She must have become aware of his scrutiny, for she turned and smiled, then gestured toward the unwinding cloudscape below.

'Look!' she shouted in his ear, 'Jupiter has a new moon!'

What is she trying to say? Floyd asked himself. Her English still isn't very good, but she couldn't possibly have made a mistake in a simple sentence like that. I'm sure I heard her correctly – yet she's pointing downward, not upward.

And then he realized that the scene immediately below them had become much brighter; he could even see yellows and greens that had been quite invisible before. Something far more brilliant than Europa was shining on the Jovian clouds.

Leonov itself, many times brighter than Jupiter's noonday sun, had brought a false dawn to the world it was leaving forever. A hundred-kilometre-long plume of incandescent plasma was trailing behind the ship, as the exhaust from the Sakharov Drive dissipated its remaining energies in the vacuum of space.

Vasili was making an announcement, but the words were completely unintelligible. Floyd glanced at his watch; yes, that would be right about now. They had achieved Jupiter escape velocity. The giant could never recapture them.

And then, thousands of kilometres ahead, a great bow of brilliant light appeared in the sky – the first glimpse of the real Jovian dawn, as full of promise as any rainbow on Earth. Seconds later the Sun leaped up to greet them – the glorious Sun, that would now grow brighter and closer every day.

A few more minutes of steady acceleration, and Leonov would be launched irrevocably on the long voyage home. Floyd felt an overwhelming sense of relief and relaxation. The immutable laws of celestial mechanics would guide him through the inner Solar System, past the tangled orbits of the asteroids, past Mars – nothing could stop him from reaching Earth.

In the euphoria of the moment, he had forgotten all about the mysterious black stain, expanding across the face of Jupiter.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

49 – Devourer of Worlds

They saw it again the next morning, ship's time, as it came around to the dayside of Jupiter. The area of darkness had now spread until it covered an appreciable fraction of the planet, and at last they were able to study it at leisure, and in detail.

'Do you know what it reminds me of?' said Katerina. 'A virus attacking a cell. The way a phage injects its DNA into a bacterium, and then multiplies until it takes over.'

'Are you suggesting,' asked Tanya incredulously, 'that Zagadka is eating Jupiter?'

'It certainly looks like it.'

'No wonder Jupiter is beginning to look sick. But hydrogen and helium won't make a very nourishing diet, and there's not much else in that atmosphere. Only a few percent of other elements.'

'Which adds up to some quintillions of tons of sulphur and carbon and phosphorus and everything else at the lower end of the periodic table,' Sasha pointed out. 'In any case, we're talking about a technology that can probably do anything that doesn't defy the laws of physics. If you have hydrogen, what more do you need? With the right know-how, you can synthesize all the other elements from it.'

'They're sweeping up Jupiter – that's for sure,' said Vasili. 'Look at this.'

An extreme close-up of one of the myriad identical rectangles was now displayed on the telescope monitor. Even to the naked eye, it was obvious that streams of gas were flowing into the two smaller faces; the patterns of turbulence looked very much like the lines of force revealed by iron filings, clustered around the ends of a bar magnet.

'A million vacuum cleaners,' said Curnow, 'sucking up Jupiter's atmosphere. But why? And what are they doing with it?'

'And how do they reproduce?' asked Max. 'Have you caught any of them in the act?'

'Yes and no,' answered Vasili. 'We're too far away to see details, but it's a kind of fission – like an amoeba.'

'You mean – they split in two, and the halves grow back to the original size?'

'Nyet. There aren't any little Zagadki – they seem to grow until they've doubled in thickness, then split down the middle to produce identical twins, exactly the same size as the original. And the cycle repeats itself in approximately two hours.'

'Two hours!' exclaimed Floyd. 'No wonder that they've spread over half the planet. It's a textbook case of exponential growth.'

'I know what they are!' said Ternovsky in sudden excitement. 'They're von Neumann machines!'

'I believe you're right,' said Vasili. 'But that still doesn't explain what they're doing. Giving them a label isn't all that much help.'

'And what,' asked Katerina plaintively, 'is a von Neumann machine? Explain, please.'

Orlov and Floyd started speaking simultaneously. They stopped in some confusion, then Vasili laughed and waved to the American.

'Suppose you had a very big engineering job to do, Katerina – and I mean big, like strip-mining the entire face of the Moon. You could build millions of machines to do it, but that might take centuries. If you were clever enough, you'd make just one machine – but with the ability to reproduce itself from the raw materials around it. So you'd start a chain reaction, and in a very short time, you'd have bred enough machines to do the job in decades, instead of millennia. With a sufficiently high rate of reproduction, you could do virtually anything in as short a period of time as you wished. The Space Agency's been toying with the idea for years – and I know you have as well, Tanya.'

'Yes: exponentiating machines. One idea that even Tsiolkovski didn't think of.'

'I wouldn't care to bet on that,' said Vasili. 'So it looks, Katerina, as if your analogy was pretty close. A bacteriophage is a von Neumann machine.'

'Aren't we all?' asked Sasha. 'I'm sure Chandra would say so.'

Chandra nodded his agreement.

'That's obvious. In fact, von Neumann got the original idea from studying living systems.'

'And these living machines are eating Jupiter!'

'It certainly looks like it,' said Vasili. 'I've been doing some calculations, and I can't quite believe the answers – even though it's simple arithmetic.'

'It may be simple to you,' said Katerina. 'Try to let us have it without tensors and differential equations.'

'No – I mean simple,' insisted Vasili. 'In fact, it's a perfect example of the old population explosion you doctors were always screaming about in the last century. Zagadka reproduces every two hours. So in only twenty hours there will be ten doublings. One Zagadka will have become a thousand.'

'One thousand and twenty-four,' said Chandra.

'I know, but let's keep it simple. After forty hours there will be a million – after eighty, a million million. That's about where we are now, and obviously, the increase can't continue indefinitely. In a couple more days, at this rate, they'll weigh more than Jupiter!'

'So they'll soon begin to starve,' said Zenia. 'And what will happen then?'

'Saturn had better look out,' answered Brailovsky. 'Then Uranus and Neptune. Let's hope they don't notice little Earth.'

'What a hope! Zagadka's been spying on us for three million years!'

Walter Curnow suddenly started to laugh.

'What's so funny?' demanded Tanya.

'We're talking about these things as if they're persons – intelligent entities. They're not – they're tools. But general-purpose tools – able to do anything they have to. The one on the Moon was a signalling device – or a spy, if you like. The one that Bowman met – our original Zagadka – was some kind of transportation system. Now it's doing something else, though God knows what. And there may be others all over the Universe,

'I had just such a gadget when I was a kid... Do you know what Zagadka really is? Just the cosmic equivalent of the good old Swiss Army knife!'