Thus, Henry and Geena laboured to collapse their shelter, their only home on the Moon, and to load up the old Apollo Rover with their survival gear.

When they were done the Rover was piled high with equipment. The collapsed shelter, a big pie-dish, was tied to the back by nylon rope.

“Like something out of The Beverly Hillbillies,” said Henry.

“I always hated that show.”

They clambered onto the Rover, and Geena pushed at the joystick.

The Rover lumbered forward.

It was a rocking and rolling ride, all the way to the rille complex. Every time it hit a mound or a depression or a crater rim — which was every few seconds — the Rover teetered precariously, obviously top-heavy.

Geena followed yesterday’s tracks, but today they rolled right past the point where they’d parked.

They approached the rim of a side rille, much smaller than Schröter’s Valley.

“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.