The calculation everybody skips
Bioregenerative life support is usually argued for on closure. The argument that matters is cost per calorie delivered, and it goes the other way for staples.
A kilogram of wheat grain holds about 3,300 kcal. In NASA's Biomass Production Chamber at Kennedy Space Center, wheat under 510 to 930 micromoles of light per square metre per second made 23 to 40 grams of dry biomass per square metre a day and took 77 to 86 days to mature, and only part of that biomass is grain. NASA's own summary of that work is that at about 40 moles of light a day, roughly 50 square metres of crops feed one person, against 20 to 25 square metres to supply their oxygen. The bay is 22 square metres. Shipping the same kilogram costs one kilogram of landed mass.
The chamber figures make the point: the staples took two to five times the daily light of lettuce and months of growth, and then milling and cooking, before a calorie appeared. At current launch and landing costs, and at the power available, shipping wins for staples by a wide margin. It loses badly for anything with a short shelf life or a vitamin content that degrades, which is exactly what the bay is sized to grow.
| Crop | kcal per kg, as eaten | Dry yield in NASA's chamber | Ships well? | Decision |
|---|---|---|---|---|
| Wheat, ricestaple | 3,300 to 3,600 | 23 to 40 g/m²/d | Yes, indefinitely | Ship it |
| Soy, pulsesprotein | 3,500 to 4,500 | 10 to 16 g/m²/d | Yes | Ship it |
| Potatostaple | 770 | 22 to 33 g/m²/d | Yes, months | Ship it |
| Lettuce, leafy greensfresh | 150 to 170 | 6 to 8 g/m²/d | No, days | Grow it |
| Tomato, pepperfresh | 180 to 310 | 13 to 20 g/m²/d | No, weeks | Grow it |
| Herbsflavour | not counted | not tested | No | Grow it |
| Radish, microgreensfast cycle | 160 | not tested | No | Grow it |
What the bay actually delivers
Twenty two square metres over six tiers, on a staggered 28 day cycle so something is ready every few days rather than everything at once. Output is budgeted at 4.1 kg a week of fresh mass. By the United States Department of Agriculture (USDA) values for these crops that is about 120 kcal a day across four crew, about one percent of their food by mass and by calories, and a much larger share of their fresh vitamins.
Why the bay glows magenta
Chlorophyll absorbs most strongly in the deep red around 660 nm and the blue around 450 nm, and least in the green, which is why leaves look green. But a leaf still absorbs most of the green that falls on it, and McCree's measurements on 22 crop species put the quantum yield of green photons close to that of red once absorbed. The case for red and blue diodes is electrical, not botanical.
A 660 nm red diode can convert electricity to photons at up to 5.5 micromoles per joule, more than any other colour, and a white diode loses part of its energy in the phosphor that makes it white. Narrowband diodes at 660 and 450 nm therefore deliver the most photons per watt at wavelengths the plant uses well. The bay looks magenta because that is the sum of deep red and blue.
A little green is added, not for the plants but for the crew. NASA's Veggie unit on the station runs one green diode for every twelve red and three blue, about six percent of its light, at about 200 micromoles per square metre per second on a 16 hour day, and the Advanced Plant Habitat carries green and white banks alongside red and blue at up to 1,000 micromoles. Under pure red and blue the plants look black and grey and inspection for disease or stress is impossible. A few percent of green costs almost nothing and makes the bay somewhere people are willing to spend time.
The bay is also part of the water and air loops
What can go wrong in a closed greenhouse
A closed agricultural system has failure modes an open field does not, and the Yuegong 1 run at Beihang University, 370 days from May 2017 with two crews of four rotating through, and Biosphere 2 are the evidence base for most of them.
| Failure | Onset | Effect | Mitigation |
|---|---|---|---|
| Pathogen introductionany crew transfer | Days | Can take a whole tier | Tier isolation, seed sterilisation, no soil |
| Nutrient imbalancegradual drift | Weeks | Yield falls before it is visible | Continuous electrical conductivity (EC) and pH, weekly assay |
| Ethylene accumulationclosed volume | Weeks | Premature senescence across every crop | Catalytic oxidiser in the bay loop |
| Pollination failureno insects | Immediate | No fruit from tomato or pepper | Manual pollination, scheduled crew task |
| Root zone anoxiapump failure | Hours | Deep water culture is unforgiving | Dual pumps, dissolved oxygen alarm |
| Light degradationdiode ageing | Years | Slow yield loss | Output measured, not assumed. Diodes replaced on measurement. |
Biosphere 2's oxygen fell from 21 percent to 14 percent over the first 16 months of closure, and the CO₂ that should have risen to match did not. Severinghaus and Broecker traced it with carbon isotopes: microbes were respiring the organic rich soils, and the CO₂ they produced was being absorbed by the structure's curing concrete, which broke the carbon accounting. The lesson is not about concrete. It is that a closed system will find an unmodelled sink, and that the only defence is measuring every flow rather than inferring any of them.
Crew time, which is the real cost
The bay draws 14 kW and takes 1.5 crew hours a day. On a base where four people have twenty productive hours between them, 1.5 hours is seven and a half percent of the entire labour budget, which is a larger cost than the electricity.
It is spent anyway, and not only for the food. Analog and orbital crews consistently rate plant care among the most valued activities available to them, and the review by Odeh and Guy collects the evidence on mood and cognition in long isolation. The bay is carried as a habitability measure that happens to also produce lettuce.
What is not yet known
The bay's contribution to oxygen and water is modelled, not measured; increment four instruments every flow so the next base's greenhouse sizing is defensible. The 4.1 kg a week is a budget, not a yield: NASA's chamber grew 6 to 8 grams of dry lettuce per square metre a day at 17 moles of light, and 22 square metres at that rate would give several times the budgeted fresh mass, so the crop mix and the crew time, not the area, set the figure. The bay's light level and photoperiod have not been fixed against the flight record, which is Veggie at about 200 micromoles for 16 hours and the Advanced Plant Habitat at up to 1,000. The 14 kW lighting load is an allocation. Whatever the mix, the calorie share stays near one percent, which is the point of section 01. A closed system will find an unmodelled sink, as Biosphere 2 did, and the only defence is measuring every flow rather than inferring any of them.
Increment plan
Increment 1, no bay
All food is shipped. The volume is landed but fitted out as stowage, because a greenhouse with nobody to tend it is dead mass.
Increment 2, two tiers
Fast cycle crops only, radish and microgreens, proving the nutrient loop and the pathogen protocol before anything slower is planted.
Increment 3, six tiers at full yield
The staggered 28 day cycle running properly, and the condensate recovery plumbed into the water loop.
Increment 4, closure measurement
Every flow instrumented, so the contribution to oxygen and water is measured rather than modelled. This is what makes the next base's greenhouse sizing defensible.
Sources and further reading
Every figure in this document traces to one of these. Agency documents and peer reviewed work first, reporting only where it is the primary record.