Keeping Humans Alive in Space

The Fundamentals of a Habitable Bubble

For humans to venture into the lethal vacuum of space, they must take a small piece of Earth with them. Every spacecraft carrying a crew is, at its core, a bubble of life, a miniature biosphere meticulously engineered to replicate the functions our planet provides for free. The complex network of hardware that achieves this is known as the Environmental Control and Life Support System, or ECLSS. It is the unseen machine, the tireless and absolutely essential technology that makes human spaceflight possible. Without it, the journey would end in minutes.

The primary mandate of any ECLSS is to create and maintain a stable, habitable environment within the sealed confines of a spacecraft. This is a far more complex task than simply providing a can of air. It involves a constant, delicate balancing act to manage the physiological needs of the crew and the harsh realities of the space environment. The system must supply breathable air, provide clean water, manage every form of waste, and control temperature and pressure within narrow, life-sustaining tolerances. It is a closed world where nothing can be taken for granted.

At the center of this world is the human crew, who are not merely passengers but integral components of the system itself. An ECLSS is designed around the constant metabolic processes of its occupants. A typical astronaut requires approximately 5 kilograms of oxygen, food, and water each day to live and work. In turn, they produce a similar amount of waste, including liquid and solid waste, and about 1 kilogram of carbon dioxide gas simply from breathing. The ECLSS must continuously process these inputs and outputs. It is a dynamic system where the machinery and the biology are locked in a life-sustaining feedback loop.

This complex task can be broken down into four fundamental pillars of life support. First is atmosphere management, the most immediate and critical function. This involves not only supplying a steady stream of oxygen but also actively removing the carbon dioxide produced by respiration, which would quickly become toxic if allowed to accumulate. It also means controlling humidity, filtering out trace chemical contaminants and microorganisms, and maintaining a safe and stable cabin pressure against the vacuum outside.

Second is water management. After oxygen, water is the most vital consumable, and by far the heaviest. An ECLSS must be able to store and distribute clean water for drinking, food preparation, and hygiene. For any mission lasting more than a few days, it must also be able to reclaim water from every available source, including wastewater from sinks and showers, urine, and even the moisture from the crew’s own breath and sweat.

Third is waste management. In the confined space of a habitat, waste is not just an inconvenience; it’s a hazard. The ECLSS must provide a way to hygienically collect, treat, and store all solid and liquid human waste, as well as general trash like food packaging and used equipment, to prevent contamination and the spread of illness.

Finally, there is thermal and pressure control. A spacecraft in orbit is subjected to brutal temperature swings, from the intense, unfiltered heat of direct sunlight to the deep, cryogenic cold of shadow. The ECLSS uses a combination of radiators, heaters, insulation, and fluid loops to maintain a comfortable internal climate. It also precisely manages the cabin’s atmospheric pressure, protecting the crew from the physiological dangers of decompression.

Underlying the design of every one of these pillars is a fundamental choice between two competing philosophies: open-loop and closed-loop systems. An open-loop system is the essence of simplicity. It operates like a scuba tank, carrying all of its consumables—oxygen, water, food—from Earth and storing or discarding all waste. This approach is reliable for short missions but is fundamentally limited by the sheer mass of supplies it can carry. A closed-loop, or regenerative, system is far more complex. It actively recycles resources, such as purifying wastewater back into drinking water or splitting exhaled carbon dioxide to recover oxygen. This dramatically reduces the need for resupply from Earth but comes at the cost of increased complexity, mass, power requirements, and potential points of failure. The entire history of life support in space can be seen as the steady, mission-driven evolution from purely open-loop systems toward the goal of a perfectly closed loop.

This evolution reveals that the ECLSS is more than just a piece of hardware; it is a primary driver of mission architecture. The choice between an open or closed system fundamentally dictates the possible scope, duration, and destination of any human spaceflight endeavor. Early, short missions like Mercury and Vostok were possible only because their open-loop systems were sufficient for flights lasting hours or a few days. They operated with a hard limit imposed by the finite amount of oxygen, water, and carbon dioxide scrubbers they could carry. The Vostok capsule, for instance, was packed with ten days’ worth of provisions not as a mission plan, but as a desperate survival contingency in case its retro-rockets failed and the cosmonaut had to wait for the orbit to naturally decay. The mission’s potential was defined by the limits of its life support.

The advent of space stations like Skylab and Salyut made this model obsolete. The logistical challenge of launching the tons of consumables required to support a crew for months on end would have been insurmountable. This necessity forced the development of the first regenerative, or closed-loop, technologies. Looking ahead, a human mission to Mars is simply impossible without a nearly fully closed-loop system, because the option for resupply doesn’t exist. The ECLSS is not a component added to the spacecraft; it is the prerequisite that makes the mission itself conceivable.

The Past: The Throwaway Era of Open-Loop Systems

The first decade of human spaceflight was a pioneering era of breathtaking firsts, all of which were accomplished inside tiny, sealed capsules. The life support systems that kept these first astronauts and cosmonauts alive were marvels of engineering for their time, but they operated on a simple, brute-force principle: carry everything you need from Earth, and throw away everything you produce. This was the throwaway era of open-loop systems, where mission duration was measured by how many tanks of oxygen and canisters of chemicals could be crammed into the available space.

First Steps in a Sealed Can: Vostok and Mercury

The two programs that kicked off the space race, the Soviet Vostok and the American Mercury, faced identical challenges in keeping a human alive in orbit for the first time. Their solutions, while similar in principle, reflected different engineering philosophies.

The Vostok spacecraft, which carried Yuri Gagarin on his historic single orbit of the Earth, was a masterpiece of simplicity and redundancy. Its ECLSS was designed for short flights and relied entirely on stored consumables. The system maintained a cabin atmosphere of mixed nitrogen and oxygen at a pressure similar to sea level on Earth, a choice that was more comfortable for the cosmonaut and safer in terms of fire risk. This breathable air was supplied from high-pressure gas bottles stored in the spacecraft’s attached service module. To handle the carbon dioxide exhaled by the cosmonaut, the system used chemical scrubbers. These likely contained a compound like potassium superoxide, which has the elegant property of releasing fresh oxygen as it chemically absorbs carbon dioxide. Thermal control was managed by circulating the cabin gas to collect heat and then rejecting it to space through radiators on the service module’s exterior. In this system, the cosmonaut’s spacesuit was not just clothing but a critical backup, a self-contained bubble that could sustain life for hours in the event of a catastrophic cabin depressurization.

Running in parallel, the American Mercury program developed a system that was functionally similar but different in key details. The most significant difference was the choice of atmosphere. To save weight on the spacecraft’s structure, Mercury engineers opted for a 100% pure oxygen atmosphere at a reduced pressure of about 5.5 pounds per square inch (psi), roughly one-third of sea-level pressure. While this decision successfully lightened the capsule, it introduced a major fire hazard, a risk that would have tragic consequences in the later Apollo program.

Like Vostok, Mercury’s oxygen was supplied from high-pressure tanks. For carbon dioxide removal the American system used replaceable canisters filled with lithium hydroxide (LiOH). When cabin air was passed through these canisters, the LiOH would react with and trap the carbon dioxide. Activated charcoal was included in the canisters to remove odors. This non-regenerative method of using disposable LiOH canisters became the standard for all early U.S. crewed missions. In a reversal of the Vostok philosophy, the Mercury ECLSS was deeply integrated with the astronaut’s spacesuit. The suit itself was the primary loop for life support, with flexible hoses connecting it to the main system to provide a constant flow of fresh oxygen for breathing and ventilation cooling. The cabin’s atmosphere served as the backup.

Pushing the Limits: The Gemini Program

Project Gemini was the essential bridge between the short, solo flights of Mercury and the complex lunar expeditions of Apollo. Its goal was to master the techniques needed for a Moon mission, including rendezvous, docking, and long-duration flight. This meant its ECLSS had to support two astronauts for missions lasting up to two weeks, a significant leap that demanded a more capable and robust system.

The Gemini ECLSS was a more sophisticated, modular design with four largely independent subsystems: an oxygen supply system, a cabin atmosphere loop, a suit loop for the astronauts, and a water management system. A key innovation for the program was the use of fuel cells to generate electrical power. By combining stored hydrogen and oxygen, these fuel cells produced electricity, with pure water as a byproduct. While this water was not initially deemed potable and was primarily used to cool spacecraft electronics, its in-flight production marked a conceptual breakthrough. For the first time, a life-sustaining consumable was being generated in space rather than simply being drawn from a tank filled on Earth. Drinking water was still carried in a pre-filled pouch, and the system for managing human waste remained basic; urine was collected and simply dumped overboard into space, a practice that would continue for decades.

To the Moon and Back: The Apollo Systems

The Apollo program, with its singular goal of landing a man on the Moon, represented the pinnacle of open-loop life support design. The complexity of the mission required not one, but two completely independent life support systems. The first was for the Command Module, the mother ship that would carry three astronauts on the three-day journey to and from the Moon. The second was a separate, much lighter system for the Lunar Module, the lander that would need to support two astronauts on the lunar surface for up to three days.

Both systems were fundamentally open-loop. They relied on tanks of stored cryogenic oxygen and, in the case of the Command Module, water produced as a byproduct of its electrical fuel cells. Both vehicles used the now-standard, non-regenerative lithium hydroxide canisters to scrub carbon dioxide from the atmosphere.

Waste management remained a rudimentary and unpleasant affair. Urine was collected in a device connected to a hose that vented the liquid directly into space. The system for solid waste was notoriously crude. It consisted of a plastic bag with an adhesive ring that an astronaut would have to tape to their buttocks. After use, a capsule of bactericide had to be broken inside the bag, which then had to be sealed and kneaded by hand to mix the contents. The entire process was difficult, time-consuming—taking up to 45 minutes—and universally loathed by the crews.

The fragility of this total reliance on consumables was thrown into stark relief during the 1970 Apollo 13 crisis. After an oxygen tank explosion crippled the Command Module, the three-man crew was forced to use the two-man Lunar Module as a “lifeboat” for the four-day trip back to Earth. The Lunar Module’s ECLSS was immediately overwhelmed. Its small LiOH canisters, designed to handle the carbon dioxide output of two men for two days, were quickly saturated by the respiration of three men. The crew faced death by asphyxiation from their own breath.

The Command Module had plenty of spare canisters, but they were square, while the Lunar Module’s system used round ones. This simple incompatibility of consumable parts nearly doomed the mission. The solution, famously jury-rigged by the crew under intense direction from Mission Control, involved using plastic bags, cardboard from a flight manual, and duct tape to build an adapter that could force air from the Lunar Module’s system through the Command Module’s square canisters. The success of this improvisation was a testament to human ingenuity, but it served as a powerful lesson: in an open-loop system, survival depends entirely on the fixed quantity of supplies you carry and your ability to physically access and use them.

Stepping Outside: The First Portable Life Support Systems

The iconic images of astronauts walking on the Moon were made possible by the Portable Life Support System (PLSS), a self-contained backpack that was, in essence, a miniature, open-loop spacecraft for a single person. The PLSS provided everything an astronaut needed to survive in the vacuum of space. It contained a tank of pressurized oxygen for breathing and to keep the suit inflated. It removed exhaled carbon dioxide with a small, disposable LiOH canister. Most importantly, it provided cooling to counteract the immense body heat generated by physical exertion. The system circulated cool water through a network of tubes woven into a Liquid Cooling Garment worn by the astronaut. This water absorbed the body heat and was then piped to a sublimator in the backpack, which exposed the heated water to the vacuum of space, causing it to freeze and then turn directly into vapor. This process of sublimation efficiently carried the heat away. This fundamental technology of a self-contained backpack providing all necessary life support functions for extravehicular activity, or EVA, set the standard for all subsequent spacesuit designs.

The design choices made in these early days had consequences that rippled through the space program for decades. The American decision to use a low-pressure, pure oxygen atmosphere for Mercury, Gemini, and Apollo was made primarily to save weight. A lower internal pressure meant the spacecraft’s structure did not have to be as robust, and therefore as heavy, to withstand the pressure difference with the vacuum of space. The Soviet Union, by contrast, chose a heavier but more Earth-like mixed-gas system for its Vostok and later Soyuz spacecraft.

The American choice had an immediate and tragic consequence in the Apollo 1 fire, where a spark during a ground test ignited the pure oxygen atmosphere, which was at a higher-than-normal pressure on the launchpad, creating an inferno. The disaster forced a complete overhaul of materials and safety procedures. A longer-term consequence was on the procedures for spacewalks. Because the cabin atmosphere was already low-pressure pure oxygen, astronauts did not need to spend hours pre-breathing to purge nitrogen from their blood before exiting the vehicle in a low-pressure spacesuit. This greatly simplified EVAs during the Apollo program.

When the United States later designed Skylab and the Space Shuttle, safety concerns and a desire for greater crew comfort led to a switch to a more Earth-like nitrogen/oxygen atmosphere. This necessary change introduced a new operational complexity that persists to this day. Astronauts must now perform lengthy pre-breathe protocols before every EVA to avoid getting decompression sickness, commonly known as “the bends.” This adds hours of overhead to every spacewalk. This demonstrates a clear path of dependency, where an initial weight-saving design choice for Mercury locked the U.S. into a specific technological path for three major programs. The eventual, necessary correction to a mixed-gas atmosphere introduced new challenges that were a direct result of that original decision. The early Soviet choice, while resulting in heavier spacecraft, avoided this entire branch of problems.

A Bridge to the Future: The First Space Stations

The arrival of the first space stations in the 1970s marked a critical turning point in the history of life support. The open-loop model, which had successfully carried humans to the Moon and back, was fundamentally unsustainable for long-term habitation in orbit. The sheer mass of consumables required for missions lasting months instead of days made the “carry-it-all” approach logistically impossible. These early orbital outposts were not just places for humans to live and work in space; they were the essential laboratories where the first steps toward regenerative, closed-loop technologies were taken.

America’s Workshop in Orbit: Skylab

Launched in 1973, Skylab was America’s first space station. Repurposed from the upper stage of a Saturn V rocket, its enormous interior volume presented new and complex challenges for environmental control. Over the course of nine months, Skylab hosted three different crews for missions lasting 28, 59, and a record-setting 84 days. A mission of this duration made the Apollo-style reliance on disposable lithium hydroxide canisters completely impractical; the number of canisters required would have been too heavy to launch and would have filled a significant portion of the station’s interior with used-up filters.

Skylab’s most significant ECLSS innovation was its move to a regenerative system for carbon dioxide removal. Instead of disposable canisters, the station used a system of two large beds filled with a porous, crystalline material called a molecular sieve, specifically a type of zeolite. The system worked in a continuous cycle. At any given time, one bed would be actively filtering the cabin air, with the zeolite crystals absorbing carbon dioxide molecules. Simultaneously, the second bed, having been saturated with CO2​, would be opened to the vacuum of space. The vacuum would effectively suck the trapped carbon dioxide out of the material, cleaning, or “regenerating,” it for the next cycle. The two beds would then swap roles. This elegant process allowed the same hardware to be used for the entire duration of the Skylab program, a major first step in closing the atmospheric loop.

In other respects, Skylab’s life support was more traditional. The station maintained a mixed-gas atmosphere of approximately 70% oxygen and 30% nitrogen, but at a reduced pressure of 5 psi to lessen structural strain. This atmosphere was supplied from a large bank of high-pressure gas tanks. Water was also stored in tanks and was not recycled. The finite supply of water, in fact, was one of the primary limiting factors for how long each crew could stay aboard the station. For spacewalks, Skylab introduced a new approach to portable life support. Instead of a fully self-contained backpack, astronauts used a long umbilical called the Astronaut Life Support Assembly (ALSA). This tether supplied oxygen and cooling water directly from the station’s main systems, reducing the need for a bulky, consumable-laden backpack and allowing for longer and less constrained work outside.

The Soviet Outposts: The Salyut Program

Running concurrently with Skylab, the Soviet Union’s Salyut program took a different and ultimately more enduring path toward long-duration spaceflight. The program, which spanned from 1971 to 1982, saw the launch of seven stations and demonstrated a steady, iterative approach to orbital habitation.

The early Salyut stations were conceptually similar to line spacecraft. Their mission duration was strictly limited by the amount of onboard consumables they carried from the launchpad. Salyut 1, the world’s first space station, was a monumental achievement, but its life support system was fundamentally an open loop. Its ability to support a crew was finite, making it essentially a single-use habitat.

The true revolution came with the second generation of stations, Salyut 6 and Salyut 7. These advanced outposts were equipped with a game-changing piece of architecture: a second docking port. This seemingly simple addition completely altered the logistics of spaceflight. It allowed the stations to be refueled and resupplied by a new fleet of automated cargo vehicles called Progress. While a crew was living and working on the station, a Progress ship could dock to the aft port, automatically pumping in fuel and transferring fresh supplies. These robotic ferries brought everything the crew needed to extend their stay: fresh food, new scientific equipment, spare parts, and, most importantly, the consumables for the life support system, including water and tanks of compressed nitrogen and oxygen.

This capability effectively transformed an open-loop system into a sustainable one, not through complex recycling technology, but through the creation of a robust and reliable logistics chain. The model of a permanently occupied station sustained by a steady stream of cargo deliveries became the fundamental blueprint for the later Mir space station and, eventually, the International Space Station. The Salyut stations also served as important platforms for early experiments in regenerative technologies. They included systems to recover usable water from atmospheric condensate and hosted a wide array of plant growth experiments—with names like Oasis, Vazon, and Phyton—to study the potential for in-space food production and biological atmosphere revitalization.

The parallel development of Skylab and the later Salyut stations highlights two distinct and, at the time, competing philosophies for achieving long-duration spaceflight. The American approach, exemplified by Skylab’s molecular sieve, focused on developing sophisticated in-situ technological regeneration. It was a self-contained, hardware-based solution designed to reduce the reliance on the supply chain by recycling a key component of the atmosphere. The Soviet approach, by contrast, focused on logistical sustainability. The key innovation of Salyut 6 was not a single piece of recycling hardware but an architectural one: the second docking port that enabled the Progress resupply vehicle. This solved the sustainability problem with routine logistics rather than with complex machinery.

Ultimately, the modern approach to life support in low-Earth orbit is a hybrid of both philosophies. The International Space Station relies on highly advanced regenerative systems that are direct descendants of the technology pioneered on Skylab, but it is also completely dependent on a continuous chain of international resupply vehicles, a concept proven by the Salyut-Progress system. This demonstrates that for a permanent human presence in space, neither approach was sufficient on its own. The reliability of the supply chain and the efficiency of onboard recycling had to be combined.

The Present: The International Space Station’s Regenerative Marvel

The International Space Station (ISS) represents the current state-of-the-art in environmental control and life support. It is the culmination of decades of lessons learned from every preceding human spaceflight program. Its ECLSS is not a single device but a complex, internationally distributed, and highly integrated suite of hardware designed to recycle air and water with an efficiency that enables the permanent human habitation of low-Earth orbit. It is a functioning, mechanical biosphere in miniature.

An International Ecosystem

The ISS ECLSS is a testament to international cooperation, with critical modules and technologies provided primarily by the United States, Russia, and Europe. This collaboration also means the system is a mosaic of different design philosophies. The U.S. segment of the station is home to the most advanced regenerative systems, focusing on achieving the highest possible efficiency in water recycling and carbon dioxide removal. The Russian segment has historically relied on simpler, more robust systems, with a greater emphasis on replenishment through the well-established Progress resupply chain. While these systems are largely compatible and interconnected, their differences reflect the distinct evolutionary paths taken by the two space programs.

Manufacturing Air from Water and Waste

The ISS maintains an Earth-like atmosphere, a carefully controlled mixture of nitrogen and oxygen at sea-level pressure (14.7 psi). This breathable air is not simply vented from storage tanks; it is actively manufactured and recycled in a remarkably elegant, closed-loop process.

The primary source of oxygen on the U.S. segment is the Oxygen Generation System (OGS). This rack of hardware uses a process called electrolysis to split molecules of recycled water into their constituent parts. The breathable oxygen gas is vented into the cabin atmosphere, while the hydrogen gas is captured as a byproduct. The Russian segment operates a similar, though less reliable, system known as Elektron.

Meanwhile, the Carbon Dioxide Removal Assembly (CDRA) constantly scrubs the air to remove the CO2​exhaled by the crew. The CDRA is a direct and advanced descendant of the system first flown on Skylab. It uses four “beds” of porous zeolite crystals. In a continuous cycle, cabin air is passed through one pair of beds, where a desiccant first removes water vapor and then the zeolite absorbs the CO2​. At the same time, the other pair of beds is exposed to the vacuum of space, which purges the previously captured CO2​ and water, regenerating the material. The two pairs of beds then swap roles, ensuring uninterrupted removal of carbon dioxide.

The true ingenuity of the system lies in how it connects these two processes. The “waste” hydrogen from the OGS and the “waste” carbon dioxide captured by the CDRA are not simply vented overboard. Instead, they are piped to another piece of hardware called the Sabatier System. Inside this reactor, in the presence of a nickel catalyst, the hydrogen and carbon dioxide react to produce two new substances: pure, potable water and methane gas.

The newly created water is fed directly back into the station’s water recycling system. This means that a hydrogen atom that started in a water molecule, was split off during oxygen generation, and then combined with a carbon atom from an astronaut’s breath, is now recovered and put back into a new water molecule. This critical step “closes the loop” on oxygen, allowing the station to recover about half of the oxygen that would otherwise be lost forever inside vented carbon dioxide molecules. The methane, for now, is considered a waste product and is vented into space.

From Yesterday’s Coffee to Tomorrow’s: The Water Recovery System

By mass, water is the most critical consumable required to support a crew in orbit. The ISS’s Water Recovery System (WRS) is an engineering marvel designed to recycle between 93% and 98% of all water used on the station. It creates a nearly closed water loop, dramatically reducing the amount of fresh water that must be launched from Earth.

The system is designed to collect water from every possible source. This includes wastewater from hygiene activities and scientific experiments, condensation collected from the cabin air which contains the crew’s sweat and respiration, and, most importantly, urine.

The recycling process is a multi-stage affair. First, urine is sent to the Urine Processor Assembly (UPA). This device uses vacuum distillation inside a spinning centrifuge—necessary to separate liquids and gases in microgravity—to pull pure water vapor away from the concentrated brine of salts and other contaminants. This recovered water is then combined with all other collected wastewater and sent to the Water Processor Assembly (WPA). The WPA is the final purification stage, using a series of specialized filtration beds and a high-temperature catalytic reactor to break down and remove any remaining organic compounds and microorganisms. The final product is water that is often purer than the tap water most people drink on Earth. It is then ready to be used for drinking, food preparation, or to be fed into the OGS to create more oxygen.

Taking Out the Trash

While air and water are recycled with incredible efficiency, solid waste management on the ISS remains an open-loop process. Human solid waste is collected in individual bags using a toilet that employs air suction instead of water to direct the waste. These bags are then sealed and stored in airtight aluminum containers. General trash, such as food packaging, used clothing, and experiment waste, is collected and compacted. Newer technologies like the Trash Compaction and Processing System (TCPS) are being tested to improve this process. The TCPS uses heat and pressure to sanitize the trash, recover any residual water for recycling, and compress the waste into dense, stackable tiles that are easier and safer to store.

Unlike air and water this solid waste is not recycled on board. All trash containers and compacted tiles are eventually loaded into a departing cargo vehicle, such as a Northrop Grumman Cygnus or a Russian Progress. After the vehicle undocks from the station, it performs a final deorbit burn and undergoes a destructive reentry, burning up completely in Earth’s atmosphere along with its cargo of trash.

The Future: Self-Sufficiency for Deep Space

Venturing beyond the relative safety of low-Earth orbit to establish a permanent presence on the Moon and send the first human missions to Mars will require a leap in life support technology as significant as the original shift from open-loop to regenerative systems. When Earth is a distant blue dot, resupply missions are years away or impossible, and there is no option for a quick return, the Environmental Control and Life Support System must evolve. It will need to be exceptionally reliable, highly autonomous, and capable of achieving nearly 100% resource closure.

The Tyranny of Distance: Challenges of Mars Missions

A human mission to Mars will be a multi-year endeavor. The ECLSS for such a journey must operate flawlessly for the entire duration without the possibility of receiving spare parts or fresh consumables from Earth. The crew will be on their own. Communication delays of up to 20 minutes each way mean that astronauts cannot rely on real-time help from Mission Control to diagnose a problem or walk them through a complex repair. This necessitates a new level of system autonomy and onboard expertise, potentially aided by AI-driven assistants that can help diagnose medical and technical issues.

The primary technological challenge is achieving near-perfect closure of the air and water loops. The highly efficient systems on the ISS are still “leaky” by deep-space standards. They require a regular resupply of water to make up for processing losses and they vent valuable hydrogen atoms into space in the form of methane. This model is simply not sustainable for a transit to Mars. Reliability is paramount; these systems can’t break down when there is no hardware store and no rescue mission on the way.

Perfecting the Machine: Next-Generation Physicochemical Systems

NASA’s Next Generation Life Support (NGLS) program is focused on developing the advanced hardware needed to meet these challenges. The goal is to create systems that are more efficient, more reliable, and less reliant on consumables.

A key area of development is SpaceCraft Oxygen Recovery (SCOR). The main objective of SCOR is to close the “methane leak” in the current air revitalization loop and enable the recovery of nearly 100% of the oxygen bound up in exhaled carbon dioxide. Two promising technologies are being pursued to achieve this. The first uses the Bosch Reaction, a process that reacts carbon dioxide and hydrogen at high temperatures to produce water and solid carbon. This avoids the creation of methane altogether, preventing the loss of hydrogen. The second technology is Methane Pyrolysis. This process would work in conjunction with a Sabatier system. It would take the methane produced by the Sabatier reaction and “crack” it at a very high temperature, breaking it down into solid carbon and hydrogen gas. This recovered hydrogen could then be fed back into the system to react with more carbon dioxide, creating a fully closed oxygen loop.

Similarly, Advanced Water Recovery systems are being designed to wring every last drop of usable water from the waste stream. Future systems will need to process the concentrated brine that is left over by the ISS’s current Urine Processor Assembly, pushing the total water recovery rate to over 98%. More advanced technologies, such as Supercritical Water Oxidation (SCWO), are also being developed. SCWO uses extremely high temperatures and pressures to break down all forms of organic waste—including solid waste and trash—into harmless byproducts like carbon dioxide and, most importantly, sterile water.

Living Off the Land: In-Situ Resource Utilization (ISRU)

Perhaps the most revolutionary concept for enabling long-term deep space missions is In-Situ Resource Utilization, or ISRU. The principle is simple: “live off the land.” Instead of carrying every necessary resource from Earth at enormous cost, ISRU aims to harvest and process local materials on the Moon or Mars to produce life-sustaining consumables.

For an ECLSS, ISRU offers the ultimate backup and a source of resupply. On the Moon, robotic systems could be sent to the permanently shadowed craters at the poles, which are now known to contain vast deposits of water ice. This ice could be mined, melted, and purified for drinking water. It could also be split via electrolysis into its constituent elements: breathable oxygen for the habitat and hydrogen and oxygen to be used as powerful, locally sourced rocket propellant.

On Mars, the atmosphere itself is a resource. It is composed of 95% carbon dioxide. The Mars Oxygen ISRU Experiment (MOXIE), a small device aboard NASA’s Perseverance rover, has already successfully demonstrated that it can use a process called solid oxide electrolysis to extract pure oxygen from the thin Martian air. Future, larger-scale versions of this technology could operate continuously, producing tons of oxygen to pressurize habitats, provide breathable air for the crew, and serve as the oxidizer needed to burn rocket fuel for the return trip to Earth.

The Ultimate Solution: Bioregenerative Life Support

The final goal for a truly self-sufficient, permanent human settlement on another world is the development of a Bioregenerative Life Support System (BLSS). This is not just a collection of machines; it is a small, carefully managed, artificial ecosystem where plants, algae, and microorganisms work in a symbiotic loop with the human crew.

In a fully realized BLSS, the different biological components would create a closed, self-sustaining circle. Plants and algae would perform photosynthesis, consuming the carbon dioxide exhaled by the crew and, in return, producing fresh oxygen and edible food. The natural process of transpiration from plant leaves would act as a highly effective water purifier. Meanwhile, a carefully selected community of microbes would break down all organic waste—including human waste and the inedible parts of plants—back into its constituent nutrients. This nutrient-rich water could then be used as a fertilizer to grow the next generation of crops, completing the loop.

This concept is not science fiction. The most advanced BLSS experiments were conducted by the Soviet Union in the 1960s and 1970s with the BIOS-3 facility in Siberia. In this sealed habitat, crews of up to three people lived for as long as 180 days. The air was regenerated almost entirely by large cultivators of Chlorella algae, and a significant portion of the crew’s food came from wheat grown under artificial lights inside the facility, achieving a remarkable degree of closure decades ago.

Today, the leading effort in this field is the European Space Agency’s MELiSSA (Micro-Ecological Life Support System Alternative) project. MELiSSA is designed as a highly compartmentalized loop of bioreactors. Different microbes are used to break down waste and convert ammonia into nitrates, creating a natural fertilizer. This is then fed to algae and higher plants, such as wheat, potatoes, and soybeans, which produce oxygen and food for the crew. The project is a systematic, long-term effort to understand and master the complex biological processes needed to create a complete, self-sustaining ecosystem for future planetary bases.

The future of life support for deep space exploration is not a simple choice between purely mechanical systems and purely biological ones. The most resilient and practical path forward lies in a sophisticated integration of both into a hybrid system. The predictable, rapid-response control offered by advanced physicochemical systems will be essential for handling immediate, mission-critical functions like maintaining air pressure and scrubbing carbon dioxide. They provide a high degree of reliability that is essential when lives are on the line.

At the same time, bioregenerative systems offer the ultimate prize: true sustainability. They can produce food, which no machine can, and can achieve nearly 100% resource recycling, dramatically reducing the immense logistical mass that would otherwise need to be launched from Earth. A future Mars habitat will likely use both. A robust physicochemical ECLSS will serve as the primary workhorse, managing the day-to-day atmospheric and water processing. In parallel, a dedicated greenhouse module—a BLSS—will operate to gradually produce fresh food, supplement the oxygen supply, and recycle nutrients. This hybrid model creates redundancy. If the BLSS suffers a crop failure, the mechanical system can handle the full life support load. If a pump or valve in the physicochemical system fails, the BLSS provides a buffer of oxygen and water production, buying the crew precious time to make repairs. This convergence of technology and biology represents the most logical and resilient path toward establishing a lasting human presence on another world.

Summary

The evolution of the environmental control and life support system is a story of humanity’s ever-expanding reach into space. It began with the simple, disposable “scuba tank” approach of the 1960s, where pioneering astronauts in Mercury, Vostok, Gemini, and Apollo capsules were sustained by a finite supply of consumables carried from Earth. This open-loop model was sufficient for the first short sprints into orbit and even for the marathon to the Moon, but its inherent limitations became clear as mission ambitions grew.

The first space stations, Skylab and Salyut, served as a critical bridge to the modern era. They were the testbeds where the fundamental unsustainability of the open-loop model for long-duration habitation forced the first steps toward regeneration. Skylab introduced regenerative carbon dioxide removal with its molecular sieve technology, while the Salyut program pioneered the concept of logistical sustainability through its automated Progress resupply vehicles.

Today, the International Space Station represents a mature fusion of these two philosophies, employing a suite of highly efficient regenerative systems to recycle air and water while remaining dependent on a steady chain of cargo deliveries. Its ECLSS is a marvel of engineering that has enabled a continuous human presence in low-Earth orbit for over two decades.

Looking forward, the challenges of sending humans to the Moon and Mars demand another great leap. The future of exploration beyond Earth’s immediate neighborhood is entirely dependent on the development of highly reliable, autonomous, and nearly fully closed-loop systems. This next generation of life support will be a hybrid, combining advanced physicochemical hardware to achieve near-perfect oxygen and water recovery with the revolutionary potential of in-situ resource utilization to “live off the land.” The ultimate goal remains the creation of self-sustaining, artificial ecosystems—bioregenerative systems that can support human life indefinitely on the hostile surfaces of other worlds. The unseen machine, once a simple matter of carrying enough bottled air, is evolving into the very heart of a new, mobile, and expanding biosphere.

What Questions Does This Article Answer?

• What are the primary functions of the Environmental Control and Life Support System (ECLSS) in space travel?
• How does ECLSS manage atmosphere, water, and waste within a spacecraft?
• What are the key challenges of creating a stable environment in space?
• What are the fundamental pillars of life support in space, and why are they crucial?
• How do open-loop and closed-loop systems differ in the context of space habitats?
• What technological evolutions have marked the transition from open-loop to closed-loop systems in space programs?
• How do the life support systems on the International Space Station (ISS) function?
• What steps are being taken to improve the efficiency of life support systems for longer space missions, such as a mission to Mars?
• How does the integration of physicochemical and bioregenerative systems shape the future of space exploration life support systems?
• What roles do international cooperation and technological innovation play in the development of modern space life support systems?