There were perfectly logical reasons why he should go on the mission – and equally logical reasons, as many of his colleagues had already pointed out, why he should not. But perhaps in the final analysis, the choice had been made by his heart, not his brain. And even here, emotion urged him in two opposite directions.

Curiosity, guilt, the determination to finish a job that had been badly botched – they all combined to drive him toward Jupiter and whatever might be waiting there. On the other hand, fear – he was honest enough to admit that – united with love of his family to keep him on Earth. Yet he had never had any real doubts; he had made his decision almost instantly, and had deflected all of Caroline's arguments as gently as he could.

And there was one other consoling thought that he had not yet risked sharing with his wife. Though he would be gone two and a half years, all but the fifty days at Jupiter would be spent in timeless hibernation. When he returned, the gap between their ages would have narrowed by more than two years.

He would have sacrificed the present so that they could share a longer future together.

5 – Leonov

The months contracted to weeks, the weeks dwindled to days, the days shrivelled to hours; and suddenly Heywood Floyd was once more at the Cape – spaceward-bound for the first time since that trip to Clavius Base and the Tycho monolith, so many years ago.

But this time he was not alone, and there was no secrecy about the mission. A few seats ahead of him rode Dr Chandra, already engaged in a dialogue with his briefcase computer, and quite oblivious to his surroundings.

One of Floyd's secret amusements, which he had never confided to anyone, was spotting similarities between human beings and animals. The resemblances were more often flattering than insulting, and his little hobby was also a very useful aid to memory.