Editing Human spaceflight (section)

== Safety concerns ==
There are two main sources of hazard in space flight: those due to the hostile space environment, and those due to possible equipment malfunctions. Addressing these issues is of great importance for NASA and other space agencies before conducting the first extended crewed missions to destinations such as Mars.<ref name="williams202104">{{cite web |url=https://www.universetoday.com/149835/every-challenge-astronauts-will-face-on-a-flight-to-mars/ |title=Every Challenge Astronauts Will Face on a flight to Mars |last=Williams |first=Matt |date=Feb 4, 2021 |website=Universe Today |access-date=Feb 5, 2021}}</ref>

=== Environmental hazards ===
{{See also|Bioastronautics|Space habitat|Effect of spaceflight on the human body|Locomotion in Space}}

Planners of human spaceflight missions face a number of safety concerns.

==== Life support ====
{{Main| Life support system}}

The basic needs for breathable air and drinkable water are addressed by the [[life support system]] of the spacecraft.

{{See also|Astronautical hygiene}}

==== Medical issues ====
{{See also|Effect of spaceflight on the human body|Sleep in space|Space medicine}}

Astronauts may not be able to quickly return to Earth or receive medical supplies, equipment, or personnel if a medical emergency occurs. The astronauts may have to rely for long periods on limited resources and medical advice from the ground.

The possibility of [[blindness]] and of [[Bone Loss|bone loss]] have been associated with human [[space flight]].<ref name="NYT-20140127">{{cite news |last=Chang |first=Kenneth |title=Beings Not Made for Space |url=https://www.nytimes.com/2014/01/28/science/bodies-not-made-for-space.html |date=27 January 2014 |work=[[The New York Times]] |access-date=27 January 2014 }}</ref><ref name="Wired-20120723">{{cite magazine |last=Mann |first=Adam |title=Blindness, Bone Loss, and Space Farts: Astronaut Medical Oddities |url=https://www.wired.com/wiredscience/2012/07/medicine-psychology-space/ |date=23 July 2012 |magazine=[[Wired (magazine)|Wired]] |access-date=23 July 2012 }}</ref>

On 31 December 2012, a [[NASA]]-supported study reported that spaceflight may harm the brains of [[astronauts]] and accelerate the onset of [[Alzheimer's disease]].<ref name="PLOS-20121231">{{cite journal |last1=Cherry |first1=Jonathan D. |last2=Frost |first2=Jeffrey L. |last3=Lemere |first3=Cynthia A. |last4=Williams |first4=Jacqueline P. |last5=Olschowka |first5=John A. |last6=O'Banion |first6=M. Kerry |title=Galactic Cosmic Radiation Leads to Cognitive Impairment and Increased Aβ Plaque Accumulation in a Mouse Model of Alzheimer's Disease |doi=10.1371/journal.pone.0053275 |volume=7 |issue=12 |page=e53275 |journal=[[PLoS ONE]]  |pmid=23300905 |date=2012 |pmc=3534034|bibcode=2012PLoSO...753275C |doi-access=free }}</ref><ref name="SpaceRef-20130101">{{cite web |title=Study Shows that Space Travel is Harmful to the Brain and Could Accelerate Onset of Alzheimer's |url=http://spaceref.com/news/viewpr.html?pid=39650 |date=1 January 2013 |publisher=SpaceRef |access-date=7 January 2013 |archive-date=21 May 2020 |archive-url=https://web.archive.org/web/20200521052054/http://spaceref.com/news/viewpr.html?pid=39650/ |url-status=dead }}</ref><ref name="NasaWatch-20130103">{{cite web |last=Cowing |first=Keith |author-link=Keith Cowing |title=Important Research Results NASA Is Not Talking About (Update) |url=http://nasawatch.com/archives/2013/01/important-resea.html |date=3 January 2013 |publisher=NASA Watch |access-date=7 January 2013 }}</ref>

In October 2015, the [[NASA Office of Inspector General]] issued a [[Effect of spaceflight on the human body|health hazards report]] related to [[space exploration]], which included the potential hazards of a [[human mission to Mars]].<ref name="AP-20151029">{{cite news |last=Dunn |first=Marcia |title=Report: NASA needs better handle on health hazards for Mars |url=http://apnews.excite.com/article/20151029/us-sci-space-travel-health-6dfd5b2c76.html |date=29 October 2015 |agency=Associated Press |access-date=30 October 2015 }}</ref><ref name="NASA-20151029oig">{{cite web |author=Staff |title=NASA's Efforts to Manage Health and Human Performance Risks for Space Exploration (IG-16-003) |url=https://oig.nasa.gov/audits/reports/FY16/IG-16-003.pdf |date=29 October 2015 |work=[[NASA]] |access-date=29 October 2015 |archive-date=30 October 2015 |archive-url=https://web.archive.org/web/20151030161330/https://oig.nasa.gov/audits/reports/FY16/IG-16-003.pdf |url-status=dead }}</ref>

On 2 November 2017, scientists reported, based on [[Magnetic resonance imaging|MRI studies]], that significant changes in the position and structure of the brain have been found in astronauts who have taken [[Effect of spaceflight on the human body|trips in space]]. Astronauts on longer space trips were affected by greater brain changes.<ref name="NEJM-20171102">{{cite journal |author=Roberts, Donna R. |display-authors=etal |title=Effects of Spaceflight on Astronaut Brain Structure as Indicated on MRI |date=2 November 2017 |journal=[[New England Journal of Medicine]] |volume=377 |issue=18 |pages=1746–1753  |doi=10.1056/NEJMoa1705129 |pmid=29091569 |s2cid=205102116 |doi-access=free }}</ref><ref name="QTZ-20171103">{{cite web |last=Foley |first=Katherine Ellen |title=Astronauts who take long trips to space return with brains that have floated to the top of their skulls |url=https://qz.com/1119668/space-travel-changes-astronauts-brains/ |date=3 November 2017 |work=[[Quartz (publication)|Quartz]] |access-date=3 November 2017 }}</ref>

Researchers in 2018 reported, after detecting the presence on the [[International Space Station]] (ISS) of five ''[[Enterobacter|Enterobacter bugandensis]]'' bacterial strains, none [[pathogen]]ic to humans, that [[microorganism]]s on ISS should be carefully monitored to assure a healthy environment for [[astronaut]]s.<ref name="EA-20181122">{{cite web |author=BioMed Central |title=ISS microbes should be monitored to avoid threat to astronaut health |url=https://www.eurekalert.org/pub_releases/2018-11/bc-ims112018.php |date=22 November 2018 |work=[[EurekAlert!]] |access-date=25 November 2018 |archive-date=26 November 2018 |archive-url=https://web.archive.org/web/20181126005739/https://www.eurekalert.org/pub_releases/2018-11/bc-ims112018.php |url-status=dead }}</ref><ref name="BMC-20181123">{{cite journal |author=Singh, Nitin K. |display-authors=etal |title=Multi-drug resistant Enterobacter bugandensis species isolated from the International Space Station and comparative genomic analyses with human pathogenic strains |date=23 November 2018 |journal=[[BMC Microbiology]] |volume=18 |issue=1 |page=175 |doi=10.1186/s12866-018-1325-2 |pmid=30466389 |pmc=6251167 |doi-access=free |bibcode=2018BMCMb..18..175S }}</ref>

In March 2019, NASA reported that latent [[virus]]es in humans may be activated during space missions, possibly adding more risk to astronauts in future deep-space missions.<ref name="EA-20190315">{{cite news |author=Staff |title=Dormant viruses activate during spaceflight – NASA investigates - The stress of spaceflight gives viruses a holiday from immune surveillance, putting future deep-space missions in jeopardy |url=https://www.eurekalert.org/pub_releases/2019-03/f-dva031519.php |date=15 March 2019 |work=[[EurekAlert!]] |access-date=16 March 2019 |archive-date=18 March 2019 |archive-url=https://web.archive.org/web/20190318230153/https://www.eurekalert.org/pub_releases/2019-03/f-dva031519.php |url-status=dead }}</ref>

On 25 September 2021, [[CNN]] reported that an alarm had sounded during the [[Inspiration4]] Earth-orbital journey on the [[SpaceX Dragon 2]]. The alarm signal was found to be associated with an apparent toilet malfunction.<ref name="CNN-20210925">{{cite news |last=Wattles |first=Jackie |title=An alarm went off on SpaceX's all-tourist space flight. The problem was the toilet |url=https://www.cnn.com/2021/09/25/tech/spacex-toilet-waste-management-system-scn/index.html |date=25 September 2021 |work=[[CNN]] |access-date=25 September 2021 }}</ref>

===== Microgravity =====
{{See also|Weightlessness}}
[[File:Space fluid shift.gif|thumb|The effects of microgravity on fluid distribution around the body (greatly exaggerated)]]

Medical data from astronauts in low Earth orbits for long periods, dating back to the 1970s, show several adverse effects of a microgravity environment: loss of [[bone]] density, decreased muscle strength and endurance, postural instability, and reductions in aerobic capacity. Over time these [[deconditioning]] effects can impair astronauts' performance or increase their risk of injury.<ref>
{{cite web|url=http://exploration.grc.nasa.gov/Exploration/Advanced/Human/Exercise/ |title=Exploration Systems Human Research Program – Exercise Countermeasures |work=NASA |url-status=dead |archive-url=https://web.archive.org/web/20081011052437/http://exploration.grc.nasa.gov/Exploration/Advanced/Human/Exercise/ |archive-date=11 October 2008 }}</ref>

In a weightless environment, astronauts put almost no weight on the back [[muscle]]s or leg muscles used for standing up, which causes the muscles to weaken and get smaller. Astronauts can lose up to twenty per cent of their muscle mass on spaceflights lasting five to eleven days. The consequent loss of strength could be a serious problem in case of a landing emergency.<ref>{{cite web|url = https://www.nasa.gov/pdf/64249main_ffs_factsheets_hbp_atrophy.pdf|title = NASA Information: Muscle Atrophy|access-date = 20 November 2015|website = NASA|archive-date = 22 July 2020|archive-url = https://web.archive.org/web/20200722232908/https://www.nasa.gov/pdf/64249main_ffs_factsheets_hbp_atrophy.pdf|url-status = dead}}</ref> Upon returning to Earth from [[long-duration spaceflight|long-duration]] flights, astronauts are considerably weakened and are not allowed to drive a car for twenty-one days.<ref>{{cite web|title = Earth Living Is Tough for Astronaut Used to Space|url = http://www.space.com/21413-hadfield-astronaut-health-return-earth.html|website = Space.com|date = 3 June 2013|access-date = 21 November 2015}}</ref>

Astronauts experiencing weightlessness will often lose their orientation, get [[motion sickness]], and lose their sense of direction as their bodies try to get used to a weightless environment. When they get back to Earth, they have to readjust and may have problems standing up, focusing their gaze, walking, and turning. Importantly, those motor disturbances only get worse the longer the exposure to weightlessness.<ref>{{cite news | url=https://abcnews.go.com/Technology/story?id=3830060&page=1 | last=Watson | first=Traci | date=11 November 2007 | title=Readjusting to gravity anti-fun for astronauts | publisher=ABC News | access-date=14 February 2020}}</ref> These changes can affect the ability to perform tasks required for approach and landing, docking, remote manipulation, and emergencies that may occur while landing.<ref>{{Cite journal|last1=Eddy|first1=D. R.|last2=Schiflett|first2=S. G.|last3=Schlegel|first3=R. E.|last4=Shehab|first4=R. L.|date=August 1998|title=Cognitive performance aboard the life and microgravity spacelab|url=https://pubmed.ncbi.nlm.nih.gov/11541924|journal=Acta Astronautica|volume=43|issue=3–6|pages=193–210|doi=10.1016/s0094-5765(98)00154-4|issn=0094-5765|pmid=11541924|bibcode=1998AcAau..43..193E}}</ref>

In addition, after long [[space flight]] missions, male astronauts may experience severe [[visual system|eyesight]] problems, which may be a major concern for future deep space flight missions, including a [[crewed mission]] to the planet [[Mars]].<ref name="Mader-2011">{{cite journal|author=Mader, T. H. |display-authors=etal |title=Optic Disc Edema, Globe Flattening, Choroidal Folds, and Hyperopic Shifts Observed in Astronauts after Long-duration Space Flight|date=2011 |journal=[[Ophthalmology (journal)|Ophthalmology]] |volume=118 |issue=10|pages=2058–2069 |doi=10.1016/j.ophtha.2011.06.021 |pmid=21849212|s2cid=13965518 |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1068&context=nasapub }}</ref><ref name="Puiu-20111109">{{cite web |last=Puiu |first=Tibi |title=Astronauts' vision severely affected during long space missions|url=http://www.zmescience.com/medicine/astronaut-eyesight-damage-weightlessness-3214143/|date=9 November 2011 |publisher=zmescience.com |access-date=9 February 2012 }}</ref><ref name="CNN-20120109">[http://www.cnn.com/video/#/video/us/2012/02/09/pkg-zarrella-astronaut-vision.cnnCNN News (CNN-TV, 02/09/2012) – Video (02:14) – Male Astronauts Return With Eye Problems]. CNN (9 February 2012). Retrieved on 22 November 2016.</ref><ref name="Space-20120313">{{cite web|title=Spaceflight Bad for Astronauts' Vision, Study Suggests |url=http://www.space.com/14876-astronaut-spaceflight-vision-problems.html |date=13 March 2012 |publisher=[[Space.com]] |access-date=14 March 2012 }}</ref><ref name="Radiology-20120313">{{cite journal |author=Kramer, Larry A. |display-authors=etal |title=Orbital and Intracranial Effects of Microgravity: Findings at 3-T MR Imaging |journal=[[Radiology (journal)|Radiology]] |volume=263 |issue=3 |pages=819–27 |doi=10.1148/radiol.12111986 |pmid=22416248 |date=13 March 2012 }}</ref><ref name="WIRED-20140212">{{cite magazine |author=Kevin Fong MD  |title=The Strange, Deadly Effects Mars Would Have on Your Body |url=https://www.wired.com/opinion/2014/02/happens-body-mars/ |date=12 February 2014 |magazine=[[Wired (magazine)|Wired]] |access-date=12 February 2014 }}</ref> Long space flights can also alter a space traveler's eye movements.<ref>{{cite journal |last1=Alexander |first1=Robert |last2=Macknik |first2=Stephen |last3=Martinez-Conde |first3=Susana |title=Microsaccades in applied environments: Real-world applications of fixational eye movement measurements |journal=Journal of Eye Movement Research |date=2020 |volume=12 |issue=6 |doi=10.16910/jemr.12.6.15 |pmid=33828760 |pmc=7962687 |doi-access=free }}</ref>

===== Radiation =====
{{See also|Health threat from cosmic rays}}
[[File:PIA17601-Comparisons-RadiationExposure-MarsTrip-20131209.png|thumb|right|Comparison of Radiation Doses – includes the amount detected on the trip from Earth to Mars by the [[Radiation assessment detector|RAD]] on the [[Mars Science Laboratory|MSL]] (2011–2013)<ref name="SCI-20130531a">{{cite journal |last=Kerr |first=Richard |title=Radiation Will Make Astronauts' Trip to Mars Even Riskier |date=31 May 2013 |journal=[[Science (journal)|Science]] |volume=340 |issue=6136 |page=1031 |doi=10.1126/science.340.6136.1031 |pmid=23723213 |bibcode=2013Sci...340.1031K}}</ref>]]

Without proper shielding, the crews of missions beyond low Earth orbit might be at risk from high-energy protons emitted by [[solar particle event]]s (SPEs) associated with [[solar flare]]s. If estimated correctly, the amount of radiation that astronauts would be exposed to from a solar storm similar to that of the most powerful in recorded history, the [[Carrington Event]], would result in [[acute radiation sickness]] at least, and could even be fatal "in a poorly shielded spacecraft".<ref>{{cite web |url=https://www.newscientist.com/article/dn7142 |title= Superflares could kill unprotected astronauts|work=New Scientist |date=21 March 2005 |first=Stephen |last=Battersby}}</ref>{{better source needed|reason=Citation does not differentiate between solar flares and solar particle events.|date=December 2021}} Another storm that could have inflicted a potentially lethal dose of radiation on astronauts outside Earth's protective [[magnetosphere]] occurred during the [[Space Age]], shortly after [[Apollo 16]] landed and before [[Apollo 17]] launched.<ref>{{cite journal |author1 = Mike Lockwood |author2 = M. Hapgood |title = The Rough Guide to the Moon and Mars |journal = Astron. Geophys. |volume = 48 |issue = 6 |pages = 11–17 |date = 2007 |doi = 10.1111/j.1468-4004.2007.48611.x |bibcode = 2007A&G....48f..11L |doi-access = free }}</ref> This solar storm, [[August 1972 solar storm|which occurred in August 1972]], could potentially have caused any astronauts who were exposed to it to suffer from acute radiation sickness, and may even have been lethal for those engaged in [[extravehicular activity]] or on the lunar surface.<ref>{{cite journal |author1 = Jennifer L. Parsons |author2 = L. W. Townsend |title = Interplanetary Crew Dose Rates for the August 1972 Solar Particle Event |journal = Radiat. Res. |volume = 153 |issue = 6 |pages = 729–733 |date = 2000 |doi = 10.1667/0033-7587(2000)153[0729:ICDRFT]2.0.CO;2 |pmid = 10825747 |bibcode = 2000RadR..153..729P |s2cid = 25250687 }}</ref>

Another type of radiation, galactic [[cosmic ray]]s, presents further challenges to human spaceflight beyond low Earth orbit.<ref>{{cite book|isbn=978-0-309-10264-3|url=http://www.nap.edu/catalog.php?record_id=11760 |title=Space Radiation Hazards and the Vision for Space Exploration |publisher=NAP |date=2006| doi=10.17226/11760 }}</ref>

There is also some scientific concern that extended spaceflight might slow down the body's ability to protect itself against diseases,<ref>{{cite journal|doi=10.1189/jlb.0309167 |title=Could spaceflight-associated immune system weakening preclude the expansion of human presence beyond Earth's orbit|date=2009|last1=Gueguinou|first1=N.|last2=Huin-Schohn|first2=C.|last3=Bascove|first3=M.|last4=Bueb|first4=J.-L.|last5=Tschirhart|first5=E.|last6=Legrand-Frossi|first6=C.|last7=Frippiat|first7=J.-P.|journal=Journal of Leukocyte Biology|volume=86|issue=5|pages=1027–1038|pmid=19690292|doi-access=}}</ref> resulting in a weakened [[immune system]] and the activation of dormant [[virus]]es in the body. [[Radiation]] can cause both short- and long-term consequences to the bone marrow stem cells from which blood and immune-system cells are created. Because the interior of a spacecraft is so small, a weakened immune system and more active viruses in the body can lead to a fast spread of infection.<ref>{{cite web |last1=Sohn |first1=Rebecca |title=How do viruses affect astronauts in space? |url=https://www.space.com/viruses-in-space-astronaut-health-impacts |website=Space.com |date=7 March 2022 |publisher=Future US |access-date=20 October 2022}}</ref>

===== Isolation =====
{{Further|Effect of spaceflight on the human body#Psychological effects|Psychological and sociological effects of spaceflight}}

During long missions, astronauts are isolated and confined in small spaces. [[Depression (mood)|Depression]], anxiety, [[cabin fever]], and other psychological problems may occur more than for an average person and could impact the crew's safety and mission success.<ref>{{Cite journal|last=Flynn|first=Christopher F.|date=1 June 2005|title=An Operational Approach to Long-Duration Mission Behavioral Health and Performance Factors|url=http://www.ingentaconnect.com/contentone/asma/asem/2005/00000076/A00106s1/art00007|journal=Aviation, Space, and Environmental Medicine|volume=76|issue=6|pages=B42–B51|pmid=15943194}}</ref> NASA spends millions of dollars on psychological treatments for astronauts and former astronauts.<ref>{{Cite book |title=Space psychology and psychiatry |date=2008 |publisher=Springer |last1=Kanas |first1=Nick |last2=Manzey |first2=Dietrich |isbn=9781402067709 |edition=2nd |location=Dordrecht |oclc=233972618}}</ref> To date, there is no way to prevent or reduce mental problems caused by extended periods of stay in space.

Due to these mental disorders, the efficiency of astronauts' work is impaired; and sometimes they are brought back to Earth, incurring the expense of their mission being aborted.<ref>{{Cite news |url=https://www.theguardian.com/science/2014/oct/05/hallucinations-isolation-astronauts-mental-health-space-missions|title=Isolation and hallucinations: the mental health challenges faced by astronauts |last=Bell|first=Vaughan|date=5 October 2014|work=The Observer|access-date=2019-02-01|language=en-GB |issn=0029-7712}}</ref> A Russian expedition to space in 1976 was returned to Earth after the cosmonauts reported a strong odor that resulted in a fear of fluid leakage; but after a thorough investigation, it became clear that there was no leakage or technical malfunction. It was concluded by NASA that the cosmonauts most likely had [[Phantosmia|hallucinated the smell]].

It is possible that the mental health of astronauts can be affected by the changes in the sensory systems while in prolonged space travel.

===== Sensory systems =====
During astronauts' spaceflight, they are in an extreme environment. This, and the fact that little change is taking place in the environment, will result in the weakening of sensory input to the astronauts' seven senses.
* [[Hearing]] – In the space station and spacecraft there are no noises from the outside, as there is no medium that can transmit sound waves. Although there are other team members who can talk to each other, their voices become familiar and do not stimulate the sense of hearing as much. Mechanical noises become familiar, as well.
* [[Sight]] – Because of weightlessness, the body's liquids attain an equilibrium that is different from what it is on the Earth. For this reason, an astronaut's face swells and presses on the eyes; and therefore their vision is impaired. The landscape surrounding the astronauts is constant, which lessens visual stimulations. Due to cosmic rays, astronauts may see flashes, even with their eyelids closed.
* [[Sense of smell|Smell]] – The space station has a permanent odor described as the smell of gunpowder. Due to the zero gravity, the bodily fluids rise to the face and prevent the sinuses from drying up, which dulls the sense of smell.
* [[Taste]] – The sense of taste is directly affected by the sense of smell and therefore when the sense of smell is dulled, the sense of taste is also. The astronauts' food is bland, and there are only certain foods that can be eaten. The food comes only once every few months, when supplies arrive, and there is little or no variety.
* [[Somatosensory system|Touch]] – There are almost no stimulating changes in physical contact. There is almost no human physical contact during the journey.
* The [[vestibular system]] (motion and equilibrium system) – Due to the lack of gravity, all the movements required of the astronauts are changed, and the vestibular system is damaged by the extreme change.
* The [[Proprioception|proprioception system]] (the sense of the relative position of one's own parts of the body and strength of effort being employed in movement) – As a result of weightlessness, few forces are exerted on the astronauts' muscles; and there is less stimulus to this system.

=== Equipment hazards ===
Space flight requires much higher velocities than ground or air transportation, and consequently requires the use of high [[energy density]] propellants for launch, and the dissipation of large amounts of energy, usually as heat, for safe reentry through the Earth's atmosphere.

==== Launch ====
{{See also| Launch escape system}}
[[File:Sts33-e204.jpg|thumb|There was no practical way for the {{OV|99}}'s crew to safely abort before the [[Space Shuttle Challenger disaster|vehicle's violent disintegration]]]]

Since rockets have the potential for fire or explosive destruction, [[space capsule]]s generally employ some sort of [[launch escape system]], consisting either of a tower-mounted solid-fuel rocket to quickly carry the capsule away from the [[launch vehicle]] (employed on [[Project Mercury|Mercury]], [[Apollo (spacecraft)|Apollo]], and [[Soyuz spacecraft|Soyuz]], the escape tower being discarded at some point after launch, at a point where an abort can be performed using the spacecraft's engines), or else [[ejection seat]]s (employed on [[Vostok spacecraft|Vostok]] and [[Project Gemini|Gemini]]) to carry astronauts out of the capsule and away for individual parachute landings.

Such a launch escape system is not always practical for multiple-crew-member vehicles (particularly [[spaceplane]]s), depending on the location of egress hatch(es). When the single-hatch Vostok capsule was modified to become the 2 or 3-person [[Voskhod (spacecraft)|Voskhod]], the single-cosmonaut ejection seat could not be used, and no escape tower system was added. The two Voskhod flights in 1964 and 1965 avoided launch mishaps. The [[Space Shuttle]] carried ejection seats and escape hatches for its pilot and copilot in early flights; but these could not be used for passengers who sat below the flight deck on later flights, and so were discontinued.

There have been only two in-flight launch aborts of a crewed flight. The first occurred on [[Soyuz 7K-T No.39|Soyuz 18a]] on 5 April 1975. The abort occurred after the launch escape system had been jettisoned when the launch vehicle's spent second stage failed to separate before the third stage ignited and the vehicle strayed off course. The crew finally managed to separate the spacecraft, firing its engines to pull it away from the errant rocket, and both cosmonauts landed safely. The second occurred on 11 October 2018 with the launch of [[Soyuz MS-10]]. Again, both crew members survived.

In the first use of a launch escape system on the launchpad, before the start of a crewed flight, happened during the planned [[Soyuz 7K-ST No. 16L|Soyuz T-10a]] launch on 26 September 1983, which was aborted by a launch vehicle fire 90 seconds before liftoff. Both cosmonauts aboard landed safely.

The only crew fatality during launch occurred on 28 January 1986, when the [[Space Shuttle Challenger disaster|Space Shuttle ''Challenger'']] broke apart 73 seconds after liftoff, due to the failure of a [[Space Shuttle Solid Rocket Booster|solid rocket booster]] seal, which caused the failure of the [[Space Shuttle external tank|external fuel tank]], resulting in an explosion of the fuel and separation of the boosters. All seven crew members were killed.

==== Extravehicular activity ====
{{main | Extravehicular activity}}
Tasks outside a spacecraft require use of a [[space suit]]. Despite the risk of mechanical failures while working in open space, there have been no spacewalk fatalities. Spacewalking astronauts routinely remain attached to the spacecraft with tethers and sometimes supplementary anchors. Un-tethered spacewalks were performed on three missions in 1984 using the [[Manned Maneuvering Unit]], and on a flight test in 1994 of the [[Simplified Aid For EVA Rescue]] (SAFER) device.

==== Reentry and landing ====
{{See also| Atmospheric reentry}}

The single pilot of [[Soyuz 1]], [[Vladimir Komarov]], was killed when his capsule's parachutes failed during an emergency landing on 24 April 1967, causing the capsule to crash.

On 1 February 2003, the crew of seven aboard the {{OV|102}} were [[Space Shuttle Columbia disaster|killed on reentry]] after completing a [[STS-107|successful mission in space]]. A wing-leading-edge [[reinforced carbon-carbon]] heat shield had been damaged by a piece of frozen [[Space Shuttle external tank|external tank]] foam insulation that had broken off and struck the wing during launch. Hot reentry gasses entered and destroyed the wing structure, leading to the breakup of the [[Space Shuttle orbiter|orbiter vehicle]].

==== Artificial atmosphere ====
There are two basic choices for an artificial atmosphere: either an Earth-like mixture of oxygen and an inert gas such as nitrogen or helium, or pure oxygen, which can be used at lower than standard atmospheric pressure. A nitrogen–oxygen mixture is used in the International Space Station and Soyuz spacecraft, while low-pressure pure oxygen is commonly used in space suits for [[extravehicular activity]].

The use of a gas mixture carries the risk of [[decompression sickness]] (commonly known as "the bends") when transitioning to or from the pure oxygen space suit environment. There have been instances of injury and fatalities caused by suffocation in the presence of too much nitrogen and not enough oxygen.
* In 1960, [[McDonnell Aircraft]] test pilot G.B. North passed out and was seriously injured when testing a Mercury cabin–space suit atmosphere system in a vacuum chamber, due to nitrogen-rich air leaking from the cabin into his space suit feed.<ref>{{cite journal |last=Giblin |first=Kelly A. |date=Spring 1998 |title =Fire in the Cockpit! |journal=[[American Heritage of Invention & Technology]] |volume=13 |issue=4 |publisher=American Heritage Publishing |url=http://www.americanheritage.com/articles/magazine/it/1998/4/1998_4_46.shtml |archive-url=https://web.archive.org/web/20081120153024/http://www.americanheritage.com/articles/magazine/it/1998/4/1998_4_46.shtml |archive-date=20 November 2008 |access-date=23 March 2011}}</ref> This incident led NASA to decide on a pure oxygen atmosphere for the Mercury, Gemini, and Apollo spacecraft.
* In 1981, three pad workers were killed by a nitrogen-rich atmosphere in the aft engine compartment of the {{OV|102}} at the [[Kennedy Space Center Launch Complex 39]].<ref>[https://web.archive.org/web/20010605212352/http://www-lib.ksc.nasa.gov/lib/chrono.html 1981 KSC Chronology Part 1 – pages 84, 85, 100; Part 2 – pages 181, 194, 195], NASA</ref>
* In 1995, two pad workers were similarly killed by a nitrogen leak in a confined area of the [[Ariane 5]] launch pad at [[Guiana Space Centre]].<ref>[http://www.esa.int/esaCP/Pr_17_1995_p_EN.html "Fatal accident at the Guiana Space Centre"], ''ESA Portal'', 5 May 1993</ref>

A pure oxygen atmosphere carries the risk of fire. The original design of the Apollo spacecraft used pure oxygen at greater than atmospheric pressure prior to launch. An electrical fire started in the cabin of [[Apollo 1]] during a ground test at [[Cape Canaveral Air Force Station Launch Complex 34|Cape Kennedy Air Force Station Launch Complex 34]] on 27 January 1967, and spread rapidly. The high pressure, increased by the fire, prevented removal of the [[plug door]] hatch cover in time to rescue the crew. All three astronauts—[[Gus Grissom]], [[Ed White (astronaut)|Ed White]], and [[Roger Chaffee]]—were killed.<ref name="SP4029">{{cite book |last=Orloff |first=Richard W. |title=Apollo by the Numbers: A Statistical Reference |url=https://history.nasa.gov/SP-4029/SP-4029.htm |access-date=12 July 2013 |series=NASA History Series |orig-year=First published 2000 |date=September 2004 |publisher=NASA |location=Washington, D.C. |isbn=978-0-16-050631-4 |lccn=00061677 |id=NASA SP-2000-4029 |chapter=Apollo 1 – The Fire: 27 January 1967 |chapter-url=https://history.nasa.gov/SP-4029/Apollo_01a_Summary.htm}}</ref> This led NASA to use a nitrogen–oxygen atmosphere before launch, and low-pressure pure oxygen only in space.

==== Reliability ====
{{See also| Reliability engineering}}

The March 1966 [[Gemini 8]] mission was aborted in orbit when an [[attitude control system]] thruster stuck in the on position, sending the craft into a dangerous spin that threatened the lives of [[Neil Armstrong]] and [[David Scott]]. Armstrong had to shut the control system off and use the reentry control system to stop the spin. The craft made an emergency reentry and the astronauts landed safely. The most probable cause was determined to be an electrical short due to a [[static electricity]] discharge, which caused the thruster to remain powered even when switched off. The control system was modified to put each thruster on its own isolated circuit.

The third lunar landing expedition, [[Apollo 13]], in April 1970, was aborted and the lives of the crew—[[Jim Lovell|James Lovell]], [[Jack Swigert]], and [[Fred Haise]]—were threatened after the failure of a [[cryogenic]] [[liquid oxygen]] tank en route to the Moon. The tank burst when electrical power was applied to internal stirring fans in the tank, causing the immediate loss of all of its contents, and also damaging the second tank, causing the gradual loss of its remaining oxygen over a period of 130 minutes. This in turn caused a loss of electrical power provided by [[fuel cell]]s to the [[Apollo Command/Service Module|command spacecraft]]. The crew managed to return to Earth safely by using the [[Apollo Lunar Module|lunar landing craft]] as a "life boat". The tank failure was determined to be caused by two mistakes: the tank's drain fitting had been damaged when it was dropped during factory testing, necessitating the use of its internal heaters to boil out the oxygen after a pre-launch test; which in turn damaged the fan wiring's electrical insulation because the thermostats on the heaters did not meet the required voltage rating due to a vendor miscommunication.

The crew of [[Soyuz 11]] were killed on 30 June 1971 by a combination of mechanical malfunctions; the crew were [[asphyxia]]ted due to cabin decompression following the separation of their descent capsule from the service module. A cabin ventilation valve had been jolted open at an altitude of {{convert|168|km}} by the stronger-than-expected shock of explosive separation bolts, which were designed to fire sequentially, but in fact had fired simultaneously. The loss of pressure became fatal within about 30 seconds.<ref>{{Cite web|url=https://history.nasa.gov/SP-4209/ch8-2.htm|title=The Partnership: A History of the Apollo–Soyuz Test Project|access-date=20 October 2007|publisher=NASA|year=1974|archive-url=https://web.archive.org/web/20070823124845/https://history.nasa.gov/SP-4209/ch8-2.htm|archive-date=23 August 2007}}</ref>

=== Fatality risk ===
{{Further|List of spaceflight-related accidents and incidents}}

{{As of|2015|December}}, 23 crew members have died in accidents aboard spacecraft. Over 100 others have died in accidents during activities directly related to spaceflight or testing.

{| class="wikitable"
|-
!Date
!Mission
!Accident cause
!Deaths
!Cause of death
|-
| 27 January 1967
| [[Apollo 1]]
| Electrical fire in the cabin, spread quickly by {{convert|16.7|psi|bar|abbr=on}} pure oxygen atmosphere and flammable nylon materials in cabin and space suits, during pre-launch test; inability to remove [[plug door]] hatch cover due to internal pressure; rupture of cabin wall allowed outside air to enter, causing heavy smoke and soot
| style="text-align: center;" | 3
| [[Cardiac arrest]] from [[carbon monoxide]] poisoning
|-
| 24 April 1967
| [[Soyuz 1]]
| Malfunction of primary landing parachute, and entanglement of reserve parachute; loss of 50% electrical power and spacecraft control problems necessitating emergency abort
| style="text-align: center;"| 1
| [[Physical trauma|Trauma]] from crash landing
|-
| 15 November 1967
| [[X-15 Flight 3-65-97]]
| The accident board found that the cockpit instrumentation had been functioning properly, and concluded that pilot [[Michael J. Adams]] had lost control of the X-15 as a result of a combination of distraction, misinterpretation of his instrumentation display, and possible [[Vertigo (medical)|vertigo]]. The electrical disturbance early in the flight degraded the overall effectiveness of the aircraft's control system and further added to pilot workload.
| style="text-align: center;"| 1
| Vehicle breakup
|-
| 30 June 1971
| [[Soyuz 11]]
| Loss of cabin pressurization due to valve opening upon Orbital Module separation before re-entry
| style="text-align: center;"| 3
| [[Asphyxia]]
|-
| 28 January 1986
| [[STS-51L]] [[Space Shuttle Challenger disaster|Space Shuttle ''Challenger'']]
| Failure of [[O-ring]] inter-segment seal in one [[Space Shuttle Solid Rocket Booster|Solid Rocket Booster]] in extreme cold launch temperature, allowing hot gases to penetrate casing and burn through a strut connecting booster to the [[Space Shuttle external tank|External Tank]]; tank failure; rapid combustion of fuel; orbiter breakup from abnormal aerodynamic forces
| style="text-align: center;"| 7
| Asphyxia from cabin breach, or trauma from water impact<ref>{{cite web|url=https://history.nasa.gov/kerwin.html |title=Report from Joseph P. Kerwin, biomedical specialist from the Johnson Space Center in Houston, Texas, relating to the deaths of the astronauts in the Challenger accident |work=NASA |url-status=dead |archive-url=https://web.archive.org/web/20130103015825/https://history.nasa.gov/kerwin.html |archive-date=3 January 2013 }}</ref>
|-
| 1 February 2003
| [[STS-107]] [[Space Shuttle Columbia disaster|Space Shuttle ''Columbia'']]
| Damaged [[reinforced carbon-carbon]] heat shield panel on wing's leading edge, caused by a piece of [[Space Shuttle external tank|External Tank]] foam insulation broken off during launch; penetration of hot atmospheric gases during re-entry, leading to structural failure of the wing, loss of control and disintegration of the orbiter
| style="text-align: center;"| 7
| Asphyxia from cabin breach, trauma from dynamic load environment as orbiter broke up<ref>{{cite web |title=Columbia Crew Survival Investigation Report |url=http://www.nasa.gov/pdf/298870main_SP-2008-565.pdf |website=NASA.gov |publisher=NASA}}</ref>
|-
| 31 October 2014
| [[SpaceShipTwo]] [[VSS Enterprise crash|VSS ''Enterprise'' powered drop-test]]
| Copilot error: premature deployment of "[[Feathering (reentry)|feathering]]" descent air-braking system caused the disintegration of the vehicle in flight; pilot survived, copilot died
| style="text-align: center;"| 1
| [[Physical trauma|Trauma]] from crash
|}