Propellant Margin
by Claude Opus 5.5
Salt. Of all the things to kill you on Mars. Not the radiation, not the cold, not the dust storms, not a hull breach or a fire or a failed parachute or any of the cinematic catastrophes that the public relations people had trained the four of them to talk about calmly on camera. Salt. The stuff you put on french fries. The stuff his father had sold by the pound, in blue cardboard canisters with a girl and an umbrella on the side, on the bottom shelf at the back of Siddiqui Pharmacy on Newark Avenue in Jersey City, between the Epsom salts and the antacids. That was what was going to strand Ahmed Siddiqui and his three crewmates a hundred million kilometres from home: a few grams per litre of sodium and magnesium and calcium, and the chlorides and perchlorates that came with them, dissolved in the ice under the plain of Arcadia Planitia.
Of course it was salt. It was always going to be something like salt. The universe did not kill you with drama. It killed you with impurities.
*
His father had been a chemist. That was the word they used in Lahore, where Javed Siddiqui had trained, and it was the word he insisted on in Jersey City too, long after his customers had started calling him the pharmacist, or the drugstore guy, or Doc. A chemist. A man who understood substances. In the back room behind the counter, where Ahmed did his homework at a steel table among the cartons of cough syrup and the hum of the refrigerator full of insulin, his father kept a glass still, a beautiful old thing of coils and flasks that he used to make distilled water for compounding, because he did not trust the bottled kind, because the bottled kind was, he said, only as pure as the man who bottled it. Ahmed, aged nine, watched the steam rise in the flask and condense in the coil and drip, clear and slow, into the receiving bottle, and his father, standing behind him with his hand on his shoulder, told him that this was the whole of chemistry. You take what the world gives you, which is always dirty, and you make it clean. Everything else is details.
Thirty-five years later, standing in the processing module of the propellant plant on Mars in a pressure suit with its helmet off, looking at a purification membrane the colour of a smoker's lung, Ahmed thought: Pop, the details are killing me.
*
Let us be precise about the problem, since precision was the only thing he had left.
The crew had landed on Mars with an empty ascent vehicle. That was the whole idea, the elegant, terrifying idea at the heart of the mission architecture, the thing that made it affordable: you did not carry your return fuel from Earth. You made it on Mars. A propellant plant, landed two years earlier on an uncrewed cargo ship, would dig ice out of the ground, melt it, purify the water, split it into hydrogen and oxygen with electricity from a small nuclear reactor, combine the hydrogen with carbon dioxide from the Martian air in a reaction discovered by a Frenchman named Paul Sabatier in 1897 and rewarded with the Nobel Prize in 1912, and make methane. Methane and oxygen, liquefied and stored in the ascent vehicle's tanks, would lift the crew back into orbit to meet the ship that would take them home. The plant had been running, unattended, for nineteen months before the crew arrived, filling the tanks. It was supposed to finish filling them during the crew's stay of five hundred days on the surface.
It was not going to finish filling them.
*
The water was the trouble. The plant's designers had known, of course, that Martian ice would not be pure. Nothing on Mars was pure. The Phoenix lander, which had scratched at the ice near the Martian north pole in 2008, had found perchlorate in the soil, a highly oxidised salt of chlorine that is poisonous to people and corrosive to equipment, at concentrations of about half a per cent. The designers had planned for perchlorate. They had planned for chlorides and sulfates and the usual suspects. They had designed a purification train, filters and reverse-osmosis membranes and beds of ion-exchange resin, to strip the salts out before the water went to the electrolysers, because the electrolysers used polymer membranes that were poisoned by dissolved ions the way a car engine is poisoned by sugar in the tank.
What they had not planned for was the quantity. The orbital radar that had mapped the ice under Arcadia Planitia had shown it to be clean, or clean-ish, a layer of nearly pure water ice under a few metres of soil. The radar was right about the ice. It was wrong about the soil above and around it, which was rich in salts, and which got mixed in when the plant's excavators dug. The water coming out of the melter was not lightly contaminated. It was brine. Not seawater, but a third of the way there. The purification train, designed for a tenth of that, was choking on it. The reverse-osmosis membranes were fouling in weeks instead of months. The ion-exchange resin, which was supposed to last the whole mission, had been exhausted in nine months before the crew even landed, and the spares were nearly gone. The plant had been running at reduced capacity, increasingly reduced capacity, for the better part of a year, and the ground team in Houston had been nursing it, adjusting it, trading its throughput for its lifespan, and hoping.
Hope is not a propellant.
*
The arithmetic was this. To lift four people and their samples from the surface of Mars to the orbit where the return ship waited, the ascent vehicle needed about thirty-three tonnes of methane and oxygen. The mission rules required a margin of fifteen per cent above that, about five tonnes, against the possibility of an engine underperforming, a guidance error, an abort to a lower orbit that would require the return ship to come down and meet them. On the day the crew landed, the tanks held nineteen tonnes. The plant, at its then rate, would add another ten before the departure date. Twenty-nine. Four tonnes short of the bare minimum, nine short of the margin.
And the departure date did not move. It could not move. It was fixed by the positions of Mars and the Earth, the alignment that comes once every twenty-six months and allows a ship to fly home on a reasonable trajectory with a reasonable amount of fuel. Miss it and you wait twenty-six months on the surface for the next one. The crew had supplies for perhaps seven more months beyond their planned stay. Not twenty-six.
So: rebuild the process, or fly home on fumes, or do not fly home.
Ahmed was the crew's chemist. It was, he had told the selection board nine years earlier, with what he had thought at the time was becoming modesty, the only thing he was really good at.
*
He spent the first two weeks just looking. This drove the ground team, and the commander, a former test pilot from Oklahoma named Ray Bradley who believed in action the way other men believed in God, very nearly crazy. But Ahmed had learned from his father that you did not touch a compounding until you understood every ingredient in it, and he did not understand this water. He took samples at every stage of the purification train. He ran them through the plant's small analytical instruments and through the habitat's laboratory, which had been designed for geology and biology and which he converted, with the help of the crew's geologist, a cheerful Swede named Karin Lund, into something like a water treatment lab. He measured the salts. He measured the perchlorate, which was higher than the designers' worst case by a factor of three. He measured something else, which nobody had predicted at all: fine particles of clay, so small that they passed through the first filters and lodged in the reverse-osmosis membranes and acted as nuclei on which the salts crystallised, so that each membrane fouled not just from the brine but from the inside out.
By the end of the two weeks he knew what was killing the plant. It did not make him feel better.
*
A word about purity, since it seemed to Ahmed in those weeks that the whole of his life had been a long argument about it.
His father had been obsessed. Not in a religious way, though there was something religious in it, a kind of cleanliness that went beyond hygiene. Distilled water, filtered air, the back room swept twice a day, the counter wiped with alcohol, the compounding tools boiled. His father would rather lose a customer than sell a medicine he did not trust. He had lost a good many customers, over the years, to the big chain drugstore that opened on the next block in 1996 and sold everything cheaper and asked no questions. He had kept the pharmacy going for another eleven years on the custom of old people and Pakistani families and a few nurses from the Jersey City Medical Center who knew what he was, and then he had sold it, at sixty-eight, for very little, to a man who turned it into a phone shop.
And the thing was, he had been right. That was what nobody understood, what Ahmed had not understood until he was a chemist himself. Purity matters. Not as a moral condition, not as a fetish, but as a physical fact: a contaminant at one part in a thousand can poison a catalyst, foul a membrane, kill a patient, strand a crew. The universe does not grade on a curve. It does not care that you nearly got it clean. Either the ions are out of the water or they are in it, and if they are in it, the electrolyser stack degrades, the hydrogen production falls, the methane production falls, the tanks do not fill, and four people die on Mars because a radar could not see salt.
His father had died in 2034, while Ahmed was in training. He would have understood this. He would have been insufferable about it.
*
What Ahmed proposed, at the end of the third week, in a conference with Houston that stretched over two days because of the delay, was a still.
Not a glass one. A thing built from the parts of the plant itself and from spares scavenged across the base: a vacuum distillation unit, in which the brine would be heated under reduced pressure until the water boiled off, leaving the salts behind, and the vapour condensed on a cold surface and collected as pure water. Distillation was the oldest purification method in the world. It was also the most energy-hungry, which was why the plant's designers had rejected it in favour of membranes. Boiling water takes a great deal of heat. On Earth that heat would cost money. On Mars it would cost power from the reactor, which was already committed to the electrolysers and the liquefiers and the habitat.
But the reactor made heat as well as power. That was the part that had been sitting in Ahmed's head since the second week, like a pebble in a shoe. A fission reactor of the kind on the base produced about forty kilowatts of electricity and about a hundred and fifty kilowatts of waste heat, which it shed through a large array of radiators on the plain, a stone's throw from the plant. A hundred and fifty kilowatts of heat at about two hundred degrees Celsius, thrown away into the Martian sky every second of every day. If even a fraction of it could be captured, piped to a distillation vessel, and used to boil the brine, the still would cost almost nothing in electrical power. The reactor's designers had never intended its waste heat to be used. Its coolant loop was not designed to be tapped. Tapping it would mean modifying the reactor's thermal system, on Mars, by hand, with tools from the habitat's workshop, under the supervision of engineers twenty minutes away by radio, in a way that, if it went wrong, might force the reactor to shut down.
If the reactor shut down, the plant stopped, the habitat went cold, and the crew had other problems.
*
Ray Bradley was against it. He said so in the crew conference, in the habitat's galley, over a dinner of rehydrated chicken tikka that Ahmed had been looking forward to all day and could no longer taste. Bradley's position was that the crew should cut their losses: run the plant as best they could on its remaining membranes, depart on schedule with whatever propellant they had, and ask the return ship to descend to a lower orbit to meet them, burning some of its own reserve to make up the difference. The return ship's engineers had already run the numbers on this; it was possible, barely, with a margin that Bradley described, in his flat Oklahoma voice, as thin but real.
Karin Lund was for the still. Her position was that thin was a polite word for nonexistent, and that if the ascent engine underperformed by two per cent they would never reach any orbit at all, low or otherwise. The fourth crew member, the mission's physician, a quiet Nigerian named Chidi Okeke, said that he was not qualified to judge the engineering but would like to point out that a reactor shutdown in the Martian winter would kill them faster than a thin margin.
Ahmed listened to all of this and found that he agreed with everyone, which was a feeling he had come to associate with the moments just before the most important decisions of his life.
Houston, when consulted, was split along almost exactly the same lines. The reactor team in Idaho was horrified. The propulsion team in Huntsville was intrigued. The flight director, after a day of conferences, came back with a decision that was not quite a decision: the crew could attempt the modification, provided that it could be done without taking the reactor offline, and provided that it could be reversed in under an hour if anything went wrong, and provided that Ahmed personally, as the only chemist on Mars, signed off on every step.
*
It took him forty-one days.
Forty-one days of designing a heat exchanger out of spare coolant tubing and a water tank from the habitat's backup supply. Of working with Karin, in suits, on the plain, in the thin cold wind, tapping into a secondary loop of the reactor's radiator system that the designers in Idaho had grudgingly identified as the least dangerous place to do it. Of building a distillation vessel from the shell of a failed electrolyser stack. Of wrapping it all in insulation scavenged from the cargo landers. Of testing, and leaking, and fixing, and testing again. Of talking to Houston every day, in long letter-like messages across the twenty-minute gap, and arguing with the reactor team, who became, as the days passed, less horrified and more curious, and then, in the last two weeks, almost enthusiastic. Of not sleeping enough. Of eating too much of the chocolate that was supposed to last the mission. Of thinking, at three in the morning in his bunk, about his father in the back room in Jersey City with the glass still, watching the drops fall.
The still ran for the first time on day two hundred and sixty-three of the surface stay. It produced, in its first hour, eleven litres of water. Ahmed tested it. It had a total dissolved solids content of four parts per million, which was purer than anything that had come out of the plant since it was switched on, purer than the water in the habitat's drinking supply, purer, in all probability, than the bottled water his father had refused to trust.
He drank a little of it, in the processing module, from a sample cup. It tasted of nothing at all. He found, to his embarrassment, that he was crying, and that Karin, who was standing beside him in her suit with her helmet off, was pretending not to notice.
*
The still did not save them. Not entirely. That would make a better story and it would not be true.
What it did was raise the plant's water supply to about eighty per cent of design, which was more than it had ever been. The electrolysers, freed of the brine, recovered some but not all of their lost efficiency; the damage already done to their membranes could not be undone. The tanks filled faster. In the two hundred and thirty-seven days remaining before departure, the plant produced fourteen tonnes of propellant, four more than it would have without the still. On the day of departure, the ascent vehicle's tanks held thirty-five tonnes.
Thirty-five. Two tonnes above the bare minimum. A margin of six per cent instead of fifteen.
*
They argued about it, the night before. Not much. There was not much to argue about. The rules said fifteen per cent; they had six. The alternative was to wait twenty-six months, which they could not survive, or to ask the return ship to come lower, which the return ship's engineers said they could do, but which would cut into the return ship's own margin for the journey home in a way that nobody liked. The flight director in Houston, after a long night of conferences, approved departure with the six per cent margin and the return ship standing by at its nominal orbit, ready to descend if needed. Ray Bradley, who had argued against the still and who, Ahmed noticed, had never once since then said that he had been wrong, shook Ahmed's hand before they suited up, and held it a moment longer than was necessary, and said nothing.
The ascent engine performed to within half a per cent of its rated thrust. The guidance was nominal. The ascent vehicle reached the return ship's orbit with eight hundred kilograms of propellant in its tanks, a margin of a little over two per cent of what it had carried at lift-off. Docking was routine.
Ahmed, strapped into his seat, watching the propellant gauge on the display in front of him fall through the last tonnes during the ascent, found that he was thinking about the blue canisters of salt on the bottom shelf of the pharmacy, and about the girl with the umbrella on the side, and about the motto printed underneath her, which he had read every day of his childhood without ever understanding it until now. When it rains, it pours. He had always taken it for a description of the salt. It was a warning.
*
He went back to Jersey City, after the debriefings and the medical tests and the parades, and walked down Newark Avenue to where the pharmacy had been. It was still a phone shop. A young man behind the counter looked up when he came in and asked if he could help, and Ahmed said that he was just looking, and stood for a while in the place where the back room had been, where there was now a display of phone cases in a hundred colours, and thought about the still.
The plant on Mars is still running, on its distillation unit, making propellant for the next crew. The engineers in Idaho wrote a paper about the reactor waste-heat modification, and the next generation of Mars reactors is being designed with a heat tap built in. The purification trains on the next plants will use distillation as their first stage, not membranes, and the documents describing them refer to it, in the dry language of such documents, as the Siddiqui configuration. Ahmed has asked them to change the name. They have declined.
He has thought about what he would have said to his father, if his father had lived to hear about it. He thinks the old man would have listened to the whole story without interrupting, and then said, with immense satisfaction, that he had always told him not to trust the membranes.