Editing International Space Station (section)

====Atmospheric control systems====
[[File:SpaceStationCycle.svg|thumb|upright=2|The interactions between the components of the ISS Environmental Control and Life Support System (ECLSS)|alt=A flowchart diagram showing the components of the ISS life support system.]]
The atmosphere on board the ISS is similar to [[Atmosphere of Earth|that of Earth]].<ref>{{cite web|last=Freudenrich|first=Craig|date=20 November 2000|title=How Space Stations Work|url=http://science.howstuffworks.com/space-station2.htm|url-status=dead|archive-url=https://web.archive.org/web/20081212014934/http://science.howstuffworks.com/space-station2.htm|archive-date=12 December 2008|access-date=23 November 2008|publisher=Howstuffworks}}</ref> Normal air pressure on the ISS is {{cvt|101.3|kPa}};<ref>{{cite web|title=5–8: The Air Up There|url=http://nasaexplores.com/show2_5_8a.php?id=04-032&gl=58|url-status=dead|archive-url=https://archive.today/20041218024247/http://nasaexplores.com/show2_5_8a.php?id=04-032&gl=58|archive-date=18 December 2004|access-date=31 October 2008|website=NASAexplores|publisher=NASA}}</ref> 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.<ref name="apollo-204-report">{{Cite report|url=https://history.nasa.gov/as204_senate_956.pdf|title=Apollo 204 Accident: Report of the Committee on Aeronautical and Space Sciences, United States Senate|last1=Anderson|first1=Clinton P.|date=30 January 1968|publisher=US Government Printing Office|location=Washington, D.C.|page=8|id=Report No. 956|url-status=live|archive-url=https://web.archive.org/web/20230810130754/https://history.nasa.gov/as204_senate_956.pdf|archive-date=10 August 2023|display-authors=etal}}</ref>{{better source needed|Source is 54 years old and does NOT conclude that 100% oxygen atmospheres should be completely avoided – the problem of Apollo 1 was the testing with pure oxygen at 16.7 psi (1.2 atm) overpressure, i.e. six times the natural partial pressure of oxygen. It explicitly states on p. 11 that "NASA has recommended by detailed review that the inflight cabin atmosphere, outside the Earth's atmosphere, should continue to be 100 percent oxygen at 5 p.s.i.A." (c. 0.3 atm). FWIW, Andy Weir's 2021 SF novel [[Project Hail Mary]] claims that an Earth-like atmosphere is maintained in near-Earth space stations to simplify evacuation in case of emergency.|date=January 2022}} Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.<ref name="spacemed">{{Cite book|last1=Davis|first1=Jeffrey R.|title=Fundamentals of Aerospace Medicine|last2=Johnson|first2=Robert|last3=Stepanek|first3=Jan|publisher=Lippincott Williams & Wilkins|year=2008|volume=XII|place=Philadelphia, Pennsylvania, USA|pages=261–264|name-list-style=amp}}</ref>

The ''[[Elektron (ISS)|Elektron]]'' system aboard ''Zvezda'' and a similar system in ''Destiny'' generate oxygen aboard the station.<ref name="OGS">{{Cite news|url=https://www.space.com/2052-air-apparent-oxygen-systems-iss.html|title=Air Apparent: New Oxygen Systems for the ISS|last=Malik|first=Tariq|date=15 February 2006|access-date=21 November 2008|url-status=live|archive-url=https://web.archive.org/web/20230814171139/https://www.russianspaceweb.com/ros.html|archive-date=14 August 2023|work=[[Space.com]]}}</ref> The crew has a backup option in the form of bottled oxygen and [[Vika oxygen generator|Solid Fuel Oxygen Generation]] (SFOG) canisters, a [[chemical oxygen generator]] system.<ref name="breath easy">{{cite web|first=Patrick L.|last=Barry|date=13 November 2000|title=Breathing Easy on the Space Station|url=https://science.nasa.gov/headlines/y2000/ast13nov_1.htm|url-status=dead|archive-url=https://web.archive.org/web/20080921141609/https://science.nasa.gov/headlines/y2000/ast13nov_1.htm|archive-date=21 September 2008|access-date=21 November 2008|publisher=NASA}}</ref> Carbon dioxide is removed from the air by the [[ISS ECLSS#Vozdukh|Vozdukh]] system in ''Zvezda''. Other by-products of human metabolism, such as methane from the intestines and ammonia from sweat, are removed by [[Activated carbon|activated charcoal]] filters.<ref name="breath easy" />

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 {{chem2|O2}} and {{chem2|H2}} by [[electrolysis]] of water and vents {{chem2|H2}} overboard. The {{cvt|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 oxygen-producing [[Vika oxygen generator|Vika]] cartridges (see also [[ISS ECLSS]]). Each 'candle' takes 5–20 minutes to decompose at {{convert|450|-|500|C}}, producing {{convert|600|L}} of {{chem2|O2}}. This unit is manually operated.<ref>{{cite web|url=http://suzymchale.com/ruspace/issrslss.html|title=RuSpace {{!}} ISS Russian Segment Life Support System|archive-url=https://web.archive.org/web/20110809155257/http://suzymchale.com/ruspace/issrslss.html|archive-date=9 August 2011|url-status=dead|website=Suzymchale.com|access-date=8 October 2011}}</ref>

The US Orbital Segment (USOS) 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 {{chem2|O2}} by electrolysis.<ref name="nasa-20001113">{{Cite web|url=https://science.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1|title=Breathing Easy on the Space Station|date=13 November 2000|publisher=[[NASA]]|access-date=8 October 2011|url-status=dead|archive-url=https://web.archive.org/web/20190311204439/https://science.nasa.gov/science-news/science-at-nasa/2000/ast13nov_1|archive-date=11 March 2019}}</ref> Hydrogen produced is combined with carbon dioxide from the cabin atmosphere and converted to water and methane.