_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 compressed it to about 600 atmospheres. There is no better shield against high-energy neutrons; they bounce off the light molecules and slow down to a speed which makes them perfectly harmless laggards which don't travel far before decaying into hydrogen themselves. This, as well as the direct radiation of the pile, turned the room hot--some 500 degrees. So what was more natural than that the same hydrogen should be circulated through pipes of chrome-vanadium steel, which is relatively impenetrable even at such temperatures, and heat the domes? There was, of course, considerable loss of energy as the compressed gas seeped through the Hill and back into the satellite's atmosphere. But the pumps maintained the pressure. It was not the most efficient system which could have been devised; it would have been ludicrous on Earth. But on Triton, terminal of nowhere, men had necessarily sacrificed some engineering excellence to the stiff requirements of transportation and labor. And after all, it had worked without a hitch for many years on Saturn's largest moon. It had worked for two years on Neptune's-- * * * * *