View source for Ion thruster

{{short description|Spacecraft engine that generates thrust by generating a jet of ions}}
{{About|a kind of reaction engine|the air propulsion concept|ionocraft}}
{{Use American English|date=November 2020}}
{{use dmy dates |date=December 2020}}

[[File:Ion Engine Test Firing - GPN-2000-000482.jpg|thumb|upright=1.2|right|The 2.3{{nbsp}}kW [[NASA Solar Technology Application Readiness|NSTAR]] ion thruster developed by [[NASA]] for the [[Deep Space 1]] spacecraft during a hot fire test at the [[Jet Propulsion Laboratory]] (1999)]]
[[File:NEXIS thruster working.jpg|thumb|upright=1.2|right|NEXIS ion engine test (2005)]]
[[File:Xenon ion engine prototype.png|thumb|upright=1.2|A prototype of a xenon ion engine being tested at NASA's Jet Propulsion Laboratory (2005)]]

An '''ion thruster''', '''ion drive''', or '''ion engine''' is a form of [[electrically powered spacecraft propulsion|electric propulsion]] used for [[spacecraft propulsion]]. An ion thruster creates a cloud of [[cation|positive ions]] from a neutral gas by ionizing it to extract some [[electron]]s from its [[atom]]s. The [[ion]]s are then accelerated using [[electricity]] to create [[thrust]]. Ion thrusters are categorized as either [[electrostatics|electrostatic]] or [[electromagnetism|electromagnetic]].

[[electrostatics|Electrostatic]] thruster ions are accelerated by the [[Coulomb force]] along the [[electric field]] direction. Temporarily stored electrons are reinjected by a ''neutralizer'' in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster. 

By contrast, [[Electromagnetism|electromagnetic]] thruster ions are accelerated by the [[Lorentz force]] to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their [[electric charge]], and are specifically referred to as [[plasma propulsion engine]]s, where the electric field is not in the direction of the acceleration.<ref name="Jahn 1968">{{cite book|last1=Jahn|first1=Robert G.|title=Physics of Electric Propulsion|date=1968|edition=1st|publisher=McGraw Hill Book Company|isbn=978-0070322448}} Reprint: {{cite book|last1=Jahn|first1=Robert G.|title=Physics of Electric Propulsion|date=2006|publisher=Dover Publications|isbn=978-0486450407}}</ref><ref name="Jahn-Choueiri 2002">{{cite book|last1=Jahn|first1=Robert G.|last2=Choueiri|first2=Edgar Y.|title=Encyclopedia of Physical Science and Technology|date=2003|edition=3rd|volume=5|publisher=Academic Press|chapter=Electric Propulsion|chapter-url=https://massless.info/images/ep-encyclopedia-2001.pdf |archive-url=https://web.archive.org/web/20221010091826/https://massless.info/images/ep-encyclopedia-2001.pdf |archive-date=2022-10-10 |url-status=live|pages=125–141|isbn=978-0122274107}}</ref>

Ion thrusters in operation typically consume 1–7&nbsp;kW of [[Electric power|power]], have [[exhaust velocity|exhaust velocities]] around 20–50&nbsp;km/s ([[Specific impulse|''I''<sub>sp</sub>]] 2000–5000{{nbsp}}s), and possess thrusts of 25–250&nbsp;mN and a [[propulsive efficiency]] 65–80%<ref name="autogenerated1">{{cite web|url=https://massless.info/images/choueiri-sciam-2009.pdf |archive-url=https://web.archive.org/web/20221010091833/https://massless.info/images/choueiri-sciam-2009.pdf |archive-date=2022-10-10 |url-status=live|title=Choueiri, Edgar Y., (2009) New dawn of electric rocket The Ion Drive}}</ref><ref name="Choueiri" /> though experimental versions have achieved {{cvt|100|kW}}, {{cvt|5|N}}.<ref>{{cite web|url=https://futurism.com/nasas-new-ion-thruster-breaks-records-could-take-humans-to-mars|title=NASA's new ion thruster breaks records, could take humans to Mars|website=futurism.com}}</ref>

The ''[[Deep Space 1]]'' spacecraft, powered by an ion thruster, changed velocity by {{cvt|4.3|km/s}} while consuming less than {{cvt|74|kg}} of [[xenon]]. The [[Dawn (spacecraft)|''Dawn'']] spacecraft broke the record, with a [[Delta-v|velocity change]] of {{cvt|11.5|km/s}}, though it was only half as efficient, requiring {{cvt|425|kg}} of xenon.<ref name='Dawn 2019'>{{cite web |url=http://www.jimhaldenwang.com/mars.htm|title=The Human Exploration of Mars|first=Jim|last=Haldenwang|work=Jim's Science Page|access-date=3 May 2019}}</ref>

Applications include control of the orientation and position of orbiting [[satellite]]s (some satellites have dozens of low-power ion thrusters), use as a main propulsion engine for low-mass robotic space vehicles (such as ''Deep Space 1'' and ''Dawn''),<ref name="autogenerated1"/><ref name="Choueiri"/> and serving as propulsion thrusters for [[crewed spacecraft]] and [[space station]]s (e.g. [[Tiangong space station|Tiangong]]).<ref name="human_ion">{{cite web |first=保淑 (Baoshu) |last=张 (Zhang) |title=配置4台霍尔电推进发动机 "天宫"掀起太空动力变革 [Hall-effect thruster for Tiangong set off space drive revolution ] |url=http://www.chinanews.com/gn/2021/06-21/9503717.shtml |website=中国新闻网 |accessdate=2021-07-18 |archive-url=https://web.archive.org/web/20210706020905/http://www.chinanews.com/gn/2021/06-21/9503717.shtml |archive-date=2021-07-06 |date=2021-06-21 |language=Chinese}}</ref>

Ion thrust engines are generally practical only in the vacuum of space as the engine's minuscule thrust cannot overcome any significant air resistance without radical design changes, as may be found in the '[[Atmosphere-breathing electric propulsion|Atmosphere Breathing Electric Propulsion]]' concept. The Massachusetts Institute of Technology (MIT) has created designs that are able to fly for short distances and at low speeds at ground level, using ultra-light materials and low drag aerofoils. An ion engine cannot usually generate sufficient thrust to achieve initial [[Takeoff|liftoff]] from any celestial body with significant surface [[gravity]]. For these reasons, spacecraft must rely on other methods such as conventional chemical rockets or [[Non-rocket spacelaunch|non-rocket launch technologies]] to reach their initial [[orbit]].

== Origins ==
[[File:SERT-1 spacecraft.jpg|thumb|upright=1.2|right|SERT-1 spacecraft]]

The first person who wrote a paper introducing the idea publicly was [[Konstantin Tsiolkovsky]] in 1911.<ref name="choueiri2">{{cite web |title=Ion Propulsion – Over 50 Years in the Making |url=http://science.nasa.gov/newhome/headlines/prop06apr99_2.htm |url-status=dead |archive-url=https://web.archive.org/web/20100327120759/http://science.nasa.gov/newhome/headlines/prop06apr99_2.htm |archive-date=2010-03-27 |work=Science@NASA}}</ref> The technique was recommended for near-vacuum conditions at high altitude, but thrust was demonstrated with ionized air streams at atmospheric pressure. The idea appeared again in [[Hermann Oberth]]'s ''Wege zur Raumschiffahrt'' (1929; ''Ways to Spaceflight''),<ref>{{Cite journal |last=Wolf |first=K. |date=1931-12-01 |title=Wege zur Raumschiffahrt |journal=Monatshefte für Mathematik und Physik |language=de |volume=38 |issue=1 |pages=A58 |doi=10.1007/BF01700815 |s2cid=115467575 |issn=1436-5081|doi-access=free }}</ref> where he explained his thoughts on the mass savings of electric propulsion, predicted its use in [[spacecraft propulsion]] and [[Spacecraft attitude control|attitude control]], and advocated electrostatic acceleration of charged gasses.<ref name="choueiri3">{{cite web |url=https://massless.info/images/choueiri-jpp-2004.pdf |archive-url=https://web.archive.org/web/20221010091826/https://massless.info/images/choueiri-jpp-2004.pdf |archive-date=2022-10-10 |url-status=live |title=A Critical History of Electric Propulsion: The First 50 Years (1906–1956) |access-date=2016-10-18 |first= E. Y. |last=Choueiri}}</ref>

A working ion thruster was built by [[Harold R. Kaufman]] in 1959 at the [[NASA]] [[Glenn Research Center]] facilities. It was similar to a gridded electrostatic ion thruster and used [[mercury (element)|mercury]] for propellant. Suborbital tests were conducted during the 1960s and in 1964, and the engine was sent into a suborbital flight aboard the [[SERT-1|Space Electric Rocket Test-1]] (SERT-1).<ref name='Ion 1964'>{{cite web |url=http://www.nasa.gov/centers/glenn/about/history/ds1.html |title=Contributions to Deep Space 1 |date=14 April 2015 |publisher=NASA}} {{PD-notice}}</ref><ref name="Cybulski">{{cite web |first1=Ronald J. |last1=Cybulski |first2=Daniel M. |last2=Shellhammer |first3=Robert R. |last3=Lovell |first4=Edward J. |last4=Domino |first5=Joseph T. |last5=Kotnik |url=https://ntrs.nasa.gov/enwiki/api/citations/19650009681/downloads/19650009681.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/enwiki/api/citations/19650009681/downloads/19650009681.pdf |archive-date=2022-10-09 |url-status=live |title=Results from SERT I Ion Rocket Flight Test |id=NASA-TN-D-2718 |publisher=[[NASA]] |date=1965}}{{PD-notice}}</ref> It successfully operated for the planned 31 minutes before falling to Earth.<ref name="Glenn">{{cite web |title=Innovative Engines – Glenn Ion Propulsion Research Tames the Challenges of 21st Century Space Travel |url=http://www.nasa.gov/centers/glenn/about/fs08grc.html |url-status=dead |archive-url=https://web.archive.org/web/20070915023928/http://www.nasa.gov/centers/glenn/about/fs08grc.html |archive-date=2007-09-15 |access-date=2007-11-19}} {{PD-notice}}</ref> This test was followed by an orbital test, SERT-2, in 1970.<ref name="sert2">{{cite web |publisher=[[NASA Glenn Research Center]] |url=http://www.grc.nasa.gov/WWW/ion/past/70s/sert2.htm |title=Space Electric Rocket Test II (SERT II) |archive-url=https://web.archive.org/web/20110927004353/http://www.grc.nasa.gov/WWW/ion/past/70s/sert2.htm |archive-date=2011-09-27 |url-status=dead |access-date=1 July 2010}}{{PD-notice}}</ref><ref>{{Cite web |date=October 25, 2010 |title=Encyclopedia Astronautica Index: 1 |url=http://www.astronautix.com/4/404page.html |access-date=2024-05-17 |website=www.astronautix.com}}</ref>

On the 12 October 1964 [[Voskhod 1]] carried out tests with ion thrusters that had been attached to the exterior of the spacecraft.<ref name="SiddiqiION">{{cite book |last=Siddiqi |first=Asif A. |url=https://history.nasa.gov/SP-4408pt1.pdf |title=Challenge To Apollo: The Soviet Union and The Space Race, 1945–1974 |date=2000 |publisher=NASA |page=423 |language=en-us}}</ref> 

An alternate form of electric propulsion, the [[Hall-effect thruster]], was studied independently in the [[United States]] and the [[Soviet Union]] in the 1950s and 1960s. Hall-effect thrusters operated on Soviet satellites from 1972 until the late 1990s, mainly used for satellite stabilization in north–south and in east–west directions. Some 100–200 engines completed missions on Soviet and [[Russia]]n satellites.<ref name="NK">{{cite web |url=http://novosti-kosmonavtiki.ru/content/numbers/198/35.shtml |title=Native Electric Propulsion Engines Today |publisher=Novosti Kosmonavtiki |date=1999
|issue=7 |archive-url=https://web.archive.org/web/20110606033558/http://www.novosti-kosmonavtiki.ru/content/numbers/198/35.shtml |archive-date=6 June 2011 |language=ru}}</ref> Soviet thruster design was introduced to the West in 1992 after a team of electric propulsion specialists, under the support of the [[Ballistic Missile Defense Organization]], visited Soviet laboratories.

== General working principle ==
Ion thrusters use beams of [[ion]]s (electrically charged atoms or molecules) to create [[thrust]] in accordance with [[momentum conservation]]. The method of accelerating the ions varies, but all designs take advantage of the [[electric charge|charge]]/[[mass]] ratio of the ions. This ratio means that relatively small potential differences can create high exhaust velocities. This reduces the amount of [[reaction mass]] or propellant required, but increases the amount of specific [[power (physics)|power]] required compared to [[chemical rocket]]s. Ion thrusters are therefore able to achieve high [[specific impulse]]s. The drawback of the low thrust is low acceleration because the mass of the electric power unit directly correlates with the amount of power. This low thrust makes ion thrusters unsuited for launching spacecraft into orbit, but effective for in-space propulsion over longer periods of time.

Ion thrusters are categorized as either [[electrostatics|electrostatic]] or [[electromagnetism|electromagnetic]]. The main difference is the method for accelerating the ions.
* Electrostatic ion thrusters use the [[Coulomb force]] and accelerate the ions in the direction of the electric field.
* Electromagnetic ion thrusters use the [[Lorentz force]] to accelerate the ions in the direction perpendicular to the electric field.

Electric power for ion thrusters is usually provided by [[solar panel]]s. However, for sufficiently large distances from the sun, [[Nuclear power in space|nuclear power]] may be used. In each case, the power supply mass is proportional to the peak power that can be supplied, and both provide, for this application, almost no limit to the energy.<ref>{{cite web |title=Ion Propulsion: Farther, Faster, Cheaper |url=https://www.nasa.gov/centers/glenn/technology/Ion_Propulsion1.html |website=NASA |access-date=4 February 2022 |archive-date=11 November 2020 |archive-url=https://web.archive.org/web/20201111185012/https://www.nasa.gov/centers/glenn/technology/Ion_Propulsion1.html |url-status=dead }}</ref>

Electric thrusters tend to produce low thrust, which results in low acceleration. Defining <math>1g = 9.81\; \mathrm{m/s^2}</math>, the [[gravity of Earth|standard gravitational acceleration of Earth]], and noting that <math>F = ma \implies a = F/m</math>, this can be analyzed. An [[NASA Solar Technology Application Readiness|NSTAR]] thruster producing a thrust force of 92&nbsp;mN<ref name=ns20070928/> will accelerate a satellite with a mass of 1{{nbsp}}[[metric ton|ton]] by 0.092{{nbsp}}N / 1000&nbsp;kg = 9.2{{e|−5}}{{nbsp}}m/s{{sup|2}} (or 9.38{{e|−6}}{{nbsp}}''g''). However, this acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets.

<math display="block">F = 2 \frac{\eta P}{g I_\text{sp}}</math>
Where:
* ''F'' is the thrust force in N,
* ''η'' is the [[efficiency]]
* ''P'' is the electrical power used by the thruster in W, and
* ''I''<sub>sp</sub> is the [[specific impulse]] in seconds.

The ion thruster is not the most promising type of [[electrically powered spacecraft propulsion]], but it is the most successful in practice to date.<ref name="Choueiri">{{cite journal|last1=Choueiri |first1=Edgar Y.|year=2009|title=New dawn of electric rocket|journal=Scientific American|volume=300|issue=2|pages=58–65|doi=10.1038/scientificamerican0209-58|pmid=19186707|bibcode=2009SciAm.300b..58C}}</ref> An ion drive would require two days to accelerate a car to highway speed in vacuum. The technical characteristics, especially [[thrust]], are considerably inferior to the prototypes described in literature,<ref name="autogenerated1"/><ref name="Choueiri"/> technical capabilities are limited by the [[space charge]] created by ions. This limits the thrust density ([[force]] per cross-sectional [[area]] of the engine).<ref name="Choueiri"/> Ion thrusters create small thrust levels (the thrust of ''Deep Space 1'' is approximately equal to the weight of one sheet of paper<ref name="Choueiri"/>) compared to conventional [[chemical rocket]]s, but achieve high [[specific impulse]], or propellant mass efficiency, by accelerating the exhaust to high speed. The [[power (physics)|power]] imparted to the exhaust increases with the square of exhaust velocity while thrust increase is linear. Conversely, chemical rockets provide high thrust, but are limited in total [[impulse (physics)|impulse]] by the small amount of [[energy]] that can be stored chemically in the propellants.<ref>{{Cite web |title=ESA Science & Technology – Electric Spacecraft Propulsion |url=https://sci.esa.int/web/smart-1/-/34201-electric-spacecraft-propulsion?fbodylongid=1535 |access-date=2024-05-17 |website=sci.esa.int}}</ref> Given the practical weight of suitable power sources, the acceleration from an ion thruster is frequently less than one-thousandth of [[standard gravity]]. However, since they operate as electric (or electrostatic) motors, they convert a greater fraction of input power into kinetic exhaust power. Chemical rockets operate as [[heat engine]]s, and [[Carnot's theorem (thermodynamics)|Carnot's theorem]] limits the exhaust velocity.

== Electrostatic thrusters ==
=== Gridded electrostatic ion thrusters ===
{{main|Gridded ion thruster}}
[[File:Ion engine.svg|thumb|upright=1.8|A diagram of how a gridded electrostatic ion engine (multipole magnetic cusp type) works]]

[[Gridded ion thruster|Gridded electrostatic ion thrusters]] development started in the 1960s<ref>{{cite journal |last1= Mazouffre|date= 2016|title= Electric propulsion for satellites and spacecraft: Established technologies and novel approaches|url= https://www.scopus.com/record/display.uri?eid=2-s2.0-84973355693&origin=inward&txGid=d2acc43c5b6bd3518f0fda0be9a7a74e|journal= Plasma Sources Science and Technology|volume= 25|issue= 3|page= 033002|doi= 10.1088/0963-0252/25/3/033002|bibcode= 2016PSST...25c3002M|s2cid= 41287361|access-date=July 29, 2021}}</ref> and, since then, they have been used for commercial satellite propulsion<ref>{{cite web|author1=|title=601 Satellite Historical Snapshot|url=https://www.boeing.com/history/products/601-satellite.page|website=Boing|date=|access-date=2021-07-26}}</ref><ref>{{Cite web|url=http://www.aerospace.org/crosslinkmag/fall-2014/electric-propulsion-at-aerospace/|title=Electric Propulsion at Aerospace {{!}} The Aerospace Corporation|website=www.aerospace.org|access-date=2016-04-10|archive-date=20 April 2016|archive-url=https://web.archive.org/web/20160420102803/http://www.aerospace.org/crosslinkmag/fall-2014/electric-propulsion-at-aerospace/|url-status=dead}}</ref><ref>{{Cite web|url=http://www.daviddarling.info/encyclopedia/X/XIPS.html|title=XIPS (xenon-ion propulsion system)|website=www.daviddarling.info|access-date=2016-04-10}}</ref> and scientific missions.<ref name="Sovey">J. S. Sovey,  V. K. Rawlin, and M. J. Patterson, "Ion Propulsion Development Projects in U. S.: Space Electric Rocket Test 1 to Deep Space 1", ''Journal of Propulsion and Power, Vol. 17'', No. 3, May–June 2001, pp. 517–526.</ref><ref>{{Cite web |url=http://www.grc.nasa.gov/WWW/ion/past/70s/sert2.htm |title=Space Electric Rocket Test |access-date=2010-07-01 |archive-url=https://web.archive.org/web/20110927004353/http://www.grc.nasa.gov/WWW/ion/past/70s/sert2.htm |archive-date=2011-09-27 |url-status=dead }}</ref> Their main feature is that the propellant [[ionization]] process is physically separated from the ion acceleration process.<ref>{{cite journal|last1=SANGREGORIO|first1=Miguel|last2=XIE|first2=Kan|date=2017|title=Ion engine grids: Function, main parameters, issues, configurations, geometries, materials and fabrication methods|journal=Chinese Journal of Aeronautics|volume=31|issue=8|pages=1635–1649 |doi=10.1016/j.cja.2018.06.005|doi-access=free}}</ref>

The ionization process takes place in the discharge chamber, where by bombarding the propellant with energetic electrons, as the energy transferred ejects valence electrons from the propellant gas's atoms. These electrons can be provided by a hot [[cathode]] [[electrical filament|filament]] and accelerated through the potential difference towards an anode. Alternatively, the electrons can be accelerated by an oscillating induced electric field created by an alternating electromagnet, which results in a self-sustaining discharge without a cathode (radio frequency ion thruster).

The positively charged ions are extracted by a system consisting of 2 or 3 multi-aperture grids. After entering the grid system near the plasma sheath, the ions are accelerated by the potential difference between the first grid and second grid (called the screen grid and the accelerator grid, respectively) to the final ion energy of (typically) 1–2&nbsp;keV, which generates thrust.

Ion thrusters emit a beam of positively charged ions. To keep the spacecraft from accumulating a charge, another [[cathode]] is placed near the engine to emit electrons into the ion beam, leaving the propellant electrically neutral. This prevents the beam of ions from being attracted (and returning) to the spacecraft, which would cancel the thrust.<ref name="Glenn"/>

Gridded electrostatic ion thruster research (past/present):
* [[NASA Solar Technology Application Readiness]] (NSTAR), 2.3&nbsp;kW, used on two successful missions
* NASA's Evolutionary Xenon Thruster ([[NEXT (ion thruster)|NEXT]]), 6.9&nbsp;kW, flight qualification hardware built; used on [[DART mission]]
* Nuclear Electric Xenon Ion System (NEXIS)
* High Power Electric Propulsion ([[HiPEP]]), 25&nbsp;kW, test example built and run briefly on the ground
* EADS Radio-frequency Ion Thruster (RIT)
* [[Dual-Stage 4-Grid]] (DS4G)<ref>{{cite press release|title=ESA and ANU make space propulsion breakthrough|publisher=ESA|date=2006-01-11|url=http://www.esa.int/esaCP/SEMOSTG23IE_index_0.html|access-date=2007-06-29}}</ref><ref>{{cite web |author=Australian National University Space Plasma, Power & Propulsion Group |date=2006-12-06 |title=ANU and ESA make space propulsion breakthrough |url=http://prl.anu.edu.au/SP3/research/SAFEandDS4G/webstory |archive-url=https://web.archive.org/web/20070627103001/http://prl.anu.edu.au/SP3/research/SAFEandDS4G/webstory |archive-date=2007-06-27 |access-date=2007-06-30 |publisher=The Australian National University}}</ref>

=== Hall-effect thrusters ===
{{Main|Hall-effect thruster}}
[[File:Wfm hall thruster.svg|thumb|upright=1.8|Schematic of a Hall-effect thruster]]

[[Hall-effect thruster]]s accelerate ions by means of an electric potential between a cylindrical anode and a negatively charged plasma that forms the cathode. The bulk of the propellant (typically xenon) is introduced near the anode, where it ionizes and flows toward the cathode; ions accelerate towards and through it, picking up electrons as they leave to neutralize the beam and leave the thruster at high velocity.

The anode is at one end of a cylindrical tube. In the center is a spike that is wound to produce a radial magnetic field between it and the surrounding tube. The ions are largely unaffected by the magnetic field, since they are too massive. However, the electrons produced near the end of the spike to create the cathode are trapped by the magnetic field and held in place by their attraction to the anode. Some of the electrons spiral down towards the anode, circulating around the spike in a [[Hall effect|Hall current]]. When they reach the anode they impact the uncharged propellant and cause it to be ionized, before finally reaching the anode and completing the circuit.<ref name="Oleson">{{cite web|url=http://gltrs.grc.nasa.gov/reports/2001/TM-2001-210676.pdf|title=Advanced Hall Electric Propulsion for Future In-Space Transportation|access-date=2007-11-21|last1=Oleson|first1=S. R.|last2=Sankovic|first2=J. M.|url-status=dead|archive-url=https://web.archive.org/web/20040122155512/http://gltrs.grc.nasa.gov/reports/2001/TM-2001-210676.pdf|archive-date=2004-01-22}} {{PD-notice}}</ref>

=== Field-emission electric propulsion ===
{{main|Field-emission electric propulsion}}

[[Field-emission electric propulsion]] (FEEP) thrusters may use [[caesium]] or [[indium]] propellants. The design comprises a small propellant reservoir that stores the liquid metal, a narrow tube or a system of parallel plates that the liquid flows through and an accelerator (a ring or an elongated aperture in a metallic plate) about a millimeter past the tube end. Caesium and indium are used due to their high atomic weights, low ionization potentials and low melting points. Once the liquid metal reaches the end of the tube, an electric field applied between the emitter and the accelerator causes the liquid surface to deform into a series of protruding cusps, or ''[[Taylor cone]]s''. At a sufficiently high applied voltage, positive ions are extracted from the tips of the cones.<ref>{{cite web |title=FEEP – Field-Emission Electric Propulsion |url=http://www.alta-space.com/index.php?page=feep |url-status=dead |archive-url=https://web.archive.org/web/20120118051025/http://www.alta-space.com/index.php?page=feep |archive-date=2012-01-18 |access-date=2012-04-27}}</ref><ref name="JPP98">{{cite web|url=http://www.alta-space.com/uploads/file/publications/feep/Marcuccio-JPP14_5_1998.pdf|title=Experimental Performance of Field Emission Microthrusters|author=Marcuccio, S.|display-authors=etal|access-date=2012-04-27|url-status=dead|archive-url=https://web.archive.org/web/20130520151812/http://www.alta-space.com/uploads/file/publications/feep/Marcuccio-JPP14_5_1998.pdf|archive-date=2013-05-20}}</ref><ref name="Nasa">{{cite web|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/11649/1/02-0194.pdf|title=In-FEEP Thruster Ion Beam Neutralization with Thermionic and Field Emission Cathodes|quote=liquid state and wicked up the needle shank to the tip where high electric fields deform the liquid and extract ions and accelerate them up to 130 km/s through 10 kV|access-date=2007-11-21|first1=Colleen|last1=Marrese-Reading|first2=Jay|last2=Polk|first3=Juergen|last3=Mueller|first4=Al|last4=Owens|url-status=dead |archive-url=https://web.archive.org/web/20061013162109/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/11649/1/02-0194.pdf|archive-date=2006-10-13}} {{PD-notice}}</ref> The electric field created by the emitter and the accelerator then accelerates the ions. An external source of electrons neutralizes the positively charged ion stream to prevent charging of the spacecraft.

== Electromagnetic thrusters ==
{{main|Plasma propulsion engine}}
{{Self-contradictory|section=about=electromagnetic thrusters|article=Electrically powered spacecraft propulsion|date=April 2018}}

=== Pulsed inductive thrusters ===
{{main|Pulsed inductive thruster}}

[[Pulsed inductive thruster]]s (PITs) use pulses instead of continuous thrust and have the ability to run on power levels on the order of megawatts (MW). PITs consist of a large [[electromagnetic coil|coil]] encircling a cone shaped tube that emits the propellant gas. [[Ammonia]] is the gas most commonly used. For each pulse, a large charge builds up in a group of capacitors behind the coil and is then released. This creates a current that moves circularly in the direction of jθ. The current then creates a magnetic field in the outward radial direction (Br), which then creates a current in the gas that has just been released in the opposite direction of the original current. This opposite current ionizes the ammonia. The positively charged ions are accelerated away from the engine due to the electric field jθ crossing the magnetic field Br, due to the Lorentz force.<ref>{{cite web|url=http://gltrs.grc.nasa.gov/reports/2003/CR-2003-212714.pdf|title=Pulsed Inductive Thruster (PIT): Modeling and Validation Using the MACH2 Code |access-date=2007-11-21|first=Pavlos G.|last=Mikellides|url-status=dead|archive-url=https://web.archive.org/web/20061010033732/http://gltrs.grc.nasa.gov/reports/2003/CR-2003-212714.pdf|archive-date=2006-10-10}} {{PD-notice}}</ref>

=== Magnetoplasmadynamic thruster ===
{{main|Magnetoplasmadynamic thruster}}

[[Magnetoplasmadynamic thruster|Magnetoplasmadynamic]] (MPD) thrusters and [[lithium Lorentz force accelerator]] (LiLFA) thrusters use roughly the same idea. The LiLFA thruster builds on the MPD thruster. [[Hydrogen]], [[argon]], [[ammonia]] and [[nitrogen]] can be used as propellant. In a certain configuration, the ambient gas in [[low Earth orbit]] (LEO) can be used as a propellant. The gas enters the main chamber where it is ionized into [[Plasma (physics)|plasma]] by the electric field between the [[anode]] and the [[cathode]]. This plasma then conducts electricity between the anode and the cathode, closing the circuit. This new current creates a magnetic field around the cathode, which crosses with the electric field, thereby accelerating the plasma due to the Lorentz force.

The LiLFA thruster uses the same general idea as the MPD thruster, though with two main differences. First, the LiLFA uses lithium vapor, which can be stored as a solid. The other difference is that the single cathode is replaced by multiple, smaller cathode rods packed into a [[Hollow cathode effect|hollow cathode]] tube. MPD cathodes are easily corroded due to constant contact with the plasma. In the LiLFA thruster, the lithium vapor is injected into the hollow cathode and is not ionized to its plasma form/corrode the cathode rods until it exits the tube. The plasma is then accelerated using the same [[Lorentz force]].<ref name="Sankaran">{{cite journal|url=https://massless.info/images/sankaran-icnta-2003.pdf |archive-url=https://web.archive.org/web/20221010091830/https://massless.info/images/sankaran-icnta-2003.pdf |archive-date=2022-10-10 |url-status=live|title=A Survey of Propulsion Options for Cargo and Piloted Missions to Mars|access-date=2016-10-18|first1=K. |last1=Sankaran|first2=L.|last2=Cassady|first3=A.D.|last3=Kodys|first4=E.Y.|last4=Choueiri|journal=Annals of the New York Academy of Sciences|year=2004|volume=1017|issue=1|pages=450–467|doi=10.1196/annals.1311.027 |pmid=15220162|bibcode=2004NYASA1017..450S|s2cid=1405279}}</ref><ref>{{cite web|url=http://gltrs.grc.nasa.gov/reports/2001/CR-2001-211114.pdf|title=High Power MPD Thruster Development at the NASA Glenn Research Center|access-date=2007-11-21|first1=Michael R.|last1=LaPointe|first2=Pavlos G.|last2=Mikellides|url-status=dead|archive-url=https://web.archive.org/web/20061011063710/http://gltrs.grc.nasa.gov/reports/2001/CR-2001-211114.pdf |archive-date=October 11, 2006}} {{PD-notice}}</ref><ref>{{cite web|url=http://dspace.mit.edu/bitstream/handle/1721.1/31061/33887503.pdf?sequence=1|title=Utilization of Ambient Gas as a Propellant for Low Earth Orbit Electric Propulsion|date=May 22, 1999|first=Buford Ray|last=Conley|url-status=dead|archive-url=https://web.archive.org/web/20110629174257/http://dspace.mit.edu/bitstream/handle/1721.1/31061/33887503.pdf?sequence=1|archive-date=June 29, 2011}}</ref>

In 2013, Russian company the [[Chemical Automatics Design Bureau]] successfully conducted a bench test of their MPD engine for long-distance space travel.<ref>{{cite web |date=17 December 2013 |title="В Воронеже создали двигатель для Марса" в блоге "Перспективные разработки, НИОКРы, изобретения" - Сделано у нас |url=http://sdelanounas.ru/blogs/44948 |publisher=Сделано у нас |language=ru}}</ref>

=== Electrodeless plasma thrusters ===
{{main|Electrodeless plasma thruster}}

[[Electrodeless plasma thruster]]s have two unique features: the removal of the anode and cathode electrodes and the ability to throttle the engine. The removal of the electrodes eliminates erosion, which limits lifetime on other ion engines. Neutral gas is first ionized by [[electromagnetic radiation|electromagnetic waves]] and then transferred to another chamber where it is accelerated by an oscillating electric and magnetic field, also known as the [[ponderomotive force]]. This separation of the ionization and acceleration stages allows throttling of propellant flow, which then changes the thrust magnitude and specific impulse values.<ref>{{cite web|url=http://www.elwingcorp.com/files/IEPC05-article.pdf|title=Development of a High Power Electrodeless Thruster|access-date=2007-11-21|first=Gregory D.|last=Emsellem |archive-url=https://web.archive.org/web/20080515145645/http://www.elwingcorp.com/files/IEPC05-article.pdf|archive-date=2008-05-15|url-status=dead}}</ref>

=== Helicon double layer thrusters ===
{{main|Helicon double-layer thruster}}

A helicon double layer thruster is a type of plasma thruster that ejects high velocity [[ionized]] gas to provide [[thrust]]. In this design, gas is injected into a tubular chamber (the ''source tube'') with one open end. [[Radio frequency]] AC power (at [[ISM band|13.56 MHz]] in the prototype design) is coupled into a specially shaped [[antenna (radio)|antenna]] wrapped around the chamber. The [[electromagnetic wave]] emitted by the antenna causes the gas to break down and form a plasma. The antenna then excites a [[helicon (physics)|helicon wave]] in the plasma, which further heats it. The device has a roughly constant [[magnetic field]] in the source tube (supplied by [[solenoid]]s in the prototype), but the magnetic field diverges and rapidly decreases in magnitude away from the source region and might be thought of as a kind of magnetic [[nozzle]]. In operation, a sharp boundary separates the high density plasma inside the source region and the low density plasma in the exhaust, which is associated with a sharp change in electrical potential. Plasma properties change rapidly across this boundary, which is known as a ''current-free electric [[double layer (plasma)|double layer]]''. The electrical potential is much higher inside the source region than in the exhaust and this serves both to confine most of the electrons and to accelerate the ions away from the source region. Enough electrons escape the source region to ensure that the plasma in the exhaust is neutral overall.

=== Variable Specific Impulse Magnetoplasma Rocket (VASIMR) ===
{{main|Variable Specific Impulse Magnetoplasma Rocket}}

The proposed [[Variable Specific Impulse Magnetoplasma Rocket]] (VASIMR) functions by using [[radio waves]] to ionize a [[propellant]] into a plasma, and then using a [[magnetic field]] to accelerate the plasma out of the back of the [[rocket engine]] to generate thrust. The VASIMR is currently being developed by [[Ad Astra Rocket Company]], headquartered in [[Houston]], [[Texas]], with help from [[Canada]]-based [[Nautel]], producing the 200&nbsp;kW RF generators for ionizing propellant. Some of the components and "plasma shoots" experiments are tested in a laboratory settled in [[Liberia, Costa Rica]]. This project is led by former NASA astronaut [[Franklin Chang-Díaz]] (CRC-USA). A 200&nbsp;kW VASIMR test engine was in discussion to be fitted in the exterior of the [[International Space Station]], as part of the plan to test the VASIMR in space; however, plans for this test onboard ISS were canceled in 2015 by [[NASA]], with a free flying VASIMR test being discussed by Ad Astra instead.<ref name="VASiMRscrapped"/> An envisioned 200&nbsp;MW engine could reduce the duration of flight from Earth to Jupiter or Saturn from six years to fourteen months, and Mars from 7 months to 39 days.<ref>{{cite web|last=Zyga|first=Lisa|date=2009|title=Plasma Rocket Could Travel to Mars in 39 Days|url=http://phys.org/news174031552.html|work=[[Phys.org]]}}</ref>

=== Microwave electrothermal thrusters ===
{{multiple image
 | align         = right
 | header        = [[Microwave electrothermal thruster]]
 | caption_align = center
 | header_align  = center
 | total_width   = 500

 | image1   = MET Sketch 1.jpg
 | width1   = 1221
 | height1  = 629
 | alt1     = Thruster components
 | caption1 = <small style="font-size:90%;">Thruster components</small>

 | image2   = MET Sketch 2.jpg
 | width2   = 1233
 | height2  = 1297 
 | alt2     = Discharge Chamber
 | caption2 = <small style="font-size:90%;">Discharge chamber</small>
 }}

Under a research grant from the [[Glenn Research Center|NASA Lewis Research Center]] during the 1980s and 1990s, Martin C. Hawley and Jes Asmussen led a team of engineers in developing a microwave electrothermal thruster (MET).<ref>{{cite news|title=Less Fuel, More Thrust: New Engines are Being Designed for Deep Space|newspaper=The Arugus-Press|location=Owosso, Michigan|page=10|volume=128|number=48
|date=26 February 1982}}</ref>

In the discharge chamber, [[microwave]] (MW) energy flows into the center containing a high level of [[ion]]s (I), causing neutral species in the gaseous [[Rocket propellant|propellant]] to ionize. Excited species flow out (FES) through the low ion region (II) to a neutral region (III) where the ions complete their [[Plasma recombination|recombination]], replaced with the flow of neutral species (FNS) towards the center. Meanwhile, energy is lost to the chamber walls through heat [[Thermal conduction|conduction]] and [[Convection (heat transfer)|convection]] (HCC), along with [[radiation]] (Rad). The remaining energy absorbed into the gaseous propellant is converted into [[thrust]].

== Radioisotope thruster ==
A theoretical propulsion system has been proposed, based on [[alpha particle]]s ({{chem|He|2+}} or {{chem|4|2|He|2+}} indicating a helium ion with a +2 charge) emitted from a [[radioisotope]] uni-directionally through a hole in its chamber. A neutralising electron gun would produce a tiny amount of thrust with high specific impulse in the order of millions of seconds due to the high relativistic speed of alpha particles.<ref>{{cite journal|title=Revisiting alpha decay-based near-light-speed particle propulsion|journal=Applied Radiation and Isotopes|volume=114|pages=14–18|doi=10.1016/j.apradiso.2016.04.005|year=2016 |last1=Zhang|first1=Wenwu|last2=Liu|first2=Zhen|last3=Yang|first3=Yang|last4=Du|first4=Shiyu|pmid=27161512|doi-access=free|bibcode=2016AppRI.114...14Z }}</ref>

A variant of this uses a graphite-based grid with a static [[Direct current|DC]] high voltage to increase thrust as [[graphite]] has high transparency to [[alpha particle]]s if it is also irradiated with short wave [[Ultraviolet|UV light]] at the correct wavelength from a solid-state emitter. It also permits lower energy and longer half-life sources which would be advantageous for a space application. [[Helium]] backfill has also been suggested as a way to increase electron mean free path.

== Comparisons ==
{| class="wikitable sortable"
|+ Test data of some ion thrusters
|-
! Thruster
! Propellant
! data-sort-type=number | Input <br/>power (kW)
! [[Specific impulse|Specific <br>impulse]] (s)
! Thrust <br/>(mN)
! Thruster <br/>mass (kg)
! Notes
|-
| [[NASA Solar Technology Application Readiness|NSTAR]]
| [[Xenon]]
| 2.3
| {{val|1700}}–{{val|3300}}<ref>{{cite web|url=http://eccentric.mae.cornell.edu/Boydgroup/jbala/IonPropulsion.html|title=Ion Propulsion|archive-url=https://web.archive.org/web/19990222082331/http://eccentric.mae.cornell.edu/Boydgroup/jbala/IonPropulsion.html|archive-date=1999-02-22}}</ref>
| 92 max.<ref name=ns20070928/>
| 8.33 <ref>{{cite journal |vauthors= Polk J, Kakuda R, Anderson J, Brophy J, Rawlin V, Patterson M, Sovey J, Hamley J |date= 2001-01-08|title=  Performance of the NSTAR ion propulsion system on the Deep Space One mission.|journal= 39th Aerospace Sciences Meeting and Exhibit|pages= 965|doi= 10.2514/6.2001-965|url= https://trs.jpl.nasa.gov/bitstream/handle/2014/12165/01-0061.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://trs.jpl.nasa.gov/bitstream/handle/2014/12165/01-0061.pdf |archive-date=2022-10-09 |url-status=live|access-date=2021-09-16}}</ref>
|Used on the ''[[Deep Space 1]]'' and ''[[Dawn (spacecraft)|Dawn]]'' space probes.
|-
| [[PPS-1350]] Hall effect || Xenon || 1.5 || {{val|1660}} || 90 || 5.3
|
|-
| [[NEXT (ion thruster)|NEXT]]<ref name=ns20070928>{{cite news|last=Shiga|first=David|title=Next-generation ion engine sets new thrust record|url=https://www.newscientist.com/article/dn12709-nextgeneration-ion-engine-sets-new-thrust-record.html|access-date=2011-02-02|newspaper=NewScientist|date=2007-09-28}}</ref>
| Xenon
| 6.9<ref name='gizmag2013'>{{cite web|url=http://www.gizmag.com/next-ion-thruster-duration-record/28067/|title=NASA's NEXT ion thruster runs five and a half years nonstop to set new record|first=David |last=Szondy|access-date=June 26, 2013}}</ref>
| {{val|4190}}<ref name='gizmag2013'/><ref name=IAC-08-C442>{{cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080047732_2008047267.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080047732_2008047267.pdf |archive-date=2022-10-09 |url-status=live|title=The NASA Evolutionary Xenon Thruster (NEXT): the next step for US deep space propulsion|first1=George R.|last1=Schmidt|first2=Michael J.|last2=Patterson|first3=Scott W.|last3=Benson}}</ref><ref name='2010 Qualification'>{{citation|first=Daniel A. |last=Herman|contribution=NASA's Evolutionary Xenon Thruster (NEXT) Project Qualifi cation Propellant Throughput Milestone: Performance, Erosion, and Thruster Service Life Prediction After 450 kg|title=57th Joint Army-Navy-NASA-Air Force (JANNAF) Propulsion Meeting|publisher=NASA - Glenn Research Center|place=Colorado Springs, Colorado, United States|date=3–7 May 2010|contribution-url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110000521.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110000521.pdf |archive-date=2022-10-09 |url-status=live|access-date=2014-03-08}} {{PD-notice}}</ref>
|<!-- 327 ? -->236 max.<ref name=ns20070928/><ref name='2010 Qualification'/>
|<13.5 <ref>{{cite journal |vauthors= Shastry R, Soulas G, Aulisio M, Schmidt G|date=2017-09-25 |title= Status of NASA's NEXT-C Ion Propulsion System Development Project|url= https://core.ac.uk/download/pdf/154737946.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://core.ac.uk/download/pdf/154737946.pdf |archive-date=2022-10-09 |url-status=live|journal= 68th International Astronautical Congress |access-date=2021-09-16}}</ref>
| Used in [[Double Asteroid Redirection Test|DART mission.]]
|-
| [[Aerojet Rocketdyne#X3 Ion Thruster|X3]]<ref name='popularmechanics2017'>{{cite web|url=https://www.popularmechanics.com/space/moon-mars/news/a28754/new-ion-thruster-breaks-records-power-thrust/|title='Mars Engine' Shatters Records for Ion Propulsion|first=Jay |last=Bennett|date=24 October 2017 |access-date=May 30, 2021}}</ref> Hall effect
| Xenon or [[krypton]]<ref name='espressoinsight2020'>{{cite web|url=https://espressoinsight.com/2020/11/25/x3-ion-thruster/|title='Deep Space Travel: X3 Ion Thruster 2021 update|date=Nov 25, 2020|access-date=May 30, 2021}}</ref>
| 102<ref name='popularmechanics2017'/>
| 1800–2650<ref name='umich2017'>{{cite web|url=https://pepl.engin.umich.edu/project/x3-nested-channel-hall-thruster/|title=X3 – Nested Channel Hall Thruster|access-date=May 30, 2021}}</ref>
| {{val|5400}}<ref name='popularmechanics2017'/>
| {{val|230}}<ref name='umich2017'/><ref name='popularmechanics2017'/>
|
|-
| NEXIS<ref>[http://en.scientificcommons.org/20787584 An overview of the Nuclear Electric Xenon Ion System (NEXIS) program (2006)] {{Webarchive |url=https://web.archive.org/web/20110522122351/http://en.scientificcommons.org/20787584|date=2011-05-22}} 2006-02-10 (Polk, Jay E., Goebel, Don, Brophy, John R., Beatty, John, Monheiser, J., Giles, D.) Scientific Commons</ref>
| Xenon
| 20.5
|
|
|
|
|-
| RIT 22<ref>[http://cs.astrium.eads.net/sp/SpacecraftPropulsion/Rita/RIT-22.html Astrium Radiofrequency Ion Thruster, Model RIT-22] EADS Astrium  {{webarchive |url=https://web.archive.org/web/20090613015624/http://cs.astrium.eads.net/sp/SpacecraftPropulsion/Rita/RIT-22.html|date=June 13, 2009}}</ref>
| Xenon
| 5
|
|
|
|
|-
| [[BHT-8000]]<ref>{{cite web|url=http://www.busek.com/index_htm_files/70000703%20BHT-8000%20Data%20Sheet%20Rev-.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.busek.com/index_htm_files/70000703%20BHT-8000%20Data%20Sheet%20Rev-.pdf |archive-date=2022-10-09 |url-status=live|title=BHT-8000 Busek Hall Effect Thruster}}</ref>
| Xenon
| 8
| {{val|2210}}
| 449
| 25
|
|-
| Hall effect
| Xenon
| 75{{citation needed|date=August 2012}}
|
|
|
|
|-
| FEEP
| [[Caesium|Liquid caesium]]
| data-sort-value=0.0006 | 6×10<sup>−5</sup>–0.06
| {{val|6000}}–{{val|10000}}<ref name="JPP98"/>
| 0.001–1<ref name="JPP98"/>
|
|
|-
| [[NPT30|NPT30-I2]]
| [[Iodine]]
| data-sort-value=0.055 | 0.034–0.066 <ref name=NPT30>{{cite journal| title = ''In-orbit demonstration of an iodine electric propulsion system''| year = 2021| doi = 10.1038/s41586-021-04015-y| last1 = Rafalskyi| first1 = Dmytro| last2 = Martínez| first2 = Javier Martínez| last3 = Habl| first3 = Lui| last4 = Zorzoli Rossi| first4 = Elena| last5 = Proynov| first5 = Plamen| last6 = Boré| first6 = Antoine| last7 = Baret| first7 = Thomas| last8 = Poyet| first8 = Antoine| last9 = Lafleur| first9 = Trevor| last10 = Dudin| first10 = Stanislav| last11 = Aanesland| first11 = Ane| journal = Nature| volume = 599| issue = 7885| pages = 411–415| pmid = 34789903| pmc = 8599014| bibcode = 2021Natur.599..411R}}</ref>
| {{val|1000}}–{{val|2500}}<ref name=NPT30/>
| 0.5–1.5<ref name=NPT30/>
| 1.2  <!-- mass kg -->
| 
|-
|[[Starlink#v1.0 (operational)|Starlink Gen1]] Hall effect<ref name=":0">{{Cite web |date=February 26, 2023 |title=SpaceX on X: "Among other enhancements, V2 minis are equipped with new argon Hall thrusters for on orbit maneuvering Developed by SpaceX engineers, they have 2.4x the thrust and 1.5x the specific impulse of our first gen thrusters. This will also be the first time ever that argon Hall thrusters are operated in space Argon Hall thruster tech specs: - 170 mN thrust - 2500 s specific impulse - 50% total efficiency - 4.2 kW power - 2.1 kg mass - Center mounted cathode" |url=https://x.com/SpaceX/status/1629948869239873538?s=20 |url-status=live |archive-url=https://web.archive.org/web/20230301003229/https://twitter.com/SpaceX/status/1629948869239873538 |archive-date=March 1, 2023 |website=[[Twitter]]}}</ref>
|[[Krypton]]<ref name=":0" />
|
|~1667
|~70.83
|
|
|-
|[[Starlink#v2.0 (initial deployment)|Starlink Gen2]] Hall effect<ref name=":0" />
|[[Argon]]<ref name=":0" />
|4.2<ref name=":0" />
|2500<ref name=":0" />
|170<ref name=":0" />
|2.1<ref name=":0" />
|Used in [[Starlink#v2.0 (initial deployment)|Starlink V2 mini]] satellites.
|-
| [[Advanced Electric Propulsion System|AEPS]]<ref name=AEPS>{{Cite journal|last=|first=|date= |title=Status of Advanced Electric Propulsion Systems for Exploration Missions |url=https://www.researchgate.net/publication/328997773|journal=Aerojet Rocketdyne |volume=|pages=|via=ResearchGate}}</ref>
| Xenon
| 13.3
| 2900
| 600
| 25
| To be used in [[Lunar Gateway]] [[Power and Propulsion Element|PPE module.]]
|}

{| class="wikitable sortable"
|+ Experimental thrusters (no mission to date)
|-
! Thruster
! Propellant
! data-sort-type=number | Input <br/>power (kW)
! [[Specific impulse|Specific <br/>impulse]] (s)
! Thrust <br/>(mN)
! Thruster <br/>mass (kg)
! Notes
|-
| Hall effect
| [[Bismuth]]
| 1.9<ref name=Bi-Szabo>Szabo, J., Robin, M., Paintal, Pote, B., S., Hruby, V., "High Density Hall Thruster Propellant Investigations", 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA Paper 2012-3853, July 2012.</ref>
| {{val|1520}} (anode)<ref name=Bi-Szabo/>
| 143 (discharge)<ref name=Bi-Szabo/>
|
|
|-
| Hall effect
| Bismuth
| 25{{citation needed|date=August 2012}}
|
|
|
|
|-
| Hall effect
| Bismuth
| 140{{citation needed|date=August 2012}}
|
|
|
|
|-
| Hall effect
| [[Iodine]]
| 0.2<ref name=I2-Szabo>{{cite journal |last1=Szabo|first1=J. |last2=Pote|first2=B. |last3=Paintal|first3=S. |last4=Robin|first4=M. |last5=Hillier|first5=A. |last6=Branam|first6=R. |last7=Huffman|first7=R. |title=Performance Evaluation of an Iodine Vapor Hall Thruster
|journal=Journal of Propulsion and Power|volume=28|issue=4|pages=848–857|doi=10.2514/1.B34291|year=2012}}</ref>
| {{val|1510}} (anode)<ref name=I2-Szabo/>
| 12.1 (discharge)<ref name=I2-Szabo/>
|
|
|-
| Hall effect
| Iodine
| 7<ref name="I2_hp-Szabo">{{cite journal |last1=Szabo |first1=J. |last2=Robin |first2=M. |last3=Paintal |first3=S. |last4=Pote |first4=B. |last5=Hruby |first5=V. |last6=Freeman |first6=C. |year=2015 |title=Iodine Plasma Propulsion Test Results at 1–10 kW |journal=IEEE Transactions on Plasma Science |volume=43 |issue=1 |pages=141–148 |bibcode=2015ITPS...43..141S |doi=10.1109/TPS.2014.2367417 |s2cid=42482511}}</ref>
| {{val|1950}}<ref name=I2_hp-Szabo/>
| 413<ref name=I2_hp-Szabo/>
|
|
|-
| [[HiPEP]]
| Xenon
| 20–50<ref name=hipepinfo/>
| {{val|6000}}–{{val|9000}}<ref name=hipepinfo>{{cite web|title=High Power Electric Propulsion Program (HiPEP)|url=http://www.grc.nasa.gov/WWW/ion/present/hipep.htm|archive-url=https://web.archive.org/web/20090305101503/http://www.grc.nasa.gov/WWW/ion/present/hipep.htm|archive-date=2009-03-05|publisher=[[NASA]]|url-status=dead|date=2008-12-22}} {{PD-notice}}</ref>
| 460–670<ref name=hipepinfo/>
|
|
|-
| [[Magnetoplasmadynamic thruster|MPDT]]
| [[Hydrogen]]
| {{val|1500}}<ref name="MPDT">{{cite web|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880020476.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880020476.pdf |archive-date=2022-10-09 |url-status=live|title=Performance and Lifetime Assessment of MPD Arc Thruster Technology|access-date=2019-05-09|author=James S. Sovey and Maris A. Mantenieks|date=January 1988 |page=11}} {{PD-notice}}</ref>
| {{val|4900}}<ref name="MPDT"/>
| {{val|26300}}{{citation needed|date=August 2012}}
|
|
|-
| [[Magnetoplasmadynamic thruster|MPDT]]
| Hydrogen
| {{val|3750}}<ref name="MPDT"/>
| {{val|3500}}<ref name="MPDT"/>
| {{val|88500}}{{citation needed|date=August 2012}}
|
|
|-
| [[Magnetoplasmadynamic thruster|MPDT]]
| Hydrogen
| {{val|7500}}{{citation needed|date=August 2012}}
| {{val|6000}}{{citation needed|date=August 2012}}
| {{val|60000}}{{citation needed|date=August 2012}}
|
|
|-
| LiLFA
| Lithium vapor
| 500
| {{val|4077}}{{citation needed|date=August 2012}}
| {{val|12000}}{{citation needed|date=August 2012}}
|
|
|-
| FEEP
| Liquid caesium
| data-sort-value=0.0006 | 6×10<sup>−5</sup>–0.06
| {{val|6000}}–{{val|10000}}<ref name="JPP98"/>
| 0.001–1<ref name="JPP98"/>
|
|
|-
| [[VASIMR]]
| [[Argon]]
| 200
| {{val|3000}}–{{val|12000}}<!-- the fiso20110119 source shows 5000 to be about the optimum Isp, but has no data on the min/max Isp. -->
| [[Approximation|Approximately]] {{val|5000}}<ref name=fiso20110119>[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/%7Efiso/telecon/Glover_1-19-11/Glover_1-19-11.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://spirit.as.utexas.edu/%7Efiso/telecon/Glover_1-19-11/Glover_1-19-11.pdf |archive-date=2022-10-09 |url-status=live|date=2011-03-11}}, Tim Glover, Future in Space Operations (FISO) Colloquium, 2011-01-19, accessed 2011-01-31.</ref>
| 620<ref>{{cite web|url=http://www.adastrarocket.com/IEPC13-149_JPSquire_submit.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.adastrarocket.com/IEPC13-149_JPSquire_submit.pdf |archive-date=2022-10-09 |url-status=live|title=VASIMR® Spaceflight Engine System Mass Study and Scaling with Power IEPC-2013-149}}</ref>
|
|-
| CAT<ref name="CAT_thruster_Space">{{cite news|last1=Mike Wall|title=New Space Engine Could Turn Tiny CubeSats into Interplanetary Explorers|url=http://www.space.com/21867-cubesat-deep-space-propulsion-kickstarter.html|access-date=June 25, 2015|website=[[Space.com]]|publisher=[[Purch]]|date=July 8, 2013}}</ref>
| Xenon, iodine, water<ref name="CAT_thruster_PEPL">{{cite web|title=PEPL Thrusters: CubeSat Ambipolar Thruster|url=http://pepl.engin.umich.edu/thrusters/CAT.html|website=pepl.engin.umich.edu|publisher=University of Michigan|access-date=June 25, 2015|archive-date=12 May 2015|archive-url=https://web.archive.org/web/20150512105036/http://pepl.engin.umich.edu/thrusters/CAT.html|url-status=dead}}</ref>
| 0.01
| 690<ref name="MARS-CAT">{{cite web|title=MARS-CAT Mission Implementation|url=http://www.marscat.space/science/implementation|archive-url=https://web.archive.org/web/20150626112412/http://www.marscat.space/science/implementation|url-status=dead|archive-date=26 June 2015|website=marscat.space|publisher=University of Houston College of Natural Sciences and Mathematics|access-date=June 25, 2015}}</ref><ref name="P4_RF_Thruster">{{cite web|title=Phase Four: Game-Changing Spacecraft propulsion|url=http://www.phasefour.io|website=phasefour.io|access-date=June 5, 2017}}</ref>
| 1.1–2 (73&nbsp;mN/kW)<ref name="CAT_thruster_PEPL"/>
| <1<ref name="CAT_thruster_PEPL"/>
|
|-
| [[Dual-Stage 4-Grid|DS4G]]
| Xenon
| 250
| {{val|19300}}
| {{val|2500}} max.
| 5
|
|-
| [[Krypton Large Impulse Thruster|KLIMT]]
| [[Krypton]]
| 0.5<ref name=ifpilm>{{cite web |url=http://www.ifpilm.pl/ifpilm.pl/en/achievements/87-krypton-hall-effect-thruster-for-space-propulsion |title=Krypton Hall effect thruster for space propulsion |archive-url=https://archive.today/20140129162249/http://www.ifpilm.pl/ifpilm.pl/en/achievements/87-krypton-hall-effect-thruster-for-space-propulsion |archive-date=2014-01-29 |work=IFPiLM.pl |access-date=2014-01-29}}</ref>
|
|
| 4<ref name=ifpilm/>
|
|-
| ID-500
| Xenon<ref>{{Cite web|date=29 January 2020|title=Transport and Energy Module: Russia's new NEP Tug|url=https://beyondnerva.com/2020/01/29/transport-and-energy-module/|website=Beyond NERVA|access-date=16 November 2020|archive-date=27 November 2020|archive-url=https://web.archive.org/web/20201127160913/http://beyondnerva.com/2020/01/29/transport-and-energy-module/|url-status=dead}}</ref>
| 32–35
| 7140
| 375–750<ref>{{Cite web|last=Teslenko|first=Vladimir|date=31 August 2015|title=Space nuclear propulsion systems are now possible only in Russia (In Russian)|url=https://www.kommersant.ru/doc/2810188|website=Kommersant}}</ref>
|34.8
| To be used in [[TEM (nuclear propulsion)|TEM]]
|}

== Lifetime ==
Ion thrusters' low thrust requires continuous operation for a long time to achieve the necessary change in velocity ([[delta-v]]) for a particular mission. Ion thrusters are designed to provide continuous operation for intervals of weeks to years.

The lifetime of electrostatic ion thrusters is limited by several processes.

=== Gridded thruster life===
In electrostatic gridded designs, charge-exchange ions produced by the beam ions with the neutral gas flow can be accelerated towards the negatively biased accelerator grid and cause grid erosion. End-of-life is reached when either the grid structure fails or the holes in the grid become large enough that ion extraction is substantially affected – e.g., by the occurrence of electron backstreaming. Grid erosion cannot be avoided and is the major lifetime-limiting factor. Thorough grid design and material selection enable lifetimes of 20,000 hours or more.

A test of the [[NASA Solar Technology Application Readiness]] (NSTAR) electrostatic ion thruster resulted in 30,472 hours (roughly 3.5 years) of continuous thrust at maximum power. Post-test examination indicated the engine was not approaching failure.<ref>{{cite web|url=http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/39521/1/05-2793.pdf|title=Destructive Physical Analysis of Hollow Cathodes from the Deep Space 1 Flight Spare Ion Engine 30,000 Hour Life Test|access-date=2007-11-21|url-status=dead|archive-url=https://web.archive.org/web/20090227050954/http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/39521/1/05-2793.pdf|archive-date=2009-02-27}} {{PD-notice}}</ref><ref name=autogenerated1/><ref name="Choueiri"/> NSTAR operated for years on ''Dawn''.

The [[NASA Evolutionary Xenon Thruster]] (NEXT) project operated continuously for more than 48,000 hours.<ref>{{cite web |title=NASA Thruster Achieves World – Record 5+ Years of Operation |url=http://spacefellowship.com/news/art34276/nasa-thruster-achieves-world-record-5-years-of-operation.html |access-date=2012-06-27}}</ref> The test was conducted in a high-vacuum test chamber. Over the course of the test, which lasted more than five and a half years, the engine consumed approximately 870 kilograms of xenon propellant. The total impulse generated would require over 10,000 kilograms of conventional rocket propellant for a similar application.

===Hall-effect thruster life===
Hall-effect thrusters suffer from strong erosion of the ceramic discharge chamber by impact of energetic ions: a test reported in 2010 <ref name="leipzig">{{cite web|url=http://www.uni-leipzig.de/~iom/muehlleithen/2010/Bundesmann_2010.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.uni-leipzig.de/~iom/muehlleithen/2010/Bundesmann_2010.pdf |archive-date=2022-10-09 |url-status=live|title=A closer look at a stationary plasma thruster}}</ref> showed erosion of around 1&nbsp;mm per hundred hours of operation, though this is inconsistent with observed on-orbit lifetimes of a few thousand hours.

The [[Advanced Electric Propulsion System]] (AEPS) is expected to accumulate about 5,000 hours and the design aims to achieve a flight model that offers a half-life of at least 23,000 hours<ref name='AEPS 2017'/> and a full life of about 50,000 hours.<ref name="Rocketdyne 2016">[http://rocket.com/article/aerojet-rocketdyne-signs-contract-develop-advanced-electric-propulsion-system-nasa Aerojet Rocketdyne Signs Contract to Develop Advanced Electric Propulsion System for NASA] Aerojet Rocketdyne Press release, 28 April 2016 Accessed: 27 July 2018.</ref>

== Propellants ==
[[Ionization energy]] represents a large percentage of the energy needed to run ion drives. The ideal propellant is thus easy to ionize and has a high mass/ionization energy ratio. In addition, the propellant should not erode the thruster to any great degree, so as to permit long life, and should not contaminate the vehicle.<ref>Sutton & Biblarz, ''Rocket Propulsion Elements'', 7th edition.</ref>

Many current designs use [[xenon]] gas, as it is easy to ionize, has a reasonably high atomic number, is inert and causes low erosion. However, xenon is globally in short supply and expensive (approximately $3,000 per kg in 2021).<ref name=Iodine-1>{{cite web| url = https://cosmosmagazine.com/space/exploration/iodine-powered-spacecraft-tested-in-orbit/| title = ''Iodine-powered spacecraft tested in orbit for the first time'' Nov 2021| date = 18 November 2021}}</ref>

Some older ion thruster designs used [[mercury (element)|mercury]] propellant. However, mercury is toxic, tended to contaminate spacecraft, and was difficult to feed accurately. A modern commercial prototype may be using mercury successfully<ref>{{cite news|last1=Elgin|first1=Ben|title=This Silicon Valley Space Startup Could Lace the Atmosphere With Mercury|url=https://www.bloomberg.com/news/articles/2018-11-19/this-space-startup-could-lace-the-atmosphere-with-toxic-mercury|access-date=19 November 2018|publisher=Bloomberg News|date=19 November 2018}}</ref> however, mercury was formally banned as a propellant in 2022 by the [[Minamata Convention on Mercury]].<ref>{{cite news |last1=Koziol |first1=Michael |title=U.N. Kills Any Plans to Use Mercury as a Rocket Propellant |url=https://spectrum.ieee.org/mercury-ion-thruster-banned |access-date=2 May 2022 |work=[[IEEE Spectrum]] |date=19 April 2022 |language=en}}</ref>

From 2018–2023, [[krypton]] was used to fuel the Hall-effect thrusters aboard [[Starlink]] internet satellites, in part due to its lower cost than conventional [[xenon]] propellant.<ref name=TC-starlink>{{cite web |url=https://techcrunch.com/2019/05/24/spacex-reveals-more-starlink-info-after-launch-of-first-60-satellites/|title=SpaceX reveals more Starlink info after launch of first 60 satellites|date=24 May 2019 |access-date=25 May 2019}}</ref> Starlink V2-mini satellites have since switched to [[argon]] Hall-effect thrusters, providing higher specific impulse.<ref>{{Cite tweet |title=Among other enhancements, V2 minis are equipped with new argon Hall thrusters for on orbit maneuvering|number=1629898794874687489|user=SpaceX |access-date=2023-02-26 |website=Twitter |language=en}}</ref>

Other propellants, such as [[bismuth]] and [[iodine]], show promise both for gridless designs such as Hall-effect thrusters,<ref name=Bi-Szabo/><ref name=I2-Szabo/><ref name=I2_hp-Szabo/> and gridded ion thrusters.<ref>{{Cite journal |last1=Grondein|first1=P. |last2=Lafleur|first2=T. |last3=Chabert|first3=P. |last4=Aanesland|first4=A. |date=March 2016|title=Global model of an iodine gridded plasma thruster |url=http://aip.scitation.org/doi/10.1063/1.4944882|journal=Physics of Plasmas |language=en |volume=23|issue=3|pages=033514 |doi=10.1063/1.4944882|bibcode=2016PhPl...23c3514G |issn=1070-664X}}</ref>

[[Iodine]] was used as a propellant for the first time in space, in the [[NPT30|NPT30-I2]] [[gridded ion thruster]] by [[ThrustMe]], on board the Beihangkongshi-1 mission launched in November 2020,<ref>{{Cite news|title=Spacety launches satellite to test ThrustMe iodine electric propulsion and constellation technologies|work=SpaceNews|url=https://spacenews.com/thrustme-2020-demonstration/}}</ref><ref>{{Cite news |title=Iodine thruster could slow space junk accumulation |url=http://www.esa.int/Applications/Telecommunications_Integrated_Applications/Iodine_thruster_could_slow_space_junk_accumulation |work=European Space Agency}}</ref><ref>{{Cite web|title=Beihangkongshi 1 (TY 20)|url=https://space.skyrocket.de/doc_sdat/beihangkongshi-1.htm|website=Gunter's Space Page}}</ref> with an extensive report published a year later in the journal ''[[Nature (journal)|Nature]]''.<ref name=Nature1 >{{cite journal |last1=Rafalskyi |first1=Dmytro |last2=Martínez Martínez |first2=Javier |last3=Habl |first3=Lui |last4=Zorzoli Rossi |first4=Elena |last5=Proynov |first5=Plamen |last6=Boré |first6=Antoine |last7=Baret |first7=Thomas |last8=Poyet |first8=Antoine |last9=Lafleur |first9=Trevor |last10=Dudin |first10=Stanislav |last11=Aanesland |first11=Ane |date=17 November 2021 |title=In-orbit demonstration of an iodine electric propulsion system |journal=Nature |volume=599 |pages=411–415 | issue=7885|doi=10.1038/s41586-021-04015-y|pmid=34789903 |pmc=8599014 |bibcode=2021Natur.599..411R |s2cid=244347528 |issn=0028-0836  |quote=''Both atomic and molecular iodine ions are accelerated by high-voltage grids to generate thrust, and a highly collimated beam can be produced with substantial iodine dissociation.''}}</ref> The CubeSat Ambipolar Thruster (CAT) used on the Mars Array of Ionospheric Research Satellites Using the CubeSat Ambipolar Thruster (MARS-CAT) mission also proposes to use solid iodine as the propellant to minimize storage volume.<ref name="MARS-CAT"/><ref name="P4_RF_Thruster"/>

[[VASIMR]] design (and other plasma-based engines) are theoretically able to use practically any material for propellant. However, in current tests the most practical propellant is [[argon]], which is relatively abundant and inexpensive.

== Energy efficiency ==
[[File:Average propulsive efficiency of rockets.png|thumb|upright=1.4|Plot of {{colorbox|#4a86e8}} instantaneous propulsive efficiency and {{colorbox|red}} overall efficiency for a vehicle accelerating from rest as percentages of the engine efficiency. Note that peak vehicle efficiency occurs at about 1.6 times exhaust velocity.]]

Ion thruster efficiency is the kinetic energy of the exhaust jet emitted per second divided by the electrical power into the device.

Overall system [[Efficient energy use|energy efficiency]] is determined by the [[propulsive efficiency]], which depends on vehicle speed and exhaust speed. Some thrusters can vary exhaust speed in operation, but all can be designed with different exhaust speeds. At the lower end of specific impulse, ''I''<sub>sp</sub>, the overall efficiency drops because ionization takes up a larger percentage energy and at the high end propulsive efficiency is reduced.

Optimal efficiencies and exhaust velocities for any given mission can be calculated to give minimum overall cost.

== Missions ==
Ion thrusters have many in-space propulsion applications. The best applications make use of the long mission interval when significant [[thrust]] is not needed. Examples of this include orbit transfers, [[Spacecraft attitude control|attitude]] adjustments, [[drag (physics)|drag]] compensation for [[low Earth orbit]]s, fine adjustments for scientific missions and cargo transport between [[propellant depot]]s, e.g., for chemical fuels. Ion thrusters can also be used for interplanetary and deep-space missions where acceleration rates are not crucial. Ion thrusters are seen as the best solution for these missions, as they require high change in velocity but do not require rapid acceleration. Continuous thrust over long durations can reach high velocities while consuming far less propellant than traditional chemical rockets.

=== Demonstration vehicles ===
==== SERT ====
Ion propulsion systems were first demonstrated in space by the [[NASA Glenn Research Center|NASA Lewis]] (now Glenn Research Center) missions [[SERT-1|Space Electric Rocket Test (SERT)-1]] and SERT-2A.<ref name="Sovey" /> A [[SERT-1]] suborbital flight was launched on 20 July 1964, and successfully proved that the technology operated as predicted in space. These were [[electrostatic ion thruster]]s using [[Mercury (element)|mercury]] and [[caesium]] as the reaction mass. SERT-2A, launched on 4 February 1970,<ref name="sert2"/><ref>[http://www.astronautix.com/craft/sert.htm SERT page] {{webarchive|url=https://web.archive.org/web/20101025005136/http://www.astronautix.com/craft/sert.htm|date=2010-10-25}} at Astronautix (Accessed July 1, 2010).</ref> verified the operation of two mercury ion engines for thousands of running hours.<ref name="sert2"/>

=== Operational missions ===
Ion thrusters are routinely used for station-keeping on commercial and military communication satellites in geosynchronous orbit. The Soviet Union pioneered this field, using [[Hall-effect thruster|stationary plasma thrusters]] (SPTs) on satellites starting in the early 1970s.

Two geostationary satellites (ESA's [[Artemis (satellite)|Artemis]] in 2001–2003<ref>{{cite web|url=http://www.esa.int/esaTE/SEM1LT0P4HD_index_0.html|title=Artemis team receives award for space rescue |access-date=2006-11-16|publisher=ESA}}</ref> and the United States military's [[AEHF-1]] in 2010–2012<ref>{{cite web|url=http://www.airforce-magazine.com/MagazineArchive/Pages/2012/January%202012/0112space.aspx|title=Rescue in Space}}</ref>) used the ion thruster to change orbit after the chemical-propellant engine failed. [[Boeing]]<ref>{{cite web |url=http://spaceflightnow.com/news/n1203/19boeing702sp/|title=Electric propulsion could launch new commercial trend|publisher=Spaceflight Now}}</ref> began using ion thrusters for station-keeping in 1997 and planned in 2013–2014 to offer a variant on their 702 platform, with no chemical engine and ion thrusters for orbit raising; this permits a significantly lower launch mass for a given satellite capability. [[AEHF-2]] used a chemical engine to raise perigee to {{cvt|16330|km}} and proceeded to [[geosynchronous orbit]] using electric propulsion.<ref name=aehf2-sfn>{{cite web |url=https://spaceflightnow.com/atlas/av031/lae/|title=Spaceflight Now &#124; Atlas Launch Report &#124; AEHF 2 communications satellite keeps on climbing|website=spaceflightnow.com}}</ref>

==== In Earth orbit ====
===== Tiangong space station =====
China's [[Tiangong space station]] is fitted with ion thrusters. Its [[Tianhe core module]] is propelled by both chemical thrusters and four Hall-effect thrusters,<ref>{{cite web|url=https://spectrum.ieee.org/everything-you-need-to-know-about-chinas-space-station-tianhe-launch|title=Three Decades in the Making, China's Space Station Launches This Week |website=IEEE |date=28 April 2021 |first=Andrew |last=Jones}}</ref> which are used to adjust and maintain the station's orbit. The development of the Hall-effect thrusters is considered a sensitive topic in China, with scientists "working to improve the technology without attracting attention". Hall-effect thrusters are created with crewed mission safety in mind with effort to prevent erosion and damage caused by the accelerated ion particles. A magnetic field and specially designed ceramic shield was created to repel damaging particles and maintain integrity of the thrusters. According to the [[Chinese Academy of Sciences]], the ion drive used on Tiangong has burned continuously for 8,240 hours without a glitch, indicating their suitability for the Chinese space station's designated 15-year lifespan.<ref>{{cite web|url=https://www.scmp.com/news/china/science/article/3135770/how-chinas-space-station-could-help-power-astronauts-mars |title=How China's space station could help power astronauts to Mars |date=2 June 2021 |first=Stephen |last=Chen}}</ref> This is the world's first Hall thruster on a human-rated mission.<ref name="human_ion" />

===== Starlink =====
[[SpaceX]]'s [[Starlink]] [[satellite constellation]] uses [[Hall-effect thruster]]s powered by [[krypton]] or [[argon]] to raise orbit, perform maneuvers, and de-orbit at the end of their use.<ref>{{cite web |url=https://techcrunch.com/2019/05/24/spacex-reveals-more-starlink-info-after-launch-of-first-60-satellites/|title=SpaceX reveals more Starlink info after launch of first 60 satellites|date=24 May 2019 |access-date=30 July 2020}}</ref>

===== GOCE =====
[[ESA]]'s [[Gravity Field and Steady-State Ocean Circulation Explorer]] (GOCE) was launched on 16 March 2009. It used ion propulsion throughout its twenty-month mission to combat the air-drag it experienced in its low orbit (altitude of 255 kilometres) before intentionally deorbiting on 11 November 2013.

==== In deep space ====
===== Deep Space 1 =====
[[NASA]] developed the [[NASA Solar Technology Application Readiness|NSTAR]] ion engine for use in interplanetary science missions beginning in the late 1990s. It was space-tested in the space probe ''[[Deep Space 1]]'', launched in 1998. This was the first use of electric propulsion as the interplanetary propulsion system on a science mission.<ref name="Sovey"/> Based on the NASA design criteria, [[HRL Laboratories|Hughes Research Labs]] developed the [[Gridded ion thruster|Xenon Ion Propulsion System]] (XIPS) for performing [[orbital station-keeping|station keeping]] on [[geosynchronous satellite]]s.<ref>{{cite journal |author1=Rawlin |first=V. K. |author2=Patterson |first2=M. J/ |author3=Gruber |first3=R. P. |date=1990 |title=Xenon Ion Propulsion for Orbit Transfer |url=https://ntrs.nasa.gov/enwiki/api/citations/19910002485/downloads/19910002485.pdf |url-status=live |journal=NASA Technical Memorandum 103193 |issue=AIAA-90-2527 |page=5 |archive-url=https://ghostarchive.org/archive/20221009/https://ntrs.nasa.gov/enwiki/api/citations/19910002485/downloads/19910002485.pdf |archive-date=2022-10-09 |access-date=25 January 2022}}</ref> [[L3Harris Electron Devices|Hughes (EDD)]] manufactured the NSTAR thruster used on the spacecraft.

===== Hayabusa and Hayabusa2 =====
The [[JAXA|Japanese Aerospace Exploration Agency's]] [[Hayabusa (spacecraft)|''Hayabusa'']] space probe was launched in 2003 and rendezvoused with the asteroid [[25143 Itokawa]]. It was powered by four xenon ion engines, which used microwave [[electron cyclotron resonance]] to ionize the propellant and an erosion-resistant carbon/carbon-composite material for its acceleration grid.<ref>{{cite web|url=http://www.ep.isas.ac.jp/muses-c/|title=小惑星探査機はやぶさ搭載イオンエンジン (Ion Engines used on Asteroid Probe Hayabusa)|access-date=2006-10-13|publisher=ISAS |language=ja|url-status=dead|archive-url=https://web.archive.org/web/20060819093452/http://www.ep.isas.ac.jp/muses-c/|archive-date=2006-08-19}}</ref> Although the ion engines on ''Hayabusa'' experienced technical difficulties, in-flight reconfiguration allowed one of the four engines to be repaired and allowed the mission to successfully return to Earth.<ref>{{cite news|first=Hiroko |last=Tabuchi |author-link=Hiroko Tabuchi |url=https://www.nytimes.com/2010/07/02/business/global/02space.html |title=Faulty Space Probe Seen as Test of Japan's Expertise |newspaper=The New York Times |date=1 July 2010}}</ref>

[[Hayabusa2]], launched in 2014, was based on Hayabusa. It also used ion thrusters.<ref>Nishiyama, Kazutaka; Hosoda, Satoshi; Tsukizaki, Ryudo; Kuninaka, Hitoshi. [https://jaxa.repo.nii.ac.jp/?action=repository_uri&item_id=15670&file_id=31&file_no=1 Operation Status of Ion Engines of Asteroid Explorer Hayabusa2], [[JAXA]], January 2017.</ref>

===== Smart 1 =====
The [[European Space Agency]]'s satellite ''[[SMART-1]]'' launched in 2003 using a [[Safran Aircraft Engines|Snecma]] [[PPS-1350]]-G Hall thruster to get from [[Geostationary transfer orbit|GTO]] to lunar orbit. This satellite completed its mission on 3 September 2006, in a controlled collision on the [[Moon]]'s surface, after a trajectory deviation so scientists could see the 3-meter crater the impact created on the visible side of the Moon.

===== Dawn =====
[[Dawn (spacecraft)|''Dawn'']] launched on 27 September 2007, to explore the asteroid [[4 Vesta|Vesta]] and the dwarf planet [[Ceres (dwarf planet)|Ceres]]. It used three ''[[Deep Space 1]]'' heritage xenon ion thrusters (firing one at a time). ''Dawn''{{'s}} ion drive is capable of accelerating from 0 to {{cvt|97|km/h}} in 4 days of continuous firing.<ref>[http://www.jpl.nasa.gov/news/features.cfm?feature=1468 The Prius of Space] {{Webarchive|url=https://web.archive.org/web/20110605015515/http://www.jpl.nasa.gov/news/features.cfm?feature=1468 |date=5 June 2011 }}, 13 September 2007, NASA Jet Propulsion Laboratory {{PD-notice}}</ref> The mission ended on 1 November 2018, when the spacecraft ran out of [[hydrazine]] chemical propellant for its attitude thrusters.<ref name="DawnEOM">{{cite web|url=https://www.nasa.gov/press-release/nasa-s-dawn-mission-to-asteroid-belt-comes-to-end|title=NASA's Dawn Mission to Asteroid Belt Comes to End|date=1 November 2018 |publisher=NASA}} {{PD-notice}}</ref>

==== LISA Pathfinder ====
[[LISA Pathfinder]] is an [[ESA]] spacecraft launched in 2015 to orbit the Sun-Earth L1 point. It does not use ion thrusters as its primary propulsion system, but uses both [[colloid thruster]]s and [[FEEP]] for precise [[Spacecraft attitude control|attitude control]] – the low thrusts of these propulsion devices make it possible to move the spacecraft incremental distances accurately. It is a test for the [[Laser Interferometer Space Antenna|LISA]] mission. The mission ended on 30 December 2017.

==== BepiColombo ====
[[ESA]]'s [[BepiColombo]] mission was launched to [[Mercury (planet)|Mercury]] on 20 October 2018.<ref name="BepiLaunch">{{cite web |url=https://www.esa.int/Our_Activities/Operations/BepiColombo_s_beginning_ends|title=BepiColombo's beginning ends|date=22 October 2018|access-date=1 November 2018|publisher=ESA}}</ref> It uses ion thrusters in combination with [[gravity assist|swing-bys]] to get to Mercury, where a chemical rocket will complete orbit insertion.

==== Double Asteroid Redirection Test ====
NASA's [[Double Asteroid Redirection Test]] (DART) was launched in 2021 and operated its [[NEXT-C]] xenon ion thruster for about 1,000 hours to reach the target asteroid on 28 September 2022.

==== Psyche ====
NASA's [[Psyche (spacecraft)|Psyche spacecraft]] was launched in 2023 and is operating its [[SPT-140]] xenon ion thruster in order to reach asteroid [[16 Psyche]] in August 2029.

=== Proposed missions ===
==== International Space Station ====
{{as of|2011|March}}, a future launch of an Ad Astra VF-200 {{nowrap|200 kW}} [[VASIMR]] electromagnetic thruster was under consideration for testing on the [[International Space Station]] (ISS).<ref name="aaESummary20100124">{{cite web|url=http://www.adastrarocket.com/EXECUTIVE%20SUMMARY240110.pdf|title=Executive summary|date=January 24, 2010|publisher=Ad Astra Rocket Company|access-date=2010-02-27|url-status=dead|archive-url=https://web.archive.org/web/20100331171616/http://www.adastrarocket.com/EXECUTIVE%20SUMMARY240110.pdf|archive-date=March 31, 2010}}</ref><ref name="dn20080807">{{cite web |url=http://dsc.discovery.com/news/2008/08/07/plasma-rocket.html|title=Plasma Rocket May Be Tested at Space Station|date=7 August 2008|first=Irene|last=Klotz|publisher=Discovery News|access-date=2010-02-27}}</ref> However, in 2015, NASA ended plans for flying the VF-200 to the ISS. A NASA spokesperson stated that the ISS "was not an ideal demonstration platform for the desired performance level of the engines". Ad Astra stated that tests of a VASIMR thruster on the ISS would remain an option after a future in-space demonstration.<ref name="VASiMRscrapped">Irene Klotz (17 March 2015). [http://sen.com/blogs/irene-klotz/nasa-nixes-ad-astra-rocket-test-on-the-space-station NASA nixes Ad Astra rocket test on the space station] ''SEN News.''</ref>

The VF-200 would have been a flight version of the [[VX-200]].<ref name="Yahoo News">{{cite web|url=https://news.yahoo.com/nasa-test-vf-200-vasimr-plasma-rocket-iss-20110310-155100-110.html|title=NASA to Test VF-200 VASIMR Plasma Rocket at the ISS|date=March 10, 2011|first=Mark|last=Whittington|publisher=Yahoo|access-date=2012-01-27}}</ref><ref name="Yahoo">{{cite news |url=http://www.dailytech.com/Commercially+Developed+Plasma+Engine+Soon+To+Be+Tested+In+Space/article12612.htm|title=Commercially Developed Plasma Engine Soon to be Tested in Space|date=August 11, 2008 |first=Jason|last=Mick|publisher=DailyTech|access-date=2010-02-27|archive-url=https://web.archive.org/web/20150222124839/http://www.dailytech.com/Commercially+Developed+Plasma+Engine+Soon+To+Be+Tested+In+Space/article12612.htm|archive-date=February 22, 2015|url-status=dead}}</ref> Since the available power from the ISS is less than 200&nbsp;kW, the ISS VASIMR would have included a trickle-charged battery system allowing for 15 minutes pulses of thrust. The ISS orbits at a relatively [[low Earth orbit|low altitude]] and experiences fairly high levels of [[atmospheric drag]], requiring [[orbital station-keeping|periodic altitude boosts]] – a high-efficiency engine (high specific impulse) for station-keeping would be valuable; theoretically VASIMR reboosting could cut fuel cost from the current US$210 million annually to one-twentieth.<ref name="aaESummary20100124"/> VASIMR could in theory use as little as 300&nbsp;kg of argon gas for ISS station-keeping instead of 7500&nbsp;kg of chemical fuel – the high exhaust velocity (high [[specific impulse]]) would achieve the same acceleration with a smaller amount of propellant, compared to chemical propulsion with its lower exhaust velocity needing more fuel.<ref name="newscientist.com">{{cite web|last=Shiga|first=David|url=https://www.newscientist.com/article/dn17918-rocket-company-tests-worlds-most-powerful-ion-engine/|title=Rocket company tests world's most powerful ion engine|publisher=New Scientist|date=2009-10-05|access-date=2019-11-16}}</ref> [[Hydrogen]] is generated by the ISS as a by-product and is vented into space.

NASA previously worked on a 50&nbsp;kW Hall-effect thruster for the ISS, but work was stopped in 2005.<ref name="newscientist.com"/>

==== Lunar Gateway ====
The [[Power and Propulsion Element|Power and Propulsion Element (PPE)]] is a module on the [[Lunar Gateway]] that provides power generation and propulsion capabilities. It is targeting launch on a commercial vehicle in January 2024.<ref>{{cite web |url=https://www.oversight.gov/sites/default/files/oig-reports/IG-21-004.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.oversight.gov/sites/default/files/oig-reports/IG-21-004.pdf |archive-date=2022-10-09 |url-status=live |title=Report No. IG-21-004: NASA's Management of the Gateway Program for Artemis Missions |pages=5–7 |work=[[Office of Inspector General (United States)|OIG]] |publisher=[[NASA]] |date=10 November 2020 |access-date=28 December 2020}}</ref> It would probably use the 50&nbsp;kW [[Advanced Electric Propulsion System]] (AEPS) under development at NASA [[Glenn Research Center]] and [[Aerojet Rocketdyne]].<ref name="AEPS 2017">Daniel A. Herman, Todd A. Tofil, Walter Santiago, Hani Kamhawi, James E. Polk, John S. Snyder, Richard R. Hofer, Frank Q. Picha, Jerry Jackson and May Allen. [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20180001297.pdf Overview of the Development and Mission Application of the Advanced Electric Propulsion System (AEPS)], NASA/TM—2018-219761 35th International Electric Propulsion Conference, Atlanta, Georgia, 8–12 October 2017, Accessed: 27 July 2018.</ref>

==== MARS-CAT ====
The MARS-CAT (Mars Array of ionospheric Research Satellites using the CubeSat Ambipolar Thruster) mission is a two 6U [[CubeSat]] concept mission to study Mars' ionosphere. The mission would investigate its plasma and magnetic structure, including transient plasma structures, magnetic field structure, magnetic activity and correlation with solar wind drivers.<ref name="MARS-CAT"/> The CAT thruster is now called the [[Radio frequency|RF]] thruster and manufactured by Phase Four.<ref name="P4_RF_Thruster"/>

==== Interstellar missions ====
[[Geoffrey A. Landis]] proposed using an ion thruster powered by a space-based laser, in conjunction with a lightsail, to propel an interstellar probe.<ref>{{cite journal |last=Landis |first=Geoffrey A. |date=1991 |title=Laser-Powered Interstellar Probe |journal=[[APS Bulletin]] |volume=36 |issue=5 |pages=1687–1688}}</ref><ref>{{cite web|url=http://www.geoffreylandis.com/laser_ion.htp |first=Geoffrey A. |last=Landis |title=Laser-powered Interstellar Probe |date=1994 |archive-url=https://web.archive.org/web/20120722013713/http://www.geoffreylandis.com/laser_ion.htp |archive-date=2012-07-22 |work=GeoffreyLandis.com}}</ref>

== Popular culture ==
* The idea of an ion engine first appeared in Donald W. Horner's ''By Aeroplane to the Sun: Being the Adventures of a Daring Aviator and his Friends'' (1910).<ref>{{cite web|url=http://sf-encyclopedia.com/entry/ion_drive|title=Themes: Ion Drive |website=Science Fiction Encyclopedia}}</ref>
* Ion propulsion is the main thrust source of the spaceship ''Kosmokrator'' in the East German/Polish science fiction film {{lang|de|[[First Spaceship on Venus|Der Schweigende Stern]]}} (1960).<ref>{{cite book |last1=Kruschel|first1=Karsten|title=Leim für die Venus – Der Science-Fiction-Film in der DDR |lang=de |trans-title=Glue for Venus – The science fiction film in the GDR |date=2007 |publisher=Heyne |isbn=978-3-453-52261-9|pages=803–888}}</ref> Minute 28:10.
* In the 1968 ''[[Star Trek: The Original Series|Star Trek]]'' episode "[[Spock's Brain]]", [[Scotty (Star Trek)|Scotty]] is repeatedly impressed by a civilization's use of ion power.<ref>{{cite web |url=http://www.chakoteya.net/StarTrek/61.htm|title=The Star Trek Transcripts – Spock's Brain|website=chakoteya.net}}</ref><ref>{{cite web|url=https://www.tor.com/2016/06/07/star-trek-the-original-series-rewatch-spocks-brain/|title=Star Trek The Original Series Rewatch: 'Spock's Brain'|first=Keith R. A.|last=DeCandido|date=June 7, 2016|website=tor.com}}</ref>
* The popular [[TIE fighter|Imperial TIE Fighter]] spacecraft from the [[Star Wars|Star Wars franchise]] are propelled by twin ion engines, hence the name.
* Ion propulsion is used by the ''Hermes'' spacecraft in the [[Andy Weir]] novel ''[[The Martian (Weir novel)|The Martian]]'' to transfer crew between Earth and Mars.<ref>{{Cite web |date=19 August 2015 |editor-last=Fox |editor-first=Steve |title=Nine Real NASA Technologies in 'The Martian' |url=https://www.nasa.gov/feature/nine-real-nasa-technologies-in-the-martian |access-date=30 June 2023 |website=NASA |archive-date=20 June 2018 |archive-url=https://web.archive.org/web/20180620125937/https://www.nasa.gov/feature/nine-real-nasa-technologies-in-the-martian/ |url-status=dead }}</ref>

== See also ==
{{colbegin|colwidth=27em}}
* [[Advanced Electric Propulsion System]]
* [[Colloid thruster]]
* [[Comparison of orbital rocket engines]]
* [[Electrically powered spacecraft propulsion]]
* [[List of spacecraft with electric propulsion]]
* [[Nano-particle field extraction thruster]]
* [[Nuclear electric rocket]]
* [[Nuclear pulse propulsion]]
* [[Plasma actuator]]
* [[Plasma propulsion engine]]
* [[Plasma speaker]]
* [[Spacecraft propulsion]]
*[[Rocket propulsion technologies (disambiguation)]]
{{colend}}

== References ==
{{reflist}}

=== Bibliography ===
{{refbegin}}
* {{cite journal|last=Lerner|first=Eric J.|author-link=Eric Lerner|date=October 2000|title=Plasma Propulsion in Space|journal=The Industrial Physicist|volume=6|issue=5|pages=16–19 |url=http://www.aip.org/tip/INPHFA/vol-6/iss-5/p16.pdf|access-date=2007-06-29|url-status=dead|archive-url=https://web.archive.org/web/20070316160623/http://www.aip.org/tip/INPHFA/vol-6/iss-5/p16.pdf |archive-date=2007-03-16}}
* [http://www.rmcybernetics.com/science/propulsion/ehdt.htm ElectroHydroDynamic Thrusters (EHDT)] RMCybernetics
{{refend}}

== External links ==
{{Commons category|Ion propulsion}}
* [http://ep.jpl.nasa.gov/ Jet Propulsion Laboratory/NASA]
* [http://www.engr.colostate.edu/ionstand/ Colorado State University Electric Propulsion & Plasma Engineering (CEPPE) Laboratory]
* [[Geoffrey A. Landis]]: [https://web.archive.org/web/20131002200923/http://www.geoffreylandis.com/laser_ion_pres.htp ''Laser-powered Interstellar Probe'']
* [http://alfven.princeton.edu/publications/choueiri-sciam-2009 Choueiri, Edgar Y. (2009) New dawn of electric rocket The Ion Drive] {{Webarchive|url=https://web.archive.org/web/20161018201525/http://alfven.princeton.edu/publications/choueiri-sciam-2009 |date=18 October 2016 }}
* [http://phys.org/news/2015-03-revolutionary-ion-spacecraft-ceres.html The revolutionary ion engine that took spacecraft to Ceres]
* [http://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/Electric%20Propulsion%20Data%20Sheets.pdf Electric Propulsion Sub-Systems] {{Webarchive|url=https://web.archive.org/web/20140107041836/http://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/Electric%20Propulsion%20Data%20Sheets.pdf |date=7 January 2014 }}
* [https://web.archive.org/web/20180218231150/http://www.fakel-russia.com/images/content/products/fakel_spd_en_print.pdf Stationary plasma thrusters]

=== Articles ===
* [https://web.archive.org/web/20170914064056/http://www.dailygalaxy.com/my_weblog/2009/04/nasa-trumps-star-trek-ion-drive-live.html "NASA Trumps Star Trek: Ion Drive Live!"] ''The Daily Galaxy'' 13 April 2009.
* [https://web.archive.org/web/20170202022602/http://www.dailygalaxy.com/my_weblog/2009/07/the-ultimate-space-gadget-nasas-ion-drive-live-a-galaxy-classic.html "The Ultimate Space Gadget: NASA's Ion Drive Live!"] ''The Daily Galaxy'', 7 July 2009.
* An early experimental ion engine is on display at the Aerospace Discovery at the [[Florida Air Museum]].

{{Spacecraft propulsion}}
{{emerging technologies|topics=yes|space=yes}}
{{portal bar|Energy|Astronomy|Spaceflight}}

[[Category:Magnetic propulsion devices]]
[[Category:Ion engines]]
[[Category:Plasma technology and applications]]
[[Category:Spacecraft propulsion]]
[[Category:Russian inventions]]