What four people need
The crew figures below are the planning values NASA life support studies use for the International Space Station (ISS): 0.84 kg of oxygen a day per crew member and about 1 kg of carbon dioxide. NASA's Baseline Values and Assumptions Document, revision 2, gives 0.895 and 1.085 kg for an 82 kg crew member, about 7 percent more, and the in situ resource utilisation (ISRU) margin covers that difference. The water figures are anchored to the station's measured recovery. The cabin runs at NASA's exploration atmosphere, 56.5 kPa (8.2 psia) at 34 percent oxygen, chosen so that prebreathe before an extravehicular activity (EVA) is short.
| Consumable | Per person per day | Four crew | Recovered | Residual |
|---|---|---|---|---|
| Oxygenmetabolic | 0.84 kg | 3.36 kg | 1.71 kg | 1.65 kg |
| Potable waterdrink and food prep | 2.0 kg | 8.00 kg | 7.84 kg | 0.16 kg |
| Hygiene waterwash and flush | 1.6 kg | 6.40 kg | 6.27 kg | 0.13 kg |
| Food, dry massshipped and grown | 0.68 kg | 2.72 kg | 0.03 kg | 2.69 kg |
| Nitrogenleak makeup | 0.04 kg | 0.16 kg | 0.00 kg | 0.16 kg |
| Total | 5.16 kg | 20.64 kg | 15.85 kg | 4.79 kg |
Of the 4.79 kg residual, 1.65 kg of oxygen comes from the ISRU plant and 3.14 kg a day comes from Earth. The oxygen recovered is 1.68 kg from Sabatier and 0.03 kg from the greenhouse; the food recovered is the greenhouse's dry matter. Carbon dioxide production runs at about 1.05 kg per person per day, so 4.2 kg for the crew. That is the input to the whole air revitalisation chain and the number the sieve beds are sized against.
The air loop, step by step
Why the oxygen loop stops at about half
This is the single most misunderstood number in closed loop life support, so it is worth being precise about.
Sabatier runs CO₂ + 4H₂ → CH₄ + 2H₂O. The two water molecules are electrolysed back to 2H₂ + O₂. Every molecule of oxygen the electrolyser makes for the crew comes with two molecules of hydrogen. Reducing one carbon dioxide takes four, so the hydrogen on hand reaches only half the carbon dioxide the crew produce, and the methane carries two of every four away for good. The loop therefore recovers about half of the oxygen, which is where the station's system stands today. NASA states the exploration requirement as a minimum of 75 percent.
There is no reactor design that fixes this. It is the stoichiometry of the reaction. The options are to supply hydrogen from outside the loop, to crack the methane and recover its hydrogen, or to make oxygen from something other than cabin air.
| Option | Closure | Cost | Verdict |
|---|---|---|---|
| Sabatier aloneflight proven, ISS | about 50% | Baseline | The gap has to be filled somehow |
| Ship hydrogenfrom Earth | about 90% | 0.34 kg/day landed | Cheap in mass, but a permanent dependency. The crew make less carbon dioxide than the oxygen they take, so it never reaches 100 |
| Methane pyrolysiscrack CH4 to carbon and H2 | 75%, NASA's minimum | Plasma pyrolysis assembly and hydrogen separation | The path NASA is on, not yet flown |
| ISRU oxygenin situ resource utilisation, from regolith | 100% independent | 1 t plant, 14 kW | The architecture chosen here |
Shipping hydrogen closes most of the loop for about 125 kg a year, which is trivial: reducing all 4.2 kg of carbon dioxide a day takes 0.76 kg of hydrogen, and electrolysis for the crew's oxygen gives back 0.42. The reason this base does not do that is that it is a construction testbed: the point is to demonstrate that the outpost can run on what is under it. A base that ships hydrogen is a very efficient camp. A base that makes its own oxygen is the beginning of an industry.
Water recovery
Water is the success story. The International Space Station reached 98 percent recovery in 2023 with the brine processor assembly, installed in March 2021 as a technology demonstration; before it the station recovered 93 to 94 percent. That is the figure this design assumes. It is achieved in two stages: a distillation assembly that handles urine at up to 87 percent recovery, and a processor that recovers what the still leaves behind as brine.
Contaminants, and the thing that actually goes wrong
Closed volumes accumulate trace contaminants: ammonia from the crew, siloxanes from materials outgassing, volatile organics from the galley and the greenhouse, and formaldehyde from adhesives. On the station these are removed by a trace contaminant control system that runs continuously and is one of the most maintenance hungry items aboard.
The lunar case is worse in one respect and better in another. Worse, because the greenhouse adds a biological source of volatiles that a station does not have. Better, because the cabin can be partially vented and repressurised from ISRU oxygen without a resupply penalty, which is an option no orbital station has.
Dual string, and what that actually means
Every function in this chain exists twice, but not identically. Identical redundancy protects against random failure and does nothing at all against a design fault or a consumable that runs out, because both strings fail the same way at the same time.
| Function | Primary | Backup | Backup is different how |
|---|---|---|---|
| CO₂ removalcontinuous | 4 bed molecular sieve | Lithium hydroxide canisters | Consumable, not regenerative. Immune to a sieve design fault. |
| Oxygen supplycontinuous | Sabatier and electrolysis | High pressure O₂ storage | Stored gas, 96 hours at full crew |
| Water recoverycontinuous | Distillation and brine | 340 L reservoir | Buffer, not a process |
| Atmosphere pressurecontinuous | N₂/O₂ makeup from store | Partial vent and repressurise | Uses ISRU oxygen, no resupply |
| Trace contaminantscontinuous | Charcoal and catalytic oxidiser | Vent and repressurise | Crude, effective, always available |
Every consumable buffer on this base is sized to at least 96 hours, because that is the time to get a crew off the surface and home if something cannot be fixed. Ninety six hours is not a comfort margin, it is the length of the rescue.
Mass and power
| Element | Mass | Power | Note |
|---|---|---|---|
| Four bed molecular sievedual string | 412 kg | 1.24 kW | Regenerated under vacuum, which is free here |
| Sabatier reactorsingle, with spare core | 118 kg | 0.42 kW | Runs hot, contributes to the thermal load |
| Oxygen generation, electrolysisdual | 296 kg | 1.86 kW | The largest single electrical load in the rack |
| Water processor and distillerdual | 484 kg | 0.92 kW | Includes the brine processor |
| Trace contaminant controldual | 96 kg | 0.18 kW | |
| Storage, tanks and plumbing | 530 kg | none | 340 L water, 96 h oxygen, nitrogen |
| Lithium hydroxide, 96 hconsumable | 78 kg | none | Replaced each resupply |
| Total | 2,014 kg | 4.62 kW | $2.0 bn landed |
What is not yet known
Methane pyrolysis, which NASA needs to reach its 75 percent minimum, is not yet flight proven and is carried as a demonstration rather than a dependency. The crew figures are the station planning values; NASA's baseline document gives 0.895 kg of oxygen and 1.085 kg of carbon dioxide per crew member per day for an 82 kg crew member, about 7 percent more, and the table should be rerun at those values even though the ISRU margin covers them. The contaminant list for a habitat with a greenhouse has not been drawn up; JSC 20584 sets the limits but says nothing about which plant volatiles will appear or how often the charcoal beds will need changing. The oxygen gap is closed by the ISRU plant, whose maturity is the open question of the ISRU document.
Increment plan
Increment 1, open loop with buffers
Sieve, oxygen store and a water processor, with consumables sized for the first rotation. Closure is around 60 percent and the resupply bill is high, which is acceptable for a first crew.
Increment 2, full closure and ISRU tie in
Sabatier, the brine processor and the ISRU oxygen feed. This is the step that takes water to 98 percent and closes the oxygen gap without shipping hydrogen.
Increment 3, greenhouse integration
The bay comes online as a real contributor rather than a science experiment: transpiration recovery into the water loop and a measurable oxygen contribution.
Increment 4, methane pyrolysis trial
Cracking the Sabatier methane to recover its hydrogen would take the loop toward NASA's 75 percent minimum. It is carried as a technology demonstration rather than a dependency.
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.