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An ion thruster, ion drive, or ion engine is a form of electric propulsion used for
spacecraft propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric ...
. It creates
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
by accelerating ions using
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as describe ...
. An ion thruster ionizes a neutral gas by extracting some
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...
s out of
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, a ...
s, creating a cloud of positive ions. Ion thrusters are categorized as either
electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...
or
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
.
Electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...
thruster ions are accelerated by the
Coulomb force Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
along the
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical 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,
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
thruster ions are accelerated by the
Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
to accelerate all species (free electrons as well as positive and negative ions) in the same direction whatever their
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons res ...
, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration. Reprint: Ion thrusters in operation typically consume 1–7 kW of
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
, have exhaust velocities around 20–50 km/s ( ''I''sp 2000–5000s), and possess thrusts of 25–250 mN and a
propulsive efficiency In aerospace engineering, concerning aircraft, rocket and spacecraft design, overall propulsion system efficiency \eta is the efficiency with which the energy contained in a vehicle's fuel is converted into kinetic energy of the vehicle, to accelera ...
65–80% though experimental versions have achieved , . The
Deep Space 1 ''Deep Space 1'' (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies. Launched on 24 October 1998, the ''Deep Space ...
spacecraft, powered by an ion thruster, changed velocity by while consuming less than of
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
. The ''Dawn'' spacecraft broke the record, with a velocity change of , though it was only half as efficient, requiring of xenon. Applications include control of the orientation and position of orbiting
satellite A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioiso ...
s (some satellites have dozens of low-power ion thrusters) and use as a main propulsion engine for low-mass robotic space vehicles (such as ''Deep Space 1'' and ''Dawn''). Ion thrust engines are practical only in the vacuum of space and cannot take vehicles through the atmosphere because ion engines do not work in the presence of ions outside the engine; additionally, the engine's minuscule thrust cannot overcome any significant air resistance. An ion engine cannot generate sufficient thrust to achieve initial
liftoff Liftoff, lift-off, or lift off may refer to: Technology * Lift-off (microtechnology), a fabrication technique * Flame lift-off, a separation of flame from burner device * Takeoff, the first moment of flight of an aerospace vehicle * Reduction ...
from any celestial body with significant surface
gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
. For these reasons, spacecraft must rely on other methods such as conventional chemical rockets or non-rocket launch technologies to reach their initial
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
.


Origins

The first person who wrote a paper introducing the idea publicly was
Konstantin Tsiolkovsky Konstantin Eduardovich Tsiolkovsky (russian: Константи́н Эдуа́рдович Циолко́вский , , p=kənstɐnʲˈtʲin ɪdʊˈardəvʲɪtɕ tsɨɐlˈkofskʲɪj , a=Ru-Konstantin Tsiolkovsky.oga; – 19 September 1935) ...
in 1911. 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 Hermann Julius Oberth (; 25 June 1894 – 28 December 1989) was an Austro-Hungarian-born German physicist and engineer. He is considered one of the founding fathers of rocketry and astronautics, along with Robert Esnault-Pelterie, Konstantin ...
's "''Wege zur Raumschiffahrt''" (Ways to Spaceflight), published in 1929, where he explained his thoughts on the mass savings of electric propulsion, predicted its use in
spacecraft propulsion Spacecraft propulsion is any method used to accelerate spacecraft and artificial satellites. In-space propulsion exclusively deals with propulsion systems used in the vacuum of space and should not be confused with space launch or atmospheric ...
and
attitude control Attitude control is the process of controlling the orientation of an aerospace vehicle with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc. Controlling vehicle ...
, and advocated electrostatic acceleration of charged gasses. A working ion thruster was built by Harold R. Kaufman in 1959 at the
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
Glenn Research Center NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
facilities. It was similar to a gridded electrostatic ion thruster and used mercury for propellant. Suborbital tests were conducted during the 1960s and in 1964, the engine was sent into a suborbital flight aboard the Space Electric Rocket Test-1 (SERT-1). It successfully operated for the planned 31 minutes before falling to Earth. This test was followed by an orbital test, SERT-2, in 1970. An alternate form of electric propulsion, the Hall-effect thruster, was studied independently in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
and the
Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nationa ...
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 Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...
n satellites. 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 The Ballistic Missile Defense Organization (BMDO) was an agency of the United States Department of Defense that began on 20 May 1974 with the responsibility for all U.S. ballistic missile defense efforts. It was renamed the Missile Defense Age ...
, visited Soviet laboratories.


General working principle

Ion thrusters use beams of ions (electrically charged atoms or molecules) to create
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
in accordance with momentum conservation. The method of accelerating the ions varies, but all designs take advantage of the
charge Charge or charged may refer to: Arts, entertainment, and media Films * '' Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * ''Charge!!'', an album by The Aqu ...
/
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...
ratio of the ions. This ratio means that relatively small potential differences can create high exhaust velocities. This reduces the amount of
reaction mass Working mass, also referred to as reaction mass, is a mass against which a system operates in order to produce acceleration. In the case of a chemical rocket, for example, the reaction mass is the product of the burned fuel shot backwards to prov ...
or propellant required, but increases the amount of specific
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
required compared to
chemical rocket A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance ...
s. Ion thrusters are therefore able to achieve high
specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
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
electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...
or
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...
. The main difference is the method for accelerating the ions. * Electrostatic ion thrusters use the
Coulomb force Coulomb's inverse-square law, or simply Coulomb's law, is an experimental law of physics that quantifies the amount of force between two stationary, electrically charged particles. The electric force between charged bodies at rest is convention ...
and accelerate the ions in the direction of the electric field. * Electromagnetic ion thrusters use the
Lorentz force In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
to accelerate the ions in the direction perpendicular to the electric field. Electric power for ion thrusters is usually provided by
solar panel A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...
s. However, for sufficiently large distances from the sun,
nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced b ...
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. Electric thrusters tend to produce low thrust, which results in low acceleration. Defining 1g = 9.81\; \mathrm, the standard gravitational acceleration of Earth, and noting that F = ma \implies a = F/m, this can be analyzed. An NSTAR thruster producing a thrust force of 92 mN will accelerate a satellite with a mass of 1
ton Ton is the name of any one of several units of measure. It has a long history and has acquired several meanings and uses. Mainly it describes units of weight. Confusion can arise because ''ton'' can mean * the long ton, which is 2,240 pounds ...
by 0.092N / 1000 kg = 9.2m/s (or 9.38''g''). However, this acceleration can be sustained for months or years at a time, in contrast to the very short burns of chemical rockets. F = 2 \frac Where: * ''F'' is the thrust force in N, * ''η'' is the
efficiency Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
* ''P'' is the electrical power used by the thruster in W, and * ''I''sp is the
specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
in seconds. The ion thruster is not the most promising type of
electrically powered spacecraft propulsion Spacecraft electric propulsion (or just electric propulsion) is a type of spacecraft propulsion technique that uses electrostatic or electromagnetic fields to accelerate mass to high speed and thus generate thrust to modify the velocity of a s ...
, but it is the most successful in practice to date. An ion drive would require two days to accelerate a car to highway speed in vacuum. The technical characteristics, especially
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
, are considerably inferior to the prototypes described in literature, technical capabilities are limited by the
space charge Space charge is an interpretation of a collection of electric charges in which excess electric charge is treated as a continuum of charge distributed over a region of space (either a volume or an area) rather than distinct point-like charges. Thi ...
created by ions. This limits the thrust density (
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
per cross-sectional
area Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an op ...
of the engine). Ion thrusters create small thrust levels (the thrust of Deep Space 1 is approximately equal to the weight of one sheet of paper) compared to conventional
chemical rocket A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordance ...
s, but achieve high
specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
, or propellant mass efficiency, by accelerating the exhaust to high speed. The
power Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
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 Impulse or Impulsive may refer to: Science * Impulse (physics), in mechanics, the change of momentum of an object; the integral of a force with respect to time * Impulse noise (disambiguation) * Specific impulse, the change in momentum per uni ...
by the small amount of
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...
that can be stored chemically in the propellants. Given the practical weight of suitable power sources, the acceleration from an ion thruster is frequently less than one-thousandth of
standard gravity The standard acceleration due to gravity (or standard acceleration of free fall), sometimes abbreviated as standard gravity, usually denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. ...
. 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 In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower stat ...
s, and Carnot's theorem limits the exhaust velocity.


Electrostatic thrusters


Gridded electrostatic ion thrusters

Gridded electrostatic ion thrusters development started in the 1960s and, since then, it has been used for commercial satellite propulsion and scientific missions.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. Their main feature is that the propellant
ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecul ...
process is physically separated from the ion acceleration process. 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 A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
filament The word filament, which is descended from Latin ''filum'' meaning " thread", is used in English for a variety of thread-like structures, including: Astronomy * Galaxy filament, the largest known cosmic structures in the universe * Solar 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 keV, which generates thrust. Ion thrusters emit a beam of positively charged ions. To keep the spacecraft from accumulating a charge, another
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
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. Gridded electrostatic ion thruster research (past/present): *
NASA Solar Technology Application Readiness The NASA Solar Technology Application Readiness (NSTAR) is a type of spacecraft ion thruster called electrostatic ion thruster. It is a highly efficient low-thrust spacecraft propulsion running on electrical power generated by solar arrays. It u ...
(NSTAR), 2.3 kW, used on two successful missions * NASA's Evolutionary Xenon Thruster (
NEXT Next may refer to: Arts and entertainment Film * ''Next'' (1990 film), an animated short about William Shakespeare * ''Next'' (2007 film), a sci-fi film starring Nicolas Cage * '' Next: A Primer on Urban Painting'', a 2005 documentary film Lit ...
), 6.9 kW, flight qualification hardware built * Nuclear Electric Xenon Ion System (NEXIS) * High Power Electric Propulsion ( HiPEP), 25 kW, test example built and run briefly on the ground * EADS Radio-frequency Ion Thruster (RIT) * Dual-Stage 4-Grid (DS4G)


Hall-effect thrusters

Hall-effect thrusters 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 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.


Field-emission electric propulsion

Field-emission electric propulsion (FEEP) thrusters may use
caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
or
indium Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts ...
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 cones''. At a sufficiently high applied voltage, positive ions are extracted from the tips of the cones. 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


Pulsed inductive thrusters

Pulsed inductive thruster A pulsed inductive thruster (PIT) is a form of ion thruster, used in spacecraft propulsion. It is a plasma propulsion engine using perpendicular electric and magnetic fields to accelerate a propellant with no electrode. Operation A nozzle r ...
s (PIT) 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 coil encircling a cone shaped tube that emits the propellant gas.
Ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
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.


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 Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
,
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...
,
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
and
nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
can be used as propellant. In a certain configuration, the ambient gas in
low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...
(LEO) can be used as a propellant. The gas enters the main chamber where it is ionized into plasma by the electric field between the
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and the
cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
. 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, 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 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 In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
. In 2013, Russian company the Chemical Automatics Design Bureau successfully conducted a bench test of their MPD engine for long-distance space travel.


Electrodeless plasma thrusters

Electrodeless plasma thruster The electrodeless plasma thruster is a spacecraft propulsion engine commercialized under the acronym "E-IMPAcT" for "Electrodeless-Ionization Magnetized Ponderomotive Acceleration Thruster". It was created by Mr. Gregory Emsellem based on technolog ...
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 waves In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) ...
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.


Helicon double layer thrusters

A helicon double layer thruster is a type of plasma thruster that ejects high velocity
ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...
gas to provide
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
. In this design, gas is injected into a tubular chamber (the ''source tube'') with one open end.
Radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the up ...
AC power (at 13.56 MHz in the prototype design) is coupled into a specially shaped
antenna Antenna ( antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two collid ...
wrapped around the chamber. The
electromagnetic wave In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) ...
emitted by the antenna causes the gas to break down and form a plasma. The antenna then excites a helicon wave in the plasma, which further heats it. The device has a roughly constant
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
in the source tube (supplied by
solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...
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 A nozzle is a device designed to control the direction or characteristics of a fluid flow (specially to increase velocity) as it exits (or enters) an enclosed chamber or pipe. A nozzle is often a pipe or tube of varying cross sectional area, ...
. 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''. 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)

The proposed
Variable Specific Impulse Magnetoplasma Rocket The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electrothermal thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic ...
(VASIMR) functions by using
radio waves Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz ( GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (s ...
to ionize a
propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the ...
into a plasma, and then using a
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
to accelerate the plasma out of the back of the
rocket engine A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accorda ...
to generate thrust. The VASIMR is currently being developed by
Ad Astra Rocket Company The Ad Astra Rocket Company is an American rocket propulsion company dedicated to the development of advanced plasma rocket propulsion technology. Located in Webster, Texas, three miles away from NASA's Johnson Space Center, the company was in ...
, headquartered in
Houston Houston (; ) is the most populous city in Texas, the most populous city in the Southern United States, the fourth-most populous city in the United States, and the sixth-most populous city in North America, with a population of 2,304,580 ...
,
Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...
, with help from
Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
-based Nautel, producing the 200 kW RF generators for ionizing propellant. Some of the components and "plasma shoots" experiments are tested in a laboratory settled in
Liberia, Costa Rica Liberia () is a district and the largest city in the Guanacaste Province of Costa Rica, located northwest of the national capital, San José, in the canton with the same name, of which it is the head city or capital. It is a major center for t ...
. This project is led by former NASA astronaut Dr. Franklin Chang-Díaz (CRC-USA). A 200 kW VASIMR test engine was in discussion to be fitted in the exterior of the
International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...
, as part of the plan to test the VASIMR in space – however plans for this test onboard ISS were canceled in 2015 by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
, with a free flying VASIMR test being discussed by Ad Astra instead. An envisioned 200 megawatt 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.


Microwave electrothermal thrusters

Under a research grant from the
NASA Lewis Research Center NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
during the 1980s and 1990s, Martin C. Hawley and Jes Asmussen led a team of engineers in developing a Microwave Electrothermal Thruster (MET). In the discharge chamber,
microwave Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ra ...
(MW) energy flows into the center containing a high level of ions (I), causing neutral species in the gaseous
propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the ...
to ionize. Excited species flow out (FES) through the low ion region (II) to a neutral region (III) where the ions complete their recombination, replaced with the flow of neutral species (FNS) towards the center. Meanwhile, energy is lost to the chamber walls through heat
conduction Conductor or conduction may refer to: Music * Conductor (music), a person who leads a musical ensemble, such as an orchestra. * ''Conductor'' (album), an album by indie rock band The Comas * Conduction, a type of structured free improvisation ...
and
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...
(HCC), along with
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
(Rad). The remaining energy absorbed into the gaseous propellant is converted into
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
.


Radioisotope thruster

A theoretical propulsion system has been proposed, based on
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...
s ( or indicating a helium ion with a +2 charge) emitted from a
radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
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. A variant of this uses a graphite-based grid with a static DC high voltage to increase thrust as
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on la ...
has high transparency to
alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...
s if it is also irradiated with short wave
UV light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
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 Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...
backfill has also been suggested as a way to increase electron mean free path.


Comparisons


Lifetime

Ion thrusters' low thrust requires continuous operation for a long time to achieve the necessary change in velocity (
delta-v Delta-''v'' (more known as " change in velocity"), symbolized as ∆''v'' and pronounced ''delta-vee'', as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such a ...
) 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 The NASA Solar Technology Application Readiness (NSTAR) is a type of spacecraft ion thruster called electrostatic ion thruster. It is a highly efficient low-thrust spacecraft propulsion running on electrical power generated by solar arrays. It u ...
(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. NSTAR operated for years on Dawn. The NASA Evolutionary Xenon Thruster (NEXT) project operated continuously for more than 48,000 hours. The test was conducted in a high vacuum test chamber. Over the course of the 5.5+ year test, 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 showed erosion of around 1 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 and a full life of about 50,000 hours.
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. (~$3,000/kg in 2021) Some older ion thruster designs used 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. Mercury was formally banned as a propellant in 2022 by the
Minamata Convention on Mercury The Minamata Convention on Mercury is an international treaty designed to protect human health and the environment from anthropogenic emissions and releases of mercury and mercury compounds. The convention was a result of three years of meetin ...
. Since 2018,
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is of ...
is used to fuel the Hall-effect thrusters aboard
Starlink Starlink is a satellite internet constellation operated by SpaceX, providing satellite Internet access coverage to 45 countries. It also aims for global mobile phone service after 2023. SpaceX started launching Starlink satellites in 2019. As ...
internet satellites, in part due to its lower cost than conventional
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
propellant. Other propellants, such as
bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...
and
iodine Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
, show promise both for gridless designs such as Hall-effect thrusters, and gridded ion thrusters.
Iodine Iodine is a chemical element with the Symbol (chemistry), symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , ...
was used as a propellant for the first time in space, in the
NPT30-I2 An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes a neutral gas by extracting some electrons out of ...
gridded ion thruster The gridded ion thruster is a common design for ion thrusters, a highly efficient low-thrust spacecraft propulsion method running on electrical power by using high-voltage grid electrodes to accelerate ions with electrostatic forces. History The ...
by
ThrustMe ThrustMe is a deep tech company that designs miniaturized aerospace thrusters for small satellites, increasing the life of satellites and making them more affordable. The company builds gridded ion thrusters (NPT30) and cold gas thrusters (I2T5) ...
, on board the Beihangkongshi-1 mission launched in November 2020, with an extensive report published a year later in the journal ''
Nature Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are ...
''. 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.
VASIMR The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electrothermal thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic ...
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 Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice a ...
, which is relatively abundant and inexpensive.


Energy efficiency

Ion thruster efficiency is the kinetic energy of the exhaust jet emitted per second divided by the electrical power into the device. Overall system energy efficiency is determined by the
propulsive efficiency In aerospace engineering, concerning aircraft, rocket and spacecraft design, overall propulsion system efficiency \eta is the efficiency with which the energy contained in a vehicle's fuel is converted into kinetic energy of the vehicle, to accelera ...
, 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''sp, 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 Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that ...
is not needed. Examples of this include orbit transfers,
attitude Attitude may refer to: Philosophy and psychology * Attitude (psychology), an individual's predisposed state of mind regarding a value * Metaphysics of presence * Propositional attitude, a relational mental state connecting a person to a propo ...
adjustments, drag compensation for
low Earth orbit A low Earth orbit (LEO) is an orbit around Earth with a period of 128 minutes or less (making at least 11.25 orbits per day) and an eccentricity less than 0.25. Most of the artificial objects in outer space are in LEO, with an altitude never m ...
s, fine adjustments for scientific missions and cargo transport between
propellant depot An orbital propellant depot is a cache of propellant that is placed in orbit around Earth or another body to allow spacecraft or the transfer stage of the spacecraft to be fueled in space. It is one of the types of space resource depots that ...
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 Lewis (now Glenn Research Center) missions Space Electric Rocket Test (SERT)-1 and SERT-2A. 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 thrusters using mercury and
caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
as the reaction mass. SERT-2A, launched on 4 February 1970, verified the operation of two mercury ion engines for thousands of running hours.


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 Stationary Plasma Thrusters (SPTs) on satellites starting in the early 1970s. Two geostationary satellites (ESA's
Artemis In ancient Greek mythology and religion, Artemis (; grc-gre, Ἄρτεμις) is the goddess of the hunt, the wilderness, wild animals, nature, vegetation, childbirth, care of children, and chastity. She was heavily identified with ...
in 2001–2003 and the United States military's AEHF-1 in 2010–2012) used the ion thruster to change orbit after the chemical-propellant engine failed.
Boeing The Boeing Company () is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, telecommunications equipment, and missiles worldwide. The company also provides leasing and ...
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 and proceeded to
geosynchronous orbit A geosynchronous orbit (sometimes abbreviated GSO) is an Earth-centered orbit with an orbital period that matches Earth's rotation on its axis, 23 hours, 56 minutes, and 4 seconds (one sidereal day). The synchronization of rotation and orbita ...
using electric propulsion.


In Earth orbit


= Tiangong space station

= China's Tiangong space station is fitted with ion thrusters.
Tianhe core module ''Tianhe'' (), officially the ''Tianhe'' core module (), is the first module to launch of the Tiangong space station. It was launched into orbit on 29 April 2021, as the first launch of the final phase of Tiangong program, part of the Chin ...
is propelled by both chemical thrusters and four Hall-effect thrusters, 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 Chinese Academy of Sciences (CAS); ), known by Academia Sinica in English until the 1980s, is the national academy of the People's Republic of China for natural sciences. It has historical origins in the Academia Sinica during the Republi ...
, the ion drive used on Tiangong has burned continuously for 8,240 hours without a glitch, indicating their suitability for Chinese space station's designated 15-year lifespan.


= Starlink

=
SpaceX Space Exploration Technologies Corp. (SpaceX) is an American spacecraft manufacturer, launcher, and a satellite communications corporation headquartered in Hawthorne, California. It was founded in 2002 by Elon Musk with the stated goal o ...
's
Starlink Starlink is a satellite internet constellation operated by SpaceX, providing satellite Internet access coverage to 45 countries. It also aims for global mobile phone service after 2023. SpaceX started launching Starlink satellites in 2019. As ...
satellite constellation A satellite constellation is a group of artificial satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global or near-global coverage, such that at any time everywhere on Earth at least one s ...
uses Hall-effect thrusters powered by
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is of ...
to raise orbit, perform maneuvers, and de-orbit at the end of their use.


= GOCE

= ESA's
Gravity Field and Steady-State Ocean Circulation Explorer The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) was the first of ESA's Living Planet Programme satellites intended to map in unprecedented detail the Earth's gravity field. The spacecraft's primary instrumentation was a hi ...
(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 The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...
developed the NSTAR ion engine for use in interplanetary science missions beginning in the late-1990s. It was space-tested in the highly successful space probe ''
Deep Space 1 ''Deep Space 1'' (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies. Launched on 24 October 1998, the ''Deep Space ...
'', launched in 1998. This was the first use of electric propulsion as the interplanetary propulsion system on a science mission. Based on the NASA design criteria, Hughes Research Labs, developed the
Xenon Ion Propulsion System An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes a neutral gas by extracting some electrons out of ...
(XIPS) for performing station keeping on
geosynchronous satellite A geosynchronous satellite is a satellite in geosynchronous orbit, with an orbital period the same as the Earth's rotation period. Such a satellite returns to the same position in the sky after each sidereal day, and over the course of a day tra ...
s. Hughes (EDD) manufactured the NSTAR thruster used on the spacecraft.


= Hayabusa and Hayabusa2

= The Japanese Aerospace Exploration Agency's ''Hayabusa'' space probe was launched in 2003 and successfully rendezvoused with the asteroid
25143 Itokawa 25143 Itokawa (provisional designation ) is a sub-kilometer near-Earth object of the Apollo group and a potentially hazardous asteroid. It was discovered by the LINEAR program in 1998 and later named after Japanese rocket engineer Hideo Itokawa ...
. It was powered by four xenon ion engines, which used microwave
electron cyclotron resonance Electron cyclotron resonance (ECR) is a phenomenon observed in plasma physics, condensed matter physics, and accelerator physics. It happens when the frequency of incident radiation coincides with the natural frequency of rotation of electrons in m ...
to ionize the propellant and an erosion-resistant carbon/carbon-composite material for its acceleration grid. 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.
Hayabusa2 is an asteroid sample-return mission operated by the Japanese state space agency JAXA. It is a successor to the '' Hayabusa'' mission, which returned asteroid samples for the first time in June 2010. ''Hayabusa2'' was launched on 3 December ...
, launched in 2014, was based on Hayabusa. It also used ion thrusters.


= Smart 1

= The
European Space Agency , owners = , headquarters = Paris, Île-de-France, France , coordinates = , spaceport = Guiana Space Centre , seal = File:ESA emblem seal.png , seal_size = 130px , image = Views in the Main Control Room (120 ...
's satellite '' SMART-1'' launched in 2003 using a
Snecma Safran Aircraft Engines, previously Snecma (''Société nationale d'études et de construction de moteurs d'aviation'') or Snecma Moteurs, is a French aerospace engine manufacturer headquartered in Courcouronnes and a subsidiary of Safran. It ...
PPS-1350-G Hall thruster to get from
GTO GTO may refer to: Entertainment * ''Great Teacher Onizuka'', a manga, anime, live-action series, and film * GameTable Online, a game portal Music bands * GTO (band), an Australian band * The GTOs, an American girl group * Giraffe Tongue Orche ...
to lunar orbit. This satellite completed its mission on 3 September 2006, in a controlled collision on the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
'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'' 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 ''Deep Space 1'' (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies. Launched on 24 October 1998, the ''Deep Space ...
'' heritage xenon ion thrusters (firing one at a time). ''Dawn'' ion drive is capable of accelerating from 0 to in 4 days of continuous firing. The mission ended on 1 November 2018, when the spacecraft ran out of hydrazine chemical propellant for its attitude thrusters.


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 thrusters and FEEP for precise
attitude control Attitude control is the process of controlling the orientation of an aerospace vehicle with respect to an inertial frame of reference or another entity such as the celestial sphere, certain fields, and nearby objects, etc. Controlling vehicle ...
– 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. 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.


Proposed missions


International Space Station

, a future launch of an Ad Astra VF-200
VASIMR The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is an electrothermal thruster under development for possible use in spacecraft propulsion. It uses radio waves to ionize and heat an inert propellant, forming a plasma, then a magnetic ...
electromagnetic thruster was under consideration for testing on the
International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...
(ISS). 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.NASA nixes Ad Astra rocket test on the space station
''SEN News'' Irene Klotz 17 March 2015
The VF-200 would have been a flight version of the VX-200. Since the available power from the ISS is less than 200 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. VASIMR could in theory use as little as 300 kg of argon gas for ISS station-keeping instead of 7500 kg of chemical fuel – the high exhaust velocity (high
specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...
) would achieve the same acceleration with a smaller amount of propellant, compared to chemical propulsion with its lower exhaust velocity needing more fuel.
Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...
is generated by the ISS as a by-product and is vented into space. NASA previously worked on a 50 kW Hall-effect thruster for the ISS, but work was stopped in 2005.


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. It would probably use the 50 kW Advanced Electric Propulsion System (AEPS) under development at NASA
Glenn Research Center NASA John H. Glenn Research Center at Lewis Field is a NASA center within the cities of Brook Park and Cleveland between Cleveland Hopkins International Airport and the Rocky River Reservation of Cleveland Metroparks, with a subsidiary facilit ...
and Aerojet Rocketdyne. The CAT thruster is now called the Radio frequency, RF thruster and manufactured by Phase Four.


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.


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). * Ion propulsion is the main thrust source of the spaceship ''Kosmokrator'' in the Eastern German/Polish science fiction movie ''First Spaceship on Venus, Der Schweigende Stern'' (1960). Minute 28:10. * In the 1968 episode of ''Star Trek: The Original Series, Star Trek'', "Spock's Brain", Scotty (Star Trek), Scotty is repeatedly impressed by a civilization's use of ion power.


See also

* 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


References


Bibliography

*
ElectroHydroDynamic Thrusters (EHDT)
RMCybernetics


External links


Jet Propulsion Laboratory/NASA

Colorado State University Electric Propulsion & Plasma Engineering (CEPPE) Laboratory
* Geoffrey A. Landis
''Laser-powered Interstellar Probe''

Choueiri, Edgar Y. (2009) New dawn of electric rocket The Ion Drive



Electric Propulsion Sub-Systems

Stationary plasma thrusters


Articles



''The Daily Galaxy'' 13 April 2009.

''The Daily Galaxy'', 7 July 2009. * An early experimental ion engine is on display at the Aerospace Discovery at the Florida Air Museum. {{portal bar, Energy, Astronomy, Spaceflight Magnetic propulsion devices Ion engines, Emerging technologies Spacecraft propulsion Russian inventions