Captain Smith had raised surprisingly few objections to the idea of passenger EVAs. He agreed that to have come all this way, and not to set foot upon the comet, was absurd.

'There'll be no problems if you follow instructions,' he said at the inevitable briefing. 'Even if you've never worn spacesuits before – and I believe that only Commander Greenburg and Dr Floyd have done so – they're quite comfortable, and fully automatic. There's no need to bother about any controls or adjustments, after you've been checked out in the airlock.

'One absolute rule: only two of you can go EVA at one time. You'll have a personal escort, of course, linked to you by five metres of safety line – though that can be played out to twenty if necessary. In addition, you'll both be tethered to the two guide-cables we've strung the whole length of the valley. The rule of the road is the same as on Earth; keep to the right! If you want to overtake anyone, you only have to unclip your buckle – but one of you must always remain attached to the line. That way, there's no danger of drifting off into space. Any questions?'

'How long can we stay out?'

'As long as you like, Ms M'Bala. But I recommend that you return just as soon as you feel the slightest discomfort. Perhaps an hour would be best for the first outing – though it may seem like only ten minutes...'

Captain Smith had been quite correct. As Heywood Floyd looked at his time-elapsed display, it seemed incredible that forty minutes had already passed. Yet it should not have been so surprising, for the ship was already a good kilometre away.

As the senior passenger – by almost any reckoning – he had been given the privilege of making the first EVA. And he really had no choice of companion.

'EVA with Yva!' chortled Mihailovich. 'How can you possibly resist! Even if,' he added with a lewd grin, 'those damn suits won't let you try all the Extravehicular Activities you'd like,'

Yva had agreed, without any hesitation, yet also without any enthusiasm. That, Floyd thought wryly, was typical. It would not be quite true to say that he was disillusioned – at his age, he had very few illusions left – but he was disappointed. And with himself rather than Yva; she was as beyond criticism or praise as the Mona Lisa – with whom she had often been compared.

The comparison was, of course, ridiculous; La Gioconda was mysterious, but she was certainly not erotic. Yva's power had lain in her unique combination of both – with innocence thrown in for good measure. Half a century later, traces of all three ingredients were still visible, at least to the eye of faith.

What was lacking – as Floyd had been sadly forced to admit – was any real personality. When he tried to focus his mind upon her, all he could visualize were the roles she had played. He would have reluctantly agreed with the critic who had once said:

'Yva Merlin is the reflection of all men's desires; but a mirror has no character.'

And now this unique and mysterious creature was floating beside him across the face of Halley's Comet, as they and their guide moved along the twin cables that spanned the Valley of Black Snow. That was his name; he was childishly proud of it, even though it would never appear on any map. There could be no maps of a world where geography was as ephemeral as weather on Earth. He savoured the knowledge that no human eye had ever before looked upon the scene around him – or ever would again.

On Mars, or on the Moon, you could sometimes -with a slight effort of imagination, and if you ignored the alien sky – pretend that you were on Earth. This was impossible here, because the towering – often overhanging – snow sculptures showed only the slightest concession to gravity. You had to look very carefully at your surroundings to decide which way was up.

The Valley of Black Snow was unusual, because it was a fairly solid structure – a rocky reef embedded in volatile drifts of water and hydrocarbon ice. The geologists were still arguing about its origin, some maintaining that it was really part of an asteroid that had encountered the comet ages ago. Corings had revealed complex mixtures of organic compounds, rather like frozen coal-tar – though it was certain that life had never played any part in their formation.

The 'snow' carpeting of the floor of the little valley was not completely black; when Floyd raked it with the beam of his flashlight it glittered and sparkled as if embedded with a million microscopic diamonds. He wondered if there were indeed diamonds on Halley: there was certainly enough carbon here. But it was almost equally certain that the temperatures and pressures necessary to create them had never existed here.

On a sudden impulse, Floyd reached down and gathered two handfuls of the snow: he had to push with his feet against the safety line to do so, and had a comic vision of himself as a trapeze artist walking a tightrope – but upside down. The fragile crust offered virtually no resistance as he buried head and shoulders into it; then he pulled gently on his tether and emerged with his handful of Halley.

He wished that he could feel it through the insulation of his gloves, as he compacted the mass of crystalline fluff into a ball that just fitted the palm of his hand. There it lay, ebony black yet giving fugitive flashes of light as he turned it from side to side.

And suddenly, in his imagination, it became the purest white – and he was a boy again, in the winter playground of his youth, surrounded with the ghosts of his childhood. He could even hear the cries of his companions, taunting and threatening him with their own projectiles of immaculate snow...

The memory was brief, but shattering, for it brought an overwhelming sensation of sadness. Across a century of time, he could no longer remember a single one of those phantom friends who stood around him; yet some, he knew, he had once loved...

His eyes filled with tears, and his fingers clenched around the ball of alien snow. Then the vision faded; he was himself again. This was not a moment of sadness, but of triumph.

'My God!' cried Heywood Floyd, his words echoing in the tiny, reverberant universe of his spacesuit, 'I'm standing on Halley's Comet – what more do I want! If a meteor hits me now, I won't have a single complaint!'

He brought up his arms and launched the snowball towards the stars. It was so small, and so dark, that it vanished almost at once, but he kept on staring into the sky.

And then, abruptly – unexpectedly – it appeared in a sudden explosion of light, as it rose into the rays of the hidden Sun. Black as soot though it was, it reflected enough of that blinding brilliance to be easily visible against the faintly luminous sky.

Floyd watched it until it finally disappeared – perhaps by evaporation, perhaps by dwindling into the distance. It would not last long in the fierce torrent of radiation overhead; but how many men could claim to have created a comet of their own?