It is a good principle in science not to believe any 'fact' – however well-attested – until it fits into some accepted frame of reference. Occasionally, of course, an observation can shatter the frame and force the construction of a new one, but that is extremely rare. Galileos and Einsteins seldom appear more than once per century, which is just as well for the equanimity of mankind.

Dr Kreuger fully accepted this principle: he would not believe his nephew's discovery until he could explain it, and as far as he could see that required nothing less than a direct Act of God. Wielding Occam's still highly serviceable razor, he thought it somewhat more probable that Rolf had made a mistake; if so, it should be fairly easy to find it.

To Uncle Paul's great surprise, it proved very difficult indeed. The analysis of radar remote-sensing observations was now a venerable and well-established art, and the experts that Paul consulted all gave the same answer, after considerable delay. They also asked: 'Where did you get that recording?'

'Sorry,' he had answered. 'I'm not at liberty to say.'

The next step was to assume that the impossible was correct, and to start searching the literature. This could be an enormous job, for he did not even know where to begin. One thing was quite certain: a brute-force, head-on attack was bound to fail. It would be just as if Roentgen, the morning after he had discovered X-rays, had started to hunt for their explanation in the physics journals of his day. The information he needed still lay years in the future.

But there was at least a sporting chance that what he was looking for was hidden somewhere in the immense body of existing scientific knowledge. Slowly and carefully, Paul Kreuger set up an automatic search programme, designed for what it would exclude as much as what it would embrace. It should cut out all Earth-related references – they would certainly number in the millions – and concentrate entirely on extraterrestrial citations.

One of the benefits of Dr Kreuger's eminence was an unlimited computer budget: that was part of the fee he demanded from the various organizations who needed his wisdom. Though this search might be expensive, he did not have to worry about the bill.

As it turned out, this was surprisingly small. He was lucky: the search came to an end after only two hours thirty-seven minutes, at the 21,456th reference.

The title was enough. Paul was so excited that his own comsec refused to recognize his voice, and he had to repeat the command for a full print-out.

Nature had published the paper in 1981 – almost five years before he was born! – and as his eyes swept swiftly over its single page he knew not only that his nephew had been right all along – but, just as important, exactly how such a miracle could occur.

The editor of that eighty-year-old journal must have had a good sense of humour. A paper discussing the cores of the outer planets was not something to grab the usual reader: this one, however, had an unusually striking title. His comsec could have told him quickly enough that it had once been part of a famous song, but that of course was quite irrelevant.

Anyway, Paul Kreuger had never heard of the Beatles, and their psychedelic fantasies.

And now Halley was too close to be seen; ironically, observers back on Earth would get a far better view of the tail, already stretching fifty million kilometres at right angles to the comet's orbit, like a pennant fluttering in the invisible gale of the solar wind.

On the morning of the rendezvous, Heywood Floyd woke early from a troubled sleep. It was unusual for him to dream – or at least to remember his dreams – and doubtless the anticipated excitements of the next few hours were responsible. He was also slightly worried by a message from Caroline, asking if he had heard from Chris lately. He had radioed back, a little tersely, that Chris had never bothered to say thank you when he had helped him get his current position on Universe's sister ship Cosmos; perhaps he was already bored with the Earth-Moon run and was looking for excitement elsewhere.

'As usual,' Floyd had added, 'we'll hear from him in his own good time.'

Immediately after breakfast, passengers and science team had gathered for a final briefing from Captain Smith. The scientists certainly did not need it, but if they felt any irritation, so childish an emotion would have been quickly swept away by the weird spectacle on the main viewscreen.

It was easier to imagine that Universe was flying into a nebula, rather than a comet. The entire sky ahead was now a misty white fog – not uniform, but mottled with darker condensations and streaked with luminous bands and brightly glowing jets, all radiating away from a central point. At this magnification, the nucleus was barely visible as a tiny black speck, yet it was clearly the source of all the phenomena around it.

'We cut our drive in three hours,' said the Captain. 'Then we'll be only a thousand kilometres away from the nucleus, with virtually zero velocity. We'll make some final observations, and confirm our landing site.'

'So we'll go weightless at 12.00 exactly. Before then, your cabin stewards will check that everything's correctly stowed. It will be just like turnaround, except that this time it's going to be three days, not two hours, before we have weight again.

'Halley's gravity? Forget it – less than one centimetre per second squared – just about a thousandth of Earth's. You'll be able to detect it if you wait long enough, but that's all. Takes fifteen seconds for something to fall a metre.

'For safety, I'd like you all here in the observation lounge, with your seat belts properly secured, during rendezvous and touchdown. You'll get the best view from here anyway, and the whole operation won't take more than an hour. We'll only be using very small thrust corrections, but they may come from any angle and could cause minor sensory disturbances.'

What the Captain meant, of course, was spacesickness – but that word, by general agreement, was taboo aboard Universe. It was noticeable, however, that many hands strayed into the compartments beneath the seats, as if checking that the notorious plastic bags would be available if urgently required.

The image on the viewscreen expanded, as the magnification was increased. For a moment it seemed to Floyd that he was in an aeroplane, descending through light clouds, rather than in a spacecraft approaching the most famous of all comets. The nucleus was growing larger and clearer; it was no longer a black dot, but an irregular ellipse – now a small, pockmarked island lost in the cosmic ocean – then, suddenly, a world in its own right.

There was still no sense of scale. Although Floyd knew that the whole panorama spread before him was less than ten kilometres across, he could easily have imagined that he was looking at a body as large as the Moon. But the Moon was not hazy around the edges, nor did it have little jets of vapour – and two large ones – spurting from its surface.

'My God!' cried Mihailovich, 'what's that?'

He pointed to the lower edge of the nucleus, just inside the terminator. Unmistakably – impossibly -a light was flashing there on the nightside of the comet with a perfectly regular rhythm: on, off, on, off, once every two or three seconds.

Dr Willis gave his patient 'I can explain it to you in words of one syllable' cough, but Captain Smith got there first.

'I'm sorry to disappoint you, Mr Mihailovich. That's only the beacon on Sampler Probe Two – it's been sitting there for a month, waiting for us to come and pick it up.'