The cautious exploration of the comet had already begun while Universe still remained in the polar shadow. First, one-man EMUs (few people now knew that stood for External Manoeuvring Unit) gently jetted over both day– and nightside, recording everything of interest. Once the preliminary surveys had been completed, groups of up to five scientists flew out in the onboard shuttle, deploying equipment and instruments at strategic spots.

The Lady Jasmine was a far cry from the primitive 'space pods' of the Discovery era, capable of operating only in a gravity-free environment. She was virtually a small spaceship, designed to ferry personnel and light cargo between the orbiting Universe and the surfaces of Mars, Moon, or the Jovian satellites. Her chief pilot, who treated her like the grande dame she was, complained with mock bitterness that flying round a miserable little comet was far beneath her dignity.

When he was quite sure that Halley – on the surface at least – held no surprises, Captain Smith lifted away from the pole. Moving less than a dozen kilometres took Universe to a different world, from a glimmering twilight that would last for months to a realm that knew the cycle of night and day. And with the dawn, the comet came slowly to life.

As the Sun crept above the jagged, absurdly close horizon, its rays would slant down into the countless small craters that pockmarked the crust. Most of them would remain inactive, their narrow throats sealed by incrustations of mineral salts. Nowhere else on Halley were such vivid displays of colour; they had misled biologists into thinking that here life was beginning, as it had on Earth, in the form of algal growths. Many had not yet abandoned that hope, though they would be reluctant to admit it.

From other craters, wisps of vapour floated up into the sky, moving in unnaturally straight trajectories because there were no winds to divert them. Usually nothing else happened for an hour or two; then, as the Sun's warmth penetrated to the frozen interior, Halley would begin to spurt – as Victor Willis had put it 'like a pod of whales'.

Though picturesque, it was not one of his more accurate metaphors. The jets from the dayside of Halley were not intermittent, but played steadily for hours at a time. And they did not curl over and fall back to the surface, but went rising on up into the sky, until they were lost in the glowing fog which they helped create.

At first, the science team treated the geysers as cautiously as if they were vulcanologists approaching Etna or Vesuvius in one of their less predictable moods. But they soon discovered that Halley's eruptions, though often fearsome in appearance, were singularly gentle and well-behaved; the water emerged about as fast as from an ordinary firehose, and was barely warm. Within seconds of escaping from its underground reservoir, it would flash into a mixture of vapour and ice crystals; Halley was enveloped in a perpetual snowstorm, falling upwards... Even at this modest speed of ejection, none of the water would ever return to its source. Each time it rounded the Sun, more of the comet's life-blood would haemorrhage into the insatiable vacuum of space.

After considerable persuasion, Captain Smith agreed to move Universe to within a hundred metres of 'Old Faithful', the largest geyser on the dayside. It was an awesome sight – a whitish-grey column of mist, growing like some giant tree from a surprisingly small orifice in a three-hundred-metre-wide crater which appeared to be one of the oldest formations on the comet. Before long, the scientists were scrambling all over the crater, collecting specimens of its (completely sterile, alas) multi-coloured minerals, and casually thrusting their thermometers and sampling tubes into the soaring water-ice-mist column itself. 'If it tosses any of you out into space,' warned the Captain, 'don't expect to be rescued in a hurry. In fact, we may just wait until you come back.'

'What does he mean by that?' a puzzled Dimitri Mihailovich had asked. As usual, Victor Willis was quick with the answer.

'Things don't always happen the way you'd expect in celestial mechanics. Anything thrown off Halley at a reasonable speed will still be moving in essentially the same orbit – it takes a huge velocity change to make a big differenc. So one revolution later, the two orbits will intersect again – and you'll be right back where you started. Seventy-six years older, of course.'

Not far from Old Faithful was another phenomenon which no-one could reasonably have anticipated. When they first observed it, the scientists could scarcely believe their eyes. Spread out across several hectares of Halley, exposed to the vacuum of space, was what appeared to be a perfectly ordinary lake, remarkable only for its extreme blackness.

Obviously, it could not be water; the only liquids which could be stable in this environment were heavy organic oils or tars. In fact, 'Lake Tuonela' turned out to be more like pitch, quite solid except for a sticky surface layer less than a millimetre thick. In this negligible gravity, it must have taken years – perhaps several trips round the warming fires of the Sun – for it to have assumed its present mirror-flatness.

Until the Captain put a stop to it, the lake became one of the principal tourist attractions on Halley's Comet. Someone (nobody claimed the dubious honour) discovered that it was possible to walk perfectly normally across it, almost as if on Earth; the surface film had just enough adhesion to hold the foot in place. Before long, most of the crew had got themselves videoed apparently walking on water...

Then Captain Smith inspected the airlock, discovered the walls liberally stained with tar, and gave the nearest thing to a display of anger that anyone had ever witnessed from him.

'It's bad enough,' he said through clenched teeth, 'having the outside of the ship coated with – soot. Halley's Comet is about the filthiest place I've ever seen...'

After that, there were no more strolls on Lake Tuonela.

In a small, self-contained universe where everyone knows everyone else, there can be no greater shock than encountering a total stranger.

Heywood Floyd was floating gently along the corridor to the main lounge when he had this disturbing experience. He stared in amazement at the interloper, wondering how a stowaway had managed to avoid detection for so long. The other man looked back at him with a combination of embarrassment and bravado, obviously waiting for Floyd to speak first.

'Well, Victor!' he said at last. 'Sorry I didn't recognize you. So you've made the supreme sacrifice, for the cause of science – or should I say your public?'

'Yes,' Willis answered grumpily. 'I did manage to squeeze into one helmet – but the damn bristles made so many scratching noises no-one could hear a word I said.'

'When are you going out?'

'Just as soon as Cliff comes back – he's gone caving with Bill Chant.'

The first flybys of the comet, in 1986, had suggested that it was considerably less dense than water -which could only mean that it was either made of very porous material, or was riddled with cavities. Both explanations turned out to be correct.

At first, the ever-cautious Captain Smith flatly forbade any cave-exploring. He finally relented when Dr Pendrill reminded him that his chief assistant Dr Chant was an experienced speleologist – indeed, that was one of the very reasons he had been chosen for the mission.

'Cave-ins are impossible, in this low gravity,' Pendrill had told the reluctant Captain. 'So there's no danger of being trapped.'

'What about being lost?'

'Chant would regard that suggestion as a professional insult. He's been twenty kilometres inside Mammoth Cave. Anyway, he'll play out a guideline.'