Clutching his equipment, trailing the rope behind him, he marched on down the slope, into the shadow of the rille wall, alone.

It was dark here, the only light scattered from the far side of the rille.

He lit his helmet torch. It cast an ellipse of light on the regolith surface before him, and back-scattering illuminated a short way beyond that. He had to keep his head down so the light showed him where his feet would come down, so he could see only a few feet ahead of himself at any time.

He was truly, he thought, approaching the heart of darkness.

At first he moved cautiously. But the slope here, sandblasted by micrometeorite rain, was still gentle. Walkback limits, he thought; he could afford to go a little faster.

He lengthened his stride. Soon he was bounding in slow-motion leaps, sailing over the rocks and dust. When he landed he sent up sprays of dust which collapsed back immediately to the surface, like handfuls of gravel.

The trick with the light now was to keep it focused on the spot he was likely to land, rather than directly under his feet; it took some coordination, but he was getting the hang of it.

He allowed himself a moment of exhilaration. Hot shit, he thought; this is the way to do a field trip.

But now the ground was getting steeper, the regolith layer thinner; the sprays raised by his footfalls were diminishing.

He tried to slow. But he’d underestimated his momentum; old Sir Isaac was pushing at his back, and the surface under his feet was slithery. He leaned back and tried to dig his heels in, but that succeeded only in tipping him over backwards.

He slid six, ten feet on his butt, deeper into the rille. In his stiff Shuttle suit it was like sliding down a ski slope in a box.

He tumbled to a halt.

He sat there in darkness, breathing hard, his torch showing him only a few yards ahead, so close was the rille’s horizon now.

His rope gathered itself up, went taut, and he got a good hard tug on the chest. Geena, telling him to behave himself.

Well, she was right. It was going to be a lot harder coming back up.

He got up and slapped ineffectually at his thighs, trying to dust himself off. He held onto the rope, and stepped cautiously, ever deeper, his feet scraping now over almost bare basalt.

He came to a place where the slope was steeper than one in one. There was a basalt ledge here, a place where one of the lava strata had become exposed; it made a place to stand. When he looked up, his torch showed him the rille wall tipping up and away from him, dauntingly steep and tall.

He wasn’t sure he could go any further in. This would have to do.