The Sentinel Part 10

The mate sighed. It still seemed hard to believe that in such circ.u.mstances two men could have behaved in so reasonable, so unemotional a manner. He could have pictured all sorts of dramatic possibilities-sudden outbursts of insanity, even attempts at murder. Yet according to McNeil nothing at all had happened. It was too bad.

McNeil was speaking again, as if to himself. "Yes, Grant behaved very well-very well indeed. It was a great pity-"

Then he seemed to lose himself in the ever-fresh, incomparable glory of the approaching planet. Not far beneath, and coming closer by kilometers every second, the snow-white crescent arms of Venus spanned more than half the sky. Down there were life and warmth and civilization-and air.

The future, which not long ago had seemed contracted to a point, had opened out again into all its unknown possibilities and wonders. But behind him McNeil could sense the eyes of his rescuers, probing, questioning-yes, and condemning too.

All his life he would hear whispers. Voices would be saying behind his back, "Isn't that the man who-?"

He did not care. For once in his life at least, he had done something of which he could feel unashamed. Perhaps one day his own pitiless self-a.n.a.lysis would strip bear the motives behind his actions, would whisper in his ear, "Altruism? Don't be a fool! You did it to bolster up your own good opinion of yourself-so much more important than anyone else's!"

But the perverse maddening voices, which all his life had made nothing seem worthwhile, were silent for the moment and he felt content. He had reached the calm at the center of the hurricane. While it lasted he would enjoy it to the full.

THE SENTINEL.

Next to "The Star" and "The Nine Billion Names of G.o.d," I suppose "The Sentinel" is my best-known short story-though not for itself, but as the seed from which 2001: A s.p.a.ce Odyssey 2001: A s.p.a.ce Odyssey sprang, twenty years after it was written in 1948. I wonder if I even noticed Christmas that year; Opus 62 bears the date 2326 December sprang, twenty years after it was written in 1948. I wonder if I even noticed Christmas that year; Opus 62 bears the date 2326 December...

Unlike most of my short stories, this one was aimed at a specific target-which it missed completely. The BBC had just announced a Short Story Compet.i.tion; I submitted "The Sentinel" hot from the typewriter, and got it back a month later.

Somehow, I've never had any luck with such contests. A few years later I wrote "The Star" specifically for a London Observer compet.i.tion on the subject "2500 A.D." It too was bounced-though the judges were perceptive enough to give an award to one Brian Aldiss.

I am continually annoyed by careless references to "The Sentinel" as "the story on which 2001 is based"; it bears about as much relation to the movie as an acorn to the resultant full-grown oak. (Considerably less, in fact, because ideas from several other stories were also incorporated.) Even the elements that Stanley Kubrick and I did actually use were considerably modified. Thus the "glittering, roughly pyramidal structure...set in the rock like a gigantic, many-faceted jewel" became-after several modifications-the famous black monolith. And the locale was moved from the Mare Crisium to the most spectacular of all lunar craters, Tycho-easily visible to the naked eye from Earth at Full Moon.

Some time after "The Sentinel" was published, I was asked if I had ever read Jack London's "The Red One" (1918). As I'd never even heard of it, I hastened to do so, and was deeply impressed by his thirty-year-earlier tale of the "Star-Born," an enormous sphere lying for ages in the jungles of Guadalca.n.a.l. I wonder if this is the first treatment of a theme which has suddenly become topical, now that the focus of the SETI debate has changed from "Where's Everyone?" to the even more puzzling "Where Are Their Artifacts?"

THE NEXT TIME YOU SEE THE FULL MOON high in the south, look carefully at its right-hand edge and let your eye travel upward along the curve of the disk. Round about two o'clock you will notice a small, dark oval: anyone with normal eyesight can find it quite easily. It is the great walled plain, one of the finest on the Moon, known as the Mare Crisium-the Sea of Crises. Three hundred miles in diameter, and almost completely surrounded by a ring of magnificent mountains, it had never been explored until we entered it in the late summer of 1996. high in the south, look carefully at its right-hand edge and let your eye travel upward along the curve of the disk. Round about two o'clock you will notice a small, dark oval: anyone with normal eyesight can find it quite easily. It is the great walled plain, one of the finest on the Moon, known as the Mare Crisium-the Sea of Crises. Three hundred miles in diameter, and almost completely surrounded by a ring of magnificent mountains, it had never been explored until we entered it in the late summer of 1996.

Our expedition was a large one. We had two heavy freighters which had flown our supplies and equipment from the main lunar base in the Mare Serenitatis, five hundred miles away. There were also three small rockets which were intended for short-range transport over regions which our surface vehicles couldn't cross. Luckily, most of the Mare Crisium is very flat. There are none of the great creva.s.ses so common and so dangerous elsewhere, and very few craters or mountains of any size. As far as we could tell, our powerful caterpillar tractors would have no difficulty in taking us wherever we wished to go.

I was geologist-or selenologist, if you want to be pedantic-in charge of the group exploring the southern region of the Mare. We had crossed a hundred miles of it in a week, skirting the foothills of the mountains along the sh.o.r.e of what was once the ancient sea, some thousand million years before. When life was beginning on Earth, it was already dying here. The waters were retreating down the flanks of those stupendous cliffs, retreating into the empty heart of the Moon. Over the land which we were crossing, the tideless ocean had once been half a mile deep, and now the only trace of moisture was the h.o.a.rfrost one could sometimes find in caves which the searing sunlight never penetrated.

We had begun our journey early in the slow lunar dawn, and still had almost a week of Earth-time before nightfall. Half a dozen times a day we would leave our vehicle and go outside in the s.p.a.ce-suits to hunt for interesting minerals, or to place markers for the guidance of future travelers. It was an uneventful routine. There is nothing hazardous or even particularly exciting about lunar exploration. We could live comfortably for a month in our pressurized tractors, and if we ran into trouble we could always radio for help and sit tight until one of the s.p.a.ces.h.i.+ps came to our rescue.

I said just now that there was nothing exciting about lunar exploration, but of course that isn't true. One could never grow tired of those incredible mountains, so much more rugged than the gentle hills of Earth. We never knew, as we rounded the capes and promontories of that vanished sea, what new splendors would be revealed to us. The whole southern curve of the Mare Crisium is a vast delta where a score of rivers once found their way into the ocean, fed perhaps by the torrential rains that must have lashed the mountains in the brief volcanic age when the Moon was young. Each of these ancient valleys was an invitation, challenging us to climb into the unknown uplands beyond. But we had a hundred miles still to cover, and could only look longingly at the heights which others must scale.

We kept Earth-time aboard the tractor, and precisely at 22.00 hours the final radio message would be sent out to Base and we would close down for the day. Outside, the rocks would still be burning beneath the almost vertical sun, but to us it was night until we awoke again eight hours later. Then one of us would prepare breakfast, there would be a great buzzing of electric razors, and someone would switch on the short-wave radio from Earth. Indeed, when the smell of frying sausages began to fill the cabin, it was sometimes hard to believe that we were not back on our own world-everything was so normal and homely, apart from the feeling of decreased weight and the unnatural slowness with which objects fell.

It was my turn to prepare breakfast in the corner of the main cabin that served as a galley. I can remember that moment quite vividly after all these years, for the radio had just played one of my favorite melodies, the old Welsh air, "David of the White Rock." Our driver was already outside in his s.p.a.ce-suit, inspecting our caterpillar treads. My a.s.sistant, Louis Garnett, was up forward in the control position, making some belated entries in yesterday's log.

As I stood by the frying pan waiting, like any terrestrial housewife, for the sausages to brown, I let my gaze wander idly over the mountain walls which covered the whole of the southern horizon, marching out of sight to east and west below the curve of the Moon. They seemed only a mile or two from the tractor, but I knew that the nearest was twenty miles away. On the Moon, of course, there is no loss of detail with distance-none of that almost imperceptible haziness which softens and sometimes transfigures all far-off things on Earth.

Those mountains were ten thousand feet high, and they climbed steeply out of the plain as if ages ago some subterranean eruption had smashed them skyward through the molten crust. The base of even the nearest was hidden from sight by the steeply curving surface of the plain, for the Moon is a very little world, and from where I was standing the horizon was only two miles away.

I lifted my eyes toward the peaks which no man had ever climbed, the peaks which, before the coming of terrestrial life, had watched the retreating oceans sink sullenly into their graves, taking with them the hope and the morning promise of a world. The sunlight was beating against those ramparts with a glare that hurt the eyes, yet only a little way above them the stars were s.h.i.+ning steadily in a sky blacker than a winter midnight on Earth.

I was turning away when my eye caught a metallic glitter high on the ridge of a great promontory thrusting out into the sea thirty miles to the west. It was a dimensionless point of light, as if a star had been clawed from the sky by one of those cruel peaks, and I imagined that some smooth rock surface was catching the sunlight and heliographing it straight into my eyes. Such things were not uncommon. When the Moon is in her second quarter, observers on Earth can sometimes see the great ranges in the Ocea.n.u.s Procellarum burning with a blue-white iridescence as the sunlight flashes from their slopes and leaps again from world to world. But I was curious to know what kind of rock could be s.h.i.+ning so brightly up there, and I climbed into the observation turret and swung our four-inch telescope round to the west.

I could see just enough to tantalize me. Clear and sharp in the field of vision, the mountain peaks seemed only half a mile away, but whatever was catching the sunlight was still too small to be resolved. Yet it seemed to have an elusive symmetry, and the summit upon which it rested was curiously flat. I stared for a long time at that glittering enigma, straining my eyes into s.p.a.ce, until presently a smell of burning from the galley told me that our breakfast sausages had made their quarter-million-mile journey in vain.

All that morning we argued our way across the Mare Crisium while the western mountains reared higher in the sky. Even when we were out prospecting in the s.p.a.ce-suits, the discussion would continue over the radio. It was absolutely certain, my companions argued, that there had never been any form of intelligent life on the Moon. The only living things that had ever existed there were a few primitive plants and their slightly less degenerate ancestors. I knew that as well as anyone, but there are times when a scientist must not be afraid to make a fool of himself.

"Listen," I said at last. "I'm going up there, if only for my own peace of mind. That mountain's less than twelve thousand feet high-that's only two thousand under Earth gravity-and I can make the trip in twenty hours at the outside. I've always wanted to go up into those hills, anyway, and this gives me an excellent excuse."

"If you don't break your neck," said Garnett, "you'll be the laughing-stock of the expedition when we get back to Base. That mountain will probably be called Wilson's Folly from now on."

"I won't break my neck," I said firmly. "Who was the first man to climb Pico and Helicon?"

"But weren't you rather younger in those days?" asked Louis gently.

"That," I said with great dignity, "is as good a reason as any for going."

We went to bed early that night, after driving the tractor to within half a mile of the promontory. Garnett was coming with me in the morning; he was a good climber, and had often been with me on such exploits before. Our driver was only too glad to be left in charge of the machine.

At first sight, those cliffs seemed completely unscaleable, but to anyone with a good head for heights, climbing is easy on a world where all weights are only a sixth of their normal value. The real danger in lunar mountaineering lies in overconfidence; a six-hundred-foot drop on the Moon can kill you just as thoroughly as a hundred-foot fall on Earth.

We made our first halt on a wide ledge about four thousand feet above the plain. Climbing had not been very difficult, but my limbs were stiff with the unaccustomed effort, and I was glad of the rest. We could still see the tractor as a tiny metal insect far down at the foot of the cliff, and we reported our progress to the driver before starting on the next ascent.

Inside our suits it was comfortably cool, for the refrigeration units were fighting the fierce sun and carrying away the body heat of our exertions. We seldom spoke to each other, except to pa.s.s climbing instructions and to discuss our best plan of ascent. I do not know what Garnett was thinking, probably that this was the craziest goose-chase he had ever embarked upon. I more than half agreed with him, but the joy of climbing, the knowledge that no man had ever gone this way before and the exhilaration of the steadily widening landscape gave me all the reward I needed.

I don't think I was particularly excited when I saw in front of us the wall of rock I had first inspected through the telescope from thirty miles away. It would level off about fifty feet above our heads, and there on the plateau would be the thing that had lured me over these barren wastes. It was, almost certainly, nothing more than a boulder splintered ages ago by a falling meteor, and with its cleavage planes still fresh and bright in this incorruptible, unchanging silence.

There were no hand-holds on the rock face, and we had to use a grapnel. My tired arms seemed to gain new strength as I swung the three-p.r.o.nged metal anchor round my head and sent it sailing up toward the stars. The first time it broke loose and came falling slowly back when we pulled the rope. On the third attempt, the p.r.o.ngs gripped firmly and our combined weights could not s.h.i.+ft it.

Garnett looked at me anxiously. I could tell that he wanted to go first, but I smiled back at him through the gla.s.s of my helmet and shook my head. Slowly, taking my time, I began the final ascent.

Even with my s.p.a.ce-suit, I weighed only forty pounds here, so I pulled myself up hand over hand without bothering to use my feet. At the rim I paused and waved to my companion, then I scrambled over the edge and stood upright, staring ahead of me.

You must understand that until this very moment I had been almost completely convinced that there could be nothing strange or unusual for me to find here. Almost, but not quite; it was that haunting doubt that had driven me forward. Well, it was a doubt no longer, but the haunting had scarcely begun.

I was standing on a plateau perhaps a hundred feet across. It had once been smooth-too smooth to be natural-but falling meteors had pitted and scored its surface through immeasurable eons. It had been leveled to support a glittering, roughly pyramidal structure, twice as high as a man, that was set in the rock like a gigantic, many-faceted jewel.

Probably no emotion at all filled my mind in those first few seconds. Then I felt a great lifting of my heart, and a strange, inexpressible joy. For I loved the Moon, and now I knew that the creeping moss of Aristarchus and Eratosthenes was not the only life she had brought forth in her youth. The old, discredited dream of the first explorers was true. There had, after all, been a lunar civilization-and I was the first to find it. That I had come perhaps a hundred million years too late did not distress me; it was enough to have come at all.

My mind was beginning to function normally, to a.n.a.lyze and to ask questions. Was this a building, a shrine-or something for which my language had no name? If a building, then why was it erected in so uniquely inaccessible a spot? I wondered if it might be a temple, and I could picture the adepts of some strange priesthood calling on their G.o.ds to preserve them as the life of the Moon ebbed with the dying oceans, and calling on their G.o.ds in vain.

I took a dozen steps forward to examine the thing more closely, but some sense of caution kept me from going too near. I knew a little of archaeology, and tried to guess the cultural level of the civilization that must have smoothed this mountain and raised the glittering mirror surfaces that still dazzled my eyes.

The Egyptians could have done it, I thought, if their workmen had possessed whatever strange materials these far more ancient architects had used. Because of the thing's smallness, it did not occur to me that I might be looking at the handiwork of a race more advanced than my own. The idea that the Moon had possessed intelligence at all was still almost too tremendous to grasp, and my pride would not let me take the final, humiliating plunge.

And then I noticed something that set the scalp crawling at the back of my neck-something so trivial and so innocent that many would never have noticed it at all. I have said that the plateau was scarred by meteors; it was also coated inches-deep with the cosmic dust that is always filtering down upon the surface of any world where there are no winds to disturb it. Yet the dust and the meteor scratches ended quite abruptly in a wide circle enclosing the little pyramid, as though an invisible wall was protecting it from the ravages of time and the slow but ceaseless bombardment from s.p.a.ce.

There was someone shouting in my earphones, and I realized that Garnett had been calling me for some time. I walked unsteadily to the edge of the cliff and signaled him to join me, not trusting myself to speak. Then I went back toward that circle in the dust. I picked up a fragment of splintered rock and tossed it gently toward the s.h.i.+ning enigma. If the pebble had vanished at that invisible barrier I should not have been surprised, but it seemed to hit a smooth, hemispherical surface and slide gently to the ground.

I knew then that I was looking at nothing that could be matched in the antiquity of my own race. This was not a building, but a machine, protecting itself with forces that had challenged Eternity. Those forces, whatever they might be, were still operating, and perhaps I had already come too close. I thought of all the radiations man had trapped and tamed in the past century. For all I knew, I might be as irrevocably doomed as if I had stepped into the deadly, silent aura of an uns.h.i.+elded atomic pile.

I remember turning then toward Garnett, who had joined me and was now standing motionless at my side. He seemed quite oblivious to me, so I did not disturb him but walked to the edge of the cliff in an effort to marshal my thoughts. There below me lay the Mare Crisium-Sea of Crises, indeed-strange and weird to most men, but rea.s.suringly familiar to me. I lifted my eyes toward the crescent Earth, lying in her cradle of stars, and I wondered what her clouds had covered when these unknown builders had finished their work. Was it the steaming jungle of the Carboniferous, the bleak sh.o.r.eline over which the first amphibians must crawl to conquer the land-or, earlier still, the long loneliness before the coming of life?

Do not ask me why I did not guess the truth sooner-the truth that seems so obvious now. In the first excitement of my discovery, I had a.s.sumed without question that this crystalline apparition had been built by some race belonging to the Moon's remote past, but suddenly, and with overwhelming force, the belief came to me that it was as alien to the Moon as I myself.

In twenty years we had found no trace of life but a few degenerate plants. No lunar civilization, whatever its doom, could have left but a single token of its existence.

I looked at the s.h.i.+ning pyramid again, and the more remote it seemed from anything that had to do with the Moon. And suddenly I felt myself shaking with a foolish, hysterical laughter, brought on by excitement and overexertion: for I had imagined that the little pyramid was speaking to me and was saying: "Sorry, I'm a stranger here myself."

It had taken us twenty years to crack that invisible s.h.i.+eld and to reach the machine inside those crystal walls. What we could not understand, we broke at last with the savage might of atomic power and now I have seen the fragments of the lovely, glittering thing I found up there on the mountain.

They are meaningless. The mechanisms-if indeed they are mechanisms-of the pyramid belong to a technology that lies far beyond our horizon, perhaps to the technology of para-physical forces.

The mystery haunts us all the more now that the other planets have been reached and we know that only Earth has ever been the home of intelligent life in our Universe. Nor could any lost civilization of our own world have built that machine, for the thickness of the meteoric dust on the plateau has enabled us to measure its age. It was set there upon its mountain before life had emerged from the seas of Earth.

When our world was half its present age, something something from the stars swept through the Solar System, left this token of its pa.s.sage, and went again upon its way. Until we destroyed it, that machine was still fulfilling the purpose of its builders; and as to that purpose, here is my guess. from the stars swept through the Solar System, left this token of its pa.s.sage, and went again upon its way. Until we destroyed it, that machine was still fulfilling the purpose of its builders; and as to that purpose, here is my guess.

Nearly a hundred thousand million stars are turning in the circle of the Milky Way, and long ago other races on the worlds of other suns must have scaled and pa.s.sed the heights that we have reached. Think of such civilizations, far back in time against the fading afterglow of Creation, masters of a universe so young that life as yet had come only to a handful of worlds. Theirs would have been a loneliness we cannot imagine, the loneliness of G.o.ds looking out across infinity and finding none to share their thoughts.

They must have searched the star cl.u.s.ters as we have searched the planets. Everywhere there would be worlds, but they would be empty or peopled with crawling, mindless things. Such was our own Earth, the smoke of the great volcanoes still staining the skies, when that first s.h.i.+p of the peoples of the dawn came sliding in from the abyss beyond Pluto. It pa.s.sed the frozen outer worlds, knowing that life could play no part in their destinies. It came to rest among the inner planets, warming themselves around the fire of the Sun and waiting for their stories to begin.

Those wanderers must have looked on Earth, circling safely in the narrow zone between fire and ice, and must have guessed that it was the favorite of the Sun's children. Here, in the distant future, would be intelligence; but there were countless stars before them still, and they might never come this way again.

So they left a sentinel, one of millions they have scattered throughout the Universe, watching over all worlds with the promise of life. It was a beacon that down the ages has been patiently signaling the fact that no one had discovered it.

Perhaps you understand now why that crystal pyramid was set upon the Moon instead of on the Earth. Its builders were not concerned with races still struggling up from savagery. They would be interested in our civilization only if we proved our fitness to survive-by crossing s.p.a.ce and so escaping from the Earth, our cradle. That is the challenge that all intelligent races must meet, sooner or later. It is a double challenge, for it depends in turn upon the conquest of atomic energy and the last choice between life and death.

Once we had pa.s.sed that crisis, it was only a matter of time before we found the pyramid and forced it open. Now its signals have ceased, and those whose duty it is will be turning their minds upon Earth. Perhaps they wish to help our infant civilization. But they must be very, very old, and the old are often insanely jealous of the young.

I can never look now at the Milky Way without wondering from which of those banked clouds of stars the emissaries are coming. If you will pardon so commonplace a simile, we have set off the fire-alarm and have nothing to do but to wait.

I do not think we will have to wait for long.

JUPITER V.

"Jupiter V," written in June 1951, belongs to that typical and often despised category of science fiction, the "gimmick" story, in which some little-known fact or natural law forms an essential part of the plot. The genre may well have originated (like much else) with Edgar Allan Poe; his "A Descent into the Maelstrom" is a cla.s.sic example.

As I wrote on its first appearance in volume form (Reach for Tomorrow, 1956): "I am by no means sure that I could write 'Jupiter V' today; it involved twenty or thirty pages of orbital calculations and should by rights be dedicated to Professor G. C. McVittie, my erstwhile tutor in applied mathematics." Twenty-seven years later I'm completely sure on this point.

But during those years something has happened that, when I wrote the story, I would have dismissed as completely incredible. The photographer covering the satellites of Jupiter for a 2044 issue of Life Life Magazine (presumably a holographic, satellite-delivered edition) was sixty-five years too late. The Voyager s.p.a.cecraft had already done the job, back in 1979... Magazine (presumably a holographic, satellite-delivered edition) was sixty-five years too late. The Voyager s.p.a.cecraft had already done the job, back in 1979...

Or at least the first part of it, for we will never never finish unravelling the complexities of the mini-solar-system formed by Jupiter and his moons. And dated though this story has been by the astonis.h.i.+ng speed of s.p.a.ce exploration (it was written, please remember, when Sputnik I was still six years in the futurel), it may yet contain some elements of truth finish unravelling the complexities of the mini-solar-system formed by Jupiter and his moons. And dated though this story has been by the astonis.h.i.+ng speed of s.p.a.ce exploration (it was written, please remember, when Sputnik I was still six years in the futurel), it may yet contain some elements of truth.

Those fuzzy, long-range shots of little Jupiter V (now officially christened Amalthea) look very, very odd indeed...

PROFESSOR F FORSTER IS SUCH A SMALL MAN that a special s.p.a.ce-suit had to be made for him. But what he lacked in physical size he more than made up-as in so often the case-in sheer drive and determination. When I met him, he'd spent twenty years pursuing a dream. What is more to the point, he had persuaded a whole succession of hard-headed business men, World Council Delegates and administrators of scientific trusts to underwrite his expenses and to fit out a s.h.i.+p for him. Despite everything that happened later, I still think that was his most remarkable achievement.... that a special s.p.a.ce-suit had to be made for him. But what he lacked in physical size he more than made up-as in so often the case-in sheer drive and determination. When I met him, he'd spent twenty years pursuing a dream. What is more to the point, he had persuaded a whole succession of hard-headed business men, World Council Delegates and administrators of scientific trusts to underwrite his expenses and to fit out a s.h.i.+p for him. Despite everything that happened later, I still think that was his most remarkable achievement....

The "Arnold Toynbee" had a crew of six aboard when we left Earth. Besides the Professor and Charles Ashton, his chief a.s.sistant, there was the usual pilot-navigator-engineer triumvirate and two graduate students-Bill Hawkins and myself. Neither of us had ever gone into s.p.a.ce before, and we were still so excited over the whole thing that we didn't care in the least whether we got back to Earth before the next term started. We had a strong suspicion that our tutor had very similar views. The reference he had produced for us was a masterpiece of ambiguity, but as the number of people who could even begin to read Martian script could be counted, if I may coin a phrase, on the fingers of one hand, we'd got the job.

As we were going to Jupiter, and not to Mars, the purpose of this particular qualification seemed a little obscure, though knowing something about the Professor's theories we had some pretty shrewd suspicions. They were partly confirmed when we were ten days out from Earth.

The Professor looked at us very thoughtfully when we answered his summons. Even under zero g he always managed to preserve his dignity, while the best we could do was to cling to the nearest handhold and float around like drifting seaweed. I got the impression-though I may of course be wrong-that he was thinking: What have I I done to deserve this? as he looked from Bill to me and back again. Then he gave a sort of "It's too late to do anything about it now" sigh and began to speak in that slow, patient way he always does when he has something to explain. At least, he always uses it when he's speaking to done to deserve this? as he looked from Bill to me and back again. Then he gave a sort of "It's too late to do anything about it now" sigh and began to speak in that slow, patient way he always does when he has something to explain. At least, he always uses it when he's speaking to us us, but it's just occurred to me-oh, never mind.

"Since we left Earth," he said, "I've not had much chance of telling you the purpose of this expedition. Perhaps you've guessed it already."

"I think I have," said Bill.

"Well, go on," replied the Professor, a peculiar gleam in his eye. I did my best to stop Bill, but have you ever tried to kick anyone when you're in free fall?

"You want to find some proof-I mean, some more more proof-of your diffusion theory of extraterrestrial culture." proof-of your diffusion theory of extraterrestrial culture."

"And have you any idea why I'm going to Jupiter to look for it?"

"Well, not exactly. I suppose you hope to find something on one of the moons."

"Brilliant, Bill, brilliant. There are fifteen known satellites, and their total area is about half that of Earth. Where would you start looking if you had a couple of weeks to spare? I'd rather like to know."

Bill glanced doubtfully at the Professor, as if he almost suspected him of sarcasm.

"I don't know much about astronomy," he said. "But there are four big moons, aren't there? I'd start on those."

"For your information, Io, Europa, Ganymede and Callisto are each about as big as Africa. Would you work through them in alphabetical order?"

"No," Bill replied promptly. "I'd start on the one nearest Jupiter and go outward."

"I don't think we'll waste any more time pursuing your logical processes," sighed the Professor. He was obviously impatient to begin his set speech. "Anyway, you're quite wrong. We're not going to the big moons at all. They've been photographically surveyed from s.p.a.ce and large areas have been explored on the surface. They've got nothing of archaeological interest. We're We're going to a place that's never been visited before." going to a place that's never been visited before."

"Not to Jupiter!" I gasped.

"Heavens no, nothing as drastic as that! But we're going nearer to him than anyone else has ever been."

He paused thoughtfully.

"It's a curious thing, you know-or you probably don't-that it's nearly as difficult to travel between Jupiter's satellites as it is to go between the planets, although the distances are so much smaller. This is because Jupiter's got such a terrific gravitational field and his moons are traveling so quickly. The innermost moon's moving almost as fast as Earth, and the journey to it from Ganymede costs almost as much fuel as the trip from Earth to Venus, even though it takes only a day and a half.

"And it's that that journey which we're going to make. No one's ever done it before because n.o.body could think of any good reason for the expense. Jupiter Five is only thirty kilometers in diameter, so it couldn't possibly be of much interest. Even some of the outer satellites, which are far easier to reach, haven't been visited because it hardly seemed worth while to waste the rocket fuel." journey which we're going to make. No one's ever done it before because n.o.body could think of any good reason for the expense. Jupiter Five is only thirty kilometers in diameter, so it couldn't possibly be of much interest. Even some of the outer satellites, which are far easier to reach, haven't been visited because it hardly seemed worth while to waste the rocket fuel."

"Then why are we we going to waste it?" I asked impatiently. The whole thing sounded like a complete wild-goose chase, though as long as it proved interesting, and involved no actual danger, I didn't greatly mind. going to waste it?" I asked impatiently. The whole thing sounded like a complete wild-goose chase, though as long as it proved interesting, and involved no actual danger, I didn't greatly mind.