. Hal had checked and rechecked the ship's orbit with infinite care, and there was no need for further speed corrections until the moment of closest approach. Yet, even knowing this, it was a strain on the nerves to watch that giant globe ballooning minute by minute. It was difficult to believe that Discovery was not plunging directly into it, and that the planet's immense gravitational field was not dragging them down to destruction. Now was the time to drop the atmospheric probes – which, it was hoped, would survive long enough to send back some information from below the Jovian cloud deck. Two stubby, bomb-shaped capsules, enclosed in ablative heat-shields, were gently nudged into orbits which for the first few thousand miles deviated scarcely at all from that of Discovery.
   But they slowly drifted away; and now, at last, even the unaided eye could see what Hal had been asserting. The ship was in a near-grazing orbit, not a collision one; she would miss the atmosphere. True, the difference was only a few hundred miles – a mere nothing when one was dealing with a planet ninety thousand miles in diameter – but that was enough.
   Jupiter now filled the entire sky; it was so huge that neither mind nor eye could grasp it any longer, and both had abandoned the attempt. If it had not been for the extraordinary variety of color – the reds and pinks and yellows and salmons and even scarlets – of the atmosphere beneath them, Bowman could have believed that he was flying low over a cloudscape on Earth.
   And now, for the first time in all their journeying, they were about to lose the Sun. Pale and shrunken though it was, it had been Discovery's constant companion since her departure from Earth, five months ago. But now her orbit was diving into the shadow of Jupiter; she would soon pass over the night side of the planet.
   A thousand miles ahead, the band of twilight was hurtling toward them; behind, the Sun was sinking swiftly into the Jovian clouds, its rays spread out along the horizon like two flaming, down-turned horns, then contracted and died in a brief blaze of chromatic glory. The night had come.
   And yet – the great world below was not wholly dark. It was awash with phosphorescence, which grew brighter minute by minute as their eyes grew accustomed to the scene. Dim rivers of light were flowing from horizon to horizon, like the luminous wakes of ships on some tropical sea. Here and there they gathered into pools of liquid fire, trembling with vast, submarine disturbances welling up from the hidden heart of Jupiter. It was a sight so awe-inspiring that Poole and Bowman could have stared for hours; was this, they wondered, merely the result of chemical and electrical forces down there in that seething caldron – or was it the by-product of some fantastic form of life? These were questions which scientists might still be debating when the newborn century drew to its close.
   As they drove deeper and deeper into the Jovian night, the glow beneath them grew steadily brighter.
   Once Bowman had flown over northern Canada during the height of an auroral display; the snow-covered landscape had been as bleak and brilliant as this. And that arctic wilderness, he reminded himself, was more than a hundred degrees warmer than the regions over which they were hurtling now.
   "Earth signal is fading rapidly," announced Hal. "We are entering the first diffraction zone."
   They had expected this – indeed, it was one of the mission's objectives, as the absorption of radio waves would give valuable information about the Jovian atmosphere. But now that they had actually passed behind the planet, and it was cutting off communication with Earth, they felt a sudden overwhelming loneliness. The radio blackout would last only an hour; then they would emerge from Jupiter's eclipsing screen, and could resume contact with the human race. That hour, however, would be one of the longest of their lives.
   Despite their relative youth, Poole and Bowman were veterans of a dozen space voyages, but now they felt like novices. They were attempting something for the first lime; never before had any ship traveled at such speeds, or braved so intense a gravitational field. The slightest error in navigation at this critical point and Discovery would go speeding on toward the far limits of the Solar System, beyond any hope of rescue.
   The slow minutes dragged by. Jupiter was now a vertical wall of phosphorescence stretching to infinity above them – and the ship was climbing straight up its glowing face. Though they knew that they were moving far too swiftly for even Jupiter's gravity to capture them, it was hard to believe that Discovery had not become a satellite of this monstrous world.
   At last, far ahead, there was a blaze of light along the horizon. They were emerging from shadow, heading out into the Sun. And at almost the same moment Hal announced: "I am in radio contact with Earth. I am also happy to say that the perturbation maneuver has been successfully completed. Our time to Saturn is one hundred and sixty-seven days, five hours, eleven minutes."
   That was within a minute of the estimate; the fly-by had been carried out with impeccable precision. Like a ball on a cosmic pool table, Discovery had bounced off the moving gravitational field of Jupiter, and had gained momentum from the impact. Without using any fuel, she had increased her speed by several thousand miles an hour.
   Yet there was no violation of the laws of mechanics; Nature always balances her books, and Jupiter had lost exactly as much momentum as Discovery had gained. The planet had been slowed down – but as its mass was a sextillion times greater than the ship's, the change in its orbit was far too small to be detectable. The time had not yet come when Man could leave his mark upon the Solar System.
   As the light grew swiftly around them, and the shrunken Sun lifted once more into the Jovian sky, Poole and Bowman reached out silently and shook each other's hands.
   Though they could hardly believe it, the first part of the mission was safely over.

20 – The World of the Gods

   But they had not yet finished with Jupiter. Far behind, the two probes that Discovery had launched were making contact with the atmosphere.
   One was never heard from again; presumably it made too steep an entry, and burned up before it could send any information. The second was more successful; it sliced though the upper layers of the Jovian atmosphere, then skimmed out once more into space. As had been planned, it had lost so much speed by the encounter that it fell back again along a great ellipse. Two hours later, it reentered atmosphere on the daylight side of the planet – moving at seventy thousand miles an hour.
   Immediately, it was wrapped in an envelope of incandescent gas, and radio contact was lost. There were anxious minutes of waiting, then, for the two watchers on the control deck. They could not be certain that the probe would survive, and that the protective ceramic shield would not burn completely away before braking had finished. If that happened, the instruments would be vaporized in a fraction of a second.
   But the shield held long enough for the glowing meteor to come to rest. The charred fragments were jettisoned, the robot thrust out its antennas and began to peer around with its electronic senses. Aboard Discovery, now almost a quarter of a million miles away, the radio started to bring in the first authentic news from Jupiter.
   The thousands of pulses pouring in every second were reporting atmospheric composition, pressure, temperature, magnetic fields, radioactivity, and dozens of other factors which only the experts on Earth could unravel. However, there was one message that could be understood instantly; it was the TV picture, in full color, sent back by the falling probe.
   The first views came when the robot had already entered the atmosphere, and had discarded its protective shell. All that was visible was a yellow mist, flecked with patches of scarlet which moved past the camera at a dizzying rate – streaming upwards as the probe fell at several hundred miles an hour.
   The mist grew thicker; it was impossible to guess whether the camera was seeing for ten inches or ten miles, because there were no details on which the eye could focus. It seemed that, as far as the TV system was concerned, the mission was a failure. The equipment had worked, but there was nothing to see in this foggy, turbulent atmosphere.
   And then, quite abruptly, the mist vanished. The probe must have fallen through the base of a high layer of cloud, and come out into a clear zone – perhaps a region of almost pure hydrogen with only a sparse scattering of ammonia crystals. Though it was still quite impossible to judge the scale of the picture, the camera was obviously seeing for miles.
   The scene was so alien that for a moment it was almost meaningless to eyes accustomed to the colors and shapes of Earth. Far, far below lay an endless sea of mottled gold, scarred with parallel ridges that might have been the crests of gigantic waves. But there was no movement; the scale of the scene was too immense to show it. And that golden vista could not possibly have been an ocean, for it was still high in the Jovian atmosphere. It could only have been another layer of cloud.
   Then the camera caught, tantalizingly blurred by distance, a glimpse of something very strange. Many miles away, the golden landscape reared itself into a curiously symmetrical cone, like a volcanic mountain. Around the summit of that cone was a halo of small, puffy clouds – all about the same size, all quite distinct and isolated. There was something disturbing and unnatural about them – if, indeed, the word "natural" could ever be applied to this awesome panorama.
   Then, caught by some turbulence in the rapidly thickening atmosphere, the probe twisted around to another quarter of the horizon, and for a few seconds the screen showed nothing but a golden blur. Presently it stabilized; the "sea" was much closer, but as enigmatic as ever. One could now observe that it was interrupted here and there with patches of darkness, which might have been holes or gaps leading to still deeper layers of the atmosphere.
   The probe was destined never to reach them. Every mile, the density of the gas around it had been doubling, the pressure mounting as it sank deeper and deeper toward the hidden surface of the planet. It was still high above that mysterious sea when the picture gave one premonitory flicker, then vanished, as the first explorer from Earth crumpled beneath the weight of the miles of atmosphere above it.
   It had given, in its brief life, a glimpse of perhaps one millionth of Jupiter, and had barely approached the planet's surface, hundreds of miles down in the deepening mists. When the picture faded from the screen, Bowman and Poole could only sit in silence, turning the same thought over in their minds.
   The ancients had, indeed, done better than they knew when they named this world after the lord of all the gods. If there was life down there, how long would it take even to locate it? And after that, how many centuries before men could follow this first pioneer – in what kind of ship?
   But these matters were now no concern of Discovery and her crew. Their goal was a still stranger world, almost twice as far from the Sun – across another half billion miles of comet-haunted emptiness.

IV – ABYSS

21 – Birthday Party

   The familiar strains of "Happy Birthday," hurled across seven hundred million miles of space at the velocity of light, died away among the vision screens and instrumentation of the control deck. The Poole family, grouped rather self-consciously round the birthday cake on Earth, lapsed into a sudden silence.
   Then Mr. Poole, Senior, said gruffly: "Well, Frank, can't think of anything else to say at the moment, except that our thoughts are with you, and we're wishing you the happiest of birthdays."
   "Take care, darling," Mrs. Poole interjected tearfully. "God bless you."
   There was a chorus of "good-byes," and the vision screen went blank. How strange to think, Poole told himself, that all this had happened more than an hour ago; by now his family would have dispersed again and its members would be miles from home. But in a way that time lag, though it could be frustrating, was also a blessing in disguise. Like every man of his age, Poole took it for granted that he could talk instantly, to anyone on Earth, whenever he pleased. Now that this was no longer true, the psychological impact was profound. He had moved into a new dimension of remoteness, and almost all emotional links had been stretched beyond the yield point.
   "Sorry to interrupt the festivities," said Hal, "but we have a problem."
   "What is it?" Bowman and Poole asked simultaneously.
   "I am having difficulty in maintaining contact with Earth. The trouble is in the AE-35 unit. My Fault Prediction Center reports that it may fail within seventy-two hours."
   "We'll take care of it," Bowman replied. "Let's see the optical alignment."
   "Here it is, Dave. It's still O.K. at the moment."
   On the display screen appeared a perfect half-moon, very brilliant against a background almost free of stars. It was covered with clouds, and showed not one geographical feature that could be recognized. Indeed, at first glance it could be easily mistaken for Venus.
   But not at a second one, for there beside it was the real Moon which Venus did not possess – a quarter the size of Earth, and in exactly the same phase. It was easy to imagine that the two bodies were mother and child, as many astronomers had believed, before the evidence of the lunar rocks had proved beyond doubt that the Moon had never been part of Earth.
   Poole and Bowman studied the screen in silence for half a minute. This image was coming to them from the long-focus TV camera mounted on the rim of the big radio dish; the cross-wires at its center showed the exact orientation of the antenna. Unless the narrow pencil beam was pointed precisely at Earth, they could neither receive nor transmit. Messages in both directions would miss their target and would shoot, unheard and unseen, out through the Solar System and into the emptiness beyond. If they were ever received, it would not be for centuries – and not by men.
   "Do you know where the trouble is?" asked Bowman.
   "It's intermittent and I can't localize it. But it appears to be in the AE-35 unit."
   "What procedure do you suggest?"
   "The best thing would be to replace the unit with a spare, so that we can check it over."
   "O.K. – let us have the hard copy."
   The information flashed on the display screen; simultaneously, a sheet of paper slid out of the slot immediately beneath it. Despite all the electronic read-outs, there were times when good old-fashioned printed material was the most convenient form of record.
   Bowman studied the diagrams for a moment, then whistled.
   "You might have told us," he said. "This means going outside the ship."
   "I'm sorry," Hal replied. "I assumed you knew that the AE-35 unit was on the antenna mounting."
   "I probably did, a year ago. But there are eight thousand subsystems aboard. Anyway, it looks a straightforward job. We only have to unlock a panel and put in a new unit."
   "That suits me fine," said Poole, who was the crew member designated for routine extravehicular activity. "I could do with a change of scenery. Nothing personal, of course."
   "Let's see if Mission Control agrees," said Bowman. He sat still for a few seconds, marshaling his thoughts, then started to dictate a message.
   "Mission Control, this is X-ray-Delta-One. At two-zero-four-five, on-board fault prediction center in our niner-triple-zero computer showed Alpha Echo three five unit as probable failure within seventy-two hours. Request check your telemetry monitoring and suggest you review unit in your ship systems simulator. Also, confirm your approval our plan to go EVA and replace Alpha Echo three five unit prior to failure. Mission Control, this is X-ray-Delta-One, two-one-zero-three transmission concluded."
   Through years of practice, Bowman could switch at a moment's notice to this jargon – which someone had once christened "Technish" – and back again to normal speech, without clashing his mental gears. Now there was nothing to do but to wait for the confirmation, which would take at least two hours as the signals made the round trip past the orbits of Jupiter and Mars.
   It came while Bowman was trying, without much success, to beat Hal at one of the geometrical pattern games stored in his memory.
   "X-ray-Delta-One, this is Mission Control, acknowledging your two-one-zero-three. We are reviewing telemetric information on our mission simulator and will advise.
   "Roger your plan to go EVA and replace Alpha-Echo three-five unit prior to possible failure. We are working on test procedures for you to apply to faulty unit."
   The serious business having been completed, the Mission Controller reverted to normal English.
   "Sorry you fellows are having a bit of trouble, and we don't want to add to your woes. But if it's convenient to you prior to EVA, we have a request from Public Information. Could you do a brief recording for general release, outlining the situation and explaining just what the AE-35 does. Make it as reassuring as you can. We could do it, of course – but it will be much more convincing in your words. Hope this won't interfere too badly with your social life. X-ray-Delta-One, this is Mission Control, two-one-five-five, transmission concluded."
   Bowman could not help smiling at the request. There were times when Earth showed a curious insensitivity and lack of tact. "Make it reassuring," indeed!
   When Poole joined him at the end of his sleep period, they spent ten minutes composing and polishing the reply. In the early stages of the mission, there had been countless requests from all the news media for interviews, discussions – almost anything that they cared to say. But as the weeks drifted uneventfully past, and the time lag increased from a few minutes to over an hour, interest had gradually slackened. Since the excitement of the Jupiter fly-by, over a month ago, they had made only three or four tapes for general release.
   "Mission Control, this is X-ray-Delta-One. Here is your press statement.
   "Earlier today, a minor technical problem occurred. Our HAL-9001 computer predicted the failure of the AE-35 unit.
   "This is a small but vital component of the communication system. It keeps our main antenna aimed at Earth to within a few thousandths of a degree. This accuracy is required, since at our present distance of more than seven hundred million miles, Earth is only a rather faint star, and our very narrow radio beam could easily miss it.
   "The antenna is kept constantly tracking Earth by motors controlled from the central computer. But those motors get their instructions via the AE-35 unit. You might compare it to a nerve center in the body, which translates the brain's instructions to the muscles of a limb. If the nerve fails to pass on the correct signals, the limb becomes useless. In our case, a breakdown of the AE-35 unit could mean that the antenna will start pointing at random. This was a common trouble with the deep-space probes of the last century. They often reached other planets, then failed to send back any information because their antenna couldn't locate Earth.
   "We don't know the nature of the fault yet, but the situation is not at all serious, and there is no need for alarm. We have two back-up AE-35s, each of which has an operational life expectancy of twenty years, so the chance that a second will fail during the course of this mission is' negligible. Also, if we can diagnose the present trouble, we may be able to repair the number one unit.
   "Frank Poole, who is specially qualified for this type of work, will go outside the ship and replace the faulty unit with the back-up. At the same time, he'll take the opportunity of checking the hull and repairing some micropunctures that have been too small to merit a special EVA.
   "Apart from this minor problem, the mission is still going uneventfully and should continue in the same manner.
   "Mission Control, this is X-ray-Delta-One, two-one-zero-four, transmission concluded."

22 – Excursion

   Discovery's extravehicular capsules or "space pods" were spheres about nine feet in diameter, and the operator sat behind a bay window which gave him a splendid view. The main rocket drive produced an acceleration of one-fifth of a gravity – just sufficient to hover on the Moon – while small attitude-control nozzles allowed for steering. From an area immediately beneath the bay window sprouted two sets of articulated metal arms or "waldoes," one for heavy duty, the other for delicate manipulation. There was also an extensible turret carrying a variety of power tools, such as screwdrivers, jack-hammers, saws, and drills.
   Space pods were not the most elegant means of transport devised by man, but they were absolutely essential for construction and maintenance work in vacuum. They were usually christened with feminine names, perhaps in recognition of the fact that their personalities were sometimes slightly unpredictable. Discovery's trio were Anna, Betty, and Clara.
   Once he had put on his personal pressure suit – his last line of defense – and climbed inside the pod, Poole spent ten minutes carefully checking the controls. He burped the steering jets, flexed the waldoes, reconfirmed oxygen, fuel, power reserve. Then, when he was completely satisfied, he spoke to Hal over the radio circuit. Though Bowman was standing by on the control deck, he would not interfere unless there was some obvious mistake or malfunction.
   "This is Betty. Start pumping sequence."
   "Pumping sequence started," repeated Hal. At once, Poole could hear the throbbing of the pumps as precious air was sucked out of the lock chamber. Presently, the thin metal of the pod's external shell made crinkling, crackling noises, then, after about five minutes, Hal reported:
   "Pumping sequence concluded."
   Poole made a final check of his tiny instrument panel. Everything was perfectly normal.
   "Open outer door," he ordered.
   Again Hal repeated his instructions; at any stage, Poole had only to call "Hold!" and the computer would stop the sequence immediately.
   Ahead, the walls of the ship slid apart. Poole felt the pod rock briefly as the last thin traces of air rushed into space. Then he was looking out at the stars – and, as it happened, at the tiny, golden disk of Saturn, still four hundred million miles away.
   "Commence pod ejection."
   Very slowly, the rail from which the pod was hanging extended itself out through the open door until the vehicle was suspended just beyond the hull of the ship.
   Poole gave a half-second burst on the main jet and the pod slid gently off the rail, becoming at last an independent vehicle pursuing its own orbit around the Sun. He now had no connection with Discovery – not even a safety line. The pods seldom gave trouble; and even if he got stranded, Bowman could easily come and rescue him.
   Betty responded smoothly to the control; he let her drift outward for a hundred feet, then checked her forward momentum and spun her round so that he was looking back at the ship. Then he began his tour of the pressure hull.
   His first target was a fused area about half an inch across, with a tiny central crater. The particle of dust that had impacted here at over a hundred thousand miles an hour was certainly smaller than a pinhead, and its enormous kinetic energy had vaporized it instantly. 'As was often the case, the crater looked as if it had been caused by an explosion from inside the ship; at these velocities, materials behaved in strange ways and the laws of common-sense mechanics seldom applied.
   Poole examined the area carefully, then sprayed it with sealant from a pressurized container in the pod's general-purpose kit. The white, rubbery fluid spread over the metal skin, hiding the crater from view. The leak blew one large bubble, which burst when it was about six inches across, then a much smaller one, then it subsided as the fast-setting cement did its work, He watched it intently for several minutes, but there was no further sign of activity. However, to make doubly certain, he sprayed on a second layer; then he set off toward the antenna.
   It took him some time to orbit Discovery's spherical pressure hull, for he never let the pod build up a speed of more than a few feet a second. He was in no hurry, and it was dangerous to move at a high velocity so near the ship. He had to keep a sharp lookout for the various sensors and instrument booms that projected from the hull at unlikely places, and he also had to be careful with his own jet blast. It could do considerable damage if it happened to hit some of the more fragile equipment.
   When at last he reached the long-range antenna, he surveyed the situation carefully. The big twenty-foot-diameter bowl appeared to be aimed directly at the Sun, for the Earth was now almost in line with the solar disk. The antenna mounting with all its orientation gear was therefore in total darkness, hidden in the shadow of the great metal saucer.
   Poole had approached it from the rear; he had been careful not to go in front of the shallow parabolic reflector, lest Betty interrupt the beam and cause a momentary, but annoying, loss of contact with Earth. He could not see anything of the equipment he had come to service until he switched on the pod's spotlights and banished the shadows.
   Beneath that small metal plate lay the cause of the trouble. The plate was secured by four locknuts, and as the entire AE-35 unit had been designed for easy replacement, Poole did not anticipate any problems.
   It was obvious, however, that he could not do the job while he remained in the space pod. Not only was it risky to maneuver so close to the delicate, and even spidery, framework of the antenna, but Betty's control jets could easily buckle the paper-thin reflecting surface of the big radio mirror. He would have to park the pod twenty feet away and go out in his suit. In any event, he could remove the unit much more quickly with his gloved hands than with Betty's remote manipulators.
   All this he reported carefully to Bowman, who double-checked every stage in the operation before it was carried out. Though this was a simple, routine job, nothing could be taken for granted in space, and no detail must be overlooked. In extravehicular activities, there was no such thing as a "minor" mistake.
   He received the O.K. for the procedure, and parked the pod some twenty feet away from the base of the antenna support. There was no danger that it would drift off into space; nevertheless, he clamped a manipulator hand over one of the many short sections of ladder rung strategically mounted on the outer hull.
   Then he checked the systems of his pressure suit, and, when he was quite satisfied, bled the air out of the pod. As Betty's atmosphere hissed away into the vacuum of space, a cloud of ice crystals formed briefly around him, and the stars were momentarily dimmed.
   There was one thing more to do before he left the pod. He switched over from manual to remote operation, putting Betty now under control of Hal. It was a standard safety precaution; though he was still secured to Betty by an immensely strong spring-loaded cord little thicker than cotton, even the best safety lines had been known to fail. He would look a fool if he needed his vehicle – and was unable to call it to his assistance by passing instructions to Hal.
   The door of the pod swung open, and he drifted slowly out into the silence of space, his safety line unreeling behind him. Take things easy – never move quickly – stop and think – these were the rules for extravehicular activity. If one obeyed them, there was never any trouble.
   He grabbed one of Betty's external handholds, and removed the spare AE-35 unit from the carry-pouch where it had been stowed, kangaroo fashion. He did not stop to collect any of the pod's collection of tools, most of which were not designed for use by human bands. All the adjustable wrenches and keys he was likely to need were already attached to the belt of his suit.
   With a gentle push, he launched himself toward the gimbaled mounting of the big dish that loomed like a giant saucer between him and the Sun. His own double shadow, thrown by Betty's spotlights, danced across the convex surface in fantastic patterns as he drifted down the twin beams. But here and there, he was surprised to notice, the rear of the great radio mirror sparkled with dazzlingly brilliant pinpoints of light.
   He puzzled over these for the few seconds of his silent approach, then realized what they were. During the voyage, the reflector must have been penetrated many times by micrometeors; he was seeing the sunlight blazing through the tiny craters. They were all far too small to have affected the system's performance appreciably.
   As he was moving very slowly, he broke the gentle impact with his outstretched arm, and grabbed hold of the antenna mounting before he could rebound. He quickly hooked his safety belt to the nearest attachment; that would give him something to brace against when he used his tools. Then he paused, reported the situation to Bowman, and considered his next step.
   There was one minor problem; he was standing – or floating – in his own light, and it was hard to see the AE-35 unit in the shadow he cast. So he ordered Hal to swing the spots off to one side, and after a little experimenting got a more uniform illumination from secondary light reflected off the back of the antenna dish.
   For a few seconds, he studied the small metal hatch with its four wire-secured locking nuts. Then, muttering to himself, "Tampering by unauthorized personnel invalidates the manufacturer's guarantee," he snipped the wires and started to untwist the nuts. They were a standard size, fitting the zero-torque wrench that he carried. The tool's internal spring mechanism would absorb the reaction as the nuts were unthreaded, so that the operator would have no tendency to spin around in reverse.
   The four nuts came off without any trouble, and Poole stowed them carefully away in a convenient pouch. (One day, somebody had predicted, Earth would have a ring like Saturn's, composed entirely of lost bolts, fasteners, and even tools that had escaped from careless orbital construction workers.) The metal cover was a little sticky, and for a moment he was afraid it might have cold-welded into place; but after a few taps it came loose, and he secured it to the antenna mounting by a large crocodile clip.
   Now he could see the electronic circuitry of the AE-35 unit. It was in the form of a thin slab, about the size of a postcard, gripped by a slot just large enough to hold it. The unit was secured in place by two locking bars, and had a small handle so that it could be easily removed.
   But it was still operating, feeding the antenna the impulses that kept it aimed at the far-off pinpoint of Earth. If it was pulled out now, all control would be lost, and the dish would slam round to its neutral or zero-azimuth position, pointing along the axis of Discovery. And this could be dangerous; it might crash into him as it rotated.
   To avoid this particular hazard, it was only necessary to cut off power to the control system; then the antenna could not move, unless Poole knocked against it himself. There was no danger of losing Earth during the few minutes it would take him to replace the unit; their target would not have shifted appreciably against the background of the stars in such a brief interval of time.
   "Hal," Poole called over the radio circuit, "I am about to remove the unit. Switch off all control power to the antenna system."

   "Antenna control power off," answered Hal.
   "Here goes. I'm pulling the unit out now."
   The card slipped out of its slot with no difficulty; it did not jam, and none of the dozens of sliding contacts stuck. Within a minute, the spare was in place.
   But Poole was taking no chances. He pushed himself gently away from the antenna mount, just in case the big dish went wild when power was restored. When he was safely out of range, he called to Hal: "The new unit should be operational. Restore control power."
   "Power on," answered Hal. The antenna remained rock steady.
   "Carry out fault prediction tests."
   Now microscopic pulses would be bouncing through the complex circuitry of the unit, probing for possible failures, testing the myriads of components to see that they all lay within their specified tolerances. This had been done, of course, a score of times before the unit had ever left the factory; but that was two years ago, and more than half a billion miles away. It was often impossible to see how solid-state electronic components could fail; yet they did.
   "Circuit fully operational," reported Hal after only ten seconds. In that time, he carried out as many tests as a small army of human inspectors.
   "Fine," said Poole with satisfaction. "Now replacing the cover."
   This was often the most dangerous part of an extravehicular operation: when a job was finished and it was merely a matter of tidying up and getting back inside the ship – that was when the mistakes were made. But Frank Poole would not have been on this mission if he had not been careful and conscientious. He took his time, and though one of the locking nuts almost got away from him, he caught it before it had traveled more than a few feet.
   Fifteen minutes later he was jetting back into the space-pod garage, quietly confident that here was one job that need not be done again.
   In this, however, he was sadly mistaken.

23 – Diagnosis

   "Do you mean to say," exclaimed Frank Poole, more surprised than annoyed, "that I did all that work for nothing?"
   "Seems like it," answered Bowman. "The unit checks out perfectly. Even under two hundred percent overload, there's no fault prediction indicated."
   The two men were standing in the tiny workshop-cum-lab in the carrousel, which was more convenient than the space-pod garage for minor repairs and exanimations. There was no danger, here, of meeting blobs of hot solder drifting down the breeze, or of completely losing small items of equipment that had decided to go into orbit. Such things could – and did – happen in the zero-gee environment of the pod bay.
   The thin, card-sized plate of the AE-35 unit lay on the bench under a powerful magnifying lens. It was plugged into a standard connection frame, from which a neat bundle of multicolored wire led to an automatic test set, no bigger than an ordinary desk computer. To check any unit it was only necessary to connect it up, slip in the appropriate card from the "trouble-shooting" library, and press a button. Usually the exact location of the fault would be indicated on a small display screen, with recommendations for action.
   "Try it yourself," said Bowman, in a somewhat frustrated voice. Poole turned the OVERLOAD SELECT switch to X-2 and jabbed the TEST button. At once, the screen flashed the notice: UNIT OK.
   "I suppose we could go on turning up the juice until we burned the thing out," he said, "but that would prove nothing. What do you make of it?"
   "Hal's internal fault predictor could have made a mistake."
   "It's more likely that our test rig has slipped up. Anyway, better safe than sorry. It's just as well that we replaced the unit, if there's the slightest doubt."
   Bowman unclipped the wafer of circuitry, and held it up to the light. The partly translucent material was veined with an intricate network of wiring and spotted with dimly visible microcomponents, so that it looked like some piece of abstract art.
   "We can't take any chances – after all, this is our link with Earth. I'll file it as N/G and drop it in the junk store. Someone else can worry about it, when we get home."
   But the worrying was to start long before that, with the next transmission from Earth.
   "X-ray-Delta-One, this is Mission Control, reference our two-one-five-five. We appear to have a slight problem.
   "Your report that there is nothing wrong with the Alpha Echo three five unit agrees with our diagnosis. The fault could lie in the associated antenna circuits, but if so that should be apparent from other tests.
   "There is a third possibility, which may be more serious. Your computer may have made an error in predicting the fault. Both our own nine-triple-zeros agree in suggesting this, on the basis of their information. This is not necessarily cause for alarm, in view of the back-up systems we have, but we would like you to watch out for any further deviations from nominal performance. We have suspected several minor irregularities in the past few days, but none have been important enough for remedial action, and they have shown no obvious pattern from which we can draw any conclusions. We are running further tests with both our computers and will report as soon as the results are available. We repeat that there is no need for alarm; the worst that can happen is that we may have to disconnect your nine-triple-zero temporarily for program analysis, and hand over control to one of our computers. The time lag will introduce problems, but our feasibility studies indicate that Earth control is perfectly satisfactory at this stage of the mission.
   "X-ray-Delta-One, this is Mission Control, two-one-five-six, transmission concluded."
   Frank Poole, who was on watch when the message came in, thought this over in silence. He waited to see if there was any comment from Hal, but the computer did not attempt to challenge the implied accusation. Well, if Hal would not raise the subject, he did not propose to do so either.
   It was almost time for the morning changeover, and normally he would wait until Bowman joined him on the control deck. But today he broke this routine, and made his way back to the carrousel.
   Bowman was already up, pouring himself some coffee from the dispenser, when Poole greeted him with a rather worried "good morning." After all these months in space, they still thought in terms of the normal twenty-four-hour cycle – though they had long since forgotten the days of the week.
   "Good morning," replied Bowman. "How's it going?" Poole helped himself to coffee. "Pretty well. Are you reasonably awake?"
   "I'm fine. What's up?"
   By this time, each knew at once when anything was amiss. The slightest interruption of the normal routine was a sign that had to be watched.
   "Well," Poole answered slowly. "Mission Control has just dropped a small bomb on us." He lowered his voice, like a doctor discussing an illness in front of the patient. "We may have a slight case of hypochondria aboard."
   Perhaps Bowman was not fully awake, after all; it took him several seconds to get the point. Then he said "Oh-I see. What else did they tell you?"
   "That there was no cause for alarm. They said that twice, which rather spoiled the effect as far as I was concerned. And that they were considering a temporary switchover to Earth control while they ran a program analysis."
   They both knew, of course, that Hal was hearing every word, but they could not help these polite circumlocutions. Hal was their colleague, and they did not wish to embarrass him. Yet at this stage it did not seem necessary to discuss the matter in private.
   Bowman finished his breakfast in silence, while Poole toyed with the empty coffee container. They were both thinking furiously, but there was nothing more to say.
   They could only wait for the next report from Mission Control – and wonder if Hal would bring up the subject himself. Whatever happened, the atmosphere aboard the ship had subtly altered. There was a sense of strain in the air – a feeling that, for the first time, something might be going wrong.
   Discovery was no longer a happy ship.