Its perpetual, but now briefly interrupted, daylight was slowly returning to Ganymede, as the disc fragmented and the rays of Lucifer poured through the widening gaps. Now the components themselves were evaporating, almost as if they needed the reinforcement of each other's contact to maintain reality.

Although it seemed like hours to the anxious watchers in Anubis City, the whole event lasted for less than fifteen minutes. Not until it was all over did anyone pay attention to Europa itself.

The Great Wall was gone: and it was almost an hour before the news came from Earth, Mars and Moon that the Sun itself had appeared to flicker for a few seconds, before resuming business as usual.

It had been a highly selective set of eclipses, obviously targeted at humankind. Nowhere else in the Solar System would anything have been noticed.

In the general excitement, it was a little longer before the world realized that TMA ZERO and TMA ONE had both vanished, leaving only their four-million-year-old imprints on Tycho and Africa.

It was the first time the Europs could ever have met humans, but they seemed neither alarmed nor surprised by the huge creatures moving among them at such lightning speed. Of course, it was not too easy to interpret the emotional state of something that looked like a small, leafless bush, with no obvious sense organs or means of communication. But if they were frightened by the arrival of Alcyone, and the emergence of its passengers, they would surely have remained hiding in their igloos.

As Frank Poole, slightly encumbered by his protective suit and the gift of shining copper wire he was carrying, walked into the untidy suburbs of Tsienville, he wondered what the Europs thought of recent events. For them, there had been no eclipse of Lucifer, but the disappearance of the Great Wall must surely have been a shock. It had stood there for a thousand years, as a shield and doubtless much more; then, abruptly, it was gone, as if it had never been...

The petabyte tablet was waiting for him, with a group of Europs standing around it, demonstrating the first sign of curiosity that Poole had ever observed in them. He wondered if Halman had somehow told them to watch over this gift from space, until he came to collect it.

And to take it back, since it now contained not only a sleeping friend but terrors which some future age might exorcise, to the only place where it could be safely stored.

It would be hard, Poole thought, to imagine a more peaceful scene – especially after the trauma of the last weeks. The slanting rays of a nearly full Earth revealed all the subtle details of the waterless Sea of Rains – not obliterating them, as the incandescent fury of the Sun would do.

The small convoy of mooncars was arranged in a semicircle a hundred metres from the inconspicuous opening at the base of Pico that was the entrance to the Vault. From this viewpoint, Poole could see that the mountain did not live up to the name that the early astronomers, misled by its pointed shadow, had given to it. It was more like a rounded hill than a sharp peak, and he could well believe that one of the local pastimes was bicycle-riding to the summit. Until now, none of those sportsmen and women could have guessed at the secret hidden beneath their wheels: he hoped that the sinister knowledge would not discourage their healthy exercise.

An hour ago, with a sense of mingled sadness and triumph, he had handed over the tablet he had brought -never letting it out of his sight – from Ganymede directly to the Moon.

'Good-bye, old friends,' he had murmured. 'You've done well. Perhaps some future generation will reawaken you. But on the whole – I rather hope not.'

He could imagine, all too clearly, one desperate reason why Halman's knowledge might be needed again. By now, surely, some message was on its way to that unknown control centre, bearing the news that its servant on Europa no longer existed. With reasonable luck, it would take 950 years, give or take a few, before any response could be expected.

Poole had often cursed Einstein in the past; now he blessed him. Even the powers behind the Monoliths, it now appeared certain, could not spread their influence faster than the speed of light. So the human race should have almost a millennium to prepare for the next encounter – if there was to be one. Perhaps by that time, it would be better prepared.

Something was emerging from the tunnel – the track-mounted, semi-humanoid robot that had carried the tablet into the Vault. It was almost comic to see a machine enclosed in the kind of isolation suit used as protection against deadly germs and here on the airless Moon! But no one was taking any chances, however unlikely they might seem. After all, the robot had moved among those carefully sequestered nightmares, and although according to its video cameras everything appeared in order, there was always a chance that some vial had leaked, or some canister's seal had broken. The Moon was a very stable environment, but during the centuries it had known many quakes and meteor impacts.

The robot came to a halt fifty metres outside the tunnel. Slowly, the massive plug that sealed the Vault swung back into place, and began to rotate in its threads, like a giant bolt being screwed into the mountain.

'All not wearing dark glasses, please close your eyes or look away from the robot!' said an urgent voice over the mooncar radio. Poole twisted round in his seat, just in time to see an explosion of light on the roof of the vehicle. When he turned back to look at Pico, all that was left of the robot was a heap of glowing slag; even to someone who had spent much of his life surrounded by vacuum, it seemed altogether wrong that tendrils of smoke were not slowly spiralling up from it.

'Sterilization completed,' said the voice of the Mission Controller. 'Thank you, everybody. Now returning to Plato City.'

How ironic – that the human race had been saved by the skilful deployment of its own insanities! What moral, Poole wondered, could one possibly draw from that?

He looked back at the beautiful blue Earth, huddling beneath its tattered blanket of clouds for protection against the cold of space. Up there, a few weeks from now, he hoped to cradle his first grandson in his arms.

Whatever godlike powers and principalities lurked beyond the stars, Poole reminded himself, for ordinary humans only two things were important – Love and Death.

His body had not yet aged a hundred years: he still had plenty of time for both.

'Their little universe is very young, and its god is still a child. But it is too soon to judge them; when We return in the Last Days, We will consider what should be saved.'