He ruffled her hair. “You think you’ll ever get to go to the Moon?”

“What for? There’s nothing there but a bunch of old rocks.”

“I thought you liked rocks. Your collection—”

“I like real rocks.”

“Moon rocks are real.”

“But they won’t let you touch “em.”

“Maybe. Anyhow, you’re wrong. About the Moon. It’s not just rocks. If you lived there you could make metals, and oxygen to breathe, and there’s silica to make glass. And with the water from the Poles, you could farm up there.”

“Water? I thought there’s no air.”

“There isn’t. But there is ice at the Poles. Deep in the dark, where the sun never shines.”

“Really? A lot of ice?”

Her father hesitated. “Well, some people think so.”

“Anyhow,” she said, “I don’t want to be a farmer.”

Her father stared up at the Moon. “You know, you’re special. I wrote your name up there, in the dust, and it will be there for a million years.”

“You told me, Dad.”

“Yeah.”

A cloud passed over the Moon’s face. It got colder.

They went indoors to watch TV.

One day, human scientists would call it the Impactor.

It had about the mass of Mars, a tenth of Earth’s. Humans would later speculate that it was a young planet in its own right.

But they were wrong. It was not a planet.

The object barrelled through the dusty plane of the Solar System.

But there seemed to be something in the way.

…And on the Moon, the Rover had jolted across the bright dust, climbing gentle slopes under the black sky, bathed in the sun’s flat light.

It must have looked strange, Jays thought, if there had been anyone around to see it. The Rover looked like a beach buggy from somebody’s home workshop. And yet here were the two of them in their shining white pressure suits, like two dough boys riding a construction-kit car, bouncing across the Moon itself. They rose up a slight incline.

Suddenly there was the rille: Schröter’s Valley, a gap in the landscape in front of them. It wound into the distance, its walls curving smoothly through shadows and sunlight. Jays could see boulders that must have been the size of apartment blocks, strewn over its floor.

Jays tried to keep the excitement out of his voice. “Look at that old rille. I’m sure I can see layering in the far side. Look, Tom. Over at one o’clock.”

Tom, distracted by the driving, said, “Let’s get up there before we do any geology.”

The Rover jolted to a stop.

Jays released his seat belt, and let it snap back into its frame. He tried to stand up. But the slope was deceptive; it was an effort to haul his suited frame out of the Rover’s lawn seat.

He took a step away from the Rover.

His suit was a stiff bubble around him, shutting out the Moon. He could hear the whir of pumps and fans in his backpack, feel the reassuring breath of oxygen over his face. The sunlight caught scuffs and scratches in his gold-tinted sun visor, creating star-bursts.

He looked up, towards the south, and there was the Earth, hanging in the sky like a blue thumb-nail. He could see a depression over the South Atlantic, a fat white swirl. Other than the Earth and the big white spotlight that was the sun, the sky was empty: save only for a single bright star that tracked across the zenith every couple of hours. That was the Apollo Command Module, waiting in orbit to take them home.

Jays, when you climb off, could you dust off our TV lens, please?

“Roger.”

He turned back to the Rover. The TV camera, operated from Houston, was a block covered by gold-coloured insulation, mounted on a bracket at the front of the Rover. He could see that dust had kicked up over the lens and insulation and cabling.

He leaned forward. He took a breath, out of instinct, as if he could just blow the thin dust layer away. But this wasn’t Earth; there was no air here, and his head was locked inside a bubble of plexiglass… He looked for the soft lens brush, and swept away the dust.

As soon as he was done the camera turned away by itself, and began to pan across the landscape in eerie silence.

It was, perhaps, the most dramatic collision event in the history of the Solar System. Humans — trying to figure out how their world and its unlikely, huge Moon had come to be — would call it the Big Whack.

The Impactor hit Earth obliquely, like a cue ball kissing its target. Earth, much more massive, absorbed the momentum of the Impactor and spun up its rotation. Mantle material vaporized and entered orbit around the Earth. Earth’s crust melted; Earth became, as if young again, a roiling ball of lava.

The orbital cloud of superheated mantle rock condensed into droplets, a dusty, rocky ring circling the Earth. But the ring was not stable. In a miniature rerun of the formation of the Solar System itself, the debris began to accrete.

It took a mere century for the debris to assemble into a new world: it was the Moon, a ball of magma glowing balefully in Earth’s sky. The remains of the Impactor were trapped in the Moon’s heart.

The new world was coming of age in a Solar System that was still very young, and huge left-over planetesimals continued to bombard its surface. Impact basins formed, wounds huge and deep, and waves of pulverized rock rushed over the surface of the Moon to form gigantic ringed structures. Immense blankets of ejecta were hurled thousands of miles over the battered ground. But so intense was the continuing flux that the formations were themselves soon shattered and covered over.

At last the flux of planetesimals began to tail off. In a moment of geological time, the last great impacts formed basins and mountains which froze forever the face of the Moon.

The Moon became a small, cold world, its evolution over a billion years after its birth, its youngest rocks more antique than Earth’s oldest.

And, far beneath the dusty plains, the remnant of the Impactor brooded, embedded in darkness.

On the Moon, Tom Barber was going through Rover read-outs for Houston. “Amp-hours 90, 92. Voltages 68, 68. Battery temperatures 101 and about 100; motor temps off scale, low. Bearing is 088, range 1.8, distance 2.5 klicks.”

Thank you…

Jays picked up a couple of sample bags from the stowage in back of the Rover, and took the geology hammer from Tom’s backpack, and his tongs from the Rover’s footpan. He pulled the gnomon out from its stowage sleeve behind his seat. In stowage, the gnomon’s three legs were folded against the central staff to make a slender sheaf. When he extracted it the legs sprang out into a spindly tripod.

Carrying his tools he loped away, towards the rille.

The regolith crunched under his feet, sinking maybe a half-inch, before he settled to a firmer footing. The dust sprayed around his feet, sinking quickly back to the surface. It was like walking on crisp, frozen snow, or maybe a cinder track.

He had to climb up a slight incline to reach the rille’s rim. He was out of breath in a few steps.

Still, he persisted.

He paused for a breath. He turned and looked back at the skeletal Rover. It looked like an ugly toy: squat and low, sitting there in a churned-up circle of dust. Its orange fenders and gold insulation were the brightest things on the surface of the Moon. A few yards beyond the Rover, Tom was working. He was gouging at the surface with a long-handled tool, taking a rake sample of the dust. His red commander’s armbands were bright.

Jays took a swig of water from the bag inside his suit. He felt his chin strap rasp against a week-old beard. He’d promised his daughter, Tracy, that he wouldn’t shave until he got home. After all, in the picture books, the explorers always came home with beards.