Watney Was Lucky

There is a movie out there called The Martian, and if you have not seen it, it is worth the evening. Matt Damon plays a botanist named Mark Watney, part of a six-person crew on the surface of Mars. A dust storm comes up, the mission gets scrubbed, and on the way to the ship Watney is hit by debris and knocked out of sight. His crew reads his suit as dead and lifts off without him. He wakes up alone on a planet with no one else on it, no way to call home, and a habitat stocked with food for a mission that was supposed to be over.

What he works out, sitting there doing arithmetic, is that rescue cannot possibly arrive before the pantry runs out. So he has to make the difference himself. He builds a garden inside the habitat, cuts up the mission’s Thanksgiving potatoes for seed stock, mixes his own waste into the Martian dirt to fertilize it, and burns rocket fuel to make water. It works. The potatoes come up, and they buy him the months he needs. It is a good scene, and it is the picture in almost everybody’s head when they think about growing food off Earth. It is also the best short course in this problem anybody has written, because almost everything in it is right, and the parts that are wrong are wrong in exactly the ways that kill you slowly.

So let me take it apart, respectfully, and use it to build the crop list I actually want.

The potato was the right call

Start with what the book got right, because it got a lot right. The potato is close to the best single crop a person in trouble can plant. It comes up in seventy to a hundred and twenty days, faster than the cereals.¹ It yields hard: a medium russet is about a hundred and sixty-eight calories and thirty-eight grams of carbohydrate, and it carries more protein than people expect, along with vitamin C, magnesium, and niacin.² It plants from a piece of itself, which means you do not need a seed supplier or a pollinator to get the next crop. Half of Europe ran on it for a reason.

If you told me I could carry exactly one crop and my problem was the next eighteen months, I would carry the potato too. Watney’s problem was the next eighteen months. Ours is not.

What the potato cannot pay

Here is the part the movie does not dwell on. A potato-only diet does not have fat in it, and fat is not a luxury item. Your body needs dietary fat to absorb vitamins A, D, E, and K, to build cell membranes, and to make hormones, and there are essential fatty acids it cannot manufacture on its own. Without any, those processes start coming apart within weeks.³ The protein is a near miss rather than a hit: potato protein is decent quality but short on lysine, methionine, cysteine, and tyrosine, and at roughly two grams per hundred, you would have to eat two and a half kilos a day to reach about fifty-two grams, which is borderline for a small adult and, because of the amino acid gaps, not fully usable. Over months you would expect muscle wasting.⁴ The potato is also low in zinc, selenium, and iodine.⁵

This is the same failure I wrote about with the rabbit, arriving from the other direction. The rabbit gives you protein with no fat. The potato gives you calories with no fat. A man can eat his fill of either one and come apart anyway, and the crew would not call it starving, because nobody would be hungry. They would just be getting weaker, and the log would say the harvest was good.

Weir knew this, which is why Watney had vitamin pills and protein rations from the mission stores.² That is the honest reading of the story: the potatoes did not feed him. They stretched a pantry that came from Earth. Take the pantry away and the arithmetic stops working.

So the first rule of the crop list is the one the potato cannot satisfy alone: something on the list has to make oil. That means peanut, soybean, sunflower, rapeseed — and it is no accident that the standing NASA candidate list for closed life support has peanut and soybean sitting right next to the potato, wheat, and rice.⁶ Those two are on there to pay the fat bill.

The waste was right too, and dangerous

Watney mixing human waste into the regolith is the real plan. On a closed habitat there is no other plan — every atom of nitrogen and phosphorus you eat has to come back around or the loop is not closed, and human excreta is where most of it goes. That is not science fiction; it is what the agencies working on this actually intend, and there are operations on Earth doing it at scale today, composting tens of thousands of gallons of human waste a month into agricultural-grade compost for spinach, peppers, sorghum, and maize.⁷

Note the word: composting. Raw excreta carries human pathogens, and what makes compost safe is the heat and microbial competition of the composting process itself, which is the step that inactivates organisms like E. coli O157:H7, Salmonella, and Listeria before anything goes on a food crop.⁸ Watney spread it raw, in a sealed tent, and ate the crop. On Earth that is how you get an outbreak in a lettuce field. In a habitat with one air volume, six people, and no hospital, it is how you get all six at once.

That is not a reason to reject the idea. It is a reason to put a composting or digestion stage between the toilet and the garden and to treat it as a pressure vessel, not a bucket — a piece of process equipment with a temperature it has to hit and hold, and a way to prove it hit it.

And the dirt was not dirt

The last thing to take from the tent is the ground itself. Martian regolith is not soil. On Earth we use the word soil to mean something with organic content in it; planetary scientists use regolith for the loose mineral blanket, and that blanket is essentially devoid of soil organic matter, which is precisely what makes growing food in it so hard.⁹ The simulants used in the labs lack organic matter and the macronutrients nitrogen, phosphorus, and sulfur, and they bring their own problems along: alkaline pH, high sodium, poor cohesion.¹⁰ A systematic review across two dozen studies is blunt about it: both lunar and Martian regolith are consistently poor substrates for growing plants.¹¹

Then there is the perchlorate. It is globally enriched in Martian regolith at levels commonly toxic to plants, and in simulant work its presence prevented germination across every treatment tested.¹² Nothing came up at all. Wash it, and you have a wash-water problem and a perchlorate-disposal problem in a place with no downstream.

The encouraging part is that regolith can be turned into something. Researchers grew alfalfa in bare basaltic simulant, then used that alfalfa to treat the same simulant, and turnip, radish, and lettuce grew significantly better in the treated batch than they did in the raw one.¹³ That is soil being made, one generation at a time, by a plant. It is the same lesson as the worm bed and the seed: the living part is the machine. Regolith plus waste is not soil. Regolith plus waste plus time plus something alive is on its way to being soil, and the first crop in that sequence is not food. It is a green manure whose whole job is to die into the ground.

The rule: everything pays rent twice

Which brings me to the standard I would hold the whole list to, and it is the one I used on the animals. Every crop has to pay in more than one loop, because there is no room for a passenger.

The loops a plant can pay into are these: calories, protein, fat, vitamins and minerals, oxygen and carbon dioxide scrubbing, water transpired and recovered clean, feed value in the half of the plant people do not eat, biomass back to the soil, and morale, which is not a soft item when the same six people eat the same thing for nine years. A crop that pays into one of those is a luxury. A crop that pays into four is infrastructure.

Some of it scores itself. Potato and sweet potato: calories, bulk, fast. Soybean and peanut: fat, protein, and both are legumes, so they are working the nitrogen side of the ledger while they do it. Wheat and rice: calories and storability, but they bring a processing bill with them — threshing, milling, extruding — which is equipment, mass, and somebody’s afternoon, and the agency people say plainly that the staple crops are a settlement-phase problem for exactly that reason.¹⁴ Leafy greens: vitamins, fast turnaround, almost no calories, and a disproportionate share of the morale. Alfalfa or clover: no human food value at all and possibly the most important thing on the list, because it makes the ground.

What it costs in floor

Now the number that decides the shape of the hull. About twenty to twenty-five square meters of crops will supply the oxygen for one person.¹⁵ Feeding that same person takes roughly forty to fifty square meters or more, and the optimization studies that chase the minimum land back into the same range — one of them lands near fifty-seven square meters per person once you force enough variety into the diet to keep it from being a single-crop trap.¹⁶

Sit with that for a second, because it is the whole argument in one line: the air is the easy half. Growing enough plants to breathe is a third of the job. The other two thirds of that floor is there so you do not slowly fall apart while breathing comfortably.

And that number assumes hybrids and optimal everything. I argued last piece for heirloom seed, which yields less. I will take that trade and pay for it in floor space, and I would rather add ten square meters of growing area to the drawing than put the habitat’s food supply on a line nobody aboard can reproduce.

The page nobody has written

So here is where this ends up, and it is not a conclusion so much as an admission. I can tell you the rule. I can tell you the loops. I can tell you which crops obviously belong and roughly how much floor it takes. What I cannot show you — what I do not believe exists anywhere — is the finished table: every candidate crop down the side, every loop across the top, scored, totaled, and argued over by people who grow things for a living, with the seed-saving burden and the processing equipment counted honestly against each one.

That table is the most important page in the manifest and nobody has written it. It goes on the punch list, and if you are the person who can fill in a column of it, you are exactly who we are looking for.

Watney got rescued. That was the plot. Nobody is coming for the barrel, and the crop list is what stands in for the rescue.

Ideas in Star Bound are published open — take them, build them better. Our open proposal.

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A note on where this comes from. I am not a metallurgist, a spacecraft engineer, or a degreed engineer of any kind. But I am a practical one, with more than forty years in heavy industrial construction: welding, millwright work, hydraulics, mining, and thirty-three years of electrical construction and commissioning on the North Slope of Alaska, in a place that punishes anything built wrong. Much of that time I have worked as a field engineer, which means I am the one standing in front of the equipment when the drawing turns out to be wrong, and the fix has to be made on the spot, with what is on hand, and has to work the first time. Anyone who has done that long enough learns that a lot of engineering gets corrected in the field by people with more experience than the people who drew it. That is the eye I bring to these subjects: not what should work on paper, but what actually gets built, and what breaks. The facts I take from other people are footnoted so you can check them and read further. The ideas built on top of those facts are my own, and I would rather see them knocked down by somebody who knows better than left standing because nobody looked.

Sources

1. Fang, “Potato’s Role in Sustainable Agriculture and Agroecology,” Molecular Soil Biology — potato matures in roughly 70 to 120 days, faster than many cereal crops — https://bioscipublisher.com/index.php/msb/article/html/4078/ ↩
2. The Prepared, “Matt Damon grew potatoes on Mars using poop. Is that realistic?” — about 168 calories and 38 g carbohydrate in a medium russet; vitamin C, magnesium, niacin; a potato-only diet is not enough protein; Watney supplemented with vitamin pills and protein rations — https://theprepared.com/blog/matt-damon-grew-potatoes-on-mars-using-poop-is-that-realistic/ ↩
3. ScienceInsights, “What Happens If You Only Eat Potatoes: The Risks” — dietary fat is required to absorb vitamins A, D, E, and K, to build cell membranes, and to produce hormones; without it these processes break down within weeks; essential fatty acids cannot be manufactured by the body — https://scienceinsights.org/what-happens-if-you-only-eat-potatoes-the-risks/ ↩
4. ScienceInsights, as above — potato protein limited in lysine, methionine, cysteine, and tyrosine; 2.1 g protein per 100 g; 2.5 kg daily yields about 52 g, borderline and incompletely usable; muscle wasting expected over months ↩
5. The Prepared, as above — potatoes are also low in zinc, chloride, selenium, and iodine ↩
6. Cavazzoni, “Top-Level Crop Models for Advanced Life Support Analysis” — the Advanced Life Support candidate crop set: dry bean, lettuce, peanut, white potato, rice, soybean, sweet potato, tomato, and wheat — https://www.researchgate.net/publication/23705293_Top-Level_Crop_Models_for_Advanced_Life_Support_Analysis ↩
7. “Reuse of human excreta,” Wikipedia — SOIL in Haiti transforms over 20,000 U.S. gallons of human excreta per month into agricultural-grade compost, used on spinach, peppers, sorghum, and maize — https://en.wikipedia.org/wiki/Reuse_of_human_excreta ↩
8. “Composting To Inactivate Foodborne Pathogens for Crop Soil Application: A Review,” Journal of Food Protection — waste feedstocks can contain human pathogens, inactivated by the heat and microbial competition of composting; outbreaks of E. coli O157:H7, Salmonella, and Listeria on raw-consumed produce are the reason it matters — https://www.sciencedirect.com/science/article/pii/S0362028X22088196 ↩
9. NASA Science, “EFRI ELiS: Bioweathering Dynamics and Ecophysiology of Microbially Catalyzed Soil Genesis of Martian Regolith” — Mars regolith is devoid of soil organic matter, making plant cultivation for life support extremely challenging — https://science.nasa.gov/biological-physical/investigations/efri-elis-kpmsg/; on the soil/regolith distinction see also “Martian regolith,” Wikipedia — https://en.wikipedia.org/wiki/Martian_regolith ↩
10. “The Potential for Lunar and Martian Regolith Simulants to Sustain Plant Growth: A Multidisciplinary Overview,” Frontiers in Astronomy and Space Sciences (2021) — simulants lack organic matter and the macronutrients N, P, and S, and exhibit alkaline pH, high sodium availability, and low cohesion — https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2021.747821/full ↩
11. “A systematic review of plant cultivation and growth in extraterrestrial regolith simulants,” Life Sciences in Space Research — across 24 studies, both Martian and lunar regolith are consistently poor substrates due to low nutrient availability, lack of organic matter, and difficult physicochemical properties — https://www.sciencedirect.com/science/article/abs/pii/S2214552426000593 ↩
12. “Perchlorate and Agriculture on Mars,” Soil Systems 5(3) — perchlorate is globally enriched in Martian regolith at levels commonly toxic to plants, and prevented germination across all plant treatments in simulant — https://www.mdpi.com/2571-8789/5/3/37 ↩
13. Kasiviswanathan et al., “Farming on Mars: Treatment of basaltic regolith soil and briny water simulants sustains plant growth,” PLOS ONE (2022) — alfalfa grown in bare simulant augmented its nutrients, significantly increasing growth and biomass of turnip, radish, and lettuce in the treated soil — https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0272209 ↩
14. NASA, “Space Crop Considerations for Human Exploration” (July 2024) — staple crops such as wheat, rice, and soybean require considerably more processing equipment (threshing, milling, extruding) and are treated as a settlement-phase capability — https://ntrs.nasa.gov/api/citations/20250001897/downloads/20231215%20Space%20Crops%20HRP%20FINAL%20STRIVES%203.pdf ↩
15. Wheeler, “Bioregenerative Life Support for Humans in Space,” NASA NTRS — about 20–25 m² of crops could provide the oxygen needs of one person — https://ntrs.nasa.gov/citations/20205008786 ↩
16. “Supplemental Food Production With Plants: A Review of NASA Research,” Frontiers in Astronomy and Space Sciences (2021) — estimates of about 40 to 50 m² or more of crop growing area per person; and “Optimized crop growth area composition for long duration spaceflight,” Life Sciences in Space Research — a minimum of 57.04 m² once per-crop area limits are imposed to keep dietary variety — https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2021.734343/full; https://pubmed.ncbi.nlm.nih.gov/34281665/ ↩