“That’s it,” Henry said. “Pull over.” Henry got out of the Rover even before they stopped, but his hoses yanked him back, and he tumbled back into his seat.

She rapped his helmet. “Do not do that again, asshole.”

“Sorry.”

They went through the complex and embarrassing ballet of getting themselves, as a joined pair, off the Rover.

Together, they walked to the rille. Geena held Henry’s hand, to ensure they didn’t separate too far. She couldn’t feel his hand, inside the thickness of his glove. It was difficult to Moonwalk, joined like this; they had to synchronize their loping.

The rille was small — only twenty or thirty yards wide, its walls deep-cut. Henry had picked it out from old low-orbit Apollo photographs. In the low sunlight, with the regolith’s tan sparkle, its eroded walls looked like a small mountain valley, she thought, somewhere above the snow line.

“There,” said Henry. He pointed along the rille. “You see that?”

She looked where he pointed. A few hundred yards along, the rille terminated; but she could see a kind of bridge of rock beneath which the valley continued, as if it entered a tunnel.

“What is it?”

“A lava tube. Our salvation. I knew there had to be one here. Maybe we can live through this after all. Come on. We haven’t much time.”

This was Henry’s latest plan. She thought it was crazy. But she had to admit, now she’d slept on it, the idea of sacrificing her life without trying was less appealing than ever.

So, with Geena clinging on to Henry’s hand, watching they didn’t foul the tubing that joined them, they loped back to the Rover and began to unload it.

He fell inexorably from the empty lunar sky, every minute dropping five thousand feet and covering sixty more miles, the shadows lengthening as he rounded the curve of the Moon.

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.