EPISODE 4: MUSHROOMS ON MARS
CLOSED LOOP SYSTEMS ON THE ISS
The International Space Station's (ISS) life support system is not yet a fully closed-loop system, but it contains several critical components that aim to recycle water, oxygen, and waste to minimize reliance on resupply missions from Earth. The ISS serves as a testbed for future closed-loop life support systems, helping researchers develop technologies that can support long-duration space missions to destinations like Mars or the Moon. Here’s an in-depth look at the current closed-loop systems on the ISS and the technology that drives them.
Components of the ISS Life Support System
The ISS’s life support system involves several interconnected systems that manage air, water, waste, and environmental control. Each of these systems plays a role in recycling essential resources like water and oxygen and maintaining the health and safety of the astronauts aboard.
1. Water Recovery and Recycling System (WRS)
The Water Recovery and Recycling System is one of the most critical components of the ISS’s life support system, enabling the crew to reuse water multiple times. In space, where water is a precious resource, the ability to recycle water is crucial for sustainability.
How it Works:
- Water Sources: The ISS recycles water from a variety of sources, including:
- Urine: Astronauts' urine is collected and processed through a system that extracts water, leaving behind waste products.
- Sweat and Condensation: Water that evaporates from astronauts' sweat or from general cabin humidity is captured, condensed, and recycled.
- Breathing: As astronauts breathe, water vapor is released into the air. This moisture is collected and filtered for reuse.
- Urine Processor Assembly (UPA): The UPA is responsible for distilling water from urine. Urine is first collected in a tank, and the UPA uses vacuum distillation to evaporate the water, leaving behind waste brine. The resulting water vapor is then condensed, filtered, and treated to remove impurities, making it safe to drink.
- Water Processor Assembly (WPA): After water is distilled from urine or collected as condensation, it enters the WPA for further filtration and purification. This system uses multiple filtration stages, including particulate filters, catalytic oxidizers, and ion exchange beds, to remove contaminants and ensure the water is safe for consumption. The WPA also adds trace amounts of iodine or silver as a disinfectant to prevent microbial growth.
Benefits of the WRS:
- Water Recycling Efficiency: The WRS recycles around 93-98% of the water used on the ISS, meaning that only a small amount of water needs to be resupplied from Earth.
- Reduced Dependency on Earth: By recycling urine, sweat, and humidity, the ISS significantly reduces the need for water resupply, which is expensive and logistically challenging.
Challenges and Future Goals:
- Complete Recycling: Currently, not all water can be recycled, particularly the waste brine left over from the UPA. Future improvements aim to develop technologies that can process this waste brine into potable water, further increasing the efficiency of the system.
- Scaling for Mars Missions: Future systems, especially for Mars missions, will need to be even more efficient, capable of recycling all available water sources with minimal waste.
2. Oxygen Generation and CO₂ Removal
Maintaining breathable air in the closed environment of the ISS is essential. The ISS life support system manages oxygen generation and carbon dioxide (CO₂) removal through a combination of processes that aim to maintain a stable atmosphere for the crew.
Oxygen Generation System (OGS):
- Electrolysis of Water: The Oxygen Generation System (OGS) on the ISS uses a process called electrolysis to split water molecules (H₂O) into oxygen (O₂) and hydrogen (H₂). The oxygen is then released into the cabin atmosphere for the astronauts to breathe, while the hydrogen is either vented into space or used in further reactions.
- Continuous Oxygen Supply: The OGS runs continuously to ensure the crew has a stable supply of oxygen. This is especially important since the ISS is a sealed environment, and oxygen cannot be naturally replenished as it would be on Earth.
Sabatier Reaction:
- Converting CO₂ into Water: The ISS uses the Sabatier reaction, a chemical process that combines hydrogen (from the electrolysis of water) with carbon dioxide (CO₂) captured from the cabin air. This reaction produces methane (CH₄) and water (H₂O). The methane is vented into space, and the water is fed back into the water recycling system for purification and reuse.
- Recycling CO₂: By recycling CO₂ into water, the ISS reduces the need for additional water resupply from Earth, closing the loop on oxygen and water production.
Carbon Dioxide Removal System (CDRA):
- Removing Excess CO₂: Astronauts produce CO₂ when they breathe, and high levels of CO₂ in a closed environment can be toxic. The **Carbon Dioxide Removal Assembly (CDRA)** filters cabin air, capturing excess CO₂ using solid sorbent beds. The CO₂ is then either vented into space or processed through the Sabatier system.
Challenges and Future Improvements:
- Efficiency in Oxygen Generation: Although the electrolysis process works well, improving the energy efficiency and reliability of the OGS is a priority for future long-duration missions.
- Complete Recycling of CO₂: In the future, more advanced systems could further recycle CO₂ into useful compounds or integrate plants into the system, which naturally remove CO₂ and produce oxygen.
3. Temperature and Humidity Control
Managing temperature and humidity is crucial for astronaut comfort and the proper functioning of equipment on the ISS. The Environmental Control and Life Support System (ECLSS) ensures that the ISS stays within the required temperature and humidity ranges.
Thermal Control System:
- The ISS uses an active thermal control system to manage the heat generated by crew activities, equipment, and solar radiation. Excess heat is dissipated using radiators on the exterior of the station.
Humidity Control:
- Condensing Humidity: The ECLSS also controls humidity levels by collecting moisture from the air, including astronauts' breath, sweat, and daily activities. This collected moisture is fed into the water recycling system, helping maintain the balance of water resources on the ISS.
Challenges:
- Managing Microgravity Effects: Without gravity, natural convection does not occur, making air circulation more challenging. The ECLSS relies on forced air circulation systems to ensure that air, temperature, and humidity are evenly distributed throughout the ISS.
4. Waste Management
The ISS produces various forms of waste, including human waste, food scraps, packaging materials, and equipment that has outlived its usefulness. Waste management is critical to ensuring the health and safety of the astronauts, as well as minimizing the environmental impact of the station.
Human Waste Processing:
- Urine Recycling: As mentioned earlier, the ISS’s urine is processed and purified into potable water. This is one of the key components of the water recovery system.
- Solid Waste Disposal: Solid human waste, such as feces, is collected in specially designed containment bags. This waste is not currently recycled but is stored in containers and eventually returned to Earth on resupply vehicles or incinerated in the atmosphere upon re-entry.
Trash and General Waste Disposal:
- Stowage and Disposal: Non-recyclable waste, such as packaging and food scraps, is stored on the ISS in cargo vehicles, such as SpaceX’s Dragon capsule or Russia’s Progress spacecraft. These vehicles carry waste back to Earth, or, in some cases, are sent into the atmosphere to burn up upon re-entry.
Future Waste Recycling Goals:
- Waste-to-Resource Technologies: Future missions will require more advanced waste recycling systems. Technologies that can convert solid waste into fuel, building materials, or other resources will be necessary to support long-duration spaceflight.
5. Food Production on the ISS
While the ISS relies on resupply missions for much of its food, there are ongoing experiments aimed at growing food in space to supplement astronauts' diets and explore the feasibility of long-term food production.
Veggie Plant Growth System:
- Growing Fresh Produce: The Veggie Plant Growth System allows astronauts to grow crops like lettuce, radishes, and mustard greens in space. The system uses LED lights and plant pillows filled with a porous growing medium to anchor the plants in microgravity.
- Psychological and Nutritional Benefits: In addition to the nutritional benefits of fresh produce, growing plants has psychological benefits for astronauts. The act of gardening and eating fresh food can improve morale during long missions.
Advanced Plant Habitat (APH):
- More Complex Plants: The APH is a more sophisticated plant growth system that allows for greater control over environmental factors like light, humidity, and nutrient delivery. This system has been used to grow more complex plants like wheat and peppers.
- Supporting Long-Term Missions: Growing food in space reduces reliance on Earth-based resupply missions and is essential for future missions to Mars or lunar bases, where resupply may not be possible for extended periods.
6. Experimental Closed-Loop Systems:
Melissa Project (ESA):
- The Micro-Ecological Life Support System Alternative (MELiSSA) project by ESA is an advanced closed-loop system that aims to recycle human waste, carbon dioxide, and organic matter into food, water, and oxygen. The system is currently being tested in labs and aboard the ISS.
- Future Applications: MELiSSA aims to be a fully autonomous life support system for long-duration missions. It includes compartments with plants, bacteria, and algae that process waste and provide essential resources. Future space missions will rely on systems like MELiSSA to ensure sustainability.
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Challenges and Future Improvements for Closed-Loop Systems on the ISS
Energy Efficiency:
- The current life support systems on the ISS require significant amounts of energy. Developing more energy-efficient systems will be essential for future missions, especially for destinations where power sources are limited, such as Mars.
Full Waste Recycling:
- The ISS does not yet have the capability to recycle solid waste (feces, food waste, packaging) into usable resources. Future advancements in waste-to-energy or waste-to-resource technologies will be critical for true closed-loop systems.
Scaling Up for Long-Duration Missions:
- The ISS’s life support system is designed for a relatively small crew in low Earth orbit. Future systems will need to be scaled up and made more robust to support larger crews for longer periods, particularly for Mars or lunar bases where resupply missions may not be feasible.
While the ISS does not yet operate a fully closed-loop system, it represents an important step toward achieving this goal for long-duration space exploration. The various systems in place on the ISS—water recovery, oxygen generation, CO₂ recycling, and waste management—are critical components of future closed-loop systems that will be essential for sustaining life on missions to the Moon, Mars, and beyond. As technology advances, these systems will evolve into more efficient, autonomous systems capable of supporting human life in space without reliance on Earth-based resupply.