On the other side of the thin partition, the ship’s plumbing once again advertised its presence with a series of soft thumps. There was an air bubble somewhere in the system, and Duncan knew, with a deadly certainty, that all the engineering skills aboard Sirius would be unable to exorcise it before the end of the voyage.

And what was that? It was a rasping, whistling sound, so irregular that no well-adjusted mechanism could possibly have produced it. As Duncan lay in the darkness, racking his brains to think of an explanation, his annoyance slowly grew to alarm. Should he call the steward and report that something had gone wrong?

He was still trying to make up his mind when a sudden explosive change in pitch and intensity left him in no doubt as to the sound’s origin. Groaning and cursing his luck, Duncan resigned himself to a sleepless night.

Dr. Chung snored..

Someone was gently shaking him. He mumbled “Go away,” then swam groggily upward from the depths of slumber.

“If you don’t hurry,” said Dr. Chung, “you’re going to miss

breakfast.”

THE LONGEST VOYAGE

1 Tis is the Captain speaking. We will be performing a final out-of-ecliptic velocity trim during the next fifteen minutes. This will be your last opportunity for a good view of Saturn, and we are orientating the ship so that it will be visible through the B Lounge windows. Thank you.”

Thank you, thought Duncan, though he was a little less grateful when he reached B Lounge. This time, too many other passengers had been tipped off by the stewards. Nevertheless, he managed to obtain a good vantage point, even though he had to stand.

Though the journey had scarcely begun, Saturn already seemed far away. The planet had dwindled to a quarter of its accustomed size; it was now only twice as large as the Moon would appear from Earth.

Yet though it had shrunk in size, it had gained in impressiveness. Sirius had risen several degrees out of the planet’s equatorial plane, and now at last he could see the rings in all their glory. Thin, concentric silver haloes, they looked so artificial that it was almost impossible to believe that they were not the work of some cosmic craftsman whose raw materials were worlds. Although at first sight they appeared to be solid, when he looked more carefully Duncan could see the planet glimmering through them, its

Ilow light contrasting strangely with their immacuaet’e, snowy whiteness. A hundred thousand kilometers below, the shadow of the rings lay in a dusky band along the equator; it could easily have been taken for an unusually dark cloud belt, rather than something whose cause lay far out in space.

The two main divisions of the rings were apparent at the most casual glance, but a more careful inspection revealed at least a dozen fainter

boundaries where there were abrupt changes in brightness between adjacent sections. Ever since the rings had been discovered, back in the seventeenth century, mathematicians like Dr. Chung had been trying to account for their structure. It had long been known that the attractions of

Saturn’s many moons segregated the billions of orbiting particles into separate bands, but the details, of the process were still unclear.

There was also a certain amount of variation within the individual bands themselves. The outermost ring, for example, showed a distinct mottling or beadiness, and a tiny clot of light was clearly visible near its eastern extremity. Was this, Duncan wondered, a moon about to be born-or the last remnants of one that had been destroyed?

Rather diffidently, he put the question to Dr. Chung.

“Both possibilities have been considered,” she said. “My studies indicate the former. That condensation may, with luck, become another satellite in a few thousand years.”

“I can’t agree, Doctor,” interjected another passenger. “It’s merely a statistical fluctuation in the particle density. They’re quite common, and seldom last more than a few years.”

“The smaller ones-yes. But this is too intense, and too near the edge of the B-ting.”

“But Vanderplas’ analysis of the Janus problem … At that moment, it became rather like the shootout in an old time Western movie. The two scientists reached simultaneously for their hip computers and then retreated, muttering equations, to the back of the lounge.

Thereafter, they completely ignored the real Saturn they had come so far to study-and which, in all probability, they would never see again.

“Captain speaking. We have concluded our velocity trim and are re orientating the ship into the plane of the ecliptic. I hope you had a good view-Saturn will be a long way off next time you see it.”

There was no perceptible sense of motion, but the great ringed globe began to creep slowly down the observation window. The passengers in

front craned forward to follow it, and there was a chorus of disappointed “Ohs” as it finally sank from view below the wide skirting that surrounded the lower part of the ship. That band of metal had one purpose only-to block any radiation from the jet that might stray forward. Even a momentary glimpse of that intolerable glare, bright as a -supernova at the moment of detonation, could cause total blindness; a few seconds’ exposure would be lethal.

Sirius was now aimed almost directly at the sun, as she accelerated toward the inner planets. While the drive was on, there could be no rear-viewing.

Duncan knew that when he next saw Saturn with his unaided eyes, it would be merely a not-very-distinguished star.

A day later, moving at three hundred kilometers a second, the ship passed another milestone. She had, of course, escaped from the planet’s gravitational field hours earlier; neither Saturn-nor, for that matter, the

Sun-could ever recapture her. The frontier that Sirius was crossing now was a purely arbitrary one: the orbit of the outermost moon.

Mnemosyne, only fifteen kilometers in diameter, could claim two modest records. It had the longest period of any satellite, taking no fewer than 1,139 days to orbit Saturn, at an average distance of twenty-one million kilometers. And it also had the longest day of any body in the Solar

System, its period of rotation being an amazing 1,143 days. Although it seemed obvious that these two facts must be connected, no one had been able to arrive at any plausible explanation of Mnemosyne’s sluggish behavior.

Purely by chance, Sirius passed within fewer than a million kilometers of the tiny world. At first, even under the highest power of the ship’s telescope, Mnemosyne was only a minute crescent showing no visible features at all, but as it swiftly grew to a half-moon, patches of light and shade merged which eventually resolved themselves into craters. It was typical of all the denser, Mercury-type satellites-as

opposed to the inner snowballs like Mimas, Enceladus, and Tethys-but to Duncan it now held a special interest. It was more to him than the last landmark on the road to Earth.

Karl was there, and had been for many weeks, with the joint Titan-Terran

Outer Satellite Survey. Indeed, that survey had been in progress as long as

Duncan could remember-the surface area of all the moons added up to a surprising number of million square kilometers-and the TTOSS team was doing a thorough job. There had been complaints about the cost, and the critics had subsided only when promised that the survey would be so thorough that it would never be necessary to go back to the outer moons again. Somehow,

Duncan doubted that the promise would be kept.

He watched the pale crescent of Mnemosyne wax to full, simultaneously dwindling astern as the ship dropped sunward, and wondered fleetingly if he should send Karl a farewell greeting. But if he did, it would only be interpreted as a taunt.

It took Duncan several days to adjust to the complicated schedule of shipboard life-a schedule dominated by the fact that the dining room (as the lounge adjacent to the cafeteria was grandly called) could seat only one third of the passengers at a time. There were consequently three sittings for each of the three main meals-so for nine hours of every day, at least a hundred people were eating, while two hundred were either thinking about the next meal or grumbling about the last. This made it very difficult for the Purser, who doubled as Entertainment Officer, to organize any shipboard activities. The fact that most of the passengers had no wish to be organized did not help him.