What was that? From beyond the portcullis of stalactites blocking further progress he could see a faint glow, like the first light of dawn. As his eyes grew adapted to the darkness, it appeared to grow brighter, and he could detect a hint of green. Now he could even see the outlines of the barrier ahead.

'What's happening?' said Greenburg anxiously.

'Nothing – just observing.'

And thinking, he might have added. There were four possible explanations.

Sunlight could be filtering down through some natural light duct – ice, crystal, whatever. But at this depth? Unlikely.

Radioactivity? He hadn't bothered to bring a counter; there were virtually no heavy elements here. But it would be worth coming back to check.

Some phosphorescent mineral – that was the one he'd put his money on. But there was a fourth possibility – the most unlikely, and most exciting, of all.

Dr Chant had never forgotten a moonless – and Luciferless – night on the shores of the Indian Ocean, when he had been walking beneath brilliant stars along a sandy beach. The sea was very calm, but from time to time a languid wave would collapse at his feet – and detonate in an explosion of light.

He had walked out into the shallows (he could still remember the feel of the water round his ankles, like a warm bath) and with every step he took there had been another burst of light. He could even trigger it by clapping his hands close to the surface.

Could similar bioluminescent organisms have evolved, here in the heart of Halley's Comet? He would love to think so. It seemed a pity to vandalize something so exquisite as this natural work of art – with the glow behind it, the barrier now reminded him of an altar screen he had once seen in some cathedral – but he would have to go back and get some explosives. Meanwhile, there was the other corridor...

'I can't get any further along this route,' he told Greenburg, 'so I'll try the other. Coming back to the junction – setting the reel on rewind.' He did not mention the mysterious glow, which had vanished as soon as he switched on his lights again.

Greenburg did not reply immediately, which was unusual; probably he was talking to the ship. Chant did not worry; he would repeat his message as soon as he had got under way again.

He did not bother, because there was a brief acknowledgement from Greenburg.

'Fine, Cliff – thought I'd lost you for a minute. Back at the chamber – now going into the other tunnel – hope there's nothing blocking that.'

This time, Greenburg replied at once.

'Sorry, Bill. Come back to the ship. There's an emergency – no, not here – everything's fine with Universe. But we may have to return to Earth at once.'

It was only a few weeks before Dr Chant discovered a very plausible explanation for the broken columns. As the comet blasted its substance away into space at each perihelion passage, its mass distribution continually altered. And so, every few thousand years, its spin became unstable, and it would change the direction of its axis – quite violently, like a top that is about to fall over as it loses energy. When that occurred, the resulting cometquake could reach a respectable five on the Richter scale.

But he never solved the mystery of the luminous glow. Though the problem was swiftly overshadowed by the drama that was now unfolding, the sense of a missed opportunity would continue to haunt him for the rest of his life.

Though he was occasionally tempted, he never mentioned it to any of his colleagues. But he did leave a sealed note for the next expedition, to be opened in 2133.

'Have you seen Victor?' said Mihailovich gleefully, as Floyd hurried to answer the Captain's summons. 'He's a broken man.'

'He'll grow it back on the way home,' snapped Floyd, who had no time for such trivialities at the moment. 'I'm trying to find out what's happened.'

Captain Smith was still sitting, almost stunned, in his cabin when he arrived. If this was an emergency affecting his own ship, he would have been a tornado of controlled energy, issuing orders right and left. But there was nothing he could do about this situation, except await the next message from Earth.

Captain Laplace was an old friend; how could he have got into such a mess? There was no conceivable accident, error of navigation, or failure of equipment that could possibly account for his predicament. Nor, as far as Smith could see, was there any way in which Universe could help him get out of it. Operations Centre was just running round and round in circles; this looked like one of those emergencies, all too common in space, where nothing could be done except transmit condolences and record last messages. But he gave no hint of his doubts and reservations when he reported the news to Floyd.

'There's been an accident,' he said. 'We've received orders to return to Earth immediately, to be fitted out for a rescue mission.'

'What kind of accident?'

'It's our sister ship, Galaxy. She was doing a survey of the Jovian satellites. And she's made a crash landing.'

He saw the look of amazed incredulity on Floyd's face.

'Yes, I know that's impossible. But you've not heard anything yet. She's stranded – on Europa.'

'Europa!'

'I'm afraid so. She's damaged, but apparently there's no loss of life. We're still awaiting details.'

'When did it happen?'

'Twelve hours ago. There was a delay before she could report to Ganymede.'

'But what can we do? We're on the other side of the Solar System. Getting back to lunar orbit to refuel, then taking the fastest orbit to Jupiter – it would be – oh, at least a couple of months!' (And back in Leonov's day, Floyd added to himself, it would have been a couple of years...)

'I know; but there's no other ship that could do anything.' -

'What about Ganymede's own inter-satellite ferries?'

'They're only designed for orbital operations.'

'They've landed on Callisto.'

'Much lower energy mission. Oh, they could just manage Europa, but with negligible payload. It's being looked into, of course.'

Floyd scarcely heard the Captain; he was still trying to assimilate this astonishing news. For the first time in half a century – and only for the second time in all history! – a ship had landed on the forbidden moon. And that prompted an ominous thought.

'Do you suppose,' he asked, 'that – whoever – whatever – is on Europa could be responsible?'

'I was wondering about that,' said the Captain glumly. 'But we've been snooping around the place for years, without anything happening.'

'Even more to the point – what might happen to us if we attempted a rescue?'

'That's the first thing that occurred to me. But all this is speculation – we'll have to wait until we have more facts. Meanwhile – this is really why I called you – I've just received Galaxy's crew manifest, and I was wondering...'

Hesitantly, he pushed the print-out across his desk. But even before Heywood Floyd scanned the list, he somehow knew what he would find.

'My grandson,' he said bleakly.

And, he added to himself, the only person who can carry my name beyond the grave.