He must fly down the visible face of the Moon, all the way to the south, before landing. His altitude would drop steadily, sixty miles, forty, twenty, ten. He imagined his trajectory unwinding, a smooth curve shaped by gravity, kissing the surface of the Moon at just the point he intended, fifty miles short of the place he intended to deliver his nuclear weapon.

He was still flying at orbital speed — three thousand miles per hour, about Mach Five — and he would keep up those speeds, accelerate in fact, all the way to the surface of the Moon. Nobody in history had ever flown so fast, so low, not even Geena.

Certainly nobody had tried to achieve a touchdown at such speeds. And yet that was what he must attempt, today.

Through the tight portholes of his Soyuz, he caught glimpses of the surface of the Moon. It was a spotlit bombing range under a black sky, fleeing under his prow, fresh craters and basin rim mountains and undulating mare plains crowding over the close horizon with an unwelcome eagerness.

His view was completely sharp, of course. There was no cloud, no layer of muddy air, to obscure his view; and at times he would lose his sense of altitude. At such moments he turned away from the windows and trusted to his instruments, his infallible electronic senses, and to the precise mathematics that had guided him here.

And now, as Arkady flew further south, a new series of mountains — a ring of them, folded and eroded — came shouldering over the horizon towards him. They straddled the Moon, as if striving to block his further progress towards the Pole.

This was, he knew, the mountainous rim of the great South Pole-Aitken Basin: the huge impact crater which straddled the South Pole of the Moon, the largest and deepest such crater in the whole of the Solar System, a walled plain as wide as the Mediterranean Sea.

His Soyuz, like a little green bug, flew over the immense, eroded shoulders of the rim mountains. The mountains stretched before him and to either side, obviously ancient, colourless as plaster-of-paris models, a five-thousand-mile-long ring of shattered and folded Moonscape.

He checked his clock. Fourteen minutes to his touchdown. He was still seventy-five thousand feet high, with almost a quarter of the Moon’s face still to traverse; yet he was already inside the great Basin.

The land beyond the rim walls was revealed now. It was battered and scarred even beyond the norm he had come to expect for this small, ancient, rocky world, every square inch of it crowded with craters and rubble. The biggest craters here were major complexes in themselves, huge and eroded, many miles across, their giant flanks punctured by smaller, brighter newcomers.

In the shadows of the mountain ring, there were places where the sun could not have shone for a billion years — perhaps the coldest places, Arkady thought, in the Solar System.