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.