Floyd looked at her in frank amazement; even now, he still found it difficult to accept her as a real human being, and never knew when she would come out with some brilliant insight or utter stupidity.

'That's a very good question, Yva. Believe me, I'm working on it.'

He was telling the truth; he could never lie to Yva Merlin. That, somehow, would be an act of sacrilege.

The first wisps of vapour were appearing over the mouth of the geyser. They shot upwards and away in their unnatural vacuum trajectories, and evaporated swiftly in the fierce Sunlight.

Old Faithful coughed again, and cleared its throat. A snowy-white – and surprisingly compact – column of ice crystals and water droplets climbed swiftly towards the sky. All one's terrestrial instincts expected it to topple and fall, but of course it did not. It continued onwards and upwards, spreading only slightly, until it merged into the vast, glowing envelope of the comet's still expanding coma. Floyd noted, with satisfaction, that the pipeline was beginning to shake as fluid rushed into it.

Ten minutes later, there was a council of war on the bridge. Captain Smith, still in a huff, acknowledged Floyd's presence with a slight nod; his Number Two, a little embarrassed, did all the talking.

'Well, it works, surprisingly well. At this rate, we can fill our tanks in twenty hours – though we may have to go out and anchor the pipe more securely.'

'What about the dirt?' someone asked.

The First Officer held up a transparent squeeze-bulb holding a colourless liquid.

'The filters got rid of everything down to a few microns, To be on the safe side, we'll run through them twice, cycling from one tank to another. No swimming pool, I'm afraid, until we pass Mars.'

That got a much needed laugh, and even the Captain relaxed a little.

'We'll run up the engines, at minimum thrust, to check that there are no operational anomalies with Halley H20. If there are, we'll forget the whole idea, and head home on good old Moon water, fob Aristarchus.'

There was one of those 'party silences' where everyone waits simultaneously for someone else to speak. Then Captain Smith broke the embarrassing hiatus.

'As you all know,' he said, 'I'm very unhappy with the whole idea. In fact – ' he changed course abruptly; it was equally well-known that he had considered sending Sir Lawrence his resignation, though in the circumstances that would have been a somewhat pointless gesture.

'But a couple of things have happened in the last few hours. The owner agrees with the project – if no fundamental objections emerge from our tests. And – this is the big surprise, and I don't know any more about it than you do – the World Space Council has not only okayed but requested that we make the diversion, underwriting any expenses incurred. Your guess is as good as mine...

'But I still have one worry -' he looked doubtfully at the little bulb of water, which Heywood Floyd was now holding up to the light and shaking gently. 'I'm an engineer, not a damn chemist. This stuff looks clean – but what will it do to the tank linings?'

Floyd never quite understood why he acted as he did; such rashness was completely uncharacteristic. Perhaps he was simply impatient with the whole debate, and wanted to get on with the job. Or perhaps he felt that the Captain needed a little stiffening of the moral fibre.

With one quick movement, he flicked open the stopcock and squirted approximately 20cc of Halley's Comet down his throat.

'There's your answer, Captain,' he said, when he had finished swallowing.

'And that,' said the ship's doctor half an hour later, 'was one of the silliest exhibitions I've ever seen. Don't you know that there are cyanides and cyanogens and God knows what else in that stuff?'

'Of course, I do,' laughed Floyd. 'I've seen the analyses – just a few parts in a million. Nothing to worry about, But I did have one surprise,' he added ruefully.

'And what was that?'

'If you could ship this stuff back to Earth, you could make a fortune selling it as Halley's Patent Purgative.'

Now that they were committed, the whole atmosphere aboard Universe had changed. There was no more argument; everyone was cooperating to the utmost, and very few people had much sleep for the next two rotations of the nucleus – a hundred hours of Earth time.

The first Halley 'day' was devoted to a still rather cautious tapping of Old Faithful, but when the geyser subsided towards nightfall the technique had been thoroughly mastered. More than a thousand tons of water had been taken aboard; the next period of daylight would be ample for the rest.

Heywood Floyd kept out of the Captain's way, not wishing to press his luck; in any event, Smith had a thousand details to attend to. But the calculation of the new orbit was not among them; that had been checked and rechecked on Earth.

There was no doubt, now, that the concept was brilliant, and the savings even greater than Jolson had claimed. By refuelling on Halley, Universe had eliminated the two major orbit changes involved in the rendezvous with Earth; she could now go straight to her goal, under maximum acceleration, saving many weeks. Despite the possible risks, everyone now applauded the scheme.

Well, almost everyone.

On Earth, the swiftly organized 'Hands off Halley!' society was indignant. Its members (a mere 236, but they knew how to drum up publicity) did not consider the rifling of a celestial body justified, even to save lives. They refused to be placated even when it was pointed out that Universe was merely borrowing material that the comet was about to lose anyway. It was, they argued, the principle of the thing. Their angry communiqués gave much needed light relief aboard Universe.

Cautious as ever, Captain Smith ran the first low-powered tests with one of the attitude-control thrusters; if this became unserviceable, the ship could manage without it, There were no anomalies; the engine behaved exactly as if it was running on the best distilled water from the lunar mines.

Then he tested the central main engine, Number One; if that was damaged, there would be no loss of manoeuvrability – only of total thrust. The ship would still be fully controllable, but, with the four remaining outboards alone, peak acceleration would be down by twenty per cent.

Again, there were no problems; even the sceptics started being polite to Heywood Floyd, and Second Officer Jolson was no longer a social outcast.

The lift-off was scheduled late in the afternoon, just before Old Faithful was due to subside. (Would it still be there to greet the next visitors in seventy-six years' time? Floyd wondered. Perhaps; there were hints of its existence even back on the 1910 photographs.)

There was no countdown, in the dramatic oldtime Cape Canaveral style. When he was quite satisfied that everything was shipshape, Captain Smith applied a mere five tons of thrust on Number One, and Universe drifted slowly upwards and away from the comet.

The acceleration was modest, but the pyrotechnics were awe-inspiring – and, to most of the watchers, wholly unexpected. Until now, the jets from the main engines had been virtually invisible, being formed entirely of highly ionized oxygen and hydrogen. Even when – hundreds of kilometres away – the gases had cooled off enough to combine chemically, there was still nothing to be seen, because the reaction gave no light in the visible spectrum.

But now, Universe was climbing away from Halley on a column of incandescence too brilliant for the eye to look upon; it seemed almost a solid pillar of flame. Where it hit the ground, rock exploded upwards and outwards; as it departed for ever, Universe was carving its signature, like cosmic graffiti, across the nucleus of Halley's Comet.