Life support

Environmental control and life support is the subsystem where the design driver is a number rather than a technology: how much mass per person per day the mission must carry, and how much of it can be recovered instead.

The requirement

Per crew member per day, at a moderate activity level:

Counting only oxygen, potable water and food, the minimum is about 6.5 kg per person per day. Hygiene water, if supplied at terrestrial expectations, multiplies that severalfold, which is why spaceflight hygiene practices are what they are.

The consequences scale brutally with mission duration:

The Mars figure is the argument for closed-loop life support stated in one number. Twenty-six tonnes of consumables is comparable to the entire payload capacity of a heavy launch vehicle, spent on nothing but keeping four people alive. Recycling is not an efficiency measure at that scale, it is the difference between a feasible architecture and an impossible one.

Air

Carbon dioxide removal is the most urgent function, because CO₂ becomes dangerous long before oxygen runs out. Partial pressure must stay below roughly 0.7 kPa for extended exposure; effects on cognition appear well below the level at which oxygen becomes a concern.

Non-regenerable lithium hydroxide is simple and mass-efficient for short missions, which is why Apollo used it. Anything longer needs a regenerable system, since the consumable mass otherwise grows without bound.

Oxygen generation on the ISS is by water electrolysis, splitting water into oxygen for the cabin and hydrogen that would otherwise be vented.

The Sabatier reaction closes part of the loop by combining that hydrogen with the carbon dioxide removed from the air:

The water returns to the water system and eventually to electrolysis; the methane is vented. This recovers roughly half the oxygen that would otherwise be lost, and the loop remains open because the carbon leaves as methane. Closing it fully requires cracking methane to recover carbon, which has been demonstrated in the laboratory and not operationally.

Water

Water is the largest mass term and therefore the most valuable to recycle.

The ISS system processes humidity condensate, urine and, more recently, the brine left over from urine processing. The urine processor uses vacuum distillation; the water processor combines filtration, catalytic oxidation and iodination. Recovery has been raised over time from roughly 75% to about 98% of water input, achieved by adding brine processing to recover what earlier stages left behind.

That last increment illustrates the economics of closure. Going from 90% to 98% recovery sounds marginal but cuts the water that must be resupplied by a factor of five, and each successive increment costs more hardware for less return. Somewhere on that curve, additional hardware mass exceeds the consumable mass it saves, and the optimum closure depends entirely on mission duration.

Short mission

Open loop wins. Hardware mass exceeds consumable savings, which is why Apollo carried water rather than recycling it.

ISS

Closure pays because resupply is expensive, but resupply exists as a backstop, so the system need not be perfect.

Mars transit

Closure is mandatory, and there is no backstop. Reliability of the recycling hardware becomes a crew survival issue rather than a logistics one.

Surface with ISRU

Local water changes the problem entirely. Extracted Martian or lunar water makes partial closure acceptable again.

Atmosphere

Cabin atmosphere is a compromise between physiology, fire safety, structural mass and extravehicular activity.

Lower pressure reduces structural mass and leak rate, and it narrows the gap to suit pressure. But a pure oxygen atmosphere is a severe fire hazard, as the Apollo 1 fire demonstrated at the cost of three lives, and the raised oxygen fraction needed to maintain partial pressure at reduced total pressure reintroduces flammability concerns.

The prebreathe problem follows directly. A suit must be at low pressure to remain flexible enough to work in, but moving a crew member from a 101 kPa cabin to a 29.6 kPa suit risks decompression sickness exactly as ascending too quickly from a dive does. The mitigation is breathing pure oxygen for hours beforehand to purge dissolved nitrogen, which costs crew time on every EVA. A reduced-pressure exploration atmosphere shortens or eliminates prebreathe, which is the main argument for it and matters greatly for a surface mission with frequent EVAs.

The rest of the subsystem

Life support is more than air and water, and the remaining functions are the ones that fail quietly.

Trace contaminant control removes the ammonia, formaldehyde, siloxanes and hundreds of other compounds that outgas from materials and metabolise from crew. In a sealed volume these accumulate, and activated charcoal and catalytic oxidation beds remove them continuously.

Particulate filtration matters more than on Earth because dust does not settle. Anything shed indoors stays airborne until a filter catches it, and on a planetary surface, abrasive regolith tracked in through an airlock is a serious problem. See Lunar surface systems.

Fire detection and suppression is complicated by the absence of buoyancy. Flames in microgravity do not form the familiar shape, spread differently and can smoulder without visible flame. Smoke does not rise, so detectors must be placed in ventilation flow paths rather than at ceilings.

Humidity control is a hard requirement rather than a comfort function. Without airflow, exhaled moisture forms a stagnant humid layer around a stationary crew member, which is why ventilation is continuous and why a sleeping crew member in still air can experience carbon dioxide buildup around their own head.

The vented methane box is where the loop stays open. Every architecture study for long-duration flight comes back to that arrow, because carbon leaving the system is what still requires resupply.

References

• Wieland, P. O. “Designing for Human Presence in Space: An Introduction to Environmental Control and Life Support Systems”, NASA RP-1324, 1994, and its later updates.
• Eckart, P. Spaceflight Life Support and Biospherics, Kluwer, 1996.
• Anderson, M. S. et al. “Life Support Baseline Values and Assumptions Document”, NASA/TP-2015-218570 and later revisions. The source for the metabolic figures above.
• Carter, L. et al. “Status of ISS Water Management and Recovery”, International Conference on Environmental Systems, published annually.
• Norcross, J. R. et al. “Effects of the 8 psia / 32% O2 Atmosphere on the Human in the Spaceflight Environment”, NASA/TM-2013-217377.