'Correct. We will decide which to use at the appropriate time.'

Poole relaxed – as far as was possible in this extraordinary situation. Halman was willing to co-operate: he still had sufficient links with his origins.

'Now, we have to get this tablet to you – physically. Its contents are too dangerous to risk sending over any radio or optical channel. I know you possess long-range control of matter: did you not once detonate an orbiting bomb? Could you transport it to Europa? Alternatively, we could send it in an auto-courier, to any point you specify.'

'That would be best: I will collect it in Tsienville. Here are the co-ordinates...

Poole was still slumped in his chair when the Bowman Suite monitor admitted the head of the delegation that had accompanied him from Earth. Whether Colonel Jones was a genuine Colonel – or even if his name was Jones – were minor mysteries which Poole was not really interested in solving; it was sufficient that he was a superb organizer and had handled the mechanics of Operation Damocles with quiet efficiency.

'Well, Frank – it's on its way. Will be landing in one hour, ten minutes. I assume that Halman can take it from there, but I don't understand how he can actually handle – is that the right word? – these tablets.'

'I wondered about that, until someone on the Europa Committee explained it. There's a well-known – though not to me! – theorem stating that any computer can emulate any other computer. So I'm sure that Halman knows exactly what he's doing. He would never have agreed otherwise.'

'I hope you're right,' replied the Colonel. 'If not – well, I don't know what alternative we have.'

There was a gloomy pause, until Poole did his best to relieve the tension.

'By the way, have you heard the local rumour about our visit?'

'Which particular one?'

'That we're a special commission sent here to investigate crime and corruption in this raw frontier township. The Mayor and the Sheriff are supposed to be running scared.'

'How I envy them,' said 'Colonel Jones'. 'Sometimes it's quite a relief to have something trivial to worry about.'

Like all the inhabitants of Anubis City (population now 56,521), Dr Theodore Khan woke soon after local midnight to the sound of the General Alarm. His first reaction was 'Not another Icequake, for Deus's sake!'

He rushed to the window, shouting 'Open' so loudly that the room did not understand, and he had to repeat the order in a normal voice. The light of Lucifer should have come streaming in, painting the patterns on the floor that so fascinated visitors from Earth, because they never moved even a fraction of a millimetre, no matter how long they waited...

That unvarying beam of light was no longer there. As Khan stared in utter disbelief through the huge, transparent bubble of the Anubis Dome, he saw a sky that Ganymede had not known for a thousand years. It was once more ablaze with stars; Lucifer had gone.

And then, as he explored the forgotten constellations, Kahn noticed something even more terrifying. Where Lucifer should have been was a tiny disc of absolute blackness, eclipsing the unfamiliar stars.

There was only one possible explanation, Khan told himself numbly. Lucifer has been swallowed by a Black Hole. And it may be our turn next.

On the balcony of the Grannymede Hotel, Poole was watching the same spectacle, but with more complex emotions. Even before the general alarm, his comsec had woken him with a message from Halman.

'It is beginning. We have infected the Monolith. But one – perhaps several – of the viruses have entered our own circuits. We do not know if we will be able to use the memory tablet you have given us. If we succeed, we will meet you in Tsienville.'

Then came the surprising and strangely moving words whose exact emotional content would be debated for generations:

'If we are unable to download, remember us.' From the room behind him, Poole heard the voice of the Mayor, doing his best to reassure the now sleepless citizens of Anubis. Though he opened with that most terrifying of official statements – 'No cause for alarm' – the Mayor did indeed have words of comfort.

'We don't know what's happening but Lucifer's still shining normally! I repeat – Lucifer is still shining! We've just received news from the interorbit shuttle Alcyone, which left for Callisto half an hour ago. Here's their view -, Poole left the balcony and rushed into his room just in time to see Lucifer blaze reassuringly on the vidscreen.

'What's happened,' the Mayor continued breathlessly, 'is that something has caused a temporary eclipse – we'll zoom in to look at it... Callisto Observatory, come in please...'

How does he know it's 'temporary'? thought Poole, as he waited for the next image to come up on the screen.

Lucifer vanished, to be replaced by a field of stars. At the same time, the Mayor faded out and another voice took over:

'– two-metre telescope, but almost any instrument will do. It's a disc of perfectly black material, just over ten thousand kilometres across, so thin it shows no visible thickness. And it's placed exactly – obviously deliberately -to block Ganymede from receiving any light.

'We'll zoom in to see if it shows any details, though I rather doubt it...'

From the viewpoint of Callisto, the occulting disc was foreshortened into an oval, twice as long as it was wide. It expanded until it completely filled the screen; thereafter, it was impossible to tell whether the image was being zoomed, as it showed no structure whatsoever.

'As I thought – there's nothing to see. Let's pan over to the edge of the thing...'

Again there was no sense of motion, until a field of stars suddenly appeared, sharply defined by the curving edge of the world-sized disc. It was exactly as if they were looking past the horizon of an airless, perfectly smooth planet.

No, it was not perfectly smooth...

'That's interesting,' commented the astronomer, who until now had sounded remarkably matter-of-fact, as if this sort of thing was an everyday occurrence. 'The edge looks jagged – but in a very regular fashion – like a saw-blade...'

A circular saw Poole muttered under his breath. Is it going to carve us up? Don't be ridiculous...

'This is as close as we can get before diffraction spoils the image – we'll process it later and get much better detail:'

The magnification was now so great that all trace of the disc's circularity had vanished. Across the vidscreen was a black band, serrated along its edge with triangles so identical that Poole found it hard to avoid the ominous analogy of a saw-blade. Yet something else was nagging at the back of his mind...

Like everyone else on Ganymede, he watched the infinitely more distant stars drifting in and out of those geometrically perfect valleys. Very probably, many others jumped to the same conclusion even before he did.

If you attempt to make a disc out of rectangular blocks -whether their proportions are 1:4:9 or any other – it cannot possibly have a smooth edge. Of course, you can make it as near a perfect circle as you like, by using smaller and smaller blocks. Yet why go to that trouble, if you merely wanted to build a screen large enough to eclipse a sun?

The Mayor was right; the eclipse was indeed temporary. But its ending was the precise opposite of a solar one.

First light broke through at the exact centre, not in the usual necklace of Bailey's Beads along the very edge. Jagged lines radiated from a dazzling pinhole – and now, under the highest magnification, the structure of the disc was being revealed. It was composed of millions of identical rectangles, perhaps the same size as the Great Wall of Europa. And now they were splitting apart: it was as if a gigantic jigsaw puzzle was being dismantled.