World Engine

by Stephen Baxter

41

  The approach to Phobos, a moon of Mars in an orbit so low that it was about where Malenfant would have chosen to put a space station, was a complicated business.

  On first arrival at Mars, the Last Small Step swept inside the orbit of Phobos itself, to only a few hundred kilometres above the surface of Mars. Indeed the ship at its lowest point was actually within the wispy outer remnants of the planet’s sparse atmosphere.

  And at that closest approach, waiting for the engine burn, the crew watched in silence as the Martian landscape fled beneath the ship’s prow.

  Rust-brown. Canyons and craters.

  Suddenly Malenfant, an astronaut who in his own time had never flown higher than Earth’s own stratosphere, was no further from the ground of Mars than he had been from Earth’s surface during his highest-altitude flights. As a space-obsessed kid he had devoured visions of Mars returned by the uncrewed space probes, which had started to reach the planet when he was just four years old. Now he was so close to Mars he could almost touch it – or so the wide-eyed kid inside him felt.

  But he could come no closer.

  Besides, this wasn’t his Mars, not the virgin Mars visited by the first Mariners, the Vikings. Not even the desolation visited by the first tentative exploratory missions of his own time. Deirdra eagerly pointed out the sites of abandoned human settlements, and Malenfant spotted geometry for himself: straight lines, neat circular arcs – palimpsests showing through dust drifts centuries deep. People had come here, come to this Mars, and had lived here a long time, and had made such a mess in the end that they had purposefully pulled back and gone home to Earth.

  But somewhere down there, he knew, were Planetary AIs like Karla on the Moon, with their own roots tapping deep into the planet’s energy sources. Silent, watchful, as the fragile human ship drifted past an empty world.

  At the moment of closest approach a final burn by the plasma engine shed most of what was left of the craft’s interplanetary velocity. Now the Step, already captured by Martian gravity, entered its preliminary orbit, a skinny ellipse that would, over thirty-six hours, take it back out to a distance of a dozen Martian diameters. There another burn would be performed, tweaking the orbit so that its next close approach to Mars would be raised to the elevation of the orbit of Phobos – and then, if the calculations worked out, the Last Small Step would make its rendezvous with Phobos itself.

  Mars receded all too quickly, for Malenfant.

  He tried to sleep.

  Four hours out from Phobos, Malenfant took to the instruments, and began a close inspection of the target.

  Deirdra joined him, and Bartholomew hovered behind them. Gershon stayed away, in his left-hand control couch. In some ways he was a classic astronaut, Malenfant thought, simulated person or not. Much more interested in the journey, the performance of his ship, than anything he might find at the destination.

  Not that Phobos was all that prepossessing. It was an irregular lump of crater-pocked rock somewhere over twenty kilometres long on its longest axis, drab, an unimpressive dark grey.

  ‘It’s not what I expected,’ Deirdra said. ‘It’s not even a proper sphere. More – potato-shaped.’

  Malenfant barked laughter. ‘You got that right. I think the theory is it’s not a proper moon at all – I mean, it and Deimos were never part of Mars, the way our Moon was created from the Earth, when the Solar System was young and Earth was hit by another body.’

  Deirdra’s eyes widened. ‘Really? I never knew that. I didn’t know the way the planets were made was so . . . violent.’

  Violent, yes, Malenfant thought. And, if the AIs were right, maybe there had been some purpose to that violence. Not now, Malenfant. Focus. Phobos.

  ‘Phobos, yeah. So maybe the moons of Mars are captured asteroids – or even two halves of a single rock that swam into Mars’s gravitational field and got broken up. This was long after the planets had formed. And yeah, Phobos is too small even to have pulled itself into a decent spherical shape.’

  Gershon grinned across from his control station. ‘Phobos does look impressive from the Martian ground, though,’ he said. ‘My ancestor Ralph was the first eyewitness to describe that. It looks about a third the size of the full Moon in Earth’s sky. But Phobos is so low it seems to shrink as it heads to the horizon, getting smaller in the sky. Almost as if it’s a flying island in the sky of Mars, not a moon at all.’

  ‘I would like to have seen that,’ Deirdra said. ‘Maybe with sound effects. Zoom.’

  ‘Of course a comparison with flying islands is appropriate,’ Gershon said now. He sounded wistful, Malenfant thought. ‘Gulliver’s Travels. Swift. The flying island, Laputa? Swift gave a good guess about Mars’s moons a century and a half before their discovery. It was just a guess, though.’ He sighed. ‘Swift was one reason I mounted my own mission to Voga, in the Step. Another story. Anyhow, when the space age came and the astronomers started to identify features on Phobos and Deimos, they used names from the book. I’ll show you.’

  He came over to Malenfant’s station, waved his unreal hand over a screen, and brought up a map, a rectangular chart in grey tones. It was an odd Mercator projection of an odder, misshapen little moon, Malenfant saw. Towards the upper and lower edges of the map, the projection smeared out the landscapes, so that craters looked like elongated ellipses, stretched east and west.

  Gershon clicked his fingers over the map, and names popped up in electric blue, hovering over the features he selected. ‘Here’s an upland called Laputa Regio, for instance. Straight out of the book. And there are prominent craters with Swift names.’ He swept his hand over the upper hemisphere. ‘Grildrig and Clustril and Flimnap and Skyresh.’

  ‘OK,’ said Deirdra. ‘But there are a few names that don’t seem to fit. Opik? Hall?’

  ‘Astronomers,’ Gershon said. ‘Hall was the discoverer of the Martian moons – in real life, I mean.’

  ‘Shklovsky?’

  ‘Russian astronomer,’ Malenfant said. ‘And we’re going to need to talk about him and his theories.’

  As the little moon swam closer, Deirdra pointed out grooves in the surface, shadows against a background already dark. ‘Those cuts are deep,’ she said. ‘And, look, they all come back from that end of Phobos.’ She waved a hand vaguely. ‘End? I bet that’s not the right word.’

  Malenfant checked some more science stuff. ‘Well, it will do. That end of Phobos is the apex, the point that leads Phobos in its orbit. It’s locked in that way by the tides of Martian gravity. And those grooves you see seem to be—’

  ‘Scars,’ Gershon put in now. ‘Every so often Phobos sails into a cloud of debris, or something, like a hail of rock. And its leading edge gets scraped.’

  Malenfant checked some more. ‘Seems the best guess is that the debris clouds are the product of impact events on Mars itself. Some big rock smashes a fresh hole in Mars, and blows up a screen of debris that poor Phobos just goes sailing into. Like an ocean liner sailing into ice floes.’ He looked again. ‘The grooves can be twenty kilometres long. A hundred metres deep. And there are twelve of them, or rather twelve families. So, maybe the product of twelve impacts on Mars? On Earth, scars like that heal. Craters get filled in, weathered, buried by continental drift and such. But a lump of rock like Phobos – make a dent in it, and it has no mechanism to fix itself. So the scar stays there for ever. And the debris, the pulverised Phobos dust, just gathers on the surface. There’s probably a layer of smashed-up dust tens of metres thick down there.’ He grinned, as Deirdra frowned at that. ‘Don’t worry. We won’t sink. Well, we shouldn’t. We brought snowshoes.’

  ‘Seriously?’

  Gershon said, ‘It doesn’t help, actually, that Phobos’s gravity, weak as it is, is weaker than it should be. Makes the surface stuff even fluffier.’

  ‘I don’t understand,’ Deirdra said. ‘Weaker?’

  ‘This is why we’re here, I guess,’ Malenfant said. ‘And why Emma II came here too.’

  Gershon said,
‘The problem is, Phobos is not a lump of rock. The density is way too low for that. Only about a third Earth’s density, for instance. That’s because it’s not a solid mass, but an aggregate of some kind. Loosely packed. Well, that’s one theory.’

  Deirdra said, ‘What’s the other theory?’

  Malenfant grinned at her. ‘That Phobos is hollow.’

  Her mouth dropped open.

  This was where Iosif Shklovsky came in, Malenfant said.

  ‘Shklovksy, as in the crater?’

  ‘Correct. That’s why he’s up here. I followed all this as Emma – my Emma – trained for her mission. The problem of Phobos first came up back in the 1960s, when I was a kid. Shklovksy was an astrophysicist who thought he could explain a slow decay in Phobos’s orbit, which had been observed since the 1940s. Phobos looked to be too light for its size; friction with Mars’s atmosphere, or maybe tidal effects, seemed to be causing it to fall out of its orbit too quickly. Kind of like Skylab would have fallen in the 1970s, if John Young’s shuttle mission hadn’t saved it.’

  ‘Or Powersat 24,’ Bartholomew put in. ‘Twenty-second century. A more familiar reference for us, Malenfant. Took out a city – Nairobi.’

  Malenfant stared at him. ‘I did not know that. OK. So Shklovsky speculated that Phobos might consist of a thin metal shell, a few centimetres thick. Maybe with a layer of meteorite rubble over the top.’

  ‘Wow.’ Deirdra thought that over. ‘This guy thought Phobos was hollow? But all that was as seen from Earth, correct? This decay. Even before the probes got there?’

  ‘Correct. The data was poor; Shklovsky didn’t get the numbers quite right. But Phobos is falling to Mars. Its orbit is shrinking in radius, by about two metres per century. That might not sound much, but it’s enough to drop Phobos so low into the atmosphere that it will break up, hit the surface of Mars, in maybe fifty million years.

  ‘I’ve met enough space scientists to know that they worry about stuff like this. Fifty million years is a long time for me and thee, but there’s evidence that Phobos is three billion years old, which is sixty times as long. Why should Phobos be so close to falling, just now? When humans are around to see it? The odds are Phobos should have fallen long ago – either that or not until the very far future, billions of years from now.’

  Deirdra nodded. ‘I think I understand. It doesn’t fit.’

  ‘Yeah,’ Malenfant said. ‘The bottom line is, Shklovsky was right, there is something wrong with Phobos. That was known even before an impossible copy of my wife showed up here. And maybe soon we will find out what.’

  ‘What about this?’ Deirdra pointed to a huge circular wound, right in the middle of the map. A crater so vast it contained lesser craters – and a brilliant blue flag. ‘The crater’s called – Stickney?’

  Gershon smiled. ‘That was the birth name of the wife of Asaph Hall, the discoverer. Kind of an ambiguous gift, though. Stickney is a mere nine kilometres across. It would be lost on the Moon, or even Earth. But, you can see on the map, the crater spans something like one seventh of the circumference of Phobos. It’s as if Earth had a crater over five thousand kilometres across. A crater wider than North America.’

  Deirdra opened her mouth, and closed it. ‘That’s amazing.’

  ‘It must have been an impact so violent that it nearly smashed Phobos to pieces altogether. Not that that’s such a challenge on a moon that’s just rubble in the first place . . .’ Malenfant pointed at the blue flag. ‘And now Stickney is the most important site on Phobos, and not just because of the world-shattering violence of its creation. Because of what that little blue flag signifies.’

  Gershon said, ‘Over the years – hell, the centuries – we sent a number of probes to Phobos. Just small scientific samplers, landers, orbiters. Never a crewed mission, there never seemed the need – once it was decided to preserve Phobos for science anyhow, rather than mine it for water and stuff. Most are just junk now. But one survives – it landed in Stickney, and it’s marked by that blue token on the map, Deirdra. Called Mini-Sat 5. The probe has just sat there for centuries, powered by sunlight and a micro fusion plant – that feeds off Phobos resources, but just a trickle – and it’s just as well it did survive . . .’ He glanced at Malenfant.

  Deirdra grinned. ‘Because that’s how we got the news about Emma?’

  ‘You got it,’ Malenfant said. ‘The probe picked up a faint signal, just regular radio, and relayed it to Earth. A signal coming from close to the probe itself.’

  ‘Maybe they used it as a landing beacon.’

  ‘That would make sense.’

  ‘And now, here we are. Almost close enough to touch. Almost . . .’

  Patience, Malenfant. He felt as eager as Deirdra sounded. But he had been a pilot long enough to know that the way to fail to reach any challenging goal, indeed the way to get yourself and your companions killed in the process, was to go rushing in without proper observation and preparation.

  So they continued to observe, and to prepare the ship for its final approach.

  And, in his head, Malenfant prepared himself.

  At last there was a soft chime.

  42

  Gershon said, ‘That’s the acquisition radar. We’re close now.’ He made his way back to his control station. ‘Malenfant, you want to come up here? Deirdra, Bartholomew, take your couches. Orbit circularisation burns coming up. And then we will go into the final approach and landing sequence.’

  Deirdra, her movements in zero G confident and competent now, swept across the cabin to her couch, strapped herself in. Bartholomew, silent, inscrutable, was already seated.

  The engines started to fire, with short, jolting bursts, and the ship swivelled and rocked in response.

  Malenfant checked his screens. The ship had swum through that adjusted ellipse of an initial orbit, to the point where it grazed the more circular path of Phobos – the moon itself now loomed large beyond the windows. But the ship was moving too rapidly: right place, wrong speed. Now the Step had to lose a lot of kinetic energy. And, more subtly, the ship had to adjust the angle of its final orbit. The Step had come sailing in from Earth in the plane of the Solar System – but Phobos orbited over the equator of Mars, which was tilted away from the solar plane, just like Earth’s equator.

  So the burn sequence was complex.

  But it went perfectly. This was a good, smart ship, and Gershon an expert pilot of it, Malenfant conceded grudgingly. The manoeuvre took half an hour or so.

  When it was done the Last Small Step sailed smoothly alongside the battered moon.

  ‘We’re only thirty-five kilometres away,’ Malenfant said. ‘Right on the money.’

  Deirdra, released briefly from her couch, came to see. ‘It’s like some huge ship,’ she said.

  ‘More like a big space station. Its gravity is feeble, even weaker than it ought to be. So this landing is going to be more like a rendezvous and docking with a station than a touchdown on the Moon, say.’

  Gershon glanced over at Malenfant. ‘Ready for the final descent burn whenever you are.’

  ‘Deirdra—’

  She was already strapping herself back in. ‘I’m ready.’

  There was a kind of rattling, audible around the base of their conical module.

  Gershon checked over his systems. ‘That’s it. We just detached from the main body, with the interplanetary engine. It will track Phobos until we are set to return, when we come back up from the surface and dock again.’

  Deirdra smiled. ‘Meanwhile we land. Just like Armstrong and Aldrin, in their lunar module.’

  Gershon snorted. ‘A more appropriate reference would be my ancestor Ralph in his Project Ares Mars Excursion Module. Same logic, yes. Four, three, two, one—’

  The lander’s burn was just a small kick, after the main ship’s big orbital-adjustment manoeuvres. But as soon as it was done, the dark cratered hide of Phobos began, it seemed to Malenfant, to swim subtly closer.

  ‘I assume,’ Barth
olomew said, ‘we are going down into Stickney.’

  ‘That’s the plan,’ Malenfant said. ‘We’re heading right into the deepest, darkest hole that ever got dug into this rubble pile.’

  ‘Where the mystery is,’ Deirdra said gleefully.

  And, perhaps, Emma, Malenfant thought.

  Gershon said, ‘Final burn. Approaching the surface.’

  Malenfant heard a series of rattling bangs from around the hull. These were verniers, smaller rockets firing in clusters, for attitude control and final adjustment of their trajectory. He imagined the view from outside, as these upper stages of the Last Small Step, separated from the arrow-shaft propulsion section, came down onto the Phobos ground, like an Apollo command and service module combination landing on its tail, with little squirts of attitude-rocket exhaust.

  All was calm. But his own heart was clattering.

  Gershon said, ‘We will be over Stickney in a few more minutes. We can use Mini-Sat 5 as a landing beacon. Then we will scope out our own final landing site. OK, Malenfant, arm the harpoons.’

  Bartholomew stared. ‘I wish I had downloaded the mission profile. Harpoons?’

  Malenfant grinned. ‘The gravity is so low that if we go in too hard, we’ll just bounce off. Hence the harpoons. Wait and see . . .’

  The attitude thrusters blipped and squirted, and the craft made gentle jerks.

  ‘Closing in,’ Gershon called. ‘Everything is nominal—’

  Another shudder, a violent one.

  Malenfant glared at Gershon. ‘What the hell?’

  Gershon stared at his screens. ‘We just got shoved sideways by the attitude system. The landing radar – I thought it glitched. According to the readings, suddenly we were flying over a hole in the ground. A deep shaft.’

  Malenfant frowned. ‘How deep?’

  ‘We’ve been taken away from it by the automatics. Descent resuming. Harpoons armed—’

  ‘How deep, damn it?’

  Gershon looked at him. ‘Too deep, Malenfant.’

  ‘What the hell does that mean?’