Chapter 4

Life Support Systems

Keeping humans alive in the hostile environment of space and on Mars requires sophisticated Environmental Control and Life Support Systems (ECLSS) that must operate flawlessly for years without resupply from Earth.

ECLSS Overview

Environmental Control and Life Support Systems encompass all the technologies and processes required to maintain a habitable environment for humans in space. Unlike aircraft that can rely on Earth's atmosphere for oxygen and cooling, spacecraft and Mars habitats must provide everything needed for human survival in an entirely self-contained package.

The International Space Station's ECLSS represents the most advanced operational life support system ever built, achieving approximately 90% recycling of water and 50% recovery of oxygen from carbon dioxide. Mars missions will need to push these numbers even higher—approaching 95-99% closure—because the mass penalty of carrying consumables for a 2-3 year mission would be prohibitive.

Atmosphere Management

Oxygen Generation

Humans require approximately 0.84 kg of oxygen per day for respiration. On the ISS, oxygen is primarily generated through electrolysis of water, splitting H₂O molecules into hydrogen and oxygen gases. The Oxygen Generation System (OGS) can produce up to 12 kg of oxygen per day, enough to support a crew of six with margin.

For Mars missions, oxygen generation must be highly reliable and efficient. Primary systems will use electrolysis, but backup systems may include chemical oxygen generators (similar to those in aircraft emergency systems) and potentially biological systems using photosynthetic organisms. The Mars atmosphere itself, containing 95% CO₂, provides a potential feedstock for oxygen production through MOXIE-type solid oxide electrolysis.

Carbon Dioxide Removal

Each person produces approximately 1 kg of carbon dioxide daily through respiration. CO₂ concentrations above 0.5% cause headaches and cognitive impairment; above 5%, CO₂ is acutely dangerous. Effective CO₂ removal is therefore critical for crew health and performance.

The ISS uses the Carbon Dioxide Removal Assembly (CDRA), which employs zeolite molecular sieves to adsorb CO₂ from the cabin atmosphere. The system operates on a two-bed regenerative cycle: while one bed adsorbs CO₂, the other is heated to release previously captured CO₂, which is then either vented to space or processed by the Sabatier reactor.

CO₂ Removal Technology	Mechanism	Recovery	TRL
Zeolite Molecular Sieves	Adsorption/desorption	Yes (to Sabatier)	9 (ISS)
Lithium Hydroxide	Chemical absorption	No (consumable)	9 (Apollo)
Metal Oxide Beds	Adsorption/thermal regen	Yes	6-7
Electrochemical	Direct electrolysis	Yes	4-5
Algae/Plants	Photosynthesis	Yes (biological)	3-4

Trace Contaminant Control

Beyond oxygen and CO₂, cabin atmospheres accumulate numerous trace contaminants from outgassing materials, human metabolism, and equipment operation. Hundreds of compounds have been detected in spacecraft atmospheres, including formaldehyde, ammonia, acetone, and various volatile organic compounds. The Trace Contaminant Control System (TCCS) uses activated charcoal beds and catalytic oxidizers to remove these substances.

Water Recovery and Management

Water is arguably the most critical consumable for human spaceflight. Each crew member requires approximately 2.5 kg of drinking water daily, plus additional water for food preparation and hygiene. Without recycling, a six-person crew would require nearly 30 tonnes of water for a Mars mission—clearly unsustainable.

ISS Water Recovery System

The ISS Water Recovery System (WRS) processes two waste streams: urine and humidity condensate from cabin air. The system achieves approximately 90% recovery of water from these sources, producing water that meets or exceeds drinking water standards.

Urine processing is particularly challenging due to the high mineral and organic content. The Urine Processor Assembly (UPA) uses vapor compression distillation: urine is heated in a rotating drum, water vapor is collected and compressed, and the compressed vapor transfers heat to incoming urine while condensing into pure water. The remaining brine is currently discarded but future systems may extract additional water.

Advanced Water Recovery

Mars missions will require even higher water recovery rates—95% or greater—to minimize the mass carried from Earth. Advanced systems under development include membrane filtration, biological treatment, and brine dewatering. Biological systems using bacteria or algae can process organic waste streams that mechanical systems cannot handle efficiently.

Thermal Control

Spacecraft and habitats must maintain comfortable temperatures despite the extreme thermal environment of space. In direct sunlight, exterior surfaces can reach over 120°C; in shadow, they can plunge below -150°C. Internal heat from electronics, lighting, and crew metabolism must be rejected to space.

Active Thermal Control

Active Thermal Control Systems (ATCS) use circulating fluid loops to collect waste heat from equipment and crew areas and transport it to external radiators. The ISS uses both ammonia (external loop) and water (internal loop) as working fluids, with heat exchangers transferring thermal energy between the two.

Radiators reject heat to space through infrared radiation—the only mechanism available in the vacuum of space. The effectiveness of radiators depends on their temperature (fourth power relationship) and area. ISS radiators cover approximately 160 m² and can reject up to 70 kW of heat. Mars missions will require similar or larger thermal rejection capabilities.

Mars Surface Thermal Challenges

Mars surface operations present unique thermal challenges. The thin atmosphere provides some convective heat transfer but limits radiative rejection compared to vacuum. Dust accumulation on radiators can degrade performance. Temperature swings between day and night (potentially 100°C or more) stress materials and seals. Habitat design must address all these factors while maintaining internal temperatures comfortable for crew.

Food Systems

Nutrition is fundamental to crew health and morale on long-duration missions. Each person requires approximately 1.8 kg of food daily, providing roughly 2,500-3,000 calories. For a six-person crew on a 2-3 year mission, this represents 12-15 tonnes of food if entirely carried from Earth.

Packaged Food

ISS crews rely on pre-packaged food sent from Earth, including freeze-dried, thermostabilized, and irradiated items. Shelf life is a significant concern; most space food is designed for 18-24 months, but Mars missions may require food stored for 3-5 years from packaging to consumption. Nutrient degradation over time, particularly vitamins, must be addressed through enhanced packaging or supplementation.

Crop Production

Growing fresh food provides nutritional, psychological, and life support benefits. Plants produce oxygen, consume CO₂, and can process wastewater. Fresh vegetables add variety and nutrition to crew diets. Experiments on ISS with the Veggie and Advanced Plant Habitat systems have successfully grown lettuce, radishes, and other crops in microgravity.

Mars surface greenhouses could eventually provide a significant fraction of crew food needs. Estimates suggest that 50-100 m² of growing area per person could provide a meaningful supplement to packaged food, with the potential for full food self-sufficiency as colony infrastructure develops.

Crop	Yield (kg/m²/day)	Calories/kg	Growth Time
Potatoes	0.15-0.25	770	90-120 days
Wheat	0.05-0.08	3,390	120-150 days
Soybeans	0.03-0.05	4,460	90-120 days
Lettuce	0.10-0.15	150	28-35 days
Tomatoes	0.08-0.12	180	60-80 days

Waste Management

Human waste management encompasses solid waste (feces), liquid waste (urine), trash, and various other waste streams. Proper waste handling is essential for crew health, system operation, and environmental control.

Human Waste Processing

Urine is valuable as a water source and is processed through the Water Recovery System. Fecal waste is more challenging; current ISS practice is to collect, stabilize (through chemical treatment or vacuum desiccation), and store it for eventual disposal. This approach will not scale to Mars missions.

Advanced waste processing systems under development include pyrolysis (thermal decomposition), wet oxidation, and biological treatment. These systems aim to recover water and nutrients from fecal waste while producing sterile, stable residue. Biological systems using carefully selected microorganisms show particular promise for integration with crop production systems.

Trash and Recyclables

ISS generates approximately 2.5 kg of trash per person per day. Current practice is to compact and store trash for disposal in departing cargo vehicles. Mars missions cannot rely on this approach; trash must be processed in place.

Trash processing options include incineration (producing CO₂ and ash), compaction and storage, recycling where possible, and creative reuse. Packaging materials might be repurposed as radiation shielding or construction material. Organic waste can feed biological processing systems.

Medical Systems

Medical care on Mars missions must address a wide range of potential health issues without the possibility of emergency evacuation to Earth. The medical system must support preventive care, diagnosis, treatment of injuries and illnesses, and potentially surgery.

Onboard Medical Capabilities

Mars mission medical systems will include comprehensive diagnostic equipment (imaging, laboratory analysis), pharmaceuticals with extended shelf life, surgical capability for emergencies, dental care, and telemedicine support from Earth (with communication delays). At least one crew member will likely be a physician with surgical training.

Health Monitoring

Continuous health monitoring through wearable sensors can detect problems before they become serious. Parameters monitored include heart rate, blood oxygen, activity levels, sleep quality, and potentially continuous glucose and hydration status. AI-assisted analysis can alert crew and ground controllers to developing health issues.

Summary

Review Questions

1. What are the primary consumables required for human survival in space? How much of each does one person need daily?
2. Explain the process of oxygen generation through water electrolysis. What are the inputs, outputs, and efficiency considerations?
3. How does the ISS remove carbon dioxide from cabin air? Why is CO₂ removal so critical for crew health?
4. Describe the water recovery system used on ISS. What processes are involved and what recovery rate is achieved?
5. What are the unique thermal control challenges for Mars surface habitats compared to spacecraft in orbit?
6. How might crop production contribute to Mars mission life support? What are the advantages beyond just food production?
7. What waste streams must Mars missions handle? How do the approaches differ from current ISS practice?
8. Why must Mars missions achieve higher life support closure rates than ISS? What are the mass implications of lower closure rates?