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would forget it and go home to dinner. In this frame of mind I was especially inventive, and I helped a lot.
    It got to the point where I even began to return to the starship in the evenings after dinner, to wander the farm alone and type some figures into the model programs to check the results. Because they had a real problem on their hands—I’d never worked on a harder one. The two ships were Deimos PRs: about forty years old, shaped like decks of cards, just over a kilometer long; powered by cesium reactor-mass, deuterium-fueled, direct-explosion rockets. The crew of forty or forty-five lived in the forward or upper part of the ships, behind the bridges. Below them were the recreational facilities, the various chambers of the farms, and the recycling plants, and below those were the huge masses of the rocket systems, and the shield that protected the crews from them. The ships were biogeocenoses, that is, enclosed ecology systems, combining biologic and technologic methods to create closures. Total closure was not possible, of course; it approached eighty percent complete for a three-year period, tailing off rapidly after that. So they were good asteroid miners, they really were. But there were loss-points that had never been satisfactorily solved, and although these were the best closed biologic life-support systems ever built, they were no starships.
    I walked in circles through the rooms of Hidalgo ’s farm, following the course of the various processes as I tried to think my way through the system. Most of the rooms were darkened, but the algae rooms still required sunglasses. Here the whole thing began. Heat and light generated by the nuclear reactions in the rocketry provided energy for the photoautotropic plants, mostly the algae chlorella pyrenoidosa and chlorella vulgaris. These were suspended in large bottles under the lights, and I thought that, despite the nutrient problems, they could be manipulated genetically or environmentally to make the gas exchange as needed.
    I took off the sunglasses and stumbled around the darkened aqua room until my sight returned. Here the excess algae was brought to feed the bottom of the food chain. Plankton and crustacea ate algae, little fish ate the plankton, big fish ate the little fish. It was the same in the barns farther along; under night lights I could make out the cages and pens for the rabbits, chickens, pigs and goats—and my nose confirmed their presence. These animals ate the plant wastes that humans didn’t use, and provided food themselves. Beyond the animals’ barn was the series of rooms planted with rows of vegetables—the farm proper—and here some lights were still on, providing a pleasant, mild illumination. I sat down against one wall and looked at a long row of cabbages. Beside me on the wall was drawn a simple schematic, left wordless like a religious token—a diagram of the system’s circular processes. Light fed algae. Algae fed plants and fish. Plants fed animals and humans, and created oxygen and water. Animals fed humans, and humans and animals created wastes which sustained microorganisms that mineralized the wastes (to an extent), making it possible to plow them back into the plants’ soil.

    The cabbages glowed in the dim light like rows of brains, working on the problem with me. The circle made by the diagram, supplemented by physiochemical operations to aid the gas exchange and the use of wastes, was nearly closed: a neat, reliable, artificial biogeocenosis. But there were two major loss points that had me stumped; and I wasn’t going to see the solution walking around the farm. One was the incomplete use of wastes. Direct use of human waste products as nutrients for plant life is limited by the build-up of chlorine ions not used by plants. Sodium chloride, for instance, is a compound used by human beings as a palatable substance, but it isn’t required in equivalent amounts by the other