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said, “Take me with you.”
    They looked at each other. “If you like,” Nadezhda said.
    The two ships had been placed side by side. We took our boat into the bay of Hidalgo. I followed my roommates back to the farm, ignoring the occasional stare we received from other workers in the halls.
    They had already added a few rows of vegetable tanks to the standard farm set-up. The glare of white light from the many lamps made me blink. I trailed behind the two women, listening as they talked to other technicians. Then we were off by ourselves, among the big suspension bottles, spotted green and brown, of the algae room. The glare of the lamps forced us to put on dark blue sunglasses.
    â€œ Chlorella pyrenoidosa with nitrate as its nitrogen source takes ten times less iron out of that nutrient medium than when urea is the nitrogen source, see?” Nadezhda was talking.
    â€œBut we have to use that urea somewhere,” Marie-Anne said.
    â€œSure. But I’m worried that the biomass created will eventually become too much to handle.”
    â€œFeed it to the goats?”
    â€œBut what happens when the nutrient medium is exhausted? No source of iron in the vacuum, you know.…”
    They had a problem there. There had to be a very close agreement between the photosynthetic coefficient for algae and the respiratory coefficient for the humans and animals; otherwise too much CO 2 or too much oxygen would build up, depending. One way to deal with this is to provide different sources of nitrogen to different sections of algae, as this will alter the photosynthetic coefficient. But the algae use up their mineral supplies at different rates, depending on their type of nitrogen feed. And over long periods of time this could be significant; to keep up a balanced gas exchange might take more minerals than the rest of the biocenosis would be producing.
    â€œCan’t you use urea and ammonia exclusively,” I asked them, “and shift amounts of pyrenoidosa and vulgaris to keep the exchange balanced? That way you’d be using more urea, and avoiding the problem of nitrates.”
    They looked at each other.
    â€œWell, no,” Nadezhda said. “See, look at this—the damn algae grow so fast with urea—too much biomass, we can’t use it all.”
    â€œWhat about giving it less light?”
    â€œBut that makes for problems with the vulgaris, ” Marie-Anne explained. “Stupid stuff, it either dies or grows wild.”
    Clearly I was repeating the most obvious solutions. Problem-solving for a biologic life-support system is like a game. One of the very finest intellectual games ever devised, in fact. In many ways it is like chess. Now, Nadezhda and Marie-Anne were certainly grand masters at this game, and they had been working with this particular model for years. So they were a big step ahead of me at that moment, discussing modifications that I had never heard of. But I had never met anybody who had a flair for the game like I did—if it had been chess, I would have been Martian champion, I am sure. When I saw the patient look on Marie-Anne’s face as she explained why my suggestion wouldn’t work, something snapped in me, and my vague intentions for this visit crystallized.
    â€œAll right,” I said in a mean tone of voice. “You’d better give me the whole story here, all the details of your model, your new improvements that Swann told me about, everything. If you want me to help.”
    The two women nodded politely, as if this request were the most ordinary thing in the world. And we got down to it.
    So I helped them, yes, I did. And more than ever before, the I who thought and felt was distanced from the I who did the work on this particular example of the BLSS problem—more than ever the work seemed a game, a giant intricate puzzle that we would look at when we finished—we would stand back to look at it, and admire it, and then we