The million-kilometre-long tendrils of magnetic force, the sudden explosion of radio waves, the geysers of electrified plasma wider than the planet Earth – they were as real and clearly visible to him as the clouds banding the planet in multi-hued glory. He could understand the complex pattern of their interactions, and realized that Jupiter was much more wonderful than anyone had ever guessed.

Even as he fell through the roaring heart of the Great Red Spot, with the lightning of its continent-wide thunderstorms detonating under him, he knew why it had persisted for centuries though it was made of gases far less substantial than those that formed the hurricanes of Earth. The thin scream of hydrogen wind faded as he sank into the calmer depths, and a sheet of waxen snowflakes – some already coalescing into barely palpable mountains of hydrocarbon foam – descended from the heights above. It was already warm enough for liquid water to exist, but there were no oceans there; this purely gaseous environment was too tenuous to support them.

He descended through layer after layer of cloud, until he entered a region of such clarity that even human vision could have scanned an area more than a thousand kilometres across. It was only a minor eddy in the vaster gyre of the Great Red Spot; and it held a secret that men had long guessed, but never proved. Skirting the foothills of the drifting foam mountains were myriad of small, sharply defined clouds, all about the same size and patterned with similar red and brown mottling. They were small only as compared with the inhuman scale of their surroundings; the very least would have covered a fair-sized city.

They were clearly alive, for they were moving with slow deliberation along the flanks of the aerial mountains, browsing off their slopes like colossal sheep. And they were calling to each other in the metre band, their radio voices faint but clear against the cracklings and concussions of Jupiter itself.

Nothing less than living gasbags, they floated in the narrow zone between freezing heights and scorching depths. Narrow, yes – but a domain far larger than all the biosphere of Earth.

They were not alone. Moving swiftly among them were other creatures so small that they could easily have been overlooked. Some of them bore an almost uncanny resemblance to terrestrial aircraft, and were of about the same size. But they too were alive – perhaps predators, perhaps parasites, perhaps even herdsmen.

A whole new chapter of evolution, as alien as that which he had glimpsed on Europa, was opening before him. There were jet-propelled torpedoes like the squids of the terrestrial oceans, hunting and devouring the huge gas-bags. But the balloons were not defenceless; some of them fought back with electric thunderbolts and with clawed tentacles like kilometre-long chainsaws.

There were even stranger shapes, exploiting almost every possibility of geometry – bizarre, translucent kites, tetrahedra, spheres, polyhedra, tangles of twisted ribbons... The gigantic plankton of the Jovian atmosphere, they were designed to float like gossamer in the uprising currents, until they had lived long enough to reproduce; then they would be swept down into the depths to be carbonized and recycled in a new generation.

He was searching a world more than a hundred times the area of Earth, and though he saw many wonders, nothing there hinted of intelligence. The radio voices of the great balloons carried only simple messages of warning or of fear. Even the hunters, who might have been expected to develop higher degrees of organization, were like the sharks in Earth's oceans – mindless automata.

And for all its breathtaking size and novelty, the biosphere of Jupiter was a fragile world, a place of mists and foam, of delicate silken threads and paper-thin tissues spun from the continual snowfall of petrochemicals formed by lightning in the upper atmosphere. Few of its constructs were more substantial than soap bubbles; its most awesome predators could be torn to shreds by even the feeblest of terrestrial carnivores.

Like Europa, but on a vastly grander scale, Jupiter was an evolutionary cul-de-sac. Intelligence would never emerge here; even if it did, it would be doomed to a stunted existence. A purely aerial culture might develop, but in an environment where fire was impossible, and solids scarcely existed, it could never even reach the Stone Age.

MISS PRINGLE RECORD

Well, Indra – Dim – I hope that came through in good shape – I still find it hard to believe. All those fantastic creatures – surely we should have detected their radio voices, even if we couldn't understand them! – wiped out in a moment, so that Jupiter could be made into a sun.

And now we can understand why. It was to give the Europs their chance. What pitiless logic: is intelligence the only thing that matters? I can see some long arguments with Ted Khan over this – The next question is: will the Europs make the grade – or will they remain forever stuck in the kindergarten – not even that – the nursery? Though a thousand years is a very short time, one would have expected some progress, but according to Dave they're exactly the same now as when they left the sea. Perhaps that's the trouble; they still have one foot – or one twig! – in the water.

And here's another thing we got completely wrong. We thought they went back into the water to sleep. It's just the other way round – they go back to eat, and sleep when they come on land! As we might have guessed from their structure – that network of branches – they're plankton feeders...

I asked Dave about the igloos they've built. Aren't they a technological advance? And he said: not really – they're only adaptations of structures they make on the sea-bed, to protect themselves from various predators – especially something like a flying carpet, as big as a football field...

There's one area, though, where they have shown initiative – even creativity. They're fascinated by metals, presumably because they don't exist in pure form in the ocean. That's why Tsien was stripped – the same thing's happened to the occasional probes that have come down in their territory. What do they do with the copper and beryllium and titanium they collect? Nothing useful, I'm afraid. They pile it all together in one place, in a fantastic heap that they keep reassembling. They could be developing an aesthetic sense – I've seen worse in the Museum of Modem Art... But I've got another theory – did you ever hear of cargo cults? During the twentieth century, some of the few primitive tribes that still existed made imitation aeroplanes out of bamboo, in the hope of attracting the big birds in the sky that occasionally brought them wonderful gifts. Perhaps the Europs have the same idea.

Now that question you keep asking me... What is Dave? And how did he – and Hal – become whatever it is they are now?

The quick answer, of course, is that they're both emulations – simulations – in the Monolith's gigantic memory. Most of the time they're inactivated; when I asked Dave about this, he said he'd been 'awake' – his actual word -for only fifty years altogether, in the thousand since his – er – metamorphosis.

When I asked if he resented this takeover of his life, he said, 'Why should I resent it? I am performing my functions perfectly.' Yes, that sounds exactly like Hal! But I believe it was Dave – if there's any distinction now.

Remember that Swiss Army knife analogy? Halman is one of this cosmic knife's myriad of components.

But he's not a completely passive tool – when he's awake, he has some autonomy, some independence – presumably within limits set by the Monolith's overriding control. During the centuries, he's been used as a kind of intelligent probe to examine, Jupiter – as you've just seen – as well as Ganymede and the Earth. That confirms those mysterious events in Florida, reported by Dave's old girl-friend, and the nurse who was looking after his mother, just moments before her death... as well as the encounters in Anubis City.