But perhaps other devices had been fixed to those scars — devices long since discarded and now beyond his imagination — perhaps the "sensors" of which the Mole spoke.

He felt a surge of irrational gratitude to the Moles. In all his crushing universe they, enigmatic as they were, represented the only element of strangeness, of otherness; they were all his imagination had to work on. The first time he had begun to speculate that things might somewhere, sometime, be other than they were here had been a hundred shifts ago when a Mole had unexpectedly asked him whether he found the Nebula air any more difficult to breathe.

"Mole," he said.

An articulated metal arm unfolded from the nose of the Mole; a camera fixed on him.

"The sky looked a bit more red today."

The transfer of nodules was not slowed but the small lens stayed steady. A red lamp somewhere on the prow of the machine began to pulse. "Please input spectrometer data."

"I don't know what you're talking about," Rees said. "And even if I did, I haven't got a 'spectrometer.' "

"Please quantify input data."

"I still don't understand," Rees said patiently.

For further seconds the machine studied him. "How red is the sky?"

Rees opened his mouth — and hesitated, stuck for words. "I don't know. Red. Darker. Not as dark as blood."

The lens lit up with a scarlet glow. "Please calibrate."

Rees imagined himself to be staring into the sky. "No, not as bright as that."

The glow scaled through a tight spectrum, through crimson to a muddy blood color.

"Back a little," Rees said. "…There. That's it, I think."

The lens darkened. The lamp on the prow, still scarlet, began to glow steady and bright. Rees was reminded of the warning light on the winch equipment and felt his flesh crawl under its blanket of weight. "Mole. What does that light mean?"

"Warning," it said in its flat voice. "Deterioration of environment life-threatening. Access to support equipment recommended."

Rees understood "threatening," but what did the rest of it mean? What support equipment? "Damn you, Mole, what are we supposed to do?"

But the Mole had no reply; patiently it continued to unload its pannier.

Rees watched, thoughts racing. The events of the last few shifts came like pieces of a puzzle to the surface of his mind.

This was a tough universe for humans. The implosion had proved that. And now, if he understood any of what the Mole had said, it seemed that the redness of the sky was a portent of doom for them all, as if the Nebula itself were some vast, incomprehensible lamp of warning.

A sense of confinement returned, its weight more crushing than the pull of the star kernel. He would never get anyone else to understand his concerns. He was just some dumb kid, and his worries were based on hints, fragments, all partially understood.

Would he still be a kid when the end came?

Scenes of apocalypse flashed through his head: he imagined dimming stars, thickening clouds, the very air souring and failing in his lungs—

He had to get back to the surface, the Belt, and onwards; he had to find out more. And in all his universe there was only one place he could go.

The Raft. Somehow he had to get to the Raft.

With a new sense of purpose, vague but burning, he turned his chair to the exit ramp.

The tree was a wheel of wood and foliage fifty yards wide. Its rotation slowing, it lowered itself reluctantly into the gravity well of the star kernel.

Pallis, the tree-pilot, was hanging by hands and feet below the knotty trunk of the tree. The star kernel and its churning Belt mine were behind his back. With a critical eye he peered up through the mat of foliage at the smoke which hung raggedly over the upper branches. The layer of smoke wasn't anywhere near thick enough: he could clearly see starlight splashing through to bathe the tree's round leaves. He moved his hands along the nearest branch, felt the uncertain quivering of the fine blade of wood. Even here, at the root of the branches, he could feel the tree's turbulent uncertainty. Two imperatives acted on the tree. It strove to flee the deadly gravity well of the star — but it also sought to escape the shadow of the smoke cloud, which drove it back into the well. A skillful woodsman should have the two imperatives in fine balance; the tree should hover in an unstable equilibrium at the required distance.

Now the tree's rotating branches bit into the air and it jerked upwards by a good yard. Pallis was almost shaken loose. A cloud of skitters came tumbling from the foliage; the tiny wheel-shaped creatures buzzed around his face and arms as they tried to regain the security of their parent.

Damn that boy—

With an angry, liquid movement of his arms he hauled himself through the foliage to the top side of the tree. The ragged blanket of smoke and steam hung a few yards above his head, attached tenuously to the branches by threads of smoke. The damp wood in at least half the fire bowls fixed to the branches had, he soon found, been consumed.

And Gover, his so-called assistant, was nowhere to be seen.

His toes wrapped around the foliage, Pallis drew himself to his full height. At fifty thousand shifts he was old by Nebula standards; but his stomach was still as flat and as hard as the trunk of one of his beloved fleet of trees, and most men would shy from the network of branch scars that covered his face, forearms and hands and flared red at moments of anger.

And this was one of those moments.

"Gover! By the Bones themselves, what do you think you are doing?"

A thin, clever face appeared above one of the bowls near the rim of the tree. Gover shook his way out of a nest of leaves and came scurrying across the platform of foliage, a pack bouncing against his narrow back.

Paliis stood with arms folded and biceps bunched. "Gover," he said softly, "I'll ask you again. What do you think you're doing?"

Gover shoved the back of his hand against his nose, pushing the nostrils out of shape; the hand came away glistening. "I'd finished," he mumbled.

Pallis leant over him. Gover's gaze slid over and away from the tree-pilot's eyes. "You're finished when I tell you so. And not before."

Gover said nothing.

"Look—" Pallis stabbed a finger at Gover's pack. "You're still carrying half your stock of wood. The fires are dying. And look at the state of the smoke screen. More holes than your damn vest. My tree doesn't know whether she's coming or going, thanks to you. Can't you feel her shuddering?

"Now, listen, Gover. I don't care a damn for you, but I do care for my tree. You cause her any more upset and I'll have you over the rim; if you're lucky the Boneys'll have you for supper, and I'll fly her home to the Raft myself. Got that?"

Gover hung before him, hands tugging listlessly at the ragged hem of his vest. Pallis let the moment stretch taut; then he hissed, "Now move it!"

With a flurry of motion Gover pulled himself to the nearest pot and began hauling wood from his pack. Soon fresh billows of smoke were rising to join the depleted cloud, and the shuddering of the tree subsided.

His exasperation simmering Pallis watched the boy's awkward movements. Oh, he'd had his share of poor assistants in the past, but in the old times most of them had been willing to learn. To try. And gradually, as hard shifts wore by, those young people had grown into responsible men and women, their minds toughening with their bodies.

But not this lot. Not the new generation.

This was his third flight with the boy Gover. And the lad was still as sullen and obstructive as when he'd first been assigned to the trees; Pallis would be more than glad to hand him back to Science.