The rocket burst lasted twenty seconds, perhaps less. Then it died. Once again the spaceman hung loose on his chain.

And then the weightlessness really cut in. To Sally it felt as if she were falling, as if her internal organs were rising inside her. She swallowed hard.

Willis, sitting silently, showed no reaction. Frank Wood whooped.

There were knocks and bangs, and the craft swivelled with tight jerks.

Al Raup produced a flask, squirted out water that hung in the air, a shimmering globe, and then closed his mouth around it. ‘OK,’ he said. ‘The noise you hear is the firing of our attitude rockets, and the manoeuvring system. The shuttle is taking itself in for its docking with the Brick Moon.’

The Brick Moon, an artificial satellite station-keeping in the position of the vanished Earth in its orbit, was the Houston of the Gap, a constant comms presence for space travellers, a place where some basic research was going on, and a link to home. They were to stay there for only a few hours, before boarding their Mars ship, the Galileo.

‘Everything’s automated,’ Raup said. ‘But because I know some of you paid no attention whatsoever to the briefings, I draw your attention to this big fat red button here.’ He pointed.

‘You understand we’re dealing with the rotation of the Earth here. I mean, the Earth you just stepped off of. When you thought you were standing still on Earth you were already moving through space, being dragged around with the surface of the world – at a speed of hundreds of miles an hour at the latitude of GapSpace. When you step between worlds you keep that momentum, and without compensating you’d be flung away through space. The first time you stepped into the Gap, incidentally, Sally, aboard that airship, it’s lucky you stepped back as quickly as you did before getting thrown around too far. Here we have to shed that velocity, so the stepper shuttle burns its rockets, and brings us to a halt relative to the Brick Moon.

‘But if we ever want to go back we need to accelerate again, to match the Earth’s spin. OK? Otherwise you’d be like a leaf in a thousand-miles-per-hour gale. So if all else fails, if I’m in capacitated and you’re out of touch with the Brick Moon, press that button and the systems will take you home. Comprende?’

‘Clear enough,’ Willis said.

‘The other likely contingencies which we may hit in the shuttle are a drop in air pressure – just close up your suits. There are sick bags in front of you, airline style. Or you may need to use the bathroom. We do have a john in here, which folds out of the wall.’ He grinned evilly. ‘But your suits do have diapers. My advice is, if you can’t hold it in, let it out—’

‘Just get on with it,’ Sally said coldly.

‘Everything’s tooty. Just relax and enjoy the ride . . .’

With a speed that was surprising, given Sally’s memories of TV images of stately dockings at the International Space Station, the stepper shuttle closed in for a rendezvous with the Brick Moon. The station was a cluster of spheres, the whole about two hundred feet across, each component sphere brightly lettered A to K. The station-keeping satellite had been hastily assembled from prefabricated sections in brick and concrete, doped to withstand the vacuum and filled with airtight pods. Sally had been amazed to learn, during the briefings, that troll labour had been used to manufacture the concrete.

In zero gravity, they scrambled out of the shuttle and through hatches. To Sally the open hatches, surfaces that had been exposed to space, smelled oddly of hot metal.

And on the other side of the entrance hovered a worker, looking strangely like a clone of Al Raup, who handed them bread and a salt dip as they drifted through. ‘Old cosmonaut traditions,’ Raup said. ‘The Russians always got more into this stuff than we ever did.’

Inside the Brick Moon, the big chambers were cluttered with stuff: various kinds of equipment, bundles of bedding and clothing, bags of garbage, bales of what looked like unopened supplies. Every wall surface seemed to be covered in Velcro; more equipment clung there, roughly shoved out of the way.

Sally bounced around, shoving off the walls, getting used to movement in these conditions. Without gravity all these curved-wall compartments felt roomy, despite the clutter. She had a feeling adjusting back to gravity would be a lot tougher.

There was a constant clatter of fans and pumps. Sally saw loose bits of paper drifting in the air currents towards grimy-looking grilles. After five minutes in the dusty air, she started sneezing violently. Dust, hanging in the air, failing to settle out without gravity.

She glimpsed only a few other people in this station during their short visit. Most people just passed through this place, exchanging one specialized craft for another, but there was some dedicated work going on here which Raup showed them perfunctorily. New kinds of materials were being tested, many of them ceramic composites; panels of the stuff were pushed out of airlocks to be tested in the conditions of space. There was a programme of medical testing, of studies of the effects of zero gravity on the human body – repeating studies that dated back to the mid twentieth century and the first spaceflights, but with much more sophisticated gear.

And there were some more intriguing, less obvious projects: on the growth of crystals in the vacuum, on the development of plant and animal life in zero gravity. Sally surprised herself by being utterly charmed by a bank of bonsai trees she found growing in reflected sunlight, vivid colours against the bleak concrete walls.

And from the windows of the Brick Moon, the Galileo could be seen to hang in empty space.

Their Mars ship was unprepossessing, just two tin cans separated by a long metallic strut, with a single flaring rocket nozzle at the base of one cylinder, a snub-nosed lander aircraft piggybacking on the side of the other, and sprawling solar-cell wings. The spinal strut was adorned with spherical fuel tanks swathed in thick silvered insulation foam; they looked like huge pearls. There was fuel enough, Sally learned, to push the Galileo to Mars and back again. The trip each way would take nine or ten weeks.

The lander was called the MEM, officially the Mars Excursion Module. The upper cylinder to which it clung was the hab module, where the three of them would live for the ride out to Mars, and back. The cladding on the hull would protect them from radiation and meteorites. Light gleamed from windows cut through the cladding, bright and cosy and warm.

They spent twelve hours in the Brick Moon. They stripped out of their pressure suits, which were checked over; they were put through brisk medical tests by an onboard doctor; they had a meal, of paste from tubes and pots, and coffee squirted from bulbs. They all used the bathroom while they were out of their suits.

Then they suited up once more, and crossed into their ship, and Sally Linsay was that bit further away from Earth, and closer to Mars.

11

EARTH WEST 1,617,524: more than a million and a half worlds from the Datum, the original world of mankind, the Armstrong and Cernan hovered in a washed-out blue sky.

And below, on a green scrape amid arid wilderness on this late January day, smoke rose from the ruins of a city.

Already, just sixteen days into the journey of the Armstrong and Cernan, Maggie was far from home. She tried to picture, in a kind of human sense, just how far. For example, they had left behind the bulk of the Long Earth’s population in just a few hours. After Step Day there had been pulses of migration outward into the Long Earth, first the early wanderers, then the purposeful trekkers, and a new wave once twain technology was available and you could ride to your destination rather than walk. Then had come the mass flight from the Datum after Yellowstone, an evacuation of millions, unplanned and unprovisioned, that had overwhelmed all that had gone before.