Poole had been surprised – and a little shocked – to discover that the crew's knowledge of Goliath's systems was very superficial. Often he had asked questions which should have been easily answered, only to be referred to the ship's own memory banks. After a while, however, he realized that the sort of in-depth training he had received in his days was no longer possible: far too many complex systems were involved for any man or woman's mind to master. The various specialists merely had to know what their equipment did, not how. Reliability depended on redundancy and automatic checking, and human intervention was much more likely to do harm than good.

Fortunately none was required on this voyage: it had been as uneventful as any skipper could have hoped, when the new sun of Lucifer dominated the sky ahead.

(Extract, text only, Tourist's Guide to Outer Solar System, v 219.3)

Even today, the giant satellites of what was once Jupiter present us with major mysteries. Why are four worlds, orbiting the same primary and very similar in size, so different in most other respects?

Only in the case of Io, the innermost satellite, is there a convincing explanation. It is so close to Jupiter that the gravitational tides constantly kneading its interior generate colossal quantities of heat – so much, indeed, that Io's surface is semi-molten. It is the most volcanically active world in the Solar System; maps of Io have a half-life of only a few decades.

Though no permanent human bases have ever been established in such an unstable environment, there have been numerous landings and there is continuous robot monitoring. (For the tragic fate of the 2571 Expedition, see Beagle 5.)

Europa, second in distance from Jupiter, was originally entirely covered in ice, and showed few surface features except a complicated network of cracks. The tidal forces which dominate Io were much less powerful here, but produced enough heat to give Europa a global ocean of liquid water, in which many strange life-forms have evolved.

In 2010 the Chinese ship Tsien touched down on Europa on one of the few outcrops of solid rock protruding through the crust of ice. In doing so it disturbed a creature of the Europan abyss and was destroyed (see Spacecraft Tsien, Galaxy, Universe).

Since the conversion of Jupiter into the mini-sun Lucifer in 2061, virtually all of Europa's ice-cover has melted, and extensive vulcanism has created several small islands.

As is well-known, there have been no landings on Europa for almost a thousand years, but the satellite is under continuous surveillance.

Ganymede, largest moon in the Solar System (diameter 5260 kilometres), has also been affected by the creation of a new sun, and its equatorial regions are warm enough to sustain terrestrial life-forms, though it does not yet have a breathable atmosphere. Most of its population is actively engaged in terraforming and scientific research; the main settlement is Anubis (pop 41,000), near the South Pole.

Callisto is again wholly different. Its entire surface is covered by impact craters of all sizes, so numerous that they overlap. The bombardment must have continued for millions of years, for the newer craters have completely obliterated the earlier ones. There is no permanent base on Callisto, but several automatic stations have been established there.

It was unusual for Frank Poole to oversleep, but he had been kept awake by strange dreams. Past and present were inextricably mixed; sometimes he was on Discovery, sometimes in the Africa Tower – and sometimes he was a boy again, among friends he had thought long-forgotten.

Where am I? he asked himself as he struggled up to consciousness, like a swimmer trying to get back to the surface. There was a small window just above his bed, covered by a curtain not thick enough to completely block the light from outside. There had been a time, around the mid-twentieth century, when aircraft had been slow enough to feature First Class sleeping accommodation: Poole had never sampled this nostalgic luxury, which some tourist organizations had still advertised in his own day, but he could easily imagine that he was doing so now.

He drew the curtain and looked out. No, he had not awakened in the skies of Earth, though the landscape unrolling below was not unlike the Antarctic. But the South Pole had never boasted two suns, both rising at once as Goliath swept towards them.

The ship was orbiting less than a hundred kilometres above what appeared to be an immense ploughed field, lightly dusted with snow. But the ploughman must have been drunk – or the guidance system must have gone crazy – for the furrows meandered in every direction, sometimes cutting across each other or turning back on themselves. Here and there the terrain was dotted with faint circles -ghost craters from meteor impacts aeons ago.

So this is Ganymede, Poole wondered drowsily. Mankind's furthest outpost from home! Why should any sensible person want to live here? Well, I've often thought that when I've flown over Greenland or Iceland in winter-time...

There was a knock on the door, a 'Mind if I come in?', and Captain Chandler did so without waiting for a reply.

'Thought we'd let you sleep until we landed – that end-of-trip party did last longer than I'd intended, but I couldn't risk a mutiny by cutting it short.'

Poole laughed.

'Has there ever been a mutiny in space?'

'Oh, quite a few but not in my time. Now we've mentioned the subject, you might say that Hal started the tradition... sorry – perhaps I shouldn't – look – there's Ganymede City!'

Coming up over the horizon was what appeared to be a criss-cross pattern of streets and avenues, intersecting almost at right-angles but with the slight irregularity typical of any settlement that had grown by accretion, without central planning. It was bisected by a broad river – Poole recalled that the equatorial regions of Ganymede were now warm enough for liquid water to exist – and it reminded him of an old wood-cut he had seen of medieval London.

Then he noticed that Chandler was looking at him with an expression of amusement... and the illusion vanished as he realized the scale of the 'city'.

'The Ganymedeans,' he said dryly, 'must have been rather large, to have made roads five or ten kilometres wide.'

'Twenty in some places. Impressive, isn't it? And all the result of ice stretching and contracting. Mother Nature is ingenious... I could show you some patterns that look even more artificial, though they're not as large as this one.'

'When I was a boy, there was a big fuss about a face on Mars. Of course, it turned out to be a hill that had been carved by sand-storms... lots of similar ones in Earth's deserts.'

'Didn't someone say that history always repeats itself? Same sort of nonsense happened with Ganymede City – some nuts claimed it had been built by aliens. But I'm afraid it won't be around much longer.'

'Why?' asked Poole in surprise.

'It's already started to collapse, as Lucifer melts the permafrost. You won't recognize Ganymede in another hundred years... there's the edge of Lake Gilgamesh – if you look carefully – over on the right-'

'I see what you mean. What's happening – surely the water's not boiling, even at this low pressure?'

'Electrolysis plant. Don't know how many skillions of kilograms of oxygen a day. Of course, the hydrogen goes up and gets lost – we hope.'

Chandler's voice trailed off into silence. Then he resumed, in an unusually diffident tone: 'All that beautiful water down there – Ganymede doesn't need half of it! Don't tell anyone, but I've been working out ways of getting some to Venus.'