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.