“I don’t know for sure. But the Moon is old, Geena. Enough time for a lot of volatiles to collect in the cold traps, where the sun never shines. Water and carbon dioxide. My models suggest there might be the equivalent of a thousandth the mass of a large cometary impactor, delivered by one process or another.”

“I’ll give you your thousandth,” she said. “So what?”

“Geena, a thousandth of a comet — if you melted it — could cover the Moon in water to a depth of a metre or so. A thousandth of a comet would contain enough carbon dioxide to form an atmosphere.”

The fans of her suit whirred patiently. She was sitting on a folded-up blanket, her suit stiff; now she came a little closer to him, as if seeking the human warmth trapped inside the layers of his suit. “So how come Prospector detected so little?”

“Because it’s logical. A working-out of the laws of physics. Solar System processes. It has to be there hidden in the deep regolith.”

“Right. And your nuke is going to melt it all.”

“Oh, no.” He sounded surprised. “Don’t you get it? To melt all that ice would take the energy of…” He thought about it. “Maybe ten times Earth’s whole nuclear arsenal.”

“So how is the bunker-buster going to work?”

She could hear the grin in his voice. “Judo.”

Suddenly, she felt weary. She just wanted done with all of this, all these schemes and plans, one way or the other.

She closed her eyes.

“You’ve got to be a believer,” he whispered gently.

Arkady checked the controls set out before him. There were instruments and switches for the main systems, a TV screen, and an optical orientation view-finder set up on a small porthole next to the panel; there were orientation controls on his right, and manoeuvring controls to his left. His small laptop, with its weapons controls, was fixed to the couch to his right.

For the nuclear device, he had rigged a simple dead man’s switch. If possible, he would set the timer after a careful emplacement. But if power was lost, the device would detonate anyway.

His craft was prepared, and the weapons it carried.

He closed his hands around the joysticks that controlled the spacecraft’s systems. There was no computer program to control what he was going to attempt, with this poor Soyuz; he must lead it perhaps to its destruction himself.

He was ready.

Two minutes left. Just ten thousand feet now. The Moon ground fled beneath him, flattening, bellying up towards him.

His descent was actually very shallow — just ninety feet in every mile, much shallower than even a civilian airliner. His path, in fact, was almost tangential to the Moon’s surface. But his speed was gigantic. The pocked ground fled beneath him, like a scarred runway in the last few feet of a descent.

One minute.