'It's also widely agreed that TMA ONE was planted on the Moon to keep track of the project – experiment – whatever it was – and to report to Jupiter – the obvious place for Solar System Mission Control. That's why another Monolith – Big Brother – was waiting there. Had been waiting four million years, when Discovery arrived. Agreed so far?'

'Yes; I've always thought that was the most plausible theory.'

'Now for the more speculative stuff. Bowman was apparently swallowed up by Big Brother, yet something of his personality seems to have survived. Twenty years after that encounter with Heywood Floyd in the second Jupiter expedition, they had another contact aboard Universe, when Floyd joined it for the 2061 rendezvous with Halley's Comet. At least, so he tells us in his memoirs – though he was well over a hundred when he dictated them.'

'Could have been senile.'

'Not according to all the contemporary accounts! Also – perhaps even more significant – his grandson Chris had some equally weird experiences when Galaxy made its forced landing on Europa. And, of course, that's where the Monolith – or a Monolith – is, right now! Surrounded by Europans...'

'I'm beginning to see what Dr Ted's driving at. This is where we came in – the whole cycle's starting over again. The Europs are being groomed for stardom.'

'Exactly – everything fits. Jupiter ignited to give them a sun, to thaw out their frozen world. The warning to us to keep our distance – presumably so that we wouldn't interfere with their development...'

'Where have I heard that idea before? Of course, Frank – it goes back a thousand years – to your own time! "The Prime Directive"! We still get lots of laughs from those old Star Trek programmes.'

'Did I ever tell you I once met some of the actors? They would have been surprised to see me now... And I've always had two thoughts about that non-interference policy. The Monolith certainly violated it with us, back there in Africa. One might argue that did have disastrous results...'

'So better luck next time – on Europa!' Poole laughed, without much humour. 'Khan used those exact words.'

'And what does he think we should do about it? Above all – where do you come into the picture?'

'First of all, we must find what's really happening on Europa – and why. Merely observing it from space is not enough.'

'What else can we do? All the probes the Medes have sent there were blown up, just before landing.'

'And ever since the mission to rescue Galaxy, crew-carrying ships have been diverted by some field of force, which no one can figure out. Very interesting: it proves that whatever is down there is protective, but not malevolent. And – this is the important point – it must have some way of scanning what's on the way. It can distinguish between robots and humans.'

'More than I can do, sometimes. Go on.'

'Well, Ted thinks there's one human being who might make it down to the surface of Europa – because his old friend is there, and may have some influence with the 'powers-that-be.'

Captain Dimitri Chandler gave a long, low whistle.

'And you're willing to risk it?'

'Yes: what have I got to lose?'

'One valuable shuttle craft, if I know what you have in mind. Is that why you've been learning to fly Falcon?'

'Well, now that you mention it... the idea had occurred to me.'

'I'll have to think it over – I'll admit I'm intrigued, but there are lots of problems.'

'Knowing you, I'm sure they won't stand in the way – once you've decided to help me.'

MISS PRINGLE LIST PRIORITY MESSAGES FROM EARTH

RECORD

Dear Indra – I'm not trying to be dramatic, but this may be my last message from Ganymede. By the time you receive it, I will be on my way to Europa.

Though it's a sudden decision – and no one is more surprised than I am – I've thought it over very carefully. As you'll have guessed, Ted Khan is largely responsible... let him do the explaining, if I don't come back. Please don't misunderstand me – in no way do I regard this as a suicide mission! But I'm ninety per cent convinced by Ted's arguments, and he's aroused my curiosity so much that I'd never forgive myself if I turned down this once-in-a-lifetime opportunity. Maybe I should say once in two lifetimes...

I'm flying Goliath's little one-person shuttle Falcon – how I'd have loved to demonstrate her to my old colleagues back at the Space Administration! Judging by past records, the most likely outcome is that I'll be diverted away from Europa before I can land. Even this will teach me something...

And if it – presumably the local Monolith, the Great Wall – decides to treat me like the robot probes it's zapped in the past, I'll never know. That's a risk I'm prepared to take.

Thank you for everything, and my very best to Joe. Love from Ganymede – and soon, I hope, from Europa.

STORE

TRANSMIT

'Europa's about four hundred thousand kay from Ganymede at the moment,' Captain Chandler informed Poole.

'If you stepped on the gas – thanks for teaching me that phrase! – Falcon could get you there in an hour. But I wouldn't recommend it: our mysterious friend might be alarmed by anyone coming in that fast.'

'Agreed and I want time to think. I'm going to take several hours, at least. And I'm still hoping...' Poole's voice trailed off into silence.

'Hoping what?'

'That I can make some sort of contact with Dave, or whatever it is, before I attempt to land.'

'Yes, it's always rude to drop in uninvited – even with people you know, let alone perfect strangers like the Europs. Perhaps you should take some gifts – what did the old-time explorers use? I believe mirrors and beads were once popular.'

Chandler's facetious tone did not disguise his real concern, both for Poole and for the valuable piece of equipment he proposed to borrow – and for which the skipper of Goliath was ultimately responsible.

'I'm still trying to decide how we work this. If you come back a hero, I want to bask in your reflected glory. But if you lose Falcon as well as yourself, what shall I say? That you stole the shuttle while we weren't looking? I'm afraid no one would buy that story. Ganymede Traffic Control's very efficient – has to be! If you left without advance notice, they'd be on to you in a microsec – well, a millisecond. No way you could leave unless I file your flight-plan ahead of time.'

'So this is what I propose to do, unless I think of something better.'

'You're taking Falcon out for a final qualification test – everyone knows you've already soloed. You'll go into a two-thousand-kilometre-high orbit above Europa – nothing unusual about that – people do it all the time, and the local authorities don't seem to object.'

'Estimated total flight time five hours plus or minus ten minutes. If you suddenly change your mind about coming home, no one can do anything about it – at least, no one on Ganymede. Of course, I'll make some indignant noises, and say how astonished I am by such gross navigational errors, etc., etc. Whatever will look best in the subsequent Court of Enquiry.'

'Would it come to that? I don't want to do anything that will get you into trouble.'

'Don't worry – it's time there was a little excitement round here. But only you and I know about this plot; try not to mention it to the crew – I want them to have – what was that other useful expression you taught me? – "plausible deniability".'

'Thanks, Dim – I really appreciate what you're doing. And I hope you'll never have to regret hauling me aboard Goliath, out round Neptune.'