World Engine

by Stephen Baxter

  Hesitation.

  ‘Those are good questions,’ Nicola said. ‘To which I am sensing we don’t have good answers right now.’

  Briggs frowned, an elegant disdain. ‘Well, I wouldn’t overstate it. We do have some suggestions, even if none is definitive. Certainly the anchoring of the Towers cannot be a simple matter. The solid crust of the world is only thirty, forty miles thick. And as you rightly say, Malenfant, crust plates tend to drift. Yet the Towers, and the linear continental feature on which they sit, have self-evidently not drifted out of line. We don’t know how this was achieved – I have to admit that. It may have been subtle. Perhaps the – what did you call them, engineers? – the hypothetical beasts who built these Towers had some way of directing the mantle currents that ultimately determine the shifting of continental plates.’

  ‘Screwing with the mantle currents of a super Earth? You call that subtle?’ Malenfant felt restless, frustrated. This orderly lecture felt like it would take for ever to get to the point he was interested in. ‘Briggs, earlier you mentioned structures under the Towers. Picked out by seismometry, right? Buried in the rock.’

  ‘Correct.’ He fumbled in a heap of paper. ‘I have records, reconstructions—’

  ‘Never mind that. Just tell me. What kind of structures, and what do they hold – shelters, machinery?’

  ‘Large, regular hollows. And, yes, large-scale machinery, of an enigmatic nature . . .’

  He produced sketches of arrangements of circular hoops, pillars, bands, and began to brag about the careful work that had gone into extracting this interpretation from scratchy seismometry data.

  Malenfant cut him off. ‘So,’ he demanded, ‘machinery to do what?’

  Briggs spread his hands. ‘Well, we can only guess—’

  ‘But,’ Lighthill put in, ‘to any of us who have been around nuclear-fusion engine technology at all – which is all of us in the King’s Space Force – and with due respect given to the Official Secrets Act, those look awfully like magnetic confinement structures to me.’

  Malenfant said carefully, ‘Tremendous fusion engines, then.’

  Briggs said reluctantly, ‘It’s not a bad guess. We also found traces of residual radioactivity, in the surrounding rocks. Just traces, but anomalous. As if the ground there had been heavily irradiated in the deep past.’

  Malenfant almost held his breath. They were getting closer to the point. ‘Irradiated. OK. So nuclear fusion energy was being released down there. Possibly. To what end?’ He glanced around at the British.

  McLaurin called down from the Harmonia. ‘Well, we talked this over. I always thought it must be some kind of global power system.’

  Briggs said, ‘A global power system to run what, Bill? There’s nothing here to run.’

  ‘Quite. You can tell we have discussed this, and failed to agree. Though Briggs here won’t even say what he thinks the Towers are.’

  Now Malenfant did hold his breath. He turned to Briggs, and exhaled noisily. ‘So tell me, Guy. What’s your idea?’

  ‘Something too crazy to be true.’ He glanced at Lighthill, almost shyly, and Malenfant saw he was fighting a kind of shyness. ‘I mean, who would build sixty-five thousand of the things and stick them head first in the ground?’

  ‘What things? What are the Towers, Guy?’

  A moment’s hesitation.

  ‘Rockets,’ Briggs said miserably. ‘I think they’re rockets! I mean, look at them. Strictly speaking the Towers themselves are just the exhaust stacks. The engines are deep underground – as revealed by our seismic soundings and so forth.

  ‘Nuclear rockets. Set on end, as if they are intended to thrust down into the ground. There’s no doubt in my mind. But it makes no sense.’

  McLaurin laughed out loud. ‘You’re right about that.’

  But Malenfant grabbed Briggs’s arm. ‘No. Listen, man, I think it makes all the sense in the world. I had the same idea. I’ve been waiting for somebody else to come up with it. And—’

  And that was when images of Bartholomew, Emma and Deirdra appeared out of nowhere.

  66

  The British were all startled by this latest manifestation of twenty-fifth-century technology – but, as Bartholomew showed them his bangle and gabbled out a hasty explanation, they quickly accepted the concept.

  Meanwhile the visitors seemed to float in the air, like angels in this planetary hell. Their originals were nearly weightless, of course, up on Melinoe.

  Deirdra said brightly, ‘Everybody seems to be very grumpy, Malenfant. But that Tower is a-ma-zing. And your discussion – we couldn’t wait any more, we had to come down and join in.’

  ‘Yeah. Fill your boots. Listen, while I’m stuck with all of you – let’s cheat. Deirdra, you may have to look stuff up . . . I’m pretty sure that the citizens of a twenty-fifth-century world state, post spaceflight, are going to know a hell of a lot more than these 2000 AD Brits about planetary formation. The natural stuff anyhow.’

  Deirdra frowned. ‘The natural stuff? As opposed to what? . . . OK, let’s hear what you want to know.’

  ‘Isn’t it obvious? Look at this world. You overheard the discussion, right? I get that a world like this must have formed close to the Sun. And now we have Briggs’s convincing proof that the planet was moved out here billions of years ago. My question is, how? How did it get out here? How does a world get moved?’

  ‘I know some of it,’ Deirdra put in eagerly. ‘I’ve already been looking it up. It’s really interesting. Can you check I’ve got it right, Emma?’

  The virtual image of twenty-first-century Emma pulled a twenty-fifth-century softscreen out of the air. ‘OK. Bartholomew, maybe you can show me how to use this damn gadget . . .’

  It was all to do with the residual protoplanetary disc – the big flat slab of nebular material, dust and ice, from which the young planets had been born in the first place, a slab orbiting an equally immature Sun at its heart.

  ‘You know about the ice line?’ Deirdra asked, her face serious.

  ‘I think so,’ Malenfant said. ‘So you start with a big cloud of dust and ice and gas, spinning around a young star. Too close in water ice can’t form, but further out – somewhere beyond the orbit of Mars for our Sun, I think – you do get ice crystals, in with the dust.’

  ‘Right. So when the planets out there form, you get big bloated worlds of ice and gas, with maybe a rocky core at the centre. They just suck it all down, in a runaway collapse. And you get Jupiter and Saturn, gas giants. Or Neptune and Uranus, ice giants. And further in, where the ice has melted and the gas dispersed, rocky worlds like Earth. The trouble is, the planets have a tendency not to stay where they are born.’

  ‘Like some humans,’ Malenfant said drily. ‘Tell me how it happens.’

  It was complicated, and it was pushing Deirdra’s knowledge, but Emma was able to help her fill in some of the gaps.

  A young planet like Jupiter could have been slowed in its initial orbit by friction with the residual protoplanetary-disc debris – perhaps directly if it was swallowing up material that it met head-on, or more subtly, if it cleared a gap by sweeping up the debris in its path, and then tidal effects caused outer lanes of dust to collapse inwards.

  ‘Anyhow Jupiter moved in,’ Emma said, consulting models on her screen. ‘Scattering the stuff of the disc, dust, fragments, embryo planets as it went, spiralling in towards the Sun. But it dragged Saturn in its wake. Huge as it is, Saturn is a lot lighter and a lot less dense, and so was comparatively easy to displace.

  ‘In some star systems migrating Jovian planets can fall all the way into their suns, it seems. But not here, not Jupiter. And it was Saturn that saved it. At some point Jupiter started pushing Saturn back out – but that acted like a lifeline to pull Jupiter away from the Sun too. The maths seems tricky. It was a kind of – resonance pump. Resonance between their orbits, which—’

  ‘OK.’ Malenfant nodded. ‘So the two giants sweep back out through the Solar System ag
ain. And that must have caused a major disruption to all the other planetary orbits. So what about Persephone?’

  ‘You can probably guess,’ Emma said. ‘There were clues, actually. Our Solar System seems to be unusual, Malenfant. Almost unique. Most planetary systems have super Earths, and the early astronomers wondered why ours didn’t.’

  Or, Malenfant mused, they should have asked what got rid of them.

  ‘But Persephone is a super Earth. It looks as if Persephone formed in the gap between Mars and Jupiter. And, yes, it could have been thrown out when the Jupiter-Saturn Grand Tack was going on – that’s what they call the final big migration in Deirdra’s timeline. Although the modelling is dubious, according to the summary report I’m looking at. It’s not clear quite how Persephone could have ended up so far from the Sun . . . and, like we said before, how come it ended up in such a neat circular orbit, all the way out here. And even so, what happened to all the other super Earths that were probably spinning around the Sun?’

  Suddenly Malenfant felt overwhelmingly weary. ‘Jeez. This damn gravity. I’m having trouble trying to think this through.’ And trying to keep hold of a supremely delicate thread of supposition and guesswork. ‘So Persephone was originally Planet Five, not Planet Nine. Remember all the old stories about Planet Five, Emma? About how it must have got smashed, leaving the rubble that was the asteroids behind? And maybe there was some super-civilisation on Planet Five that went to war and destroyed its whole world . . . Well, maybe those old stories had something in them after all.’

  Emma frowned. ‘What do you mean?’

  ‘Look – you talked about these mechanistic, chaotic theories of the formation of the Solar System. Migrating gas giants and unstable orbits. So what if it wasn’t just mechanistic? OK, so Jupiter and Saturn sailing back and forth made a huge mess of everything. But suppose you were some conscious agent, who wanted things to come out a certain way. Couldn’t you use all that instability, all that chaos, to achieve your own ends? Suppose you wanted Persephone out of the inner System, for some reason.’

  Nicola walked over. ‘I hesitate to butt in on all this hypothesising. But yes, that’s possible, Malenfant. If a system is inherently unstable, a small deflection of the right kind can make a big difference.’

  ‘I know about this,’ Deirdra said, butting back in herself. ‘Like a sand heap. Add one more grain at the top, in just the right place, and it all collapses.’

  ‘Right,’ said Malenfant doggedly. ‘Right. It all collapses. So you could conceivably have pushed a planet, say, from one place to another, and started a cascade that finished up with the result you wanted – like the ejection of a super Earth. Or many super Earths . . .’ He had a vision of the young Solar System as a pool table, the balls scattering everywhere, with one cue ball being nudged this way, not that, so lesser worlds, maybe super Earths in themselves, were clattered off the table altogether . . .

  A cue ball with rockets like tacks stuck around its equator.

  He glanced over at that nearest Tower, a slim line to Heaven, picked out in the light of the pinprick Sun.

  Lighthill seemed to know what Malenfant was thinking. ‘Bloody hell,’ said the Wing Commander.

  ‘Briggs,’ Malenfant said, ‘suppose you’re right. Suppose this Tower is a rocket of some kind. And the world is rimmed by sixty-five thousand of those beasts. What is a rocket for, save to push stuff? What could you push with an engine like that?’

  Briggs just stared. Then he started to dig around in the cabin’s clutter. ‘Nicola, where in hell is my slide rule?’

  Deirdra suddenly laughed, in wonder. ‘You know why I wanted to come out here, Malenfant, with you? It wasn’t just curiosity. Not just wanting an adventure. I’m not that much of a kid.’

  Emma said seriously, ‘Deirdra, I don’t think you are any kind of a kid at all.’

  ‘It was because I hoped to find what we did find. I mean, not the detail, but . . . If you couldn’t find it, then who?’

  ‘Find what?’

  ‘I’m still not sure. But think about it.’ She pointed in the direction of the Sun. ‘Earth is that way. My Earth.’ She glanced over her shoulder. ‘Shiva, that way. And in the middle . . . here we are. Almost in the way. On a planet with big rockets on it.’

  There was a stunned silence.

  ‘Here we are indeed,’ Malenfant murmured, his brain churning.

  Emma watched him warily. ‘I know you, Malenfant. I know a version of you anyhow. You’re coming up with one of your schemes, aren’t you?’

  Briggs frowned. ‘What is she muttering about, man?’

  Malenfant just stared at him. ‘Sorry. My thoughts are running ahead of me. One question, Guy. These rocket engines. Do you think they still work?’

  He consulted scribbled notes on the back of an old note – paper was precious to this crew, Malenfant knew. ‘Hard to tell. It’s not as if we’ve tried them. But – well, I think so. They may be billions of years old, but they are pretty simple technology. Evidently robust. And they’ve just sat out here in the dark and cold, minding their own business. Yes, I think they will work.’ Briggs looked up and grinned. ‘I can even think of a way to trigger them.’

  ‘Bloody hell,’ Lighthill said again.

  Bartholomew appeared out of nowhere. ‘Reid Malenfant, are you contemplating moving this planet? We forbid it! Every indication shows—’

  Deirdra snapped her fingers. ‘Off!’

  Bartholomew froze.

  Stopped speaking. Stood straight. Then, his body at attention, his head slumped forward, and he stood inert.

  His image winked out of existence.

  Emma stared at Deirdra, aghast. ‘What did you do?’

  ‘My mother set up the option. Just in case his ethical constraints went wrong and he stopped us doing something really important.’

  ‘Wrong in whose judgement?’

  Deirdra shrugged. ‘Well – mine.’ She looked at Malenfant. ‘So what next?’

  ‘Bloody hell!’ said Lighthill.

  67

  Malenfant’s suggestion sent Guy Briggs into a fever of speculation. ‘It’s as if you had unblocked a drain in my head, man. Just let me think it through . . .’ He locked himself inside one of the lander’s tiny sleep compartments with records, reference books, slide rule.

  Lighthill, a good commander Malenfant conceded, ordered the overheated group to break up, to take time for coffee, bathroom or ‘a smoke’.

  Briggs re-emerged after half an hour, eyes rimmed red with fatigue, bundles of scribbled-on paper under his arm.

  Then they got together again, for more discussion of this strange mix of engineering, archaeology and geology, the three disciplines entwined. As, Malenfant realised, humans struggling to make sense of engines older than some worlds.

  Briggs stumbled through their understanding so far. Beneath each Tower, essentially, as discovered by clumsy British seismometry, was a sealed chamber, like a sizable pharaoh’s tomb – and a surrounding cloud, visible in the soundings, of what looked like fuel tanks, spherical shells like sunken balloons, all embedded in a network of pipes and ducts, themselves of tremendous capacity. Shadows suggested still more enigmatic machinery deeper down, as well as a mighty substructure beneath each Tower, anchoring the mechanisms to the deep bedrock. But the British visitors lacked the resources to explore any deeper.

  ‘Some of it seems obvious,’ Malenfant said, tracing shadowy lines on the engineer’s sketched pencil plan. ‘Surely this big sealed box is the nuclear engine compartment. It holds the hoops and lines we thought were for magnetic confinement. And this seems to be part of a chain of connections between the Towers.’

  ‘We think so,’ Briggs said, nodding. ‘We think that when one engine fires it triggers the next, in a cascade. See the feed here? They seem to be linked in loops of more than a couple of thousand each . . .’

  Malenfant tapped the central box. ‘The engine itself, though—’

  ‘Fusion technology,’ Briggs said
briskly. ‘As previously guessed. We’re confident about that, or the physicist johnnies are. They can even tell the fusion flavour from the contamination of the layers around. Or some such. The primary feedstock must have been deuterium.’

  ‘That’s an isotope of hydrogen,’ Malenfant said. ‘Present in sea water! Is that what the piping is for? To deliver deuterium from the planet’s oceans? OK. That makes sense. But what about the propellant? Big as they are, if these Towers are just rockets you would need to shove something out of the nozzle to—’

  ‘Slow down,’ Lighthill advised. ‘You’re second-guessing our resident genius here.’

  Briggs grinned, self-effacing. ‘You’re right, though, Malenfant. I did think through the figures. I estimate that each Tower, each rocket, when burning, would have needed more than a couple of tons of deuterium fuel every second. And fifteen thousand tons per second of hydrogen, or similar numbers for some equivalent propellant . . .’

  Even as Malenfant was trying to absorb these staggering numbers, he had to grin. ‘All this was needed to do what, Guy?’

  He sighed. ‘That’s the part I’m wrestling with. All that’s needed to move the planet.’

  Malenfant looked around. He wasn’t surprised by this conclusion. Most people were, judging by their expressions.

  Deirdra just said, ‘At last we’re getting to the point.’

  Briggs spread out his notes on a table-top; they curled and shifted in the gentle breezes of the air conditioning.

  ‘Look,’ Briggs said, ‘you can guess a lot of it just from the basic set-up. The laws of thermodynamics.

  ‘We know how much energy it would take to hoik a planet the mass of Persephone out of the Sun’s gravity well. Newton could have worked that one out from his laws of gravitation. And we know how long it took, a million years; the geology shows that. So, energy divided by time – we know what the power must have been. Turns out at – well, I won’t quote you raw horsepower numbers. About as much as ten thousand times the power Earth receives from sunlight. Or, something like a hundred-thousandth of the Sun’s total power output. Might not sound like much, but stars are one heck of a lot bigger than anything you do on a planetary scale. Even this.’