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.

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.