For the first forty-eight hours of the voyage, Heywood Floyd could not really believe the comfort, the spaciousness – the sheer extravagance of Universe's living arrangements. Yet most of his fellow passengers took them for granted; those who had never left Earth before assumed that all spaceships must be like this.

He had to look back at the history of aeronautics to put matters in the right perspective. In his own lifetime, he had witnessed – indeed, experienced – the revolution that had occurred in the skies of the planet now dwindling behind him. Between the clumsy old Leonov and the sophisticated Universe lay exactly fifty years. (Emotionally, he couldn't really believe that – but it was useless arguing about arithmetic.)

And just fifty years had separated the Wright Brothers from the first jet airliners. At the beginning of that half-century, intrepid aviators had hopped from field to field, begoggled and windswept on open chairs; at its end, grandmothers had slumbered peacefully between continents at a thousand kilometres an hour.

So he should not, perhaps, have been astonished at the luxury and elegant decor of his stateroom, or even the fact that he had a steward to keep it tidy. The generously sized window was the most startling feature of his suite, and at first he felt quite uncomfortable thinking of the tons of air pressure it was holding in check against the implacable, and never for a moment relaxing, vacuum of space.

The biggest surprise, even though the advance literature should have prepared him for it, was the presence of gravity. Universe was the first spaceship ever built to cruise under continuous acceleration, except for the few hours of the mid-course 'turnaround'. When her huge propellant tanks were fully loaded with their five thousand tons of water, she could manage a tenth of a gee – not much, but enough to keep loose objects from drifting around. This was particularly convenient at mealtimes – though it took a few days for the passengers to learn not to stir their soup too vigorously.

Forty-eight hours out from Earth, the population of Universe had already stratified itself into four distinct classes.

The aristocracy consisted of Captain Smith and his officers. Next came the passengers; then crew – non-commissioned and stewards. And then steerage...

That was the description that the five young space scientists had adopted for themselves, first as a joke but later with a certain amount of bitterness. When Hoyd compared their cramped and jury-rigged quarters with his own luxurious cabin, he could see their point of view, and soon became the conduit of their complaints to the Captain.

Yet all things considered, they had little to grumble about; in the rush to get the ship ready, it had been touch and go as to whether there would be any accommodation for them and their equipment. Now they could look forward to deploying instruments around – and on – the comet during the critical days before it rounded the Sun, and departed once more to the outer reaches of the Solar System. The members of the science team would establish their reputations on this voyage, and knew it. Only in moments of exhaustion, or fury with misbehaving instrumentation, did they start complaining about the noisy ventilating system, the claustrophobic cabins, and occasional strange smells of unknown origin.

But never the food, which everyone agreed was excellent. 'Much better,' Captain Smith assured them, 'than Darwin had on the Beagle.'

To which Victor Willis had promptly retorted:

'How does he know? And by the way, Beagle's commander cut his throat when he got back to England.'

That was rather typical of Victor, perhaps the planet's best-known science communicator – to his fans – or 'pop-scientist' – to his equally numerous detractors. It would be unfair to call them enemies; admiration for his talents was universal, if occasionally grudging. His soft, mid-Pacific accent and expansive gestures on camera were widely parodied, and he had been credited (or blamed) for the revival of full-length beards. 'A man who grows that much hair,' critics were fond of saying, 'must have a lot to hide.'

He was certainly the most instantly recognizable of the six VIPs – though Floyd, who no longer regarded himself as a celebrity, always referred to them ironically as 'The Famous Five'. Yva Merlin could often walk unrecognized on Park Avenue, on the rare occasions when she emerged from her apartment. Dimitri Mihailovich, to his considerable annoyance, was a good ten centimetres below average height; this might help to explain his fondness for thousand-piece orchestras – real or synthesized -but did not enhance his public image.

Clifford Greenburg and Margaret M'Bala also fell into the category of 'famous unknowns' – though this would certainly change when they got back to Earth. The first man to land on Mercury had one of those pleasant, unremarkable faces that are very hard to remember; moreover the days when he had dominated the news were now thirty years in the past. And like most authors who are not addicted to talk shows and autographing sessions, Ms M'Bala would be unrecognized by the vast majority of her millions of readers.

Her literary fame had been one of the sensations of the forties. A scholarly study of the Greek pantheon was not usually a candidate for the best-seller lists, but Ms M'Bala had placed its eternally inexhaustible myths in a contemporary space-age setting. Names which a century earlier had been familiar only to astronomers and classical scholars were now part of every educated person's world picture; almost every day there would be news from Ganymede, Callisto, Io, Titan, Japetus – or even more obscure worlds like Carme, Pasiphaë, Hyperion, Phoebe...

Her book would have been no more than modestly successful, however, had she not focused on the complicated family life of Jupiter-Zeus, Father of all the Gods (as well as much else). And by a stroke of luck, an editor of genius had changed her original title, The View from Olympus, to The Passions of the Gods. Envious academics usually referred to it as Olympic Lusts, but invariably wished they had written it.

Not surprisingly, it was Maggie M – as she was quickly christened by her fellow passengers – who first used the phrase Ship of Fools. Victor Willis adopted it eagerly, and soon discovered an intriguing historical resonance. Almost a century ago, Katherine Anne Porter had herself sailed with a group of scientists and writers aboard an ocean liner to watch the launch of Apollo 17, and the end of the first phase of lunar exploration.

'I'll think about it,' Ms M'Bala had remarked ominously, when this was reported to her. 'Perhaps it's time for a third version. But I won't know, of course, until we get back to Earth...'

It was many months before Rolf van der Berg could once again turn his thoughts and energies towards Mount Zeus. The taming of Ganymede was a more than full-time job, and he was away from his main office at Dardanus Base for weeks at a time, surveying the route of the proposed Gilgamesh-Osiris monorail.

The geography of the third and largest Galilean moon had changed drastically since the detonation of Jupiter – and it was still changing. The new sun that had melted the ice of Europa was not as powerful here, four hundred thousand kilometres further out – but it was warm enough to produce a temperate climate at the centre of the face forever turned towards it. There were small, shallow seas – some as large as Earth's Mediterranean – up to latitudes forty north and south. Not many features still survived from the maps generated by the Voyager missions back in the twentieth century. Melting permafrost and occasional tectonic movements triggered by the same tidal forces operating on the two inner moons made the new Ganymede a cartographer's nightmare.