Life support loop for a four-person surface habitat

Question

What must four people import each year to breathe, drink and eat?

Assumptions

• Crew4 people, 365 days on the surface (our assumption) → 1,460 crew-days
• Oxygen0.895 kg per person per day, long-mission nominal, range 0.61–1.08 (BVAD Rev 2, Table 3-31)
• Water4.48 kg per person per day: drinking 2.00, food rehydration 0.50, urinal flush 0.50, hygiene 1.98 (BVAD Rev 2, Table 4-20, Early Planetary Base)
• Food2.39 kg per person per day including packaging (BVAD Rev 2)
• CO₂ produced1.085 kg per person per day (BVAD Rev 2, Table 3-31)
• Water recovery98%, as demonstrated on the ISS in 2023
• MOXIE best rate12 g/h at ≥98% purity; 16 runs, 122 g total

Why 0.895 and not 0.82: BVAD 2022 uses 0.895 kg of oxygen per person per day, up from 0.82 in the 2015 and 2018 editions (0.816 nominal), because it assumes more daily exercise. Older studies citing about 0.82 used the pre-2022 baseline.

The numbers

Yearly need  M = r × N × D   (N × D = 4 × 365 = 1,460 crew-days)

Crew O₂:  0.895 kg × 4 = 3.58 kg O₂/day
          0.895 × 1,460 ≈ 1,307 kg O₂/yr  (BVAD band 0.61–1.08 → 891–1,577 kg)

Open loop, nothing recycled:
  O₂     0.895 × 1,460  ≈  1,307 kg
  Water  4.48  × 1,460  ≈  6,541 kg
  Food   2.39  × 1,460  ≈  3,489 kg
  Total                 ≈   11.3 t/yr

With the loop:
  Water makeup  6,541 × (1 − 0.98)  ≈    131 kg
  O₂            made locally        ≈      0 kg imported
  Food          unchanged           ≈  3,489 kg
  Total                             ≈    3.6 t/yr

Local O₂ rate:  1,307 kg / 8,760 h ≈ 149 g/h continuous ≈ 12.4 × MOXIE’s best hour
CO₂ to remove:  1.085 × 4 ≈ 4.34 kg/day

Sabatier closure (stoichiometry):
  CO₂ + 4 H₂ → CH₄ + 2 H₂O ;  2 H₂O → 2 H₂ + O₂
  0.895 kg O₂ = 28.0 mol → 56 mol H₂ → 14.0 mol CO₂ reduced → 14.0 mol O₂ back
  recovered fraction = 14.0 / 28.0 = 50 %  (H₂ leaves in the vented CH₄)

SOXE energy floor:  CO₂ → CO + ½ O₂, ΔH° ≈ +283 kJ/mol → ≈ 4.9 kWh per kg O₂
  0.149 kg/h × 4.9 kWh/kg ≈ 0.73 kW continuous (excludes compressor, heaters, losses)

Result

Modeled: open-loop imports are about 11.3 t a year for four people (3.58 kg of O₂ a day). Recycling 98% of the water and making all the oxygen locally cuts that to about 3.6 t, and about 96% of what is left is food.

Trade-offs

Open problems

• No solid-oxide unit has run continuously on Mars for a year.
• At MOXIE’s roughly 10% electrical-to-oxygen efficiency the 0.73 kW floor becomes about 7 kW; a full-scale unit is expected to do far better. Treat 1–7 kW as the band until a scaled unit runs.
• The 98% recovery figure assumes ISS-style spares; spares mass is not counted. Airlock gas loss per EVA is not in this model.
• Food is about 3.5 t a year. Crop yield per square metre and per kilowatt under Martian conditions is not established.

What would change the answer

• Size oxygen production to also cover airlock losses and leakage (BVAD nominal 0.02 kg/day per module), then use the excess to buffer outages.

Sources

1. Ewert, Chen, Powell, Life Support Baseline Values and Assumptions Document (BVAD), NASA/TP-2015-218570/REV2 (2022)
2. Anderson, Ewert, Keener, Wagner, BVAD, NASA/TP-2015-218570 (2015)
3. BVAD, NASA/TP-2015-218570/REV1 (2018)
4. NASA, “NASA Achieves Water Recovery Milestone on International Space Station” (2023)
5. NASA, “NASA’s Oxygen-Generating Experiment MOXIE Completes Mars Mission” (2023)
6. NASA, Increased Oxygen Recovery from Sabatier Systems (NTRS 20150016511)
7. Hoffman et al., “Mars Oxygen ISRU Experiment (MOXIE): Preparing for human Mars exploration,” Science Advances (2022)
8. NIST Chemistry WebBook, carbon dioxide thermochemistry (ΔHf°)