'So do I. But it won't be much longer now – one way or the other.'

She glanced briefly toward the situation display, where the figure 20 was slowly flashing. It was the most unnecessary bit of information in the entire ship, since everyone knew by heart the number of days until the launch window opened.

And the assault on Zagadka was scheduled.

For the second time, Heywood Floyd was looking the other way when it happened. But it would have made no difference in any case; even the vigilant monitor camera showed only a faint blur between one full frame and the subsequent blank one.

Once more he was on duty aboard Discovery, sharing the graveyard shift with Sasha over on Leonov. As usual, the night had been totally uneventful; the automatic systems were performing their jobs with their normal efficiency. Floyd would never have believed, a year ago, that he would one day orbit Jupiter at a distance of a few hundred thousand kilometres and give it barely a glance – while trying; not very successfully, to read The Kreutzer Sonata in the original. According to Sasha, it was still the finest piece of erotic fiction in (respectable) Russian literature, but Floyd had not yet progressed far enough to prove that. And now he never would.

At 0125 he was distracted by a spectacular, though not unusual, eruption on the terminator of Io. A vast umbrella-shaped cloud expanded into space, and started to shower its debris back on to the burning land below. Floyd had seen dozens of such eruptions, but they never ceased to fascinate him. It seemed incredible that so small a world could be the seat of such titanic energies.

To get a better view, he moved around to one of the other observation windows. And what he saw there – or, rather, what he did not see there – made him forget about Io, and almost everything else.

When he had recovered, and satisfied himself that he was not suffering – again? – from hallucinations, he called the other ship.

'Good morning, Woody,' yawned Sasha. 'No – I wasn't asleep. How are you getting on with old Tolstoi?'

'I'm not. Take a look outside and tell me what you see.'

'Nothing unusual, for this part of the cosmos. Io doing its thing. Jupiter. Stars. Oh my God!'

'Thanks for proving I'm sane. We'd better wake the skipper.'

'Of course. And everyone else. Woody – I'm scared.'

'You'd be a fool not to be. Here we go. Tanya? Tanya? Woody here. Sorry to wake you up – but your miracle's happened. Big Brother has gone. Yes – vanished. After three million years, he's decided to leave.

'I think he must know something that we don't.'

It was a sombre little group that gathered, during the next fifteen minutes, for a hasty conference in the wardroom-cum-observation lounge. Even those who had just gone to sleep were instantly awake, as they sipped thoughtfully from bulbs of hot coffee – and kept glancing at the shockingly unfamiliar scene outside Leonov's windows, to convince themselves that Big Brother had indeed vanished.

'It must know something that we don't.' That spontaneous phrase of Floyd's had been repeated by Sasha and now hung silently, ominously, in the air. He had summed up what everyone was now thinking – even Tanya.

It was still too early to say 'I told you so' – nor did it really matter whether that warning had any validity. Even if it was perfectly safe to stay, there was no point in doing so. With nothing to investigate, they might as well go home, just as quickly as possible. Yet it was not quite as simple as that.

'Heywood,' said Tanya, 'I'm now prepared to take that message, or whatever it was, much more seriously. I'd be stupid not to after what's happened. But even if there is danger here, we still have to weigh one risk against another. Coupling Leonov and Discovery together, operating Discovery with that huge off-axis load, disconnecting the ships in a matter of minutes so we can fire our engines at the right moment; no responsible captain would take such chances without very good – I'd say overwhelming – reasons. Even now, I don't have such reasons. I've only got the word of ... a ghost. Not very good evidence in a court of law.'

'Or a court of inquiry,' said Walter Curnow, in an unusually quiet voice, 'even if we all backed you up.'

'Yes, Walter – I was thinking of that. But if we get home safely, that will justify everything – and if we don't, it hardly matters, does it? Anyway, I'm not going to decide now. As soon as we've reported this, I'm going back to bed. I'll give you my decision in the morning after I've slept on it. Heywood, Sasha, will you come up to the bridge with me? We have to wake up Mission Control, before you go back on watch.'

The night had not yet finished with its surprises. Somewhere around the orbit of Mars, Tanya's brief report passed a message going in the opposite direction.

Betty Fernandez had talked at last, Both the CIA and the National Security Agency were furious; their combined blandishments, appeals to patriotism, and veiled threats had failed completely – yet the producer of a sleazy gossip network had succeeded, thereby making himself immortal in the annals of Videodom.

It was half luck, half inspiration. The news director of 'Hello, Earth!' had suddenly realized that one of his staff bore a striking likeness to David Bowman; a clever makeup artist had made it perfect. José Fernandez could have told the young man that he was taking a terrible risk, but he had the good fortune that often favours the brave. Once he had got his foot inside the door, Betty had capitulated. By the time she had – quite gently – thrown him out, he had obtained essentially the whole story. And to do him credit, he had presented it with a lack of leering cynicism quite uncharacteristic of his network. It got him that year's Pulitzer.

'I wish,' Floyd said rather wearily to Sasha, 'she'd talked earlier. It would have saved me a lot of trouble. Anyway, that settles the argument. Tanya can't possibly have any doubts now. But we'll leave it until she wakes up – don't you agree?'

'Of course – it's not urgent, even though it's certainly important. And she'll need the sleep. I have a feeling none of us will get much from now on.'

I'm sure you're right, thought Floyd. He felt very tired, but even if he had not been on duty he would have found it impossible to sleep. His mind was too active, analysing the events of this extraordinary night, trying to anticipate the next surprise.

In one way, he felt an enormous sense of relief: All uncertainty about their departure was surely ended; Tanya could have no further reservations.

But a much greater uncertainty remained. What was happening?

There was only one experience in Floyd's life that matched the situation. As a very young man, he had once gone canoeing with some friends down a tributary of the Colorado River – and they had lost their way.

They had been swept faster and faster between the canyon walls, not completely helpless, but with only enough control to avoid being swamped. Ahead might be rapids – perhaps even a waterfall; they did not know. And in any case, there was little they could do about it.

Once again, Floyd felt himself in the grip of irresistible forces, sweeping him and his companions toward an unknown destiny. And this time the dangers were not only invisible; they might be beyond human comprehension.