Editing Human spaceflight (section)

=== 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>