It was very simple – a hollow tube just a metre long, with a footpad at one end and a retaining loop at the other. At the touch of a button, it could telescope out to five or six times its normal length, and the internal shock-absorbing system allowed a skilled operator to perform the most amazing manoeuvres. The footpad could also become a claw or hook if necessary; there were many other refinements, but that was the basic design. It looked deceptively easy to use; it wasn't.

The airlock pumps finished recycling; the EXIT sign came on; the outer doors opened, and they drifted slowly into the void.

Discovery was windmilling about two hundred metres away, following them in orbit around Io, which filled half the sky. Jupiter was invisible, on the other side of the satellite. This was a matter of deliberate choice; they were using Io as a shield to protect them from the energies raging back and forth in the flux-tube that linked the two worlds. Even so, the radiation level was dangerously high; they had less than fifteen minutes before they must get back to shelter.

Almost immediately, Curnow had a problem with his suit. 'It fitted me when I left Earth,' he complained. 'But now I'm rattling around inside like a pea in a pod.'

'That's perfectly normal, Walter,' said Surgeon-Commander Rudenko, breaking into the radio circuit. 'You lost ten kilos in hibernation, which you could very well afford to miss. And you've already put three of them back.'

Before Curnow had time to think of a suitable retort, he found himself gently but firmly jerked away from Leonov.

'Just relax, Walter,' said Brailovsky. 'Don't use your thrusters, even if you start tumbling. Let me do all the work.'

Curnow could see the faint puffs from the younger man's backpack, as its tiny jets drove them toward Discovery. With each little cloud of vapour there came a gentle tug on the towline, and he would start moving toward Brailovsky; but he never caught up with him before the next puff came. He felt rather like a yo-yo – now making one of its periodic comebacks on Earth – bouncing up and down on its string.

There was only one safe way to approach the derelict, and that was along the axis around which it was slowly revolving. Discovery's centre of rotation was approximately amidships, near the main antenna complex, and Brailovsky was heading directly toward this area, with his anxious partner in tow. How will he stop both of us in time? Curnow asked himself.

Discovery was now a huge, slender dumbbell slowly flailing the entire sky ahead of them. Though it took several minutes to complete one revolution, the far ends were moving at an impressive speed. Curnow tried to ignore them, and concentrated on the approaching – and immobile – centre.

'I'm aiming for that,' said Brailovsky. 'Don't try to help, and don't be surprised at anything that happens.'

Now, what does he mean by that? Curnow asked himself, while preparing to be as unsurprised as possible.

Everything happened in about five seconds. Brailovsky triggered his broomstick, so that it telescoped out to its full length of four metres and made contact with the approaching ship. The broomstick started to collapse, its internal spring absorbing Brailovsky's considerable momentum; but it did not, as Curnow had fully expected, bring him to rest beside the antenna mount. It immediately expanded again, reversing the Russian's velocity so that he was, in effect, reflected away from Discovery just as rapidly as he had approached. He flashed past Curnow, heading out into space again, only a few centimetres away. The startled American just had time to glimpse a large grin before Brailovsky shot past him.

A second later, there was a jerk on the line connecting them, and a quick surge of deceleration as they shared momentum. Their opposing velocities had been neatly cancelled; they were virtually at rest with respect to Discovery. Curnow had merely to reach out to the nearest handhold, and drag them both in.

'Have you ever tried Russian roulette?' he asked, when he had got his breath back.

'No – what is it?'

'I must teach you sometime. It's almost as good as this for curing boredom.'

'I hope you're not suggesting, Walter, that Max would do anything dangerous?'

Dr Rudenko sounded as if she was genuinely shocked, and Curnow decided it was best not to answer; sometimes the Russians did not understand his peculiar sense of humour. 'You could have fooled me,' he muttered under his breath, not loud enough for her to hear.

Now that they were firmly attached to the hub of the windmilling ship, he was no longer conscious of its rotation – especially when he fixed his gaze upon the metal plates immediately before his eyes. The ladder stretching away into the distance, running along the slender cylinder that was Discovery's main structure, was his next objective. The spherical command module at its far end seemed several light-years away, though he knew perfectly well that the distance was only fifty metres.

'I'll go first,' said Brailovsky, reeling in the slack on the line linking them together. 'Remember – it's downhill all the way from here. But that's no problem – you can hold on with one hand. Even at the bottom, gravity's only about a tenth gee. And that's – what do you say? – chickenshit.'

'I think you mean chickenfeed. And if it's all the same to you, I'm going feet first. I never liked crawling down ladders the wrong way up – even in fractional gravity.'

It was essential, Curnow was very well aware, to keep up this gently bantering tone; otherwise he would be simply overwhelmed by the mystery and danger of the situation. There he was, almost a billion kilometres from home, about to enter the most famous derelict in the entire history of space exploration; a media reporter had once called Discovery the Marie Celeste of space, and that was not a bad analogy. But there was also much that made his situation unique; even if he tried to ignore the nightmare moonscape filling half the sky, there was a constant reminder of its presence at hand. Every time he touched the rungs of the ladder, his glove dislodged a thin mist of sulphur dust.

Brailovsky, of course, was quite correct; the rotational gravity caused by the ship's end-over-end tumbling was easily countered. As he grew used to it, Curnow even welcomed the sense of direction it gave him.

And then, quite suddenly, they had reached the big, discoloured sphere of Discovery's control and life-support module. Only a few metres away was an emergency hatch – the very one, Curnow realized, that Bowman had entered for his final confrontation with Hal.

'Hope we can get in,' muttered Brailovsky. 'Pity to come all this way and find the door locked.'

He scraped away the sulphur obscuring the AIRLOCK STATUS display panel.

'Dead, of course. Shall I try the controls?'

'Won't do any harm – but nothing will happen.'

'You're right. Well, here goes with manual...

It was fascinating to watch the narrow hairline open in the curved wall, and to note the little puff of vapour dispersing into space, carrying with it a scrap of paper. Was that some vital message? They would never know; it spun away, tumbling end over end without losing any of its initial spin as it disappeared against the stars.

Brailovsky kept turning the manual control for what seemed a very long time, before the dark, uninviting cave of the airlock was completely open. Curnow had hoped that the emergency lights, at least, might still be operating. No such luck.

'You're boss now, Walter. Welcome to US territory.'

It certainly did not look very welcoming as he clambered inside, flashing the beam of his helmet light around the interior. As far as Curnow could tell, everything was in good order. What else had he expected? he asked himself, half angrily.

Closing the door manually took even longer than opening it, but there was no alternative until the ship was powered up again. Just before the hatch was sealed, Curnow risked a glance at the insane panorama outside.