It was a striking composition – and a glimpse of the future that even then lay only three years ahead. On the flight of Apollo 8, Anders, Borman and Lovell were to see this splendid sight with their unaided eyes, as they watched Earth rise above the farside on Christmas Day, 1968.

Heywood Floyd admired the painting, but he also regarded it with mixed feelings. He could not forget that it was older than everybody else on the ship – with one exception.

He was already nine years old when Alexei Leonov had painted it.

Even now, more than three decades after the revelations of the first Voyager flybys, no one really understood why the four giant satellites differed so wildly from one another. They were all about the same size, and in the same part of the Solar System – yet they were totally dissimilar, as if children of a different birth.

Only Callisto, the outermost, had turned out to be much as expected. When Leonov raced past at a distance of just over 100,000 kilometres, the larger of its countless craters were clearly visible to the naked eye. Through the telescope, the satellite looked like a glass ball that had been used as a target by high-powered rifles; it was completely covered with craters of every size, right down to the lower limit of visibility. Callisto, someone had once remarked, looked more like Earth's Moon than did the Moon itself.

Nor was this particularly surprising. One would have expected a world out here – at the edge of the asteroid belt – to have been bombarded with the debris left over from the creation of the Solar System. Yet Ganymede, the satellite next door, had a totally different appearance. Though it had been well peppered with impact craters in the remote past, most of them had been ploughed over – a phrase that seemed peculiarly appropriate. Huge areas of Ganymede were covered with ridges and furrows, as if some cosmic gardener had dragged a giant rake across them. And there were light-coloured streaks, like trails that might have been made by slugs fifty kilometres across. Most mysterious of all were long, meandering bands, containing dozens of parallel lines. It was Nikolai Ternovsky who decided what they must be – multilane superhighways, laid out by drunken surveyors. He even claimed to have detected over-passes and cloverleaf intersections.

Leonov had added some trillions of bits of information about Ganymede to the store of human knowledge, before it crossed the orbit of Europa. That icebound world, with its derelict and its dead, was on the other side of Jupiter, but it was never far from anyone's thoughts.

Back on Earth, Dr Chang was already a hero and his countrymen had, with obvious embarrassment, acknowledged countless messages of sympathy. One had been sent in the name of Leonov's crew – after, Floyd gathered, considerable redrafting in Moscow. The feeling on board the ship was ambiguous – a mixture of admiration, regret, and relief. All astronauts, irrespective of their national origins, regarded themselves as citizens of space and felt a common bond, sharing each other's triumphs and tragedies. No one on Leonov was happy because the Chinese expedition had met with disaster; yet at the same time, there was a muted sense of relief that the race had not gone to the swiftest.

The unexpected discovery of life on Europa had added a new element to the situation – one that was now being argued at great length both on Earth and aboard Leonov. Some exobiologists cried 'I told you so!', pointing out that it should not have been such a surprise after all. As far back as the 1970s, research submarines had found teeming colonies of strange marine creatures thriving precariously in an environment thought to be equally hostile to life – the trenches on the bed of the Pacific. Volcanic springs, fertilizing and warming the abyss, had created oases of life in the deserts of the deep.

Anything that had happened once on Earth should be expected millions of times elsewhere in the Universe; that was almost an article of faith among scientists. Water – or at least ice – occurred on all the moons of Jupiter. And there were continuously erupting volcanoes on Io – so it was reasonable to expect weaker activity on the world next door. Putting these two facts together made Europan life seem not only possible, but inevitable – as most of nature's surprises are, when viewed with 20/20 hindsight.

Yet that conclusion raised another question, and one vital to Leonov's mission. Now that life had been discovered on the moons of Jupiter – did it have any connection with the Tycho monolith, and the still more mysterious artifact in orbit near Io?

That was a favourite subject to debate in the Six O'Clock Soviets. It was generally agreed that the creature encountered by Dr Chang did not represent a high form of intelligence – at least, if his interpretation of its behaviour was correct. No animal with even elementary powers of reasoning would have allowed itself to become a victim of its instincts, attracted like a moth to the candle until it risked destruction.

Vasili Orlov was quick to give a counter-example that weakened, if it did not refute, that argument.

'Look at whales and dolphins,' he said. 'We call them intelligent – but how often they kill themselves in mass strandings! That looks like a case where instinct overpowers reason.'

'No need to go to the dolphins,' interjected Max Brailovsky. 'One of the brightest engineers in my class was fatally attracted to a blonde in Kiev. When I heard of him last, he was working in a garage. And he'd won a gold medal for designing spacestations. What a waste!'

Even if Dr Chang's Europan was intelligent, that of course did not rule out higher forms elsewhere. The biology of a whole world could not be judged from a single specimen.

But it had been widely argued that advanced intelligence could never arise in the sea; there were not enough challenges in so benign and unvarying an environment. Above all, how could marine creatures ever develop a technology without the aid of fire?

Yet perhaps even that was possible; the route that humanity had taken was not the only one. There might be whole civilizations in the seas of other worlds.

Still, it seemed unlikely that a space-faring culture could have arisen on Europa without leaving unmistakable signs of its existence in the form of buildings, scientific installations, launching sites, or other artifacts. But from pole to pole, nothing could be seen but level ice and a few outcroppings of bare rock.

No time remained for speculations and discussions when Leonov hurtled past the orbits of Io and tiny Mimas. The crew was busy almost non-stop, preparing for the encounter and the brief onset of weight after months in free-fall. All loose objects had to be secured before the ship entered Jupiter's atmosphere, and the drag of deceleration produced momentary peaks that might be as high as two gravities.

Floyd was lucky; he alone had time to admire the superb spectacle of the approaching planet, now filling almost half the sky. Because there was nothing to give it scale, there was no way that the mind could grasp its real size. He had to keep telling himself that fifty Earths would not cover the hemisphere now turned toward him.

The clouds, colourful as the most garish sunset on Earth, raced so swiftly that he could see appreciable movement in as little as ten minutes. Great eddies were continually forming along the dozen or so bands that girdled the planet, then rippling away like swirls of smoke. Plumes of white gas occasionally geysered up from the depths, to be swept away by the gales caused by the planet's tremendous spin. And perhaps strangest of all were the white spots, sometimes spaced as regularly as pearls on a necklace, which lay along the tradewinds of the middle Jovian latitudes.