question: How does the ISS not run out of oxygen?
comment: Electrolysis of water (H2O) is the main method to generate oxygen aboard the ISS. Water is split into oxygen (O2) and hydrogen (H2). The oxygen is vented into the breathable cabin air system, known as the Oxygen Generation System, while the explosive hydrogen is vented externally.
passages: The US Orbital Segment has redundant supplies of oxygen, from a pressurised storage tank on the "Quest" airlock module delivered in 2001, supplemented ten years later by ESA-built Advanced Closed-Loop System (ACLS) in the "Tranquility" module (Node 3), which produces by electrolysis. Hydrogen produced is combined with carbon dioxide from the cabin atmosphere and converted to water and methane.
  
  Section::::Systems.:Power and thermal control.
   In 2011, American news outlet CBS news and news magazine spaceflightnow reported "The OGA over the past six months has not been running well because the water that's been fed to it is just slightly too acidic," said station Flight Director Chris Edelen. "For the past several months, the station crew has been using oxygen brought up aboard visiting Progress supply ships, a European cargo craft and the Russian Elektron oxygen generator while awaiting delivery of the OGA repair equipment. The OGA, like the Elektron, uses electricity to split water molecules into hydrogen and oxygen. "
  
  Section::::Atmosphere.:Sabatier system.
   The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters.
  
  Carbon dioxide is removed from the air by the Russian "Vozdukh" system in "Zvezda," one Carbon Dioxide Removal Assembly (CDRA) located in the U.S. Lab module, and one CDRA in the U.S. Node 3 module. Other by-products of human metabolism, such as methane from the intestines and ammonia from sweat, are removed by activated charcoal filters or by the Trace Contaminant Control System (TCCS).
  
  Section::::Water recovery systems.
   The "Elektron" system aboard "Zvezda" and a similar system in "Destiny" generate oxygen aboard the station. The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters, a chemical oxygen generator system. Carbon dioxide is removed from the air by the Vozdukh system in "Zvezda". Other by-products of human metabolism, such as methane from the intestines and ammonia from sweat, are removed by activated charcoal filters.
   Section::::Atmosphere.:Oxygen generating system.
   Section::::Atmosphere.:Vika.
  
  The Vika or TGK oxygen generator, also known as Solid Fuel Oxygen Generation (SFOG) when used on the ISS, is a chemical oxygen generator originally developed by Roscosmos for Mir, and it provides an alternate oxygen generating system. It uses canisters of solid lithium perchlorate, which are burned to create gaseous oxygen. Each canister can supply the oxygen needs of one crewmember for one day.
  
  Section::::Atmosphere.:Vozdukh.
  
  Another Russian system, Vozdukh (Russian "Воздух", meaning "air"), removes carbon dioxide from the air based on the use of regenerable absorbers of carbon dioxide gas.
  
  Section::::Temperature and Humidity Control.
   By 15 June, the primary Russian computers were back online, and communicating with the US side of the station by bypassing a circuit, but secondary systems remained offline. NASA reported that without the computer that controls the oxygen levels, the station had 56 days of oxygen available.
   Elektron is a Russian Electrolytic Oxygen Generator, which was also used on Mir. It uses electrolysis to produce oxygen. This process splits water molecules reclaimed from other uses on board the station into oxygen and hydrogen via electrolysis. The oxygen is vented into the cabin and the hydrogen is vented into space. The three Russian Elektron oxygen generators on board the International Space Station have been plagued with problems, frequently forcing the crew to use backup sources (either bottled oxygen or the Vika system discussed below). To support a crew of six, NASA added the oxygen generating system discussed above.
   Part of the ROS atmosphere control system is the oxygen supply. Triple-redundancy is provided by the Elektron unit, solid fuel generators, and stored oxygen. The primary supply of oxygen is the Elektron unit which produces and by electrolysis of water and vents overboard. The 1 kW system uses approximately one litre of water per crew member per day. This water is either brought from Earth or recycled from other systems. "Mir" was the first spacecraft to use recycled water for oxygen production. The secondary oxygen supply is provided by burning -producing Vika cartridges (see also ISS ECLSS). Each 'candle' takes 5–20 minutes to decompose at 450–500 °C, producing 600 litres of . This unit is manually operated.
   The ISS has two water recovery systems. "Zvezda" contains a water recovery system that processes water vapor from the atmosphere that could be used for drinking in an emergency but is normally fed to the Elektron system to produce oxygen. The American segment has a Water Recovery System installed during STS-126 that can process water vapour collected from the atmosphere and urine into water that is intended for drinking. The Water Recovery System was installed initially in "Destiny" on a temporary basis in November 2008 and moved into "Tranquility" (Node 3) in February 2010.
   Oxygen generators on board the International Space Station produce oxygen from water using electrolysis; the hydrogen produced was previously discarded into space. As astronauts consume oxygen, carbon dioxide is produced, which must then be removed from the air and discarded as well. This approach required copious amounts of water to be regularly transported to the space station for oxygen generation in addition to that used for human consumption, hygiene, and other uses—a luxury that will not be available to future long-duration missions beyond low Earth orbit.
   The Sabatier system closes the loop in the ECLSS by combining waste hydrogen from the Oxygen generating system and carbon dioxide from the station atmosphere using the Sabatier reaction and preserving these valuable chemicals. The outputs of this reaction are water, and methane. The water is recycled to reduce the total amount of water that must be carried to the station from Earth, and the methane is vented overboard by the now shared hydrogen vent line installed for the Oxygen generating system.
  
  Section::::Atmosphere.:Elektron.
   The atmosphere on board the ISS is similar to the Earth's. Normal air pressure on the ISS is 101.3 kPa (14.7 psi); the same as at sea level on Earth. An Earth-like atmosphere offers benefits for crew comfort, and is much safer than a pure oxygen atmosphere, because of the increased risk of a fire such as that responsible for the deaths of the Apollo 1 crew. Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.
   The gross mass of the station changes over time. The total launch mass of the modules on orbit is about (). The mass of experiments, spare parts, personal effects, crew, foodstuff, clothing, propellants, water supplies, gas supplies, docked spacecraft, and other items add to the total mass of the station. Hydrogen gas is constantly vented overboard by the oxygen generators.
  
  Section::::Structure.
  
  The ISS is a third generation modular space station. Modular stations can allow modules to be added to or removed from the existing structure, allowing greater flexibility.
   Recharging the high-pressure tanks was accomplished by the Space Shuttle fleet until its retirement. When an orbiter was docked to the station's Pressurized Mating Adapters (PMA-2 or PMA-3), oxygen was routed through pressure lines from the PMAs to the "Quest" Airlock. The pumping of the oxygen from the docked spacecraft tanks into "Quest"s high-pressure tank was accomplished by the Oxygen Recharge Compressor Assembly (ORCA).
  
  After the retirement of the Space Shuttle fleet, the Nitrogen Oxygen Recharge System (NORS) and spacecraft from the Commercial Orbital Transportation Services (COTS) program will take over this task.
  
  Section::::Construction.
   Early on January 1, 2005, the Elektron generator, repaired in 2004, failed again, and the crew had to rely again on onboard oxygen.
  
  Section::::2006 – Venting of gas.
   For the ISS Dragon cargo flights, the ISS's Canadarm2 grapples its Flight-Releasable Grapple Fixture and berths Dragon to the station's US Orbital Segment using a Common Berthing Mechanism. The CRS Dragon does not have an independent means of maintaining a breathable atmosphere for astronauts and instead circulates in fresh air from the ISS. For typical missions, Dragon is planned to remain berthed to the ISS for about 30 days.
   The ISS has two rack sized CDRAs (Carbon Dioxide Removal Assemblies) for redundancy with the Lab CDRA primary and the Node 3 CDRA as a powered off backup because of issues with sticking valves in the Node 3. It can step in as a backup if necessary but is not the preferred CDRA because of the problems with the sticking Air Selector Valves (ASVs). The Russian segment has its own carbon dioxide scrubbers but they are not sufficient for the whole of the ISS.
   ISS ECLSS
  
  The International Space Station Environmental Control and Life Support System (ECLSS) is a life support system that provides or controls atmospheric pressure, fire detection and suppression, oxygen levels, waste management and water supply. The highest priority for the ECLSS is the ISS atmosphere, but the system also collects, processes, and stores waste and water produced and used by the crew—a process that recycles fluid from the sink, shower, toilet, and condensation from the air. The Elektron system aboard "Zvezda" and a similar system in "Destiny" generate oxygen aboard the station.
   Section::::Atmospheric components studied by SAGE III on ISS.:Nitrogen dioxide, nitrogen trioxide, and chlorine dioxide.
   Astronauts aboard the International Space Station preparing for extra-vehicular activity (EVA) "camp out" at low atmospheric pressure, , spending eight sleeping hours in the Quest airlock chamber before their spacewalk. During the EVA they breathe 100% oxygen in their spacesuits, which operate at , although research has examined the possibility of using 100% O at in the suits to lessen the pressure reduction, and hence the risk of DCS.
  
  Section::::Treatment.
   In CELSS, air is initially supplied by external supply, but is maintained by the use of foliage plants, which create oxygen in photosynthesis (aided by the waste-byproduct of human respiration, CO). Eventually, the main goal of a CELSS environment is to have foliage plants take over the complete and total production of oxygen needs; this would make the system a "closed", instead of "controlled", system.
  
  Section::::Components of CELSS.:Food / consumables production.
   Assembly resumed in 2006 with the arrival of STS-115 with "Atlantis", which delivered the station's second set of solar arrays. Several more truss segments and a third set of arrays were delivered on STS-116, STS-117, and STS-118. As a result of the major expansion of the station's power-generating capabilities, more pressurised modules could be accommodated, and the "Harmony" node and "Columbus" European laboratory were added. These were soon followed by the first two components of "Kibō". In March 2009, STS-119 completed the Integrated Truss Structure with the installation of the fourth and final set of solar arrays. The final section of "Kibō" was delivered in July 2009 on STS-127, followed by the Russian "Poisk" module. The third node, "Tranquility", was delivered in February 2010 during STS-130 by the Space Shuttle "Endeavour", alongside the Cupola, followed in May 2010 by the penultimate Russian module, "Rassvet". "Rassvet" was delivered by Space Shuttle "Atlantis" on STS-132 in exchange for the Russian Proton delivery of the US-funded "Zarya" module in 1998. The last pressurised module of the USOS, "Leonardo", was brought to the station in February 2011 on the final flight of "Discovery", STS-133. The Alpha Magnetic Spectrometer was delivered by "Endeavour" on STS-134 the same year.
   Section::::Structure.:Pressurised modules.:"Tranquility".
  
  Tranquility, also known as Node 3, is the third and last of the station's US nodes, it contains an additional life support system to recycle waste water for crew use and supplements oxygen generation. Like the other US nodes, it has six berthing mechanisms, five of which are currently in use. The first one connects to the station's core via the "Unity" module, others host the "Cupola", the PMA docking port #3, the "Leonardo" PMM and the Bigelow Expandable Activity Module. The final zenith port remains free.
  
  Section::::Structure.:Pressurised modules.:"Columbus".
   Electrical power is provided by two steerable solar power arrays on each module, which use photovoltaic cells to convert sunlight into electricity. Energy is stored to power the station when it passes into the Earth's shadow. Resupply ships will replenish fuel for the station's propulsion engines for station keeping, to counter the effects of atmospheric drag.
  
  Section::::Structure.:Docking.
  
raw_presuppositions: ISS should run out of oxygen.
raw_corrections: ISS turns water into oxygen and hydrogen.