Introduction

by ARTHUR C. CLARKE

I have already described, in the Introduction to Venus Prime 4, the story of my life-long fascination with the greatest of all planets. Only since 1979, however, has it been discovered—to the delighted amazement of astronomers—that Jupiter’s wonders are matched by those of its many satellites.

In 1610, Galileo Galilei turned his newly invented “optic tube" upon the planet Jupiter. He was not surprised to see that—unlike the stars—its showed a perceptible disc, but during the course of the next few weeks he made a discov­ery that demolished the medieval image of the universe. In that world-picture, everything—including Sun and Moon—revolved around a central Earth. But Jupiter had four faint sparks of light revolving around it. Earth was not the only planet with a moon. To make matters even worse—Jupiter had not one, but four companions. No wonder that some of Galileo’s more intransigent colleagues refused to look through his diabolical invention. Anyway, they argued, if Jupiter’s satellites were that small, they didn’t really matter, and the heck with them . . .

Until the 19th century, the four “Galilean" moons—Io, Europa, Ganymede, and Callisto—remained as no more than featureless pinpoints even through the most powerful telescopes. Their regular movements (in periods ranging from a mere 42 hours for Io, up to 17 days for distant Cal­listo) around their giant master made them a source of con­tinual delight to generations of astronomers, amateur and professional. A good pair of modern binoculars—rigidly supported—will show them easily, as they swing back and forth along Jupiter’s equatorial plane. Usually three or four will be visible, but on rare occasions Jupiter will appear as moonless as Galileo’s opponents would have wished, because all four of the satellites will be eclipsed by the planet, or inconspicuously transiting its face.

There was no reason to suppose, before the Space Age opened, that the four Galilean satellites would be very dif­ferent from our own Moon—that is, airless, cratered deserts where nothing ever moved except the shadows cast by the distant sun. In fact, this proved to be true for the outermost satellite, Callisto: it is so saturated with craters of all sizes that there is simply no room for any more.

This was about the only unsurprising result of the 1979 Voyager missions, undoubtedly the most successful in the history of space exploration. For the three inner moons proved to be wildly different from Callisto, and from each other.

Io is pockmarked with volcanoes—the first active ones ever discovered beyond the Earth—blasting sulphurous vapors a hundred kilometers into space. Europa is a frozen ice-pack from pole to pole, covered with the intricate traceries of frac­tured floes. And Ganymede—larger than Mercury, and not much smaller than Mars—is most bizarre of all. Much of its surface looks as if scraped by gigantic combs, leaving multi­ple groves meandering for thousands of kilometers. And there are curious pits from which emerge tracks that might have been made by snails the size of an Olympic stadium.

If you want to know more about these weird places, I refer you to the numerous splendidly illustrated volumes that were inspired by the Voyager missions. Stanley Kurbick and I never dreamed, back in the mid-Sixties, that within a dozen years we would be seeing closeups of the places we were planning to send our astronauts: we thought such knowledge would not be available until at least 2001. Without the Voyagers, I could never have written Odyssey Two. Thank you, NASA and JPL.

In addition to its quartet of almost planet-sized moons, the Voyager spaceprobes discovered that Jupiter also has Saturn-like rings—though they are much less spectacular—and at least a dozen smaller satellites. As befits such a giant, it is a mini-solar system in its own right, whose exploration may take many centuries—and many lives.

The short story “Jupiter V," the genesis of this novel, takes place on a satellite which was discovered by a sharp-eyed astronomer, E. E. Barnard, back in 1982. Now offi­cially christened Amalthea, Jupiter V was long believed to be the moon closest to Jupiter, but even smaller and closer satellites were detected by the Voyagers. There may be scores, or hundreds, or thousands more; some day we’ll have to answer the question: “How small can a lump of rock be and still qualify as a moon?"

Written in 1951, and later published in the collection Reach for Tomorrow (1956), “Jupiter V" is one of the few stories whose origins I can pinpoint exactly. Its first inspi­ration (explicitly mentioned in the original version) was Chesley Bonestall’s wonderful series of astronomical paint­ings, featured in a 1944 issue of Life magazine.* Later reprinted in the volume edited by Willy Ley, The Conquest of Space (1949), they must have made thousands of people realize for the first time that the other planets and satellites of the Solar System were real places, which one day we might visit.

Chesley’s paintings—when published in The Conquest of Space—inspired legions of young space cadets, and this somewhat older one. Little did I know, to coin a phrase, that one day I would collaborate with Chesley on a book about the exploration of the outer planets (Beyond Jupiter [1972]: see the Introduction to Venus Prime 4). How glad I am that Chesley—who died, still painting furiously, at the age of 99—lived to see the reality behind his imagination.

The second input to “Jupiter V" was somewhat more sophisticated. In 1949, during my final year at King’s Col­lege, London, my applied mathematics instructor Dr. G. C. McVittie gave a lecture which made an indelible impression on me. It was on the apparently unpromising subject of perturbation theory—i.e., what happens to an orbiting body when some external force alters its velocity. At that date, nothing could have seemed of less practical importance; today, it is the basis of the multi-billion dollar communica­tions satellite industry, and all space rendezvous missions.

The conclusions that “Mac" illustrated on the blackboard were surprising, and often counterintuitive; who would have thought that one way to make a satellite go faster was to slow it down? Over the next few decades, I used pertur­bation theory in a number of tales besides “Jupiter V" and it plays a vital role, though in very different ways, in the finales of both 2010 and 2061.

In March 1989 the Royal Astronomical Society, of which Dr. McVittie had long been a leading Fellow, gave a special symposium in his memory, and I took pains to tell the orga­nizers about his contribution to my own career.

But back to Jupier V—Amalthea. In 1951 I felt perfectly safe in making it anything I wished, for it was inconceiv­able that we would get a good look at it during the twenti­eth century. Yet that was just one of the feats accomplished in the Voyager missions.

Well, perhaps not a good look, but Voyager’s slightly blurred image, though from several thousand kilometers away, completely demolished my description: “There were faint crisscrossing lines on the surface of the satellite, and suddenly my eye grasped their full pattern. For it was a pattern; those lines covered Five with the same geometrical accuracy as the lines of latitude and longitude divide up a globe of the Earth. . . ."

I’m not worried: the real Amalthea looks even weirder. It’s a delicate shade of pink—probably as a result of spray­ing by sulphur dust spewed out from nearby Io. And it has a matched pair of prominent white spots, looking very much like protruding eyes.

Maybe that’s just what they are. . . .

  Arthur C. Clarke
Colombo, Sri Lanka