Editing Artificial gravity

{{Short description|Use of circular rotational force to mimic gravity}}
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[[File:The Agena Target Docking Vehicle at a distance of approximately 80 feet from the Gemini-11 spacecraft.jpg|thumb|[[Gemini 11]] tethered in 1966 the GATV-5006 [[Agena target vehicle]] performing various tests including a first artificial gravity test in a [[microgravity]] environment.]]
[[File:Nautilus-X ISS demo 1.png|thumb|Proposed [[Nautilus-X]] International space station centrifuge demo concept, 2011]]

'''Artificial gravity''' is the creation of an [[inertial force]] that mimics the effects of a [[Gravity|gravitational]] force, usually by [[Circular motion|rotation]].<ref name="iaaweb.org">{{Cite book |editor-last2=Yajima |editor-first2=Kazuyoshi |editor-last3=Paloski |editor-first3=William |editor-first1=Laurence |editor-last1=Young |url=https://iaaweb.org/iaa/Scientific%20Activity/Study%20Groups/SG%20Commission%202/sg22/sg22finalreportr.pdf |title=Artificial Gravity Research to enable Human Space Exploration |date=September 2009 |isbn=978-2-917761-04-5 |access-date=February 23, 2022 |archive-url=https://web.archive.org/web/20161013004743/http://iaaweb.org/iaa/Scientific%20Activity/Study%20Groups/SG%20Commission%202/sg22/sg22finalreportr.pdf |archive-date=October 13, 2016 |url-status=dead |publisher=[[International Academy of Astronautics]] }}</ref> 
Artificial gravity, or '''rotational gravity''', is thus the appearance of a [[centrifugal force]] in a [[rotating frame of reference]] (the transmission of [[centripetal acceleration]] via [[normal force]] in the non-rotating frame of reference), as opposed to the force experienced in [[linear acceleration]], which by the [[equivalence principle]] is indistinguishable from gravity.
In a more general sense, "artificial gravity" may also refer to the effect of linear acceleration, e.g. by means of a [[rocket engine]].<ref name="iaaweb.org"/>

Rotational simulated gravity has been used in simulations to help astronauts train for extreme conditions.<ref>{{cite journal | pmid = 18619137 | volume=79 | issue=7 | title=Space medicine at the NASA-JSC, neutral buoyancy laboratory | date=July 2008 | journal=[[Aerospace Medicine and Human Performance|Aviation, Space, and Environmental Medicine]] | pages=732–733 | last1 = Strauss | first1 = Samuel | issn = 0095-6562 | lccn = 75641492 | oclc = 165744230}}</ref> 
Rotational simulated gravity has been proposed as a solution in [[human spaceflight]] to the adverse [[Effect of spaceflight on the human body|health effects caused by prolonged weightlessness]].<ref name="clement2015">{{Cite tech report |last1=Clément |first1=Gilles |last2=Charles |first2=John B. |last3=Norsk |first3=Peter |last4=Paloski |first4=William H. |url=https://ntrs.nasa.gov/citations/20150009486 |title=Human Research Program Human Health Countermeasures Element: Evidence Report - Artificial Gravity |date=February 15, 2015 |publisher=[[NASA]] |archive-url=https://web.archive.org/web/20240312223508/https://ntrs.nasa.gov/api/citations/20150009486/downloads/20150009486.pdf |archive-date=March 12, 2024 |hdl=2060/20150009486 |url-status=live }}</ref> 
However, there are no current practical outer space applications of artificial gravity for humans due to concerns about the size and cost of a spacecraft necessary to produce a useful [[centripetal force]] comparable to the gravitational field strength on Earth (''g'').<ref name="popularmechanics.com">{{Cite magazine |last1=Feltman |first1=Rachel |date=May 3, 2013 |title=Why Don't We Have Artificial Gravity? |url=https://www.popularmechanics.com/space/rockets/a8965/why-dont-we-have-artificial-gravity-15425569/ |url-status=live |archive-url=https://web.archive.org/web/20220101150056/https://www.popularmechanics.com/space/rockets/a8965/why-dont-we-have-artificial-gravity-15425569/ |archive-date=January 1, 2022 |access-date=February 23, 2022 |magazine=[[Popular Mechanics]] |issn=0032-4558 |oclc=671272936 }}</ref>
Scientists are concerned about the effect of such a system on the inner ear of the occupants. The concern is that using centripetal force to create artificial gravity will cause disturbances in the inner ear leading to nausea and disorientation. The adverse effects may prove intolerable for the occupants.<ref name="ncbi.nlm.nih.gov">{{Cite journal|last1=Clément|first1=Gilles R.|last2=Bukley|first2=Angelia P.|last3=Paloski|first3=William H.|date=2015-06-17|title=Artificial gravity as a countermeasure for mitigating physiological deconditioning during long-duration space missions|journal=Frontiers in Systems Neuroscience|volume=9|page=92|doi=10.3389/fnsys.2015.00092|issn=1662-5137|pmc=4470275|pmid=26136665|doi-access=free}}</ref>

==Centrifugal force==
{{see|Absolute rotation|Bucket argument}}
[[File:Artificial Gravity Space Station - GPN-2003-00104.jpg|thumb|Artificial gravity space station. 1969 NASA concept. A drawback is that the astronauts would be moving between higher gravity near the ends and lower gravity near the center.]]
In the context of a rotating space station, it is the radial force provided by the spacecraft's hull that acts as centripetal force. Thus, the "gravity" force felt by an object is the [[centrifugal force]] perceived in the [[rotating frame of reference]] as pointing "downwards" towards the hull.

By [[Newton's third law]], the value of [[Little g|little ''g'']] (the perceived "downward" acceleration) is equal in magnitude and opposite in direction to the centripetal acceleration. It was tested with satellites like [[Bion 3]] (1975) and [[Bion 4]] (1977); they both had [[centrifuge]]s on board to put some specimens in an artificial gravity environment.

===Differences from normal gravity===
[[Image:ArtificialGravity.gif|right|thumb|Balls in a rotating spacecraft]]

From the perspective of people rotating with the habitat, artificial gravity by rotation behaves similarly to normal gravity but with the following differences, which can be mitigated by increasing the radius of a space station.
* Centrifugal force varies with distance: Unlike real gravity, the apparent force felt by observers in the habitat pushes radially outward from the axis, and the centrifugal force is directly proportional to the distance from the axis of the habitat. With a small radius of rotation, a standing person's head would feel significantly less gravity than their feet.<ref name="symp1973">{{Cite conference |last=Stone |first=Ralph W. |date=August 1970 |title=An Overview of Artificial Gravity |url=https://ntrs.nasa.gov/api/citations/19740010641/downloads/19740010641.pdf#page=35 |conference=Fifth Symposium on the Role of the Vestibular Organs in Space Exploration |location=Naval Aerospace Medical Institute, Pensacola, Florida |publisher=[[NASA]] |page=25 |oclc=4200952 |id=SP-314 |archive-url=https://web.archive.org/web/20240415123635/https://ntrs.nasa.gov/api/citations/19740010641/downloads/19740010641.pdf#page=35 |archive-date=April 15, 2024 |hdl=2060/19740010641 |url-status=live }}</ref> Likewise, passengers who move in a space station experience changes in apparent weight in different parts of the body.<ref name="davis1994">{{Cite journal |last1=Davis |first1=BL |last2=Cavanagh |first2=PR |last3=Perry |first3=JE |date=September 1994 |title=Locomotion in a rotating space station: a synthesis of new data with established concepts |journal=Gait & Posture |volume=2 |issue=3 |pages=157–165 |doi=10.1016/0966-6362(94)90003-5 |issn=0966-6362 |pmid=11539277 }}</ref>
* The [[Coriolis effect]] gives an apparent force that acts on objects that are moving relative to a rotating reference frame. This apparent force acts at right angles to the motion and the rotation axis and tends to curve the motion in the opposite sense to the habitat's spin. If an [[astronaut]] inside a rotating artificial gravity environment moves towards or away from the axis of rotation, they will feel a force pushing them in or against the direction of spin. These forces act on the [[semicircular canals]] of the inner ear and can cause [[dizziness]].<ref name="alfred1969">{{Cite tech report |last1=Larson |first1=Carl Alfred |url=https://ntrs.nasa.gov/api/citations/19690029825/downloads/19690029825.pdf |title=Rotating Space Station Stabilization Criteria for Artificial Gravity |date=October 1969 |publisher=[[NASA]] |id=NASA-TN-D-5426 |archive-url=https://web.archive.org/web/20240815073410/https://ntrs.nasa.gov/api/citations/19690029825/downloads/19690029825.pdf |archive-date=August 15, 2024 |url-status=live |hdl=2060/19690029825 }}</ref> Lengthening the period of rotation (lower spin rate) reduces the Coriolis force and its effects. It is generally believed that at 2&nbsp;[[Revolutions per minute|rpm]] or less, no adverse effects from the Coriolis forces will occur, although humans have been shown to adapt to rates as high as 23&nbsp;[[Revolutions per minute|rpm]].<ref name=hecht2002>{{cite journal |title=Adapting to artificial gravity (AG) at high rotational speeds |journal = Life in Space for Life on Earth|volume = 23|pages = P1-5|author=Hecht, H. |author2=Brown, E. L. |author3=Young, L. R. |name-list-style=amp|publisher=Proceedings of "Life in space for life on Earth". 8th European Symposium on Life Sciences Research in Space. 23rd Annual International Gravitational Physiology Meeting |date=June 2–7, 2002 |issue = 1|pmid = 14703662|display-authors=etal|bibcode = 2002ESASP.501..151H}}</ref>
* Changes in the rotation axis or rate of a spin would cause a disturbance in the artificial gravity field and stimulate the semicircular canals (refer to above). Any movement of mass within the station, including a movement of people, would shift the axis and could potentially cause a dangerous wobble. Thus, the rotation of a space station would need to be adequately stabilized, and any operations to deliberately change the rotation would need to be done slowly enough to be imperceptible.<ref name="alfred1969"/> One possible solution to prevent the station from wobbling would be to use its liquid water supply as [[ballast]] which could be pumped between different sections of the station as required. 

[[File: RotationSpeedOfCentrifuge.svg|thumb|Speed in [[Revolutions per minute|rpm]] for a centrifuge of a given radius to achieve a given ''g''-force]]

===Human spaceflight===
The [[Gemini 11]] mission attempted in 1966 to produce artificial gravity by rotating the capsule around the [[Agena Target Vehicle]] to which it was attached by a 36-meter tether. They were able to generate a small amount of artificial gravity, about 0.00015&nbsp;''g'', by firing their side thrusters to slowly rotate the combined craft like a slow-motion pair of [[bolas]].<ref name=" Gatland1976">{{Cite book|first=Kenneth|last=Gatland|title=Manned Spacecraft, Second Revision|place=New York, NY, USA|publisher=MacMillan|year=1976|pages=180–182|isbn=978-0-02-542820-1}}</ref> The resultant force was too small to be felt by either astronaut, but objects were observed moving towards the "floor" of the capsule.<ref name="Clément G 2007">{{Cite book |editor-last=Clément |editor-first=Gilles |editor-last2=Bukley |editor-first2=Angie |title=Artificial Gravity |date=May 28, 2007 |isbn=978-0-387-70714-3 |series=Space Technology Library |location=New York |doi=10.1007/0-387-70714-X |eissn=2542-8896 |issn=0924-4263 |publisher=[[Springer Science+Business Media|Springer]] }}</ref>

==== Health benefits ====
[[File:Jsc2004e18862.jpg|thumb|Artificial gravity has been suggested for interplanetary journeys to Mars]]
Artificial gravity has been suggested as a solution to various health risks associated with spaceflight.<ref name="ncbi.nlm.nih.gov" /> In 1964, the [[Soviet]] space program believed that a human could not survive more than 14 days in space for fear that the [[heart]] and [[blood vessels]] would be unable to adapt to the weightless conditions.<ref>{{cite journal|jstor=3947769|title=Weightlessness Obstacle to Space Survival|date=April 4, 1964|journal=The Science News-Letter|volume=86|issue=7|pages=103}}</ref> This fear was eventually discovered to be unfounded as spaceflights have now lasted up to 437 consecutive days,<ref>{{cite news|url=https://www.npr.org/sections/thetwo-way/2017/04/24/525374569/astronaut-peggy-whitson-sets-new-nasa-record-for-most-days-in-space|title=Astronaut Peggy Whitson Sets NASA Record For Most Days In Space|newspaper=NPR|date=April 24, 2017|access-date=April 4, 2018|last1=Chappell|first1=Bill}}</ref> with missions aboard the International Space Station commonly lasting 6 months. However, the question of human safety in space did launch an investigation into the physical effects of prolonged exposure to weightlessness. In June 1991, the Spacelab Life Sciences 1 on the [[Space Shuttle]] flight [[STS-40]] flight performed 18 experiments on two men and two women over nine days. In an environment without gravity, it was concluded that the response of [[white blood cells]] and [[muscle]] mass decreased. Additionally, within the first 24 hours spent in a weightless environment, [[blood volume]] decreased by 10%.<ref name="jstor.org">{{cite journal|jstor=1311819|title=Artificial Gravity and Space Travel|first=Leonard|last=David|date=April 4, 1992|journal=BioScience|volume=42|issue=3|pages=155–159|doi=10.2307/1311819}}</ref><ref name="popularmechanics.com"/><ref name="iaaweb.org"/> Long periods of weightlessness can cause brain swelling and eyesight problems.<ref>{{Cite web | url=https://phys.org/news/2012-03-prolonged-space-brain-eye-abnormalities.html |title = Prolonged space travel causes brain and eye abnormalities in astronauts}}</ref> Upon return to Earth, the effects of prolonged weightlessness continue to affect the human body as fluids pool back to the lower body, the [[heart rate]] rises, a drop in [[blood pressure]] occurs, and there is a reduced tolerance for [[exercise]].<ref name="jstor.org"/>

Artificial gravity, for its ability to [[mimic]] the behavior of gravity on the human body, has been suggested as one of the most encompassing manners of combating the physical effects inherent in weightless environments. Other measures that have been suggested as symptomatic treatments include exercise, diet, and [[Pingvin exercise suit|Pingvin suits]]. However, criticism of those methods lies in the fact that they do not fully eliminate health problems and require a variety of solutions to address all issues. Artificial gravity, in contrast, would remove the weightlessness inherent in space travel. By implementing artificial gravity, space travelers would never have to experience weightlessness or the associated side effects.<ref name="iaaweb.org"/> Especially in a modern-day six-month journey to [[Mars]], exposure to artificial gravity is suggested in either a continuous or intermittent form to prevent extreme debilitation to the astronauts during travel.<ref name="ncbi.nlm.nih.gov" />

====Proposals====
[[File:Nasa mars artificial gravity 1989.jpg|thumb|Rotating Mars spacecraft – 1989 NASA concept]]
Several proposals have incorporated artificial gravity into their design:
* Discovery II: a 2005 vehicle proposal capable of delivering a 172-metric-ton crew to Jupiter's orbit in 118 days. A very small portion of the 1,690-metric-ton craft would incorporate a centrifugal crew station.<ref>{{cite web|access-date=2011-09-28|author1=Craig H. Williams|author2=Leonard A. Dudzinski|author3=Stanley K. Borowski|author4=Albert J. Juhasz|date=March 2005|location=Cleveland, Ohio|publisher=NASA|title=Realizing "2001: A Space Odyssey": Piloted Spherical Torus Nuclear Fusion Propulsion|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050160960_2005161052.pdf}}</ref>
* [[Nautilus-X|Multi-Mission Space Exploration Vehicle]] (MMSEV): a 2011 [[NASA]] proposal for a long-duration crewed space transport vehicle; it included a rotational artificial gravity [[space habitat]] intended to promote crew health for a crew of up to six persons on missions of up to two years in duration. The [[torus|torus-ring]] [[centrifuge]] would utilize both standard metal-frame and [[Inflatable space habitat|inflatable]] spacecraft structures and would provide 0.11 to 0.69&nbsp;[[Standard gravity|''g'']] if built with the {{convert|40|ft}} diameter option.<ref name=fiso20110126b>[http://spirit.as.utexas.edu/~fiso/telecon/Holderman-Henderson_1-26-11/Holderman_1-26-11.ppt NAUTILUS – X:  Multi-Mission Space Exploration Vehicle] {{Webarchive|url=https://web.archive.org/web/20110304044259/http://spirit.as.utexas.edu/~fiso/telecon/Holderman-Henderson_1-26-11/Holderman_1-26-11.ppt |date=March 4, 2011 }}, Mark L. Holderman, ''Future in Space Operations (FISO) Colloquium'', 2011-01-26. Retrieved 2011-01-31</ref><ref name=stn20110128>
[http://www.hobbyspace.com/nucleus/index.php?itemid=26786 NASA NAUTILUS-X: multi-mission exploration vehicle includes a centrifuge, which would be tested at ISS] {{webarchive|url=https://web.archive.org/web/20110225085854/http://hobbyspace.com//nucleus/index.php?itemid=26786 |date=February 25, 2011 }}, ''RLV and Space Transport News'', 2011-01-28. Retrieved 2011-01-31</ref>
* [[Nautilus-X#ISS centrifuge demonstration|ISS Centrifuge Demo]]: a 2011 NASA proposal for a demonstration project preparatory to the final design of the larger torus centrifuge space habitat for the Multi-Mission Space Exploration Vehicle. The structure would have an outside diameter of {{convert|30|ft}} with a ring interior cross-section diameter of {{convert|30|in}}. It would provide 0.08 to 0.51&nbsp;''g'' partial gravity. This test and evaluation centrifuge would have the capability to become a Sleep Module for the ISS crew.<ref name=fiso20110126b/><!-- the ISS Centrifuge Demo is described on pages 15–21 of the fiso20110126b ref -->
[[Image:Tempo-3-0003.jpg|thumb|right|Artist’s rendering of TEMPO³ in orbit]]
* [[Mars Direct]]: A plan for a crewed [[Mars]] mission created by NASA engineers [[Robert Zubrin]] and [[David Baker (aerospace engineer)|David Baker]] in 1990, later expanded upon in Zubrin's 1996 book ''[[The Case for Mars]]''. The "Mars Habitat Unit", which would carry astronauts to Mars to join the previously launched "Earth Return Vehicle", would have had artificial gravity generated during flight by tying the spent upper stage of the booster to the Habitat Unit, and setting them both rotating about a common axis.<ref>{{cite web|url=http://www.nss.org/resources/books/non_fiction/NF_037_caseformars.html|title=NSS Review: The Case for Mars|website=www.nss.org|access-date=April 4, 2018|archive-date=January 11, 2018|archive-url=https://web.archive.org/web/20180111165837/http://www.nss.org/resources/books/non_fiction/NF_037_caseformars.html|url-status=dead}}</ref>
* The proposed [[Tempo3]] mission rotates two halves of a spacecraft connected by a tether to test the feasibility of simulating gravity on a crewed mission to Mars.<ref>[http://members.marssociety.org/TMQ/TMQ-V1-I1.pdf The Mars Quarterly] {{Webarchive|url=https://web.archive.org/web/20170421044642/http://members.marssociety.org/TMQ/TMQ-V1-I1.pdf|date=April 21, 2017}} pg15-Tom Hill</ref>
* The [[Mars Gravity Biosatellite]] was a proposed mission meant to study the effect of artificial gravity on mammals. An artificial gravity field of 0.38&nbsp;''g'' (equivalent to [[Mars]]'s surface gravity) was to be produced by rotation (32 rpm, radius of ca. 30&nbsp; cm). Fifteen mice would have orbited Earth ([[Low Earth orbit]]) for five weeks and then land alive.<ref name="iac07">Korzun, Ashley M.; Wagner, Erika B.; et al. (2007). [http://smartech.gatech.edu/bitstream/1853/26717/1/IAC-07-A1.9.-A2.7.05.pdf Mars Gravity Biosatellite: Engineering, Science, and Education]. 58th [[International Astronautical Congress]].</ref> However, the program was canceled on 24 June 2009, due to a lack of funding and shifting priorities at NASA.<ref>{{cite web|url=http://www.spaceref.com/news/viewsr.html?pid=31612|archive-url=https://archive.today/20120914085936/http://www.spaceref.com/news/viewsr.html?pid=31612|url-status=dead|archive-date=September 14, 2012|title=The Mars Gravity Biosatellite Program Is Closing Down|website=www.spaceref.com|date=June 24, 2009|access-date=April 4, 2018}}</ref>
* Vast Space is a private company that proposes to build the world's first artificial gravity space station using the rotating spacecraft concept.<ref>{{Cite web |last=Werner |first=Debra |date=2022-09-15 |title=Vast Space to develop artificial-gravity space station |url=https://spacenews.com/vast-space-intro/ |access-date=2023-09-17 |website=SpaceNews |language=en-US}}</ref>

==== Issues with implementation ====
Some of the reasons that artificial gravity remains unused today in [[spaceflight]] trace back to the problems inherent in [[implementation]]. One of the realistic methods of creating artificial gravity is the centrifugal effect caused by the [[centripetal force]] of the floor of a rotating structure pushing up on the person. In that model, however, issues arise in the size of the spacecraft. As expressed by John Page and Matthew Francis, the smaller a spacecraft (the shorter the radius of rotation), the more rapid the rotation that is required. As such, to simulate gravity, it would be better to utilize a larger spacecraft that rotates slowly. 

The requirements on size about rotation are due to the differing forces on parts of the body at different distances from the axis of rotation. If parts of the body closer to the rotational axis experience a force that is significantly different from parts farther from the axis, then this could have adverse effects. Additionally, questions remain as to what the best way is to initially set the rotating motion in place without disturbing the stability of the whole spacecraft's orbit. At the moment, there is not a ship massive enough to meet the rotation requirements, and the costs associated with building, maintaining, and [[launching]] such a craft are extensive.<ref name="popularmechanics.com"/>

In general, with the small number of negative health effects present in today's typically shorter spaceflights, as well as with the very large cost of [[research]] for a technology which is not yet really needed, the present day development of artificial gravity technology has necessarily been stunted and sporadic.<ref name="iaaweb.org"/><ref name="jstor.org"/>  

As the length of typical space flights increases, the need for artificial gravity for the passengers in such lengthy spaceflights will most certainly also increase, and so will the knowledge and resources available to create such artificial gravity, most likely also increase.  In summary, it is probably only a question of time, as to how long it might take before the conditions are suitable for the completion of the development of artificial gravity technology, which will almost certainly be required at some point along with the eventual and inevitable development of an increase in the average length of a spaceflight.<ref>[https://www.nasa.gov/podcasts/houston-we-have-a-podcast/artificial-gravity/ Artificial Gravity, Houston We Have a Podcast] NASA.gov. By Gary Jordan and Bill Paloski. March 26, 2021. Retrieved February 11, 2024.</ref>

====In science fiction====
Several science fiction novels, films, and series have featured artificial gravity production.

* In the movie ''[[2001: A Space Odyssey (film)|2001: A Space Odyssey]]'', a rotating centrifuge in the ''Discovery'' spacecraft provides artificial gravity. 
* The 1999 television series ''[[Cowboy Bebop]]'', a rotating ring in the ''Bebop'' spacecraft creates artificial gravity throughout the spacecraft.
* In the novel ''[[The Martian (Weir novel)|The Martian]]'', the ''Hermes'' spacecraft achieves artificial gravity by design; it employs a ringed structure, at whose periphery forces around 40% of Earth's gravity are experienced, similar to Mars' gravity.
** In the novel ''[[Project Hail Mary]]'' by the same author, weight on the titular ship ''Hail Mary'' is provided initially by engine thrust, as the ship is capable of constant acceleration up to {{math|2 ''ɡ''}} and is also able to separate, turn the crew compartment inwards, and rotate to produce {{math|1 ''ɡ''}} while in orbit.
* The movie ''[[Interstellar (film)|Interstellar]]'' features a spacecraft called the ''Endurance'' that can rotate on its central axis to create artificial gravity, controlled by retro thrusters on the ship.
* The 2021 film ''[[Stowaway (2021 film)|Stowaway]]'' features the upper stage of a launch vehicle connected by 450-meter long [[Space tether|tethers]] to the ship's main hull, acting as a counterweight for [[inertia]]-based artificial gravity.<ref>{{cite web |last1=Kiang |first1=Jessica |title=Review: Anna Kendrick is lost, and found, in space in smart sci-fi 'Stowaway' |url=https://www.latimes.com/entertainment-arts/movies/story/2021-04-22/stowaway-netflix-review |website=Los Angeles Times |date=April 22, 2021 |access-date=25 April 2021}}</ref>
* The series [[The Expanse (TV series)|The Expanse]] utilizes both rotational gravity and linear thrust gravity in various space stations and spaceships. Notably, Tycho Station and the [[Generation ship]] ''LDSS Nauvoo'' use rotational gravity. Linear gravity is provided by a fictitious 'Epstein Drive', which killed its creator Solomon Epstein during its maiden flight due to high gravity injuries. 
* In the television series ''[[For All Mankind (TV series)|For All Mankind]]'', the space hotel ''Polaris'', later renamed ''Phoenix'' after being purchased and converted into a space vessel by Helios Aerospace for their own Mars mission, features a wheel-like structure controlled by thrusters to create artificial gravity, whilst a central axial hub operates in zero gravity as a docking station.

==Linear acceleration==
{{see|Equivalence principle}}
Linear acceleration is another method of generating artificial gravity, by using the thrust from a spacecraft's engines to create the illusion of being under a gravitational pull. A spacecraft under constant acceleration in a straight line would have the appearance of a gravitational pull in the direction opposite to that of the acceleration, as the thrust from the engines would cause the spacecraft to "push" itself up into the objects and persons inside of the vessel, thus creating the feeling of weight. This is because of [[Newton’s third law|Newton's third law]]: the weight that one would feel standing in a linearly accelerating spacecraft would not be a true gravitational pull, but simply the reaction of oneself pushing against the craft's hull as it pushes back. Similarly, objects that would otherwise be free-floating within the spacecraft if it were not accelerating would "fall" towards the engines when it started accelerating, as a consequence of [[Newton's first law]]: the floating object would remain at rest, while the spacecraft would accelerate towards it, and appear to an observer within that the object was "falling".

To emulate artificial gravity on Earth, spacecraft using linear acceleration gravity may be built similar to a skyscraper, with its engines as the bottom "floor". If the spacecraft were to accelerate at the rate of 1&nbsp;''g''—Earth's gravitational pull—the individuals inside would be pressed into the hull at the same force, and thus be able to walk and behave as if they were on Earth.

This form of artificial gravity is desirable because it could functionally create the illusion of a gravity field that is uniform and unidirectional throughout a spacecraft, without the need for large, spinning rings, whose fields may not be uniform, not unidirectional with respect to the spacecraft, and require constant rotation. This would also have the advantage of relatively high speed: a spaceship accelerating at 1&nbsp;''g'', 9.8&nbsp;m/s<sup>2</sup>, for the first half of the journey, and then decelerating for the other half, could reach [[Mars]] within a few days.<ref>{{cite book|last1=Clément|first1=Gilles|url=https://books.google.com/books?id=YUcjOsG0hi0C|title=Artificial Gravity|last2=Bukley|first2=Angelia P.|publisher=Springer New York|year=2007|isbn=978-0-387-70712-9|page=35}} [https://books.google.com/books?id=YUcjOsG0hi0C&pg=PA35 Extract of page 35]</ref> Similarly, a hypothetical [[space travel using constant acceleration]] of 1&nbsp;''g'' for one year would reach [[relativistic speed]]s and allow for a round trip to the nearest star, [[Proxima Centauri]]. As such, low-impulse but long-term linear acceleration has been proposed for various interplanetary missions. For example, even heavy (100 [[ton]]) cargo payloads to Mars could be transported to Mars in {{nowrap|27 months}} and retain approximately 55 percent of the [[Low Earth orbit|LEO]] vehicle mass upon arrival into a Mars orbit, providing a low-gravity gradient to the spacecraft during the entire journey.<ref name="fiso201101192">[http://spirit.as.utexas.edu/~fiso/telecon/Glover_1-19-11/Glover_1-19-11.pdf VASIMR VX-200 Performance and Near-term SEP Capability for Unmanned Mars Flight] {{Webarchive|url=https://web.archive.org/web/20110311141639/http://spirit.as.utexas.edu/~fiso/telecon/Glover_1-19-11/Glover_1-19-11.pdf|date=March 11, 2011}}, Tim Glover, Future in Space Operations (FISO) Colloquium, pp. 22, 25, 2011-01-19. Retrieved 2011-02-01</ref>

This form of gravity is not without challenges, however. At present, the only practical engines that could propel a vessel fast enough to reach speeds comparable to Earth's gravitational pull require [[Rocket engine#Chemically powered|chemical]] [[Spacecraft propulsion#Reaction engines|reaction rockets]], which expel [[reaction mass]] to achieve thrust, and thus the acceleration could only last for as long as a vessel had fuel. The vessel would also need to be constantly accelerating and at a constant speed to maintain the gravitational effect, and thus would not have gravity while stationary, and could experience significant swings in ''g''-forces if the vessel were to accelerate above or below 1&nbsp;''g''. Further, for point-to-point journeys, such as Earth-Mars transits, vessels would need to constantly accelerate for half the journey, turn off their engines, perform a 180° flip, reactivate their engines, and then begin decelerating towards the target destination, requiring everything inside the vessel to experience weightlessness and possibly be secured down for the duration of the flip.

A propulsion system with a very high [[specific impulse]] (that is, good efficiency in the use of [[reaction mass]] that must be carried along and used for propulsion on the journey) could accelerate more slowly producing useful levels of artificial gravity for long periods of time. A variety of [[Spacecraft propulsion#Electromagnetic propulsion|electric propulsion]] systems provide examples. Two examples of this long-duration, [[Thrust-to-weight ratio|low-thrust]], high-impulse propulsion that have either been practically used on spacecraft or are planned in for near-term in-space use are [[Hall effect thruster]]s and [[Variable Specific Impulse Magnetoplasma Rocket]]s (VASIMR). Both provide very high [[specific impulse]] but relatively low thrust, compared to the more typical chemical reaction rockets. They are thus ideally suited for long-duration firings which would provide limited amounts of, but long-term, milli-''g'' levels of artificial gravity in spacecraft.{{Citation needed|date=February 2011}}

In a number of science fiction plots, acceleration is used to produce artificial gravity for [[Interstellar travel|interstellar]] spacecraft, propelled by as yet [[theoretical]] or [[Spacecraft propulsion#Hypothetical methods|hypothetical]] means.

This effect of linear acceleration is well understood, and is routinely used for 0&nbsp;''g'' cryogenic fluid management for post-launch (subsequent) in-space firings of [[upper stage]] rockets.<ref name="goff20092">{{cite web|author=Jon Goff|display-authors=etal|year=2009|title=Realistic Near-Term Propellant Depots|url=http://selenianboondocks.com/wp-content/uploads/2009/09/NearTermPropellantDepots.pdf|access-date=2011-02-07|publisher=American Institute of Aeronautics and Astronautics|quote=Developing techniques for manipulating fluids in microgravity, which typically fall into the category known as settled propellant handling. Research for cryogenic upper stages dating back to the Saturn S-IVB and Centaur found that providing a slight acceleration (as little as 10<sup>−4</sup> to 10<sup>−5</sup>&nbsp;''g'' of acceleration) to the tank can make the propellants assume a desired configuration, which allows many of the main cryogenic fluid handling tasks to be performed in a similar fashion to terrestrial operations. The simplest and most mature settling technique is to apply thrust to the spacecraft, forcing the liquid to settle against one end of the tank.}}</ref>

[[Roller coaster]]s, especially [[launched roller coasters]] or those that rely on [[electromagnetic propulsion]], can provide linear acceleration "gravity", and so can relatively high acceleration vehicles, such as [[sports car]]s. Linear acceleration can be used to provide [[air-time]] on roller coasters and other thrill rides.

==Simulating lunar gravity==
{{see also |Gravitation of the Moon#Simulating lunar gravity}}
In January 2022, China was reported by the [[South China Morning Post]] to have built a small ({{convert|60|cm|lk=on}} [[diameter]]) research facility to simulate low [[lunar gravity]] with the help of [[magnet]]s.<ref name=Futurism01a>{{cite web |author=|authorlink=|title=China building "Artificial Moon" that simulates low gravity with magnets|url=https://futurism.com/the-byte/china-artificial-moon-magnets|website=Futurism.com|publisher=Recurrent Ventures|access-date=17 January 2022 |date=January 12, 2022|language=|quote=Interestingly, the facility was partly inspired by previous research conducted by Russian physicist Andrew Geim in which he floated a frog with a magnet. The experiment earned Geim the Ig Nobel Prize in Physics, a satirical award for unusual scientific research. It's cool that a quirky experiment involving floating a frog could lead to something approaching an honest-to-God antigravity chamber.}}</ref><ref name=scmp2022-01-12a>{{cite web |first1=Stephen | last1 = Chen|authorlink=|title=China has built an artificial moon that simulates low-gravity conditions on Earth|url=https://www.scmp.com/news/china/science/article/3162972/china-has-built-artificial-moon-simulates-low-gravity-conditions|website=|publisher=[[South China Morning Post]]|access-date=17 January 2022 |date=12 January 2022 |language=|quote=It is said to be the first of its kind and could play a key role in the country’s future lunar missions. The magnetic field supported the landscape and was inspired by experiments to levitate a frog.}}</ref> The facility was reportedly partly inspired by the work of [[Andre Geim]] (who later shared the 2010 [[Nobel Prize in Physics]] for his research on [[graphene]]) and [[Michael Berry (physicist)|Michael Berry]], who both shared the [[Ig Nobel Prize]] in Physics in [[List_of_Ig_Nobel_Prize_winners#2000|2000]] for the [[magnetic levitation]] of a frog.<ref name=Futurism01a/><ref name=scmp2022-01-12a/>

==Graviton control or generator==
{{Main|Graviton}}

==Speculative or fictional mechanisms==
{{Main|Gravitational shielding|Anti-gravity}}

In science fiction, artificial gravity (or cancellation of gravity) or "paragravity"<ref>''Collision Orbit'', 1942 by [[Jack Williamson]]</ref><ref>''[[Pale Blue Dot (book)|Pale Blue Dot]]: A Vision of the Human Future in Space'' by [[Carl Sagan]], Chapter 19</ref> is sometimes present in spacecraft that are neither rotating nor accelerating. At present, there is no confirmed technique as such that can simulate gravity other than actual rotation or acceleration. There have been many claims over the years of such a device. [[Eugene Podkletnov]], a Russian engineer, has claimed since the early 1990s to have made such a device consisting of a spinning superconductor producing a powerful "[[Gravitomagnetism|gravitomagnetic]] field." In 2006, a research group funded by [[European Space Agency|ESA]] claimed to have created a similar device that demonstrated positive results for the production of gravitomagnetism, although it produced only 0.0001&nbsp;''g''.<ref>{{cite web|url=https://gsp.esa.int/article-view/-/wcl/Fd1ZihgaGrwB/10192/towards-a-new-test-of-general-relativity-|title=Toward a new test of general relativity?|publisher=Esa.int|access-date=2013-08-06|archive-url=https://web.archive.org/web/20171228000202/https://gsp.esa.int/article-view/-/wcl/Fd1ZihgaGrwB/10192/towards-a-new-test-of-general-relativity-|archive-date=December 28, 2017|url-status=dead}}</ref>

==See also==
{{col div|colwidth=30em}}
* {{annotated link|Non-inertial reference frame}}
* {{annotated link|Anti-gravity}}
* {{annotated link|Gravitational shielding}}
* {{annotated link|Electrogravitics}}
* {{annotated link|Coriolis effect}}
* {{annotated link|Centrifuge Accommodations Module}}
* {{annotated link|Fictitious force}}
* {{annotated link|Rotating wheel space station}}
* {{annotated link|Space habitat}}
* {{annotated link|Space travel under constant acceleration}}
* {{annotated link|Stanford torus}}
{{colend}}

==References==
{{Reflist|30em}}

==External links==
{{Commons category|Artificial gravity}}
* [http://www.artificial-gravity.com/ List of peer review papers on artificial gravity]
* [https://www.youtube.com/watch?v=zruVHZCAf24/ TEDx talk about artificial gravity]
* [http://www.projectrho.com/rocket/rocket3u.html Overview of artificial gravity in Sci-Fi and Space Science] {{Webarchive|url=https://web.archive.org/web/20100527233439/http://www.projectrho.com/rocket/rocket3u.html |date=May 27, 2010 }}
* [https://web.archive.org/web/20030116203809/http://www.nas.nasa.gov/About/Education/SpaceSettlement/teacher/materials/ringworld/ringworld.html NASA's Java simulation of artificial gravity]
* [http://selenianboondocks.com/2010/11/variable-gravity-research-facility-xgrf/ Variable Gravity Research Facility (xGRF)], concept with tethered rotating satellites, perhaps a [[Bigelow Aerospace|Bigelow]] [[B330|expandable module]] and a spent [[upper stage]] as a counterweight

{{Space medicine}}

{{DEFAULTSORT:Artificial Gravity}}
[[Category:Fictional technology|Gravity]]
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[[Category:Space colonization]]
[[Category:Scientific speculation]]
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