‘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!’
VII
LUCIFER RISING
50
Farewell to Jupiter
It was not easy to compose the message, especially after the one he had just sent to his lawyer. Floyd felt like a hypocrite; but he knew it had to be done to minimize the pain that was inevitable on both sides.
He was sad, but no longer disconsolate. Because he was coming back to Earth in an aura of successful achievement – even if not precisely heroism – he would be bargaining from a position of strength. No one – no one – would be able to take Chris away from him.
‘My dear Caroline [it was no longer ‘My dearest’], I am on my way home. By the time you get this, I’ll already be in hibernation. Only a few hours from now, as it will seem to me, I’ll open my eyes – and there will be the beautiful blue Earth hanging in space beside me.
‘Yes, I know it will still be many months for you, and I’m sorry. But we knew that’s the way it would be before I left; as it is, I’m getting back weeks ahead of schedule because of the change in the mission plan.
‘I hope we can work something out. The main question is: What’s best for Chris? Whatever our own feelings, we must put him first. I know I’m willing to do so, and I’m sure you are.’
Floyd switched off the recorder. Should he say what he had intended: ‘A boy needs his father?’ No – it would not be tactful, and might only make matters worse. Caroline might well retort that between birth and four years old it was the mother who mattered most to a child – and if he had believed otherwise, he should have stayed on Earth.
‘… Now about the house. I’m glad the Regents have taken that attitude, which will make it much easier for both of us. I know we both loved the place, but it will be too big now and will bring back too many memories. For the time being, I’ll probably get an apartment in Hilo: I hope I can find some permanent place as quickly as possible.
‘That’s one thing I can promise everyone – I won’t leave Earth again. I’ve had enough of space travelling for one lifetime. Oh, perhaps the Moon, if I really have to – but of course that’s just a weekend excursion.
‘And talking of moons, we’ve just passed the orbit of Sinope, so we’re now leaving the Jovian system. Jupiter is more than twenty million kilometres away, and is barely larger than our own Moon.
‘Yet even from this distance, you can tell that something terrible has happened to the planet. Its beautiful orange colour has vanished; it’s a kind of sickly grey, only a fraction of its former brilliance. No wonder it’s only a faint star now in the sky of Earth.
‘But nothing else has happened, and we’re well past the deadline. Could the whole thing have been a false alarm or a kind of cosmic practical joke? I doubt if we’ll ever know. Anyway, it’s brought us home ahead of schedule, and I’m grateful for that.
‘Goodbye for the present, Caroline – and thank you for everything. I hope we can still be friends. And my dearest love, as ever, to Chris.’
When he had finished, Floyd sat quietly for a while in the tiny cubicle he would not need much longer. He was just about to carry the audio chip up to the bridge for transmission, when Chandra came drifting in.