Editing Human spaceflight (section)

===== 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>