Just as 2010: Odyssey Two was not a direct sequel to 2001: A Space Odyssey, so this book is not a linear sequel to 2010. They must all be considered as variations on the same theme, involving many of the same characters and situations, but not necessarily happening in the same universe.

Developments since Stanley Kubrick suggested in 1964 (five years before men landed on the Moon!) that we should attempt 'the proverbial good science-fiction movie' make total consistency impossible, as the later stories incorporate discoveries and events that had not even taken place when the earlier books were written. 2010 was made possible by the brilliantly successful 1979 Voyager flybys of Jupiter, and I had not intended to return to that territory until the results of the even more ambitious Galileo Mission were in.

Galileo would have dropped a probe into the Jovian atmosphere, while spending almost two years visiting all the major satellites. It should have been launched from the space shuttle in May 1986, and would have reached its objective by December 1988. So around 1990 I hoped to take advantage of the flood of new information from Jupiter and its moons...

Alas, the Challenger tragedy eliminated that scenario; Galileo – now sitting in its clean room at the Jet Propulsion Laboratory – must now find another launch vehicle. It will be lucky if it arrives at Jupiter merely seven years behind schedule.

I have decided not to wait.

Colombo, Sri Lanka,

April 1987

'For a man of seventy, you're in extremely good shape,' remarked Dr Glazunov, looking up from the Medcom's final print-out. 'I'd have put you down as not more than sixty-five.'

'Happy to hear it, Oleg. Especially as I'm a hundred and three – as you know perfectly well.'

'Here we go again! Anyone would think you've never read Professor Rudenko's book.'

'Dear old Katerina! We'd planned a get-together on her hundredth birthday. I was so sorry she never made it – that's what comes of spending too much time on Earth.'

'Ironic, since she was the one who coined that famous slogan "Gravity is the bringer of old age."'

Dr Heywood Floyd stared thoughtfully at the ever-changing panorama of the beautiful planet, only six thousand kilometres away, on which he could never walk again. It was even more ironic that, through the most stupid accident of his life, he was still in excellent health when virtually all his old friends were dead.

He had been back on Earth only a week when, despite all the warnings and his own determination that nothing of the sort would ever happen to him, he had stepped off that second-storey balcony. (Yes, he had been celebrating: but he had earned it – he was a hero on the new world to which Leonov had returned.) The multiple fractures had led to complications, which could best be handled in the Pasteur Space Hospital.

That had been in 2015. And now – he could not really believe it, but there was the calendar on the wall – it was 2061.

For Heywood Floyd, the biological clock had not merely been slowed down by the one-sixth Earth gravity of the hospital; twice in his life it had actually been reversed. It was now generally believed – though some authorities disputed it – that hibernation did more than merely stop the ageing process; it encouraged rejuvenation. Floyd had actually become younger on his voyage to Jupiter and back.

'So you really think it's safe for me to go?'

'Nothing in this Universe is safe, Heywood. All I can say is that there are no physiological objections. After all, your environment will be virtually the same aboard Universe as it is here. She may not have quite the standard of – ah – superlative medical expertise we can provide at Pasteur, but Dr Mahindran is a good man. If there's any problem he can't cope with, he can put you into hibernation again, and ship you back to us, COD.'

It was the verdict that Floyd had hoped for, yet somehow his pleasure was alloyed with sadness. He would be away for weeks from his home of almost half a century, and the new friends of his later years. And although Universe was a luxury liner compared with the primitive Leonov (now hovering high above Farside as one of the main exhibits at the Lagrange Museum) there was still some element of risk in any extended space voyage. Especially like the pioneering one on which he was now preparing to embark.

Yet that, perhaps, was exactly what he was seeking – even at a hundred and three (or, according to the complex geriatric accounting of the late Professor Katerina Rudenko, a hale and hearty sixty-five.) During the last decade, he had become aware of an increasing restlessness and a vague dissatisfaction with a life that was too comfortable and well-ordered.

Despite all the exciting projects now in progress around the Solar System – the Mars Renewal, the establishment of the Mercury Base, the Greening of Ganymede – there had been no goal on which he could really focus his interests and his still considerable energies. Two centuries ago, one of the first poets of the Scientific Era had summed up his feelings perfectly, speaking through the lips of Odysseus/Ulysses:

'Three suns', indeed! It was more than forty:

Ulysses would have been ashamed of him. But the next verse – which he knew so well – was even more appropriate:

'To seek, to find...' Well, now he knew what he was going to seek, and to find – because he knew exactly where it would be. Short of some catastrophic accident, there was no way in which it could possibly elude him.

It was not a goal he had ever consciously had in mind, and even now he was not quite sure why it had become so suddenly dominant. He would have thought himself immune to the fever which was once again infecting mankind – for the second time in his life! – but perhaps he was mistaken. Or it could have been that the unexpected invitation to join the short list of distinguished guests aboard Universe had fired his imagination, and awakened an enthusiasm he had not even known he possessed.