The **Environmental Control and Life Support System (ECLSS)** onboard the **International Space Station (ISS)** is a highly sophisticated system that maintains a safe, habitable environment for astronauts by managing critical life support functions. Its primary role is to ensure a sustainable atmosphere, provide clean water, manage waste, and regulate temperature within the station. As a closed-loop system, the ECLSS enables the ISS to support long-duration missions with minimal resupply from Earth, especially in terms of air and water. ### Key Functions of the ECLSS The ECLSS on the ISS performs several essential tasks: 1. **Air Revitalization and Oxygen Supply** 2. **Water Recovery and Management** 3. **Temperature and Humidity Control** 4. **Waste Management** 5. **Fire Detection and Suppression** ### 1. **Air Revitalization and Oxygen Supply** A continuous supply of breathable air is vital to the survival of the ISS crew. The ECLSS maintains the proper levels of oxygen, removes carbon dioxide (CO₂), and controls the levels of other trace gases in the atmosphere. #### Oxygen Generation System (OGS) The ISS’s oxygen is primarily generated by the **Oxygen Generation System (OGS)**, which is a part of the ECLSS. It works by electrolysis, splitting water molecules (H₂O) into oxygen (O₂) and hydrogen (H₂): - **Electrolysis Process**: Water from the **Water Recovery System (WRS)** is fed into the OGS. The water is split into oxygen and hydrogen using **electricity**. The oxygen is then released into the cabin for the astronauts to breathe, while the hydrogen is vented into space or used in other systems. - **Backup Oxygen Supply**: In addition to the OGS, the ISS has backup systems that can release stored oxygen. For instance, the **Solid Fuel Oxygen Generation (SFOG)** system, also known as **candles**, chemically produces oxygen when ignited. This provides a secondary oxygen source in case the OGS is offline. #### Carbon Dioxide Removal Since humans continuously exhale **carbon dioxide (CO₂)**, it's crucial to remove CO₂ from the ISS atmosphere to prevent buildup that could harm the crew. - **Carbon Dioxide Removal Assembly (CDRA)**: The **CDRA** uses a series of sorbent beds to capture and remove CO₂ from the air. Air from the station's atmosphere is pumped through a **zeolite**-based bed, which absorbs the CO₂. The CO₂ is then vented into space. The CDRA works continuously to keep CO₂ levels within safe limits. - **Russian Vozdukh System**: The Russian **Vozdukh system** is an additional CO₂ scrubber, and it works similarly to the CDRA by using adsorption technology to capture CO₂ and release it into space. It acts as a redundancy in the CO₂ removal process. #### Trace Contaminant Control The **Trace Contaminant Control System (TCCS)** removes low concentrations of trace gases that can accumulate from the crew’s activities, equipment, or materials on the ISS. These contaminants could include volatile organic compounds (VOCs) like acetone, ammonia, and ethylene. ### 2. **Water Recovery and Management** The **Water Recovery and Management** system is a critical part of the ECLSS and allows the ISS to recover up to **93-94%** of the water onboard. The **Water Recovery System (WRS)** purifies wastewater, urine, sweat, and cabin humidity into potable water. - **Urine Processor Assembly (UPA)**: The UPA distills urine to extract water, using vacuum distillation and filtration techniques. It’s a key component of the water recycling loop on the ISS. - **Water Processor Assembly (WPA)**: The WPA processes water from multiple sources, including recovered urine, condensate from cabin air, and hygiene water. It filters out impurities, breaks down organic contaminants, and chemically disinfects the water to ensure it is safe to drink. Water on the ISS is used for: - Drinking - Rehydrating food - Personal hygiene (handwashing, brushing teeth) - Experimental use (especially in plant growth systems) ### 3. **Temperature and Humidity Control** The ECLSS controls the **temperature** and **humidity** levels aboard the ISS to ensure they remain within safe and comfortable limits for the crew. Without gravity, natural convection doesn’t occur, so active systems are needed to maintain airflow and temperature regulation. #### Temperature Control System (TCS) The **Temperature Control System** ensures that the various modules and compartments of the ISS remain at optimal temperatures for both the crew and equipment. - **Heat Exchangers**: Internal heat generated by electrical systems, life support systems, and crew activities is captured by **heat exchangers**. These transfer the heat to an **External Active Thermal Control System (EATCS)**, which uses **ammonia loops** to dissipate heat into space via large radiators. - **Cold Plates**: Many sensitive pieces of equipment are cooled using **cold plates** connected to the thermal control loop. These cold plates are essentially surfaces through which heat from equipment can be transferred into the cooling system. #### Humidity Control The **Condensing Heat Exchanger** system removes excess moisture from the cabin air to keep humidity levels within safe and comfortable limits. This also serves as a primary source for water recovery in the **Water Recovery System (WRS)**. The moisture collected from the air is routed to the WPA for purification and reuse. Maintaining proper humidity is critical in space because too much humidity can lead to condensation, which could damage electronic equipment and lead to mold or bacterial growth. Too little humidity can cause dehydration and health issues for the crew. ### 4. **Waste Management** The ECLSS also handles waste produced by the crew, including **solid waste**, **liquid waste**, and **gaseous waste**. - **Solid Waste**: Solid human waste is collected and compacted in containers that are eventually returned to Earth for disposal or incinerated in Earth’s atmosphere when waste containers are ejected on resupply vehicles like **Cygnus** or **Progress**. - **Liquid Waste**: Liquid waste (mainly urine) is sent to the **Urine Processor Assembly (UPA)** to extract water, with the remaining brine being either stored or processed further for even more water recovery. ### 5. **Fire Detection and Suppression** Fire is a serious hazard in the confined space of the ISS, where microgravity could cause flames to behave unpredictably. The ECLSS includes several fire safety measures: - **Fire Detection**: Smoke detectors are located throughout the station. In microgravity, smoke doesn’t rise, so the detectors rely on airflow systems to bring air (and any possible smoke) to the sensors. - **Fire Suppression**: The ISS is equipped with **portable fire extinguishers** that use **carbon dioxide** to put out fires without leaving damaging residue that could harm sensitive equipment. Fire suppression is also carefully designed not to deplete oxygen levels too drastically. ### Redundancy and Reliability The ISS is designed with **multiple redundancies** to ensure continuous operation in case of failures. For example: - Both the U.S. and Russian segments have independent **air revitalization systems**. - Multiple sources of oxygen exist (OGS, SFOG, Russian oxygen tanks). - Water purification has **backup filters** and storage systems in case of failures. ### Total Volume of Resources Managed The ECLSS system manages air and water volumes to support a crew of typically **six astronauts** on the ISS. On average: - **Air**: The ISS has an atmosphere pressurized to approximately **14.7 psi**, similar to Earth's sea level pressure, with a volume of air in the main pressurized modules being around **915 cubic meters**. - **Water**: The ISS holds approximately **400 liters (105 gallons)** of water at any given time, which is circulated through the system for drinking, hygiene, and food preparation. ### Future Developments NASA and its partners are continuously working on improving the ECLSS in preparation for future deep space missions, such as those to **Mars**. The lessons learned from the ISS’s ECLSS will inform the development of even more efficient life support systems for long-term space exploration where resupply missions are impossible. - **Brine Processor Assembly**: An ongoing development aims to further improve water recovery rates from **urine brine**, potentially increasing water recovery to nearly **98-99%**. - **Closed-loop Oxygen Recycling**: Current research is exploring how to create more closed-loop oxygen systems that can recover even more oxygen from carbon dioxide. ### Conclusion The **Environmental Control and Life Support System (ECLSS)** aboard the ISS is a marvel of engineering that keeps astronauts alive in the hostile environment of space. By maintaining breathable air, recycling water, controlling temperature and humidity, and managing waste, the ECLSS ensures the long-term habitability of the station. This system not only sustains life aboard the ISS but also provides critical insights for future space exploration and potential human habitats on the Moon, Mars, or beyond.