'... What delightful news about the baby dolphin! I can just imagine how excited Chris was when the proud parents brought it into the house. You should have heard the ohs and ahs of my shipmates when they saw the videos of them swimming together, and Chris riding on its back. They suggest we call it Sputnik, which means companion as well as satellite.

'Sorry it's been quite a while since my last message, but the newscasts will have given you an idea of the huge job we've had to do. Even Captain Tanya's given up all pretence of a regular schedule; each problem has to be fixed as it comes along, by whoever is on the spot. We sleep when we can't stay awake any longer.

'I think we can all be proud of what we've done. Both ships are operational and we've nearly finished our first round of tests on Hal. In a couple of days we'll know if we can trust him to fly Discovery when we leave here to make our final rendezvous with Big Brother.

'I don't know who first gave it that name – the Russians, understandably, aren't keen on it. And they've waxed quite sarcastic about our official designation TMA-2, pointing out to me – several times – that it's the best part of a billion kilometres from Tycho. Also that Bowman reported no magnetic anomaly, and that the only resemblance to TMA-1 is the shape. When I asked them what name they preferred, they came up with Zagadka, which means enigma. It's certainly an excellent name; but everyone smiles when I try to pronounce it, so I'll stick to Big Brother.

'Whatever you call the thing, it's only ten thousand kilometres away now, and the trip won't take more than a few hours. But that last lap has us all nervous, I don't mind telling you.

'We'd hoped that we might find some new information aboard Discovery. That's been our only disappointment, though we should have expected it. Hal, of course, was disconnected long before the encounter, and so has no memories of what happened; Bowman has taken all his secrets with him. There's nothing in the ship's log and automatic recording systems that we didn't already know.

'The only new item we discovered was purely personal – a message that Bowman had left for his mother. I wonder why he never sent it; obviously, he did expect – or hope – to return to the ship after that last EVA. Of course, we've had it forwarded to Mrs Bowman – she's in a nursing home, somewhere in Florida, and her mental condition is poor, so it may not mean anything to her.

'Well, that's all the news this time. I can't tell you how much I miss you... and the blue skies and green seas of Earth. All the colours here are reds and oranges and yellows – often as beautiful as the most fantastic sunset, but after a while one grows sick for the cool, pure rays at the other end of the spectrum.

'My love to you both – I'll call again just as soon as I can.'

Nikolai Temovsky, Leonov's control and cybernetics expert, was the only man aboard who could talk to Dr Chandra on something like his own terms. Although Hal's principal creator and mentor was reluctant to admit anyone into his full confidence, sheer physical exhaustion had forced him to accept help. Russian and Indo-American had formed a temporary alliance, which functioned surprisingly well. Most of the credit for this went to the good-natured Nikolai, who was somehow able to sense when Chandra really needed him, and when he preferred to be alone. The fact that Nikolai's English was much the worst on the ship was totally unimportant, since most of the time both men spoke a computerese wholly unintelligible to anyone else.

After a week's slow and careful reintegration, all of Hal's routine, supervisory functions were operating reliably. He was like a man who could walk, carry out simple orders, do unskilled jobs, and engage in low-level conversation. In human terms, he had an Intelligence Quotient of perhaps 50; only the faintest outlines of his original personality had yet emerged.

He was still sleepwalking; nevertheless, in Chandra's expert opinion, he was now quite capable of flying Discovery from its close orbit around Io up to the rendezvous with Big Brother.

The prospect of getting an extra seven thousand kilometres away from the burning hell beneath them was welcomed by everyone. Trivial though that distance was in astronomical terms, it meant that the sky would no longet be dominated by a landscape that might have been imagined by Dante or Hieronymus Bosch. And although not even the most violent eruptions had blasted any material up to the ships, there was always the fear that Io might attempt to set a new record. As it was, visibility from Leonov's observation deck was steadily degraded by a thin film of sulphur, and sooner or later someone would have to go out and clean it off.

Only Curnow and Chandra were aboard Discovery when Hal was given the first control of the ship. It was a very limited form of control; he was merely repeating the program that had been fed into his memory, and monitoring its execution. And the human crew was monitoring him: if any malfunction occurred, they would take over immediately.

The first burn lasted for ten minutes; then Hal reported that Discovery had entered the transfer orbit. As soon as Leonov's radar and optical tracking confirmed that, the other ship injected itself into the same trajectory. Two minor in-course corrections were made; then, three hours and fifteen minutes later, both arrived uneventfully at the first Lagrange point, L. 1 – 10,500 kilometres up, on the invisible line connecting the centres of Io and Jupiter.

Hal had behaved impeccably, and Chandra showed unmistakable traces of such purely human emotions as satisfaction and even joy. But by that time, everyone's thoughts were elsewhere; Big Brother, alias Zagadka, was only a hundred kilometres away.

Even from that distance, it already appeared larger than the Moon as seen from Earth, and shockingly unnatural in its straight-edged, geometrical perfection. Against the background of space it would have been completely invisible, but the scudding Jovian clouds 350,000 kilometres below showed it up in dramatic relief. They also produced an illusion that, once experienced, the mind found almost impossible to refute. Because there was no way in which its real location could be judged by the eye, Big Brother often looked like a yawning trapdoor set in the face of Jupiter.

There was no reason to suppose that a hundred kilometres would be 'safer than ten, or more dangerous than a thousand; it merely seemed psychologically right for a first reconnaissance. From that distance, the ship's telescopes could have revealed details only centimetres across -but there were none to be seen. Big Brother appeared completely featureless; which, for an object that had, presumably, survived millions of years of bombardment by space debris, was incredible.

When Floyd stared through the binocular eyepiece, it seemed to him that he could reach out and touch those smooth, ebon surfaces – just as he had done on the Moon, years ago. That first time, it had been with the gloved hand of his spacesuit. Not until the Tycho monolith had been enclosed in a pressurized dome had he been able to use his naked hand.

That had made no difference; he did not feel that he had ever really touched TMA-1. The tips of his fingers had seemed to skitter over an invisible barrier, and the harder he pushed, the greater the repulsion grew. He wondered if Big Brother would produce the same effect.

Yet before they came that close, they had to make every test they could devise and report their observations to Earth. They were in much the same position as explosives experts trying to defuse a new type of bomb, which might be detonated by the slightest false move. For all that they could tell, even the most delicate of radar probes might trigger some unimaginable catastrophe.