335 SYSTEMS: ENVIRONMENTAL CONTROL AND LIFE SUPPORT SYSTEM—SUPPORTING THE HUMAN ELEMENT OF THE ISS CHAPTER 19 Atmosphere Control and Supply/Atmosphere Revitalization On the ISS, O2 and N2 levels are maintained to values typical of those on the surface of Earth at sea level. Dry air consists of about 78% N2 and 20% O2, by volume. Human respiration takes O2 into the lungs, which is absorbed in the body, and releases CO2 gaseous N2, being inert, is not consumed in the process. Hypoxia, and eventually death, will occur if the O2 levels drop too low. In certain cases, humans can survive for limited intervals with lower levels of O2. The O2 level may be dropped to the equivalent altitude of 3048 m (10,000 ft) for up to a day in a contingency. As the crew members breathe, O2 levels need to be replenished over time. In a perfect system, N2 would never need to be resupplied however, a small amount of leakage occurs on the ISS, as well as lost gas, when vestibules are depressed to allow vehicles to depart, thus necessitating replenishing. It is important to keep the level of O2 high enough for the crew to adequately breathe, but not so high as to create a flammability risk, as O2 is a highly flammable gas. If the concentration of O2 is kept lower than approximately 24%, the risk of a spark causing a combustion event is fairly low at the atmospheric pressure on the ISS. Short-term exceptions are allowed during preparation for spacewalks (see Chapter 17). Two ways to get these critical gases on the ISS include delivering the gasses in a tank or generating them in situ. Various vehicles—Russian Progress, European Space Agency’s Automated Transfer Vehicle, Space Shuttle, Japan Aerospace Exploration Agency’s H-II Transfer Vehicle (HTV), Dragon, and Cygnus— transport O 2 , N 2 , or air (a mixture of N 2 and O 2 ) to the ISS. Progress and the Automated Transfer Vehicle have large storage tanks. A valve is opened for a predetermined amount of time to bleed some O 2 , N 2 , or air into the main cabin whenever the atmosphere on the ISS requires more gas. Three O 2 tanks are situated outside of the airlock. One tank is used to resupply the atmosphere, whereas the second and third are primarily used for the Extravehicular Activity (EVA) Mobility Unit (EMU) (see Chapter 17) and are intended to be used only for the general atmosphere in an emergency. A fourth O 2 tank is stowed outside on the ISS truss. This tank can be accessed via an EVA, if the tank is required. The Dragon, Cygnus, and HTV vehicles can also bring up O 2 and N 2 tanks, which are called the Nitrogen and Oxygen Resupply Systems tanks. The tanks can either be vented directly to the cabin, as above, or be used to resupply the external O 2 and N 2 tanks outside of the airlock for future use. Transporting O 2 to the ISS is costly therefore, it is better to generate O 2 in situ where possible. Both the USOS and the RS have generators that can produce O 2 from water using electrolysis, which is the process of splitting water molecules into hydrogen (H 2 ) and O 2 using electricity. Having two independent systems provides redundancy if one suffers a problem. The Oxygen Generation Assembly (OGA) (Figure 1) performs this task on the USOS. Figure 1. Astronaut Dan Burbank works on the OGA during Expedition 30. Finally, O 2 can be supplied by the Solid Oxygen Generator where solid “candles” are burned, thereby producing O 2 as a by-product. Candles have been used in places such as submarines for years. These candles are used in an extreme contingency case due to flammability risk (see Dragonfly, 1998). The ETHOS flight controllers monitor the O 2 levels closely and work with their Russian counterparts to ensure the right levels are always available. The Pressure Control Assembly (PCA) monitors the total pressure of the cabin air. Similar sensors are present in the Columbus Module and Japanese Experiment Module (JEM). Not only does the PCA measure the