that till I must. There's got to be some rational explanation."

"Leakage in the pipes?" ventured Catherine.

"We'd know that. Nor does it account for the radiation. No, it's—" His voice twisted up on him, and he groped out a cigaret. "It's something natural."

"What is natural?" said Alemán. "How do we know, leetle creeping things as we are, living only by the grace of God? We 'ave come one long way from home." His vision strayed to the viewport with a kind of horror.

Yes, thought Gilchrist in the chilled darkness of his mind, yes, we have come far. Four and a half billion kilometers further out from the sun. The planet-sized moon of a world which could swallow ours whole without noticing. A thin hydrogen atmosphere, glaciers of nitrogen which turn to rivers when it warms up, ammonia snow, and a temperature not far above absolute zero. What do we know? What is this arrogance of ours which insists that the truth on Earth is also the truth on the rim of space?

No!

He stood up, shuddering with cold, and said slowly: "We'd better go see Dr. Vesey. He has to know, and maybe they haven't thought to check the radiation. And then—"

Catherine stood waiting.

"Then we have to think our way out of this mess," he finished lamely. "Let's, uh, start from the beginning. Think back how th-th-the heating plant works."

Down in the bowels of the Hill was a great man-made cave. It had been carved out of the native iron, with rough pillars left to support the roof; walls and ceiling were lined with impermeable metal, but the floor was in its native state—who cared if there was seepage downward?

The pile sat there, heart and life of the station.

It was not a big one, just sufficient to maintain man on Triton. Part of its energy was diverted to the mercury-vapor turbines which furnished electricity. The rest went to heat the domes above.

Now travel across trans-Jovian spaces is long and costly; even the smallest saving means much. Very heavy insulation against the haze of neutrons which the pile emitted could scarcely be hauled from Earth, nor had there been any reason to spend time and labor manufacturing it on Triton.

Instead, pumps sucked in the hydrogen air and 

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