World Engine

by Stephen Baxter

  ‘And one heck of an amount to deliver with fusion rockets,’ Malenfant said.

  ‘Correct. This is neglecting any boosts that may have been got from slingshotting around Jupiter, and so forth, so it’s a conservative estimate. It is a big, clumsy way to do it – but it’s possible.’

  ‘OK. And you said you would have more than one Tower blasting at any one time.’

  ‘Correct.’ Briggs grinned. ‘Hold on to your hats. Turns out you need somewhere over two thousand Towers to light up together, to generate the right thrust – to deliver that power number I quoted.’

  Malenfant pursed his lips. ‘Doesn’t that match the number you worked out regarding the ignition clustering? A couple of thousand? The groups that seem linked to fire together . . .’

  ‘Exactly,’ Briggs said. ‘Exactly.’

  Lighthill frowned. ‘Spill the beans, Guy.’

  ‘This is where I got stuck in my reasoning, back in the sleep cupboard over there. Once I became convinced that these structures could be rockets, well – it seemed unreasonable to me, because they point every which way. What are they for? How are they used? I mean, if they had all been crammed together at one pole, all thrusting in the same direction . . .’ His grin returned. ‘But you can get thrust in a single direction consistent with this equatorial arrangement – if you light up selections of the Towers in sequence.’ He grabbed another scrap of much-used paper, drew a rough circle, and hastily sketched Towers all around, like bristles on a hedgehog. ‘You fire up a neighbourhood – two thousand or so at a time. But all in one location. So you get thrust that way.’ He drew a fat arrow to indicate a push away from the burning Towers, down into the ground, through the planet’s core.

  Malenfant saw it. The Towers were like rockets on a test stand, pointing down, not up – and shoving at the planet itself.

  ‘But an hour later the world has turned, and now you light up the next neighbourhood of two thousand towers. See? Whichever cluster is now positioned where the first stuff was.’

  ‘Ah,’ Malenfant said. ‘So at any time, as the world turns, you get a uniform thrust in the same direction.’

  Briggs dumped the paper. ‘That, ladies and gentlemen, is how you move a planet.’

  ‘I will resist the temptation to applaud,’ Nicola said drily. ‘There are a hell of a lot of big numbers hidden behind your hand-waving, Guy.’

  ‘Well, you have that right.’

  ‘You said that each Tower needs fifteen thousand tons per second of hydrogen propellant. But you have over two thousand burning at any moment. And they had to keep burning for a million years, from what you say. In total that’s, mm . . . I can’t handle the orders of magnitude in my head.’

  ‘It works out at something like a thousand times the mass of all of Earth’s oceans,’ Briggs said grimly. ‘This is a bigger world, bigger oceans, but not that big. I mean, there are feeding tubes to the extant oceans, frozen as they are. But I suspect they are primarily for extracting the deuterium fusion fuel – of which we would only need less than one copy of Earth’s oceans.’

  ‘Oh, modest, then, by comparison. So, where did they get the propellant?’

  ‘I’m not sure,’ Briggs admitted. ‘But the resource is out there in the Solar System. Maybe they mined the gas giants. The fuel load is only a thousandth the mass of Jupiter, say. Though that’s a bloody big amount, I admit! If you could somehow mine that – and send the fuel out over a thousand astronomical units, and keep that up for a million years . . .’

  Lighthill put in, ‘There’s also plenty of water ice floating around even out here in the comet cloud too, though you’d have to seek it out. Maybe they tethered comet nuclei, threw them in towards the migrating planet, where they were scooped up, broken down, fed to the engines.’

  ‘That could work,’ Briggs conceded. He sighed. ‘Even my non-boggleable mind boggles when I think about this stuff. But the point is, as you say, Geoff, the material is there; I suppose they found a way.’

  Lighthill grinned. ‘The details to be left as an exercise to the student. Although one needs a way to fire up the system in the first place.’

  McLaurin called down, ‘Now there I might be able to help, at last. I’ve been studying aspects of the upper Towers. And I have a strong suspicion that all one needs to do is to chuck in a firework . . .’

  They spoke on. Malenfant half-listened, as more astounding numbers were unearthed about the planet-moving scheme. Such as, the thrust developed at the base of each Tower would be about as much as the full weight of the Earth’s crust, forty kilometres thick, pressing on the upper mantle . . .

  Then, inside his head, Deirdra gasped.

  In retrospect, Malenfant would think he should have jumped out of his skin. Because everything was about to change.

  ‘Malenfant.’

  He muttered his reply. ‘Yeah?’

  ‘I’ve been thinking.’

  He smiled. ‘About what?’

  ‘About Shiva. And Persephone, and Earth. How they are all lined up, more or less.’

  ‘Yes . . .’

  ‘As you noticed before. Which is why the British came out here, to use that line-up to get to Shiva.’ He felt a twinge of curiosity, of anticipation. He had learned to marvel at this young person’s intuition, and where it led her. ‘Go on.’

  ‘As things stand, when Shiva approaches, it won’t come all that close to Persephone, will it?’

  ‘Correct. I checked it out after we talked. Right now Persephone is about eighty astronomical units out from that direct line from Shiva to Sun. In years to come it will get closer to that line, following its own orbit – in fact it will get to within just a few tens of millions of kilometres at closest approach to Shiva as it passes, in five hundred years. Half an astronomical unit away. A very close approach on the scale of the Solar System out here, but – close, but no cigar. The mass of Shiva is so great that it won’t be deflected by the encounter, though Persephone will be, a little.’

  ‘All that will happen in five hundred years.’

  ‘Yeah.’

  ‘Hmm. And meanwhile Guy is saying these engines on Persephone could push it across a thousand astronomical units in a million years. That’s what the builders of this place seem to have achieved.’

  ‘So?’

  ‘So, how long would it take to push Persephone half an astronomical unit? An extra half, on top of its orbital motion.’

  And Malenfant was electrified. ‘Five hundred years. Shit. Let me think about it.’

  So he did, sitting silently, until the conversation began to run down.

  Then he stood up. Knowing what he had to do.

  He clapped Lighthill on the shoulder. ‘Look. I know you guys came out here as soldiers of the Empire. But as discoverers, not appropriators, right? As expedition commander you are more Captain Cook than Clive of India.’

  Lighthill shrugged. ‘Not an entirely apt comparison, Malenfant, but it will do.’

  ‘And you do have a mission here.’

  He frowned. ‘As you know. Which is to refuel, and go on to investigate Shiva.’

  ‘True enough. But – well, can’t that wait?’

  ‘Wait for what?’

  ‘For another crew to go out and explore. Some other time.’

  ‘What are you getting at?’

  ‘How would you like to save a world?’

  It took them a day to argue about how it could be done. The targeting would have to be pretty precise, but Deirdra’s brainstormed estimates were about right. Even more crucially, it turned out that Persephone’s axis of rotation was pretty much at right angles to the plane of the Solar System. An equatorial push would indeed send the planet towards the future position of Shiva.

  Yes, it could be done.

  A day to argue about whether it should be done.

  A week to get ready to do it.

  And, if they were right, Malenfant thought, they were about to change the destinies of worlds.

  68

&nb
sp; By the morning of Operation Blue Touch Paper – and Malenfant saw that Lighthill had recorded it that way in his handwritten RASF log-book – everybody, save Briggs and McLaurin in the Charon lander, had been evacuated to the rather crowded base on Melinoe.

  And in the habitat’s main control room, everybody followed the data feeds as Briggs and McLaurin inched their way down towards the top of the Tower that had been chosen as the target for their crude attempt to revive the ancient world-moving mechanism. This Tower, dubbed Tower One, was close to the map-makers’ meridian, just north of the Malebolge archipelago.

  It was a slow process. Briggs and McLaurin went cautiously, if only because they had a couple of twenty-kiloton-strength nuclear devices on board the lander. One as the trigger, one as a spare. Bombs which the British crew had thrown together from ‘spare engine parts’, as they put it, spin-offs from the nuclear drive systems of their craft.

  The speed and efficiency with which the British had been able to achieve this bomb-making had convinced Malenfant that it was a pre-rehearsed contingency. He supposed it made sense, if they had to be prepared to defend territory out in space from encroachment from some rival: the Germans, the French – even, God forbid, the Americans. But if that contingency was remote, it was wasteful of mass allowances to send out craft on deep-space journeys purposefully armed with dedicated weapons that would likely never be used. Better to find a way to put together quickly something almost as effective, from ‘spare engine parts’. Although, as previously noted, he thought, they probably hauled small arms at least, if they had hand grenades aboard.

  And, during the preparations, it had been bemusing for him to watch Nicola Mott working on this stuff. Nicola, who in Malenfant’s life had been a wide-eyed dreamer from England who had been thrilled to get anywhere close to a space programme – and now here was another Nicola, part of a crew that was preparing to drop a nuclear weapon into an alien artefact, without, apparently, a moral qualm.

  These different British really were tough-minded, he realised.

  As the descent continued, Deirdra drifted over to Malenfant and put her hand in his. ‘I can’t bear it. Watching these bleeps on the radar screens. The steady voices counting down. It’s like waiting for somebody to be executed.’

  ‘I know.’ He squeezed her hand back, and nodded over at Bartholomew, who still stood inert, his head dropped to his chest. ‘You want us to wake up the doc? He might have something to calm your stomach, at least. But of course he’d probably try to put a stop to the whole experiment.’

  ‘No . . . Is it always like this?’

  ‘You mean, edge of the envelope stuff? I guess so. Every space mission anyhow. Especially the first time you try something new. Like Columbus, I guess, with ships full of sailors who thought they might sail off the edge of the world . . . The difference is, now we get to see Columbus live on TV. Time seems to crawl. And you wonder how everybody else can be so damn calm. Of course they aren’t, inside they are just as churned up as you are.’

  McLaurin’s clipped tones sounded from the radio speaker, heavy with static. Coming up on target.

  Nicola was handling the moon station’s communications. ‘Roger to that, Charon.’

  Five minutes to position. Checking solar angle . . .

  Deirdra frowned. ‘Solar angle?’

  Lighthill pursed his lips. ‘Let’s remember what we are trying to achieve here.’ He glanced out of the ports, checking the position of the Sun. Then he floated in the air, his back to the pinpoint light of the distant star. ‘Imagine I am flying over the equator of Persephone. The world turns beneath me. The Sun is behind me. Somewhere out there ahead of me, further out, is Shiva. Plummeting towards me, but not quite, along its straight line to the Sun. My orbit is carrying me towards that line, but at Shiva’s closest approach I will be just offset from that direct line – by less than an astronomical unit. I need to move to my right to be in the firing line, in five centuries, when Shiva passes by.

  ‘So I need to light up the planetary rockets on the left side,’ and he held out that hand. ‘The Tower group which, in fact, is just coming into the sunlight, into dawn. After we kick-start the process with our own nuke, we think a whole group of Towers will fire up, one every second or so – because of the self-triggering system, the underground connections between the Towers. Well, that’s our best guess as to what will happen. Fire one, and that will trigger the ignition of the next in line. And so on—’

  ‘Until around two thousand are all firing,’ Deirdra said.

  ‘That’s it. Firing together, so as to thrust the planet to the right, towards the Sun–Shiva line.

  ‘As the planet turns, so – we think – the Towers will start to shut down at the front of that row of fire, one by one, while another lights up at the far end. The next group of two thousand plus taking the torch. And that process repeats over and over. So you always have two thousand Towers firing in roughly the same direction in space. A nice steady thrust towards Shiva’s trajectory line. Has to be that way.

  ‘And, if we have done our sums right, in just a few hundred years, when Shiva reaches Persephone’s orbit – there will be Persephone in front of it.

  ‘Blam! We aren’t expecting Shiva to be shattered, but it should suffer a deflection. And from out here, a thousand times further from the Sun than Earth, even a small deflection might be enough to avert a collision with Neptune – and, presumably, save the inner Solar System. Because the geometry is such that without such a collision Shiva will miss the inner System altogether.’

  Listening in, Guy Briggs called from the Charon. If any of you chaps are having twinges of conscience about the fate of Persephone, now’s the time to speak up. Look, if we do succeed in colliding it with Shiva, the planet will be badly damaged, if not shattered altogether. And remember that wherever Malenfant’s cosmic engineers got the hydrogen propellant from to bring Persephone out here in the first place, we will be extracting it from what’s left of the planet’s oceans: ice with a dribble of water at the base . . .

  ‘Oceans that still, let me remind you, host life,’ put in Josh Morris.

  True enough, Josh. Your choice – our choice.

  Deirdra surprised Malenfant by speaking up first. ‘This world is clearly messed up already. It should have been the centrepiece of the Solar System. It should have been richer in life than Earth, even. It’s not that the life that survives here doesn’t count. Surely every life form counts. But the greater good is to sacrifice that life, or chance sacrificing it, to save Earth.’ Then she seemed to lose her nerve. ‘That’s how I feel about it anyhow.’

  Emma patted her back. ‘And it is your world we are trying to save . . . You speak for it very well.’

  Now the Charon was making its final approach to the Tower, and Malenfant heard Guy Briggs call out distances and timings to Bill McLaurin. Twenty seconds. Last chance to pull out . . . Ten seconds. Five. Two.

  One.

  Lighthill leaned close to a microphone. ‘Bombs away, Commander.’

  Bombs away, sir.

  69

  Malenfant, peering through thick glass view ports, saw it all.

  Saw the initial flare as the British bomb’s fire spewed out of the mouth of the chosen Tower. A spark that itself dazzled Malenfant.

  Then a pause. Malenfant found he was holding his breath.

  Until more atomic fire roared out of the spout of the Tower.

  And not just a spark this time. It was like a tremendous lantern, suspended twenty kilometres over the plains and frozen seas of Persephone – a glow much brighter than the distant Sun. Suddenly, in that eerie nuclear dawn, Malenfant could see the sweep of the great equatorial continent, its plains and uneroded mountains sprawling, the glitter of frozen lakes of water or air. And, on the smoother sheets of the frozen oceans, the rocky islands of Malebolge – like a vaster Aegean, he thought. Yes – yes, this could have been a world, vibrant and rich, far richer than Earth, he saw it for himself, if only it had not been torn
away from the warmth of its Sun.

  All this in one and a third seconds.

  And then the second Tower, away from the Sun, flared too.

  And then a third.

  And a fourth.

  Blinded by the unfiltered glare, Malenfant had to turn away.

  ‘My God,’ said Josh Morris, sounding aghast. ‘What have we done?’ And he laughed.

  Bill McLaurin’s voice was distorted by static. Operation Blue Touch Paper appears to have been a success, Wing Commander.

  ‘I can bloody well see that, Bill. Get your backsides out of there pronto.’

  Acknowledged.

  Malenfant knew the drill now. Persephone was suddenly a lethal place to be close to, as high-energy electromagnetic radiation flooded the equatorial belt where the Towers stood, and the planet’s own magnetic field trapped lethal products of the nuclear blasts – most of it high-energy alpha particles, hydrogen atoms drawn from Persephone’s frozen seas and now stripped of their electrons and hurled at relativistic speeds. So the Charon was to retreat to orbit.

  And meanwhile the crews, on Melinoe and the Charon, watched as the events of the fire-up of Persephone’s planetary engines continued. They had a global view; Malenfant knew the British, in the course of their explorations here, had planted a few crude satellites in orbit, equipped with television cameras and other instruments, and on the ground too there were instrument emplacements, patient, automated observers.

  So now, over the next hour, Malenfant watched as one Tower lit up after another, a relentless march of fire along the crests of the mighty structures. Each ignition occurred a little more than a second after its neighbour – a pace, Malenfant thought, like a slow walk, the lumbering gait of a giant, stamping along that vast picket fence. Giants walking. That seemed to be a lingering image, for him. Here and now, he was following their footprints across the Solar System, so it seemed.