What the feedstock actually is
In situ resource utilisation (ISRU) means making what the base needs from what is already under it. Apollo, Luna and Chang'e have returned samples from eleven sites, the last from the far side in June 2024, and the composition is well characterised. Mare regolith is basaltic, highland regolith is anorthositic, and both are dominated by oxides. There is no processing step needed to concentrate the resource, because the resource is the bulk material.
| Oxide | Mare, by mass | Highland, by mass | Oxygen fraction | Note |
|---|---|---|---|---|
| SiO₂silica | 45.4% | 45.0% | 53% | The bulk of the melt |
| Al₂O₃alumina | 14.9% | 24.6% | 47% | Higher in the highlands |
| FeOiron oxide | 14.1% | 5.7% | 22% | The alloy comes mostly from here |
| CaOlime | 11.8% | 15.8% | 29% | |
| MgOmagnesia | 9.2% | 7.5% | 40% | |
| TiO₂titania | 3.9% | 0.6% | 40% | Mare only, and useful |
| Oxygen, total | 44.6% | 44.3% | Call it 43 percent after volatiles |
Polar water ice gets the attention. The Lunar Crater Observation and Sensing Satellite (LCROSS) measured 5.6 plus or minus 2.9 percent water by mass in the ejecta it threw up from Cabeus, the shadowed floors run from about 25 K at the coldest to below 110 K, and the first attempt to drill for ice on the surface, PRIME 1 (the Polar Resources Ice Mining Experiment) on IM 2 in March 2025, ran its TRIDENT drill (The Regolith and Ice Drill for Exploring New Terrain) through its full range of motion but never reached the ground because the lander was on its side. The grade at any particular spot is unknown. Oxygen in regolith is everywhere, at a known concentration, in material a rover can scoop anywhere on the site. For an outpost that needs oxygen every single day, availability beats grade.
Molten regolith electrolysis
Regolith is heated above 1,600 °C until it is a conductive melt, and a current is passed through it. Oxygen evolves at the anode as a gas and metal collects at the cathode. There is no reagent, no consumable, and nothing to ship: the only inputs are soil and electricity.
The competing process, hydrogen reduction of ilmenite, runs at a gentler 900 °C but only attacks the iron titanium oxide fraction, which is a few percent of the soil, and less in the highlands. A 2023 system study that sized whole plants for the three leading processes found hydrogen reduction competitive only where the feed holds 7.5 percent ilmenite or more, which the anorthositic rim does not. It needs hydrogen shipped from Earth and it wastes most of the material it handles. Molten regolith electrolysis (MRE) takes everything.
The third route, carbothermal reduction, heats a spot of regolith with methane present and pulls the oxygen out as carbon monoxide, then as water. NASA's Carbothermal Reduction Demonstration ran a reactor built by Sierra Space in the Dirty Thermal Vacuum Chamber at Johnson in April 2023 and extracted oxygen from simulant, the first time that had been done in vacuum, and Sierra Space ran a standalone automated version in the same chamber in 2024. It is the more mature process today. MRE is chosen here because it needs no carbon loop, because its metal output feeds construction, and because Blue Origin's Blue Alchemist reported silicon, metals and oxygen from simulant by MRE in February 2023, passed its critical design review under a 35 million dollar NASA Tipping Point award in September 2025, and is due to demonstrate autonomous operation in a simulated lunar environment in 2026.
Excavate and screen
A rover scrapes the top 100 mm at the excavation face and screens out anything above 5 mm. The plant takes about 14 kg a day, so one shift a week at a modest 14 kg an hour keeps the hopper ahead of it.
Feed and preheat
Screened fines go to a hopper and down a screw conveyor. Waste heat from the cell walls preheats the charge, which is worth about 18 percent of the total energy.
Melt and hold
The cell holds above 1,600 °C. Getting there is the expensive part; holding is cheap, which is why the plant runs continuously rather than in batches.
Electrolyse
Direct current through the melt. Oxygen at the anode, molten metal at the cathode. The anode material is the hard engineering problem: it has to survive oxygen at 1,600 degrees.
Tap and cast
Oxygen is cooled, dried and compressed to storage. The metal is tapped and cast into ingots, or poured directly into paver moulds for the berms.
Return the slag
What is left is a glassy slag that goes back out as fill or aggregate. Nothing is stockpiled as waste, because on a site with no waste disposal there is no such thing as waste.
The energy account
What comes out besides oxygen
| Output | Rate | Composition | Where it goes |
|---|---|---|---|
| Oxygenthe product | 1.80 kg/day | 99.2% O₂ after drying | Life support makeup, and extravehicular activity (EVA) recharge |
| Iron silicon alloycathode metal | 2.40 kg/day | Fe with Si, Al, Ti | Printer feedstock, brackets, repair stock |
| Glassy slagresidue | 9.20 kg/day | Ca and Mg silicates | Cast into pavers for the berms |
| Fines below 20 µmscreened out | 0.60 kg/day | as excavated | Returned to the face, never stockpiled |
| Feed | 14.0 kg/day | One rover shift covers a week |
The alloy is the underrated output. At 2.4 kg a day it is about 880 kg a year of structural metal that did not have to be launched, and it arrives in exactly the place the construction gantry needs it.
The plant
The plant is rated at about 1,000 kg of oxygen a year at 14 kW, the figure Schreiner's model gives for a 400 kg reactor, and budgeted at 1.8 kg a day, about two thirds of that, which covers the 1.65 kg a day life support gap with margin for anode rebuilds. Three cells rather than one, for the same reason everything else here is plural: a cell with a failed anode can be taken offline and rebuilt while the other two carry the load at reduced rate. Anode life is the least certain number in this whole document.
Molten regolith electrolysis has been demonstrated at laboratory scale with simulant, most publicly by Blue Origin's Blue Alchemist. It has never run on the Moon, never run for years, and the anode problem is not solved. This is the highest technical risk item in the architecture, and the reason increment two carries a full oxygen store as backup rather than relying on the plant from day one.
Failure modes
| Failure | Effect | Buffer | Response |
|---|---|---|---|
| Anode erosionexpected, gradual | Cell efficiency falls | Two other cells | Scheduled rebuild. Consumable anodes are carried as spares. |
| Feed blockagefines bridging in the hopper | Cell starves | Hours | Vibrator on the hopper wall, then manual clearing on EVA. |
| Melt freezepower loss at the cell | Cell is scrap | Minutes | Cells hold on the essential bus. A frozen cell is drilled out, which is a multi day job. |
| Oxygen line contaminationcarbon carryover | Product off spec | 96 h O₂ store | Inline analyser diverts to vent until it is back in spec. |
| Excavator downthe rover | No feed | 6 days of screened stock | Screened fines are kept ahead, deliberately, for exactly this. |
What is not yet known
Molten regolith electrolysis has run only on simulant, so anode life, the behaviour of the rim's highland feedstock in the melt, and the split of the energy account between electrolysis and wall loss are all estimates. The 14 kW and the 1,000 kg a year come from a parametric model, not a built machine, and the yield of 1.8 kg of oxygen from 14 kg of feed has not been shown on real regolith. Anode life is the least certain number in this whole document, and it is the reason increment two carries a full oxygen store as backup. Carbothermal reduction is further along, with vacuum tests in 2023 and 2024, and is the fallback if the anode problem does not close. Whether the ice next door is worth mining is unknown until a drill reaches it; PRIME 1 did not.
Increment plan
Increment 1, no plant
Oxygen is shipped and stored. The site is surveyed and the excavation face is chosen on composition, not convenience.
Increment 2, one cell
A single cell at partial duty, proving the process and the anode life on the real feedstock rather than on simulant. Oxygen store still covers the crew.
Increment 3, three cells
Full 1.8 kg a day, which closes the life support gap entirely. This is the point the base stops importing oxygen.
Increment 4, metal into construction
The alloy tap is plumbed to the printer feedstock hopper, and the slag goes into paver moulds instead of back to the face.
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.