Phase Space. Stephen Baxter

tanks continued at a stubborn couple of per cent a month, despite the new generation of supercritical water oxidizers they used to reduce their solid wastes.

      It’s still about pea plants and plumbing, he thought dismally.

      He started clambering into his skinsuit, hauling the heavy fabric over his useless legs.

      He took a last glance around the glass-wall displays of his hab module. It shocked him when the displays showed him he was the only one of Ra’s two thousand inhabitants on the surface.

      Well, hell, it suited him out here, even if it had stranded him in this lonely assignment, monitoring the systems that watched near-Ra space. Most of the inhabitants of Ra had been born up here, and lived their lives encased in the fused-regolith walls of old mine shafts. They didn’t know any better.

      Greenberg, though, preferred to keep a weather eye on the stars, unchanged since his Iowa boyhood. He even liked seeing Earth swim past on its infrequent close approaches, like a blue liner on a black ocean, approaching and receding. It made him nostalgic. Even if he couldn’t go home any more.

      Suited up, he shut down the glass-wall displays. The drab green walls of the hab module were revealed, with their equipment racks and antique bathroom and galley equipment and clumsy-looking up-down visual cues. This was just an old Space Station module, dragged out here and stuck to the surface of Ra-Shalom, covered over with a couple of metres of regolith. He was tethered in the rim shadow of Helin Crater, the place he’d landed on that first jaunt in the Ehricke. And that had been all of a hundred years ago, my God.

      He pushed his way through the diaphragm lock set in the floor of his hab module. He was at the top of one of Shaft Two, one of the earliest they’d dug out, with those first clumsy drill-blast-muck miners. It was a rough cylinder ten metres across, lined with regolith glass and hung with lamps and tethers; it descended beneath him, branching and curving.

      He grabbed hold of a wall spider, told it his destination, and let it haul him on down into the tunnel along its stay wires.

      Thus, clinging to his metal companion, he descended into the heart of Ra-Shalom.

      His legs dangled uselessly, and so he set his suit to tuck them up to his chest. He was thinking of taking the surgeons’ advice, and opting for amputation. What the hell. He was a hundred and fifty years old, give or take; he wasn’t going to start complaining.

      Anyhow, apart from that, the surgeons were preserving him pretty well. They were treating him to a whole cocktail of growth hormones and DHEA and melatonin treatments and beta-carotene supplements, not to mention telomere therapy and the glop those little nano-machines had painted on the surface of his shrivelled-up brain to keep him sharp.

      These guys were good at keeping you alive.

      This asteroid was small. A stable population was important, and a heavy investment in training needed a long payback period to be effective. So the birth rate was low, and a lot of research was directed to human longevity.

      He understood the logic. But still, he missed the sound of children playing, every now and again. The youngsters here didn’t seem to mind that, which made them a little less than human, in his view. But maybe that was part of the adjustment humans were having to make, as they learned to live off-Earth.

      In fact he missed his own kids, his daughters, even though, astonishingly, they were now both old ladies themselves.

      The surgeons had even managed to repair some of the cumulative microgravity damage he’d suffered over the years. For instance, his skeletal and cardiac muscles were deeply atrophied. Until they found a way to stabilize it, his bone calcium had continued to wash out in his urine, at a half per cent a month. At last, the surgeons said, the inner spongy bone, the trabeculae, had vanished altogether, without hope of regeneration.

      He never had been too conscientious about his time in the treadmills. It had left him a cripple, on Earth.

      So, at age eighty, he’d left Earth.

      Even then they had been closing down the cans – the early stations starting with Mir and the Space Station, that had relied completely on materials brought up from Earth. In retrospect it just didn’t make sense to haul material up from Earth at great expense, when it was already here, just floating around in the sky, in rocks like Ra.

      So he’d come back to come out to Ra-Shalom, the place that had made him briefly famous.

      He suspected the surgeons liked to have him around, as a control experiment. The youngsters were heavily treated from birth, up here, to enable them to endure a lifetime of microgravity. Not a one of them could land on Earth, of course, or even Mars. But not too many of them showed a desire to do any such thing.

      The wall spider, scuttling busily, brought him to the mine face, the terminus of Shaft Seven. It was a black, dusty wall, like a coal face. There was dust everywhere, floating in the air.

      There were five or six people here, in their brightly coloured skinsuits, scraping their way around the stalled miner. Their suits were seamless and without folds, to guard against the dust. They were all tall, their limbs spindly as all hell, their skeletal structures pared down as far as they would go.

      The miner itself clung to the walls with a dozen fat legs, with the balky rock splitter itself held out on a boom before the face. It was a radial-axial design with a percussive drill, powered by hydraulics, with a drill feed, a radial splitter and a loader. But for now it was inert.

      One of the youngsters came up to him. It was Gita Weissman, Mike’s granddaughter. She grinned through her translucent faceplate; her skinsuit was what they used to call Day-Glo orange.

      ‘Dust,’ she said. ‘It’s always the dust.’

      ‘Yeah.’

      ‘Grab a pump. We want to get this baby back on line or we’ll miss quota again.’

      He started to prepare a vacuum-pump tube.

      The ‘dust’ was surface rock flour: half of it invisible to the naked eye, abrasive, and electrostatically sticky. Despite their best efforts it had gotten all the way through the interior workings of Ra, coating every surface.

      A lot of Earthbound experience was worthless up here. No machine, for example, which used its own weight for leverage was going to be any use. Nevertheless, some terrestrial technologies, like coal gasification, had proven to be good bases for development of systems that gave a low capital investment and a fast payback.

      Greenberg remembered how they’d celebrated when the ore processors had first started up, and water had come trickling out of crushed and heated asteroid cinder. It had touched, he supposed, something deep and human, some atavistic response to the presence of water here, the stuff of life in this ancient rock from space.

      Whatever, it had been one terrific party.

      And this rock, and many others like it, had proven to be as rich as those old sci-fi-type dreamers, who Greenberg used to laugh at, had hoped. Ra was fat with water – twenty per cent of its mass, locked up in hydrate minerals and in subsurface ice. It exported kerogen, a tarry petrochemical compound found in oil shales, which contained a good balance of nutrients: primordial soup, they called it. Ra pumped out hydrogen, methane, kerosene and methanol for propellants, and carbon monoxide, hydrogen and methane combinations to support metal processing …

      And so on.

      Ra was just a big volatiles warehouse floating around in the sky. And with the big surface mass drivers that Greenberg called softball pitchers, Ra products were shipped to places that were volatile-poor – like Mars, lacking nitrogen, and the Moon, dry as a desert. It was a lot cheaper to export them from a rock floating around up here than from all the way at the bottom of Earth’s gravity well.

      To Greenberg’s great surprise, Ra’s inhabitants had become rich.

      The first justification for opening up the rocks had been to make them serve as short-term resource factories