Not until Leonov was well on the way home would small explosive charges be detonated, in the hope that the waves propagated through Big Brother would reveal something about its interior structure. This last measure had been hotly debated, both by those who argued that it would generate no results at all – and those who feared it would produce altogether too many.

For a long time, Floyd had wavered between the two viewpoints; now the matter seemed only of trivial importance.

The time for final contact with Big Brother – the great moment that should have been the climax of the expedition – was on the wrong side of the mysterious deadline. Heywood Floyd was convinced that it belonged to a future that would never exist; but he could get no one to agree with him.

And that was the least of his problems. Even if they did agree, there was nothing that they could do about it.

Walter Curnow was the last person he would have expected to resolve the dilemma. For Walter was almost the epitome of the sound, practical engineer, suspicious of flashes of brilliance and technological quick-fixes. No one would ever accuse him of being a genius; and sometimes it required genius to see the blindingly obvious.

'Consider this purely as an intellectual exercise,' he had begun, with most uncharacteristic hesitancy. 'I'm quite prepared to be shot down.'

'Go on,' answered Floyd. 'I'll hear you out politely. That's the least I can do – everyone's been very polite to me. Too polite, I'm afraid.'

Curnow gave a lopsided grin.

'Can you blame them? But if it's any consolation, at least three people now take you quite seriously, and are wondering what we should do.'

'Does that three include you?'

'No; I'm sitting on the fence, which is never terribly comfortable. But in case you're right – I don't want to wait here and take whatever's coming. I believe there's an answer to every problem, if you look in the right place.'

'I'll be delighted to hear it. I've been looking hard enough. Presumably not in the right place.'

'Perhaps. If we want to make a quick getaway – say in fifteen days, to beat that deadline – we'll need an extra delta-vee of about thirty kilometres a second.'

'So Vasili calculates. I haven't bothered to check, but I'm sure he's right. After all, he got us here.'

'And he could get us away – if we had the additional propellant.'

'And if we had a Star Trek beam transporter, we could get back to Earth in an hour.'

'I'll try and rig one up the next time I have a spare moment. But meanwhile, may I point out that we have several hundred tons of the best possible propellant, only a few metres away in Discovery's fuel tanks.'

'We've been through that dozens of times. There's absolutely no way of transferring it to Leonov. We've no pipelines – no suitable pumps. And you can't carry liquid ammonia around in buckets, even in this part of the Solar System.'

'Exactly. But there's no need to do so.'

'Eh?'

'Burn it right where it is. Use Discovery as a first stage, to boost us home.'

If anyone except Walter Curnow had made the suggestion, Floyd would have laughed at him. As it was, his mouth dropped open and it was several seconds before he could think of a suitable comment. What finally emerged was: 'Damn. I should have thought of that.'

Sasha was the first they approached. He listened patiently, pursed his lips, then played a rallentando on his computer keyboard. When the answers flashed up, he nodded thoughtfully.

'You're right. It would give us the extra velocity we need to leave early. But there are practical problems -'

'We know. Fastening the ships together. The off-axis thrust when only Discovery's drive is operating. Cutting loose again at the critical moment. But there are answers to all of these.'

'I see you've been doing your homework. But it's a waste of time. You'll never convince Tanya.'

'I don't expect to – at this stage,' Floyd answered. 'But I'd like her to know that the possibility exists. Will you give us moral support?'

'I'm not sure. But I'll come along to watch; it should be interesting.'

Tanya listened more patiently than Floyd had expected, but with distinct lack of enthusiasm. However, by the time he had finished, she showed what could only be called reluctant admiration.

'Very ingenious, Heywood -,

'Don't congratulate me. All the credit should go to Walter. Or the blame.'

'I don't imagine there will be much of either; it can never be more than a – what did Einstein call that sort of thing? – "thought experiment". Oh, I suspect it would work – in theory, at least. But the risks! So many things could go wrong. I'd only be prepared to consider it if we had absolute and positive proof that we were in danger. And with all respect, Heywood, I see not the slightest evidence of that.'

'Fair enough; but at least you now know that we have another option. Do you mind if we work out the practical details – just in case?'

'Of course not – as long as it doesn't interfere with the preflight checkout. I don't mind admitting that the idea does intrigue me. But it's really a waste of time; there's no way I'd ever approve it. Unless David Bowman appeared to me personally.'

'Would you even then, Tanya?'

Captain Orlova smiled, but without much humour. 'You know, Heywood – I'm really not sure. He'd have to be very persuasive.'

44 – Vanishing Trick

It was a fascinating game in which everyone joined – but only when off duty. Even Tanya contributed ideas to the 'thought experiment', as she continued to call it.

Floyd was perfectly well aware that all the activity was generated not by fear of an unknown danger that only he took seriously, but by the delightful prospect of returning to Earth at least a month earlier than anyone had imagined. Whatever the motive, he was satisfied. He had done his best, and the rest was up to the Fates.

There was one piece of luck, without which the whole project would have been stillborn. The short, stubby Leonov, designed to drill safely through the Jovian atmosphere during the braking manoeuvre, was less than half the length of Discovery and so could be neatly piggybacked on the larger vessel, And the midships antenna mount would provide an excellent anchor point – assuming that it was strong enough to take the strain of Leonov's weight while Discovery's drive was operating.

Mission Control was sorely puzzled by some of the requests flashed back to Earth during the next few' days. Stress analyses of both ships, under peculiar loads; effects of off-axis thrusts; location of unusually strong or weak points in the hulls – these were only some of the more esoteric problems the perplexed engineers were asked to tackle. 'Has something gone wrong?' they inquired anxiously.

'Not at all,' Tanya replied. 'We're merely investigating possible options. Thank you for your cooperation. End of transmission.'

Meanwhile, the programme went ahead as planned. All systems were carefully checked in both ships, and readied for the separate voyages home; Vasili ran simulations on return trajectories and Chandra fed them to Hal when they had been debugged – getting Hal to make a final check in the process. And Tanya and Floyd worked amicably together orchestrating the approach to Big Brother like generals planning an invasion.

It was what he had come all the way to do, yet Floyd's heart was no longer in it. He had undergone an experience he could share with no one – even those who believed him. Though he carried out his duties efficiently, much of the time his mind was elsewhere.

Tanya understood perfectly.

'You're still hoping for that miracle to convince me, aren't you?'

'Or deconvince me – that would be equally acceptable. It's the uncertainty that I dislike.'