CHAPTER 19 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS 334 Astronauts drink water that has been recycled from urine and sweat on the International Space Station. If Command & Data Handling is the brains of the International Space Station (ISS), the Environmental systems are the heart and lungs of the vehicle. Adding humans to a spacecraft brings a significant overhead in terms of life support. An atmosphere must be provided for humans to survive in the harsh frontier that is space living in a space suit would be neither practical nor supportable for any significant amount of time. A fundamental aspect of the ISS is to provide a shirtsleeve environment in which astronauts can conduct research. To this end, the Environmental Control and Life Support System (ECLSS) provides a breathable atmosphere at a normal atmospheric pressure. This means the system provides oxygen (O2) and nitrogen (N2) at the same ratio as on Earth while removing contaminates such as carbon dioxide (CO2) and other impurities in the gas. As on Earth, water is the most critical consumable after O2. Water is required for survival, rehydrating food, bathing, and waste removal. Early space stations such as Skylab required the astronauts to carry with them all the gas and water, which were consumed. On the United States On-orbit Segment (USOS), the goal is to recycle and generate as much of these commodities in as closed loop as possible. For example, drinking water turns into perspiration and urine. In turn, sweat is collected out of the atmosphere by a dehumidifier while urine is separated from other waste and stored. The sweat and urine are then carefully processed back into drinking water. These types of technologies and processes will need to be well established and reliable if humans are to travel to Mars. The ISS life support system is designed to handle seven crew members routinely and can support a surge of up to 11 for a brief period of time. Pressure on the ISS is normally maintained between 724 to 770 mm Hg (14.0 to 4.9 psi), which is equivalent to what is experienced around sea level on Earth. For crew comfort, temperatures are maintained at 22ºC to 26ºC (72ºF to 79ºF). Humidity can be controlled to whatever level is desired, but it is generally kept low (~45%) for crew comfort and to minimize water condensing on critical surfaces of the space station. The Environmental and Thermal Operating Systems (ETHOS) flight controller is responsible for monitoring these systems continuously ETHOS carefully tracks O 2 , CO 2 , and every drop of water. Many of these same systems are found on the Russian Segment (RS) therefore, careful coordination is required between the two teams. Since most emergencies (i.e., fire, loss of atmosphere, chemical spill) affect the atmosphere, ETHOS has one of the most critical jobs on the flight control team: emergency response. If any of these problems occur, ETHOS guides the flight control team and astronauts through the procedures, thereby ensuring crew safety. Of all the areas in which the flight control team trains, emergencies get the most attention. Fortunately, to date, no serious problems have occurred, mainly due to the vigilance of the ETHOS team.