There was another possibility. After all these years, he could still remember what an anticlimax the 1985/6 encounter had been to the general public. Now was a chance – the last for him, and the first for humanity – to more than make up for any previous disappointment.

Back in the twentieth century, only flybys had been possible. This time, there would be an actual landing, as pioneering in its way as Armstrong's and Aldrin's first steps on the Moon.

Dr Heywood Floyd, veteran of the 2010-15 mission to Jupiter, let his imagination fly outwards to the ghostly visitor once again returning from the deeps of space, gaining speed second by second as it prepared to round the Sun. And between the orbits of Earth and Venus the most famous of all comets would meet the still uncompleted space-liner Universe, on her maiden flight.

The exact point of rendezvous was not yet settled, but his decision was already made.

'Halley – here I come...' whispered Heywood Floyd.

It is not true that one must leave Earth to appreciate the full splendour of the heavens. Not even in space is the starry sky more glorious than when viewed from a high mountain, on a perfectly clear night, far from any source of artificial illumination. Even though the stars appear brighter beyond the atmosphere, the eye cannot really appreciate the difference; and the overwhelming spectacle of half the celestial sphere at a single glance is something that no observation window can provide.

But Heywood Floyd was more than content with his private view of the Universe, especially during the times when the residential zone was on the shadow side of the slowly revolving space hospital. Then there would be nothing in his rectangular field of view but stars, planets, nebulae – and occasionally, drowning out all else, the unblinking glare of Lucifer, new rival to the Sun.

About ten minutes before the beginning of his artificial night, he would switch off all the cabin lights – even the red emergency standby – so that he could become completely dark-adapted. A little late in life for a space engineer, he had learned the pleasures of naked-eye astronomy, and could now identify virtually any constellation, even if he could glimpse only a small portion of it.

Almost every 'night' that May, as the comet was passing inside the orbit of Mars, he had checked its location on the star charts. Although it was an easy object with a good pair of binoculars, Floyd had stubbornly resisted their aid; he was playing a little game, seeing how well his ageing eyes would respond to the challenge. Though two astronomers on Mauna Kea already claimed to have observed the comet visually, no-one believed them, and similar assertions from other residents of Pasteur had been treated with even greater scepticism.

But tonight, a magnitude of at least six was predicted; he might be in luck. He traced the line from Gamma to Epsilon, and stared towards the apex of an imaginary equilateral triangle set upon it – almost as if he could focus his vision across the Solar System by a sheer effort of will.

And there it was! – just as he had first seen it, seventy-six years ago, inconspicuous but unmistakable. If he had not known exactly where to look, he would not even have noticed it, or would have dismissed it as some distant nebula.

To his naked eye it was merely a tiny, perfectly circular blob of mist; strain as he would, he was unable to detect any trace of a tail. But the small flotilla of probes that had been escorting the comet for months had already recorded the first outbursts of dust and gas that would soon create a glowing plume across the stars, pointing directly away from its creator, the Sun,

Like everyone else, Heywood Floyd had watched the transformation of the cold, dark – no, almost black – nucleus as it entered the inner Solar System. After seventy years of deepfreeze, the complex mixture of water, ammonia and other ices was beginning to thaw and bubble. A flying mountain, roughly the shape – and size – of the island of Manhattan was turning on a cosmic spit every fifty-three hours; as the heat of the Sun seeped through the insulating crust, the vaporizing gases were making Halley's Comet behave like a leaking steam-boiler. Jets of water vapour, mixed with dust and a witch's brew of organic chemicals, were bursting out from half a dozen small craters; the largest – about the size of a football field – erupted regularly about two hours after local dawn. It looked exactly like a terrestrial geyser, and had been promptly christened 'Old Faithful'.

Already, he had fantasies of standing on the rim of that crater, waiting for the Sun to rise above the dark, contorted landscape which he already knew well through the images from space. True, the contract said nothing about passengers – as opposed to crew and scientific personnel – going outside the ship when it landed on Halley.

On the other hand, there was also nothing in the small print that specifically forbade it.

They'll have a job to stop me, thought Heywood Floyd. I'm sure I can still handle a spacesuit. And if I'm wrong...

He remembered reading that a visitor to the Taj Mahal had once remarked: 'I'd die tomorrow for a monument like this.'

He would gladly settle for Halley's Comet.

Even apart from that embarrassing accident, the return to Earth had not been easy.

The first shock had come soon after revival, when Dr Rudenko had woken him from his long sleep. Walter Curnow was hovering beside her, and even in his semi-conscious state he could tell that something was wrong; their pleasure at seeing him awake was a little too exaggerated, and failed to conceal a sense of strain. Not until he was fully recovered did they let him know that Dr Chandra was no longer with them.

Somewhere beyond Mars, so imperceptibly that the monitors could not pinpoint the time, he had simply ceased to live. His body, set adrift in space, had continued unchecked along Leonov's orbit, and had long since been consumed by the fires of the Sun.

The cause of death was totally unknown, but Max Brailovsky expressed a view that, highly unscientific though it was, not even Surgeon-Commander Katerina Rudenko attempted to refute.

'He couldn't live without Hal.'

Walter Curnow, of all people, added another thought.

'I wonder how Hal will take it?' he asked. 'Something out there must be monitoring all our broadcasts. Sooner or later, he'll know.'

And now Curnow was gone too – so were they all except little Zenia. He had not seen her for twenty years, but her card arrived punctually every Christmas. The last one was still pinned above his desk; it showed a troika laden with gifts speeding through the snows of a Russian winter, watched by extremely hungry-looking wolves.

Forty-five years! Sometimes it seemed only yesterday that Leonov had returned to Earth orbit, and the applause of all mankind. Yet it had been a curiously subdued applause, respectful but lacking genuine enthusiasm. The mission to Jupiter had been altogether too much of a success; it had opened a Pandora's box, the full contents of which had yet to be disclosed.

When the black monolith known as Tycho Magnetic Anomaly One had been excavated on the Moon, only a handful of men knew of its existence. Not until after Discovery's ill-fated voyage to Jupiter did the world learn that, four million years ago, another intelligence had passed through the Solar System, and left its calling card. The news was a revelation – but not a surprise; something of the sort had been expected for decades.

And it had all happened long before the human race existed. Although some mysterious accident had befallen Discovery out round Jupiter, there was no real evidence that it involved anything more than a shipboard malfunction. Although the philosophical consequences of TMA 1 were profound, for all practical purposes mankind was still alone in the Universe.