'... Hello, Dimitri. This is Woody, switching to Key Two in fifteen seconds... Hello, Dimitri – multiply Keys Three and Four, take cube root, add pi squared and use nearest integer as Key Five. Unless your computers are a million times faster than ours – and I'm damn sure they're not – no one can decrypt this, on your side or mine. But you may have some explaining to do; anyway, you're good at that.

'By the way, my usual excellent sources told me about the failure of the latest attempt to persuade old Andrei to resign; I gather that your delegation had no more luck than the others, and you're still saddled with him as President. I'm laughing my head off; it serves the Academy right. I know he's over ninety, and growing a bit – well, stubborn. But you won't get any help from me, even though I'm the world's – sorry, Solar System's – leading expert on the painless removal of elderly scientists.

'Would you believe that I'm still slightly drunk? We felt we deserved a little party, once we'd successfully rendez – rendezvous, damn, rendezvoused with Discovery. Besides, we had two new crew members to welcome aboard. Chandra doesn't believe in alcohol – it makes you too human – but Walter Curnow more than made up for him, Only Tanya remained stone-cold sober, just as you'd expect.

'My fellow Americans – I sound like a politician, God help me – came out of hibernation without any problems, and are both looking forward to starting work. We'll all have to move quickly; not only is time running out, but Discovery seems to be in very bad shape. We could hardly believe our eyes when we saw how its spotless white hull had turned a sickly yellow.

'Io's to blame, of course. The ship's spiralled down to within three thousand kilometres, and every few days one of the volcanoes blasts a few megatons of sulphur up into the sky. Even though you've seen the movies, you can't really imagine what it's like to hang above that inferno; I'll be glad when we can get away, even though we'll be heading for something much more mysterious – and perhaps far more dangerous.

'I flew over Kilauea during the '06 eruption; that was mighty scary, but it was nothing – nothing – compared to this. At the moment, we're over the nightside, and that makes it worse. You can see just enough to imagine a lot more. It's as close to Hell as I ever want to get.

'Some of the sulphur lakes are hot enough to glow, but most of the light comes from electrical discharges. Every few minutes the whole landscape seems to explode, as if a giant photoflash has gone off above it. And that's probably not a bad analogy; there are millions of amps flowing in the flux-tube linking Io and Jupiter, and every so often there's a breakdown. Then you get the biggest lightning flash in the Solar System, and half our circuit-breakers jump out in sympathy.

'There's just been an eruption right on the terminator, and I can see a huge cloud expanding up toward us, climbing into the sunlight. I doubt if it will reach our altitude, and even if it does it will be harmless by the time it gets here. But it looks ominous – a space monster, trying to devour us.

'Soon after we got here, I realized that Io reminded me of something; it took me a couple of days to work it out, and then I had to check with Mission Archives because the ship's library couldn't help – shame on it. Do you remember how I introduced you to The Lord of the Rings, when we were kids back at that Oxford conference? Well, Io is Mordor: look up Part Three. There's a passage about "rivers of molten rock that wound their way... until they cooled and lay like twisted dragon-shapes vomited from the tormented earth." That's a perfect description: how did Tolkien know, a quarter century before anyone ever saw a picture of Io? Talk about Nature imitating Art.

'At least we won't have to land there: I don't think that even our late Chinese colleagues would have attempted that. But perhaps one day it may be possible; there are areas that seem fairly stable, and not continually inundated by sulphur floods.

'Who would have believed that we'd come all the way to Jupiter, greatest of planets – and then ignore it. Yet that's what we're doing most of the time; and when we're not looking at Io or Discovery, we're thinking about the Artifact.

'It's still ten thousand kilometres away, up there at the libration point, but when I look at it through the main telescope it seems close enough to touch. Because it's so completely featureless, there's no indication of size, no way the eye can judge it's really a couple of kilometres long. If it's solid, it must weigh billions of tons.

'But is it solid? It gives almost no radar echo, even when it's square-on to us. We can see it only as a black silhouette against the clouds of Jupiter, three hundred thousand kilometres below. Apart from its size, it looks exactly like the monolith we dug up on the Moon.

'Well, tomorrow we'll go aboard Discovery, and I don't know when I'll have time or opportunity to speak to you again. But there's one more thing, old friend, before I sign off.

'It's Caroline. She's never really understood why I had to leave Earth, and in a way I don't think she'll ever quite forgive me. Some women believe, that love isn't the only thing – but everything. Perhaps they're right... anyway, it's certainly too late to argue now.

'Try and cheer her up when you have a chance. She talks about going back to the mainland. I'm afraid that if she does...

'If you can't get through to her, try to cheer up Chris. I miss him more than I care to say.

'He'll believe Uncle Dimitri – if you say that his father still loves him, and will be coming home just as quickly as he can.'

Even in the best of circumstances, it is not easy to board a derelict and uncooperative spaceship. Indeed, it can be positively dangerous.

Walter Curnow knew that as an abstract principle; but he did not really feel it in his bones until he saw the entire hundred-metre length of Discovery turning end-over-end, while Leonov kept at a safe distance. Years ago, friction had braked the spin of Discovery's carousel, thus transferring its angular momentum to the rest of the structure. Now, like a drum-majorette's baton at the height of its trajectory, the abandoned ship was slowly tumbling along its orbit.

The first problem was to stop that spin, which made Discovery not only uncontrollable but almost unapproachable. As he suited up in the airlock with Max Brailovsky, Curnow had a very rare sensation of incompetence, even inferiority; it was not his line of business. He had already explained gloomily, 'I'm a space engineer, not a space monkey'; but the job had to be done. He alone possessed the skills that could save Discovery from Io's grasp. Max and his colleagues, working with unfamiliar circuit diagrams and equipment, would take far too long. By the time they had restored power to the ship and mastered its controls, it would have plunged into the sulphurous firepits below.

'You're not scared, are you?' asked Max, when they were about to put on their helmets.

'Not enough to make a mess in my suit. Otherwise, yes.' Max chuckled. 'I'd say that's about right for this job. But don't worry – I'll get you there in one piece, with my – what do you call it?'

'Broomstick. Because witches are supposed to ride them.'

'Oh yes. Have you ever used one?'

'I tried once, but mine got away from me. Everyone else thought it was very funny.'

There are some professions which have evolved unique and characteristic tools – the longshoreman's hook, the potter's wheel, the bricklayer's trowel, the geologist's hammer. The men who had to spend much of their time on zero-gravity construction projects had developed the broomstick.