Editing Spacecraft propulsion

{{short description|Method used to accelerate spacecraft}}
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[[File:Shuttle Main Engine Test Firing.jpg|thumb|A remote camera captures a close-up view of an [[RS-25]] during a test firing at the [[John C. Stennis Space Center]] in [[Hancock County, Mississippi]].]]
[[File:Engine of APOLLO Eagle.png|thumb|[[Bipropellant rocket]] engines of the [[Apollo Lunar Module]] [[reaction control system]] (RCS)]]

'''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 entry.

Several methods of pragmatic spacecraft propulsion have been developed, each having its own drawbacks and advantages. Most satellites have simple reliable chemical thrusters (often [[monopropellant rocket]]s) or [[resistojet rocket]]s for orbital station-keeping, while a few use [[Reaction wheel|momentum wheels]] for attitude control. Russian and antecedent Soviet bloc satellites have used [[Spacecraft electric propulsion|electric propulsion]] for decades,<ref>{{Cite web|url=http://fluid.ippt.gov.pl/sbarral/hall.html|title=Electric Propulsion Research at Institute of Fundamental Technological Research|date=16 August 2011|archive-url=https://web.archive.org/web/20110816154150/http://fluid.ippt.gov.pl/sbarral/hall.html|archive-date=16 August 2011}}</ref> and newer Western geo-orbiting spacecraft are starting to use them for north–south station-keeping and orbit raising. Interplanetary vehicles mostly use chemical rockets as well, although a few have used electric propulsion such as [[ion thruster]]s and [[Hall-effect thruster]]s. Various technologies need to support everything from small satellites and robotic deep space exploration to space stations and human missions to Mars.

Hypothetical in-space propulsion technologies describe propulsion technologies that could meet future space science and exploration needs. These propulsion technologies are intended to provide effective exploration of the Solar System and may permit mission designers to plan missions to "fly anytime, anywhere, and complete a host of science objectives at the destinations" and with greater reliability and safety. With a wide range of possible missions and candidate propulsion technologies, the question of which technologies are "best" for future missions is a difficult one; expert opinion now holds that a portfolio of propulsion technologies should be developed to provide optimum solutions for a diverse set of missions and destinations.<ref name="meyer">{{cite web |url=http://www.nasa.gov/pdf/501329main_TA02-ID_rev3-NRC-wTASR.pdf |title=In-space propulsion systems roadmap |last=Meyer |first=Mike |date=April 2012 |website=nasa.gov |access-date=Feb 1, 2021 |page=9 |archive-date=October 9, 2022 |archive-url=https://ghostarchive.org/archive/20221009/http://www.nasa.gov/pdf/501329main_TA02-ID_rev3-NRC-wTASR.pdf |url-status=dead }}</ref><ref name="mason">Mason, Lee S. "[http://www.mdcampbell.com/Mason2006.pdf A practical approach to starting fission surface power development.]" proceedings of International Congress on Advances in Nuclear Power Plants (ICAPP'06), American Nuclear Society, La Grange Park, Illinois, 2006b, paper. Vol. 6297. 2006.</ref><ref name="leone">{{cite news |last=Leone |first=Dan |date=May 20, 2013 |title=NASA Banking on Solar Electric Propulsion's Slow but Steady Push |url=http://www.spacenews.com/article/civil-space/35395space-technology-and-innovation-nasa-banking-on-solar-electric-propulsion%E2%80%99s |archive-url=https://archive.today/20130720074025/http://www.spacenews.com/article/civil-space/35395space-technology-and-innovation-nasa-banking-on-solar-electric-propulsion%E2%80%99s |archive-date=July 20, 2013 |access-date=February 1, 2021 |newspaper=Space News |publisher=SpaceNews, Inc}}</ref>

==Purpose and function==
{{more citations needed section | date = July 2023}}
[[Space exploration]] is about reaching the destination safely (mission enabling), quickly (reduced transit times), with a large quantity of [[Payload (air and space craft)|payload]] mass, and relatively inexpensively (lower cost). The act of reaching the destination requires an in-space propulsion system, and the other metrics are modifiers to this fundamental action.{{Sfn|Meyer|2012|p=5}}<ref name="leone" /> Propulsion technologies can significantly improve a number of critical aspects of the mission.

When launching a spacecraft from Earth, a propulsion method must overcome a higher [[gravity drag|gravitational]] pull to provide a positive net acceleration.<ref name="beginners_guide">{{cite web |last=Benson |first=Tom |title=Guided Tours: Beginner's Guide to Rockets |url=http://exploration.grc.nasa.gov/education/rocket/guided.htm |archive-url=https://web.archive.org/web/20130814022045/http://exploration.grc.nasa.gov/education/rocket/guided.htm |archive-date=2013-08-14 |access-date=2007-08-02 |publisher=NASA}}</ref> When in space, the purpose of a [[propulsion system]] is to change the velocity, or ''v'', of a spacecraft.<ref>{{cite web |last=Zobel |first=Edward A. |year=2006 |title=Summary of Introductory Momentum Equations |url=http://id.mind.net/~zona/mstm/physics/mechanics/momentum/introductoryProblems/momentumSummary2.html |archive-url=https://web.archive.org/web/20070927025532/http://id.mind.net/~zona/mstm/physics/mechanics/momentum/introductoryProblems/momentumSummary2.html |archive-date=September 27, 2007 |access-date=2007-08-02 |publisher=Zona Land}}</ref>

In-space propulsion begins where the [[upper stage]] of the [[launch vehicle]] leaves off, performing the functions of [[rocket propulsion|primary propulsion]], [[reaction engine|reaction control]], [[orbital station-keeping|station keeping]], [[Spacecraft attitude control|precision pointing]], and [[orbital maneuver]]ing. The [[rocket engine|main engine]]s used in [[space]] provide the primary propulsive force for [[Geostationary orbit|orbit transfer]], [[Hohmann transfer orbit|planetary trajectories]], and extra [[List of landings on extraterrestrial bodies|planetary landing]] and [[wikt:ascent|ascent]]. The reaction control and orbital maneuvering systems provide the propulsive force for orbit maintenance, position control, station keeping, and spacecraft attitude control.{{Sfn|Meyer|2012|p=5}}<ref name="mason" /><ref name="leone" />

In orbit, any additional [[Impulse (physics)|impulse]], even tiny, will result in a change in the orbit path, in two ways:<ref name="z677">{{cite web | title=In-Space Propulsion Technology Products for NASA's Future Science and Exploration Missions | url=https://ntrs.nasa.gov/api/citations/20110016163/downloads/20110016163.pdf | access-date=2024-08-03}}</ref>
* Prograde/retrograde (i.e. acceleration in the tangential/opposite in tangential direction), which increases/decreases altitude of orbit.
* Perpendicular to orbital plane, which changes [[orbital inclination change|orbital inclination]].{{citation needed|date = July 2023}}
Earth's surface is situated fairly deep in a [[gravity well]]; the [[escape velocity]] required to leave its orbit is 11.2 kilometers/second.<ref>{{Cite web |date=2024-02-23 |title=Escape velocity {{!}} Definition, Formula, Earth, Moon, & Facts {{!}} Britannica |url=https://www.britannica.com/science/escape-velocity |access-date=2024-04-20 |website=www.britannica.com |language=en}}</ref> Thus for destinations beyond, propulsion systems need enough propellant and to be of high enough efficiency. The same is true for other planets and moons, albeit some have lower gravity wells.

As human beings evolved in a gravitational field of "one ''g''" (9.81m/s²), it would be most comfortable for a human spaceflight propulsion system to provide that acceleration continuously,{{according to whom|date = July 2023}} (though human bodies can tolerate much larger accelerations over short periods).<ref>{{Cite web |title=The Jump to Light Speed Is a Real Killer |url=https://www.scientificamerican.com/article/star-wars-science-light-speed/ |access-date=2024-04-20 |website=Scientific American |language=en}}</ref> The occupants of a rocket or spaceship having such a propulsion system would be free from the ill effects of [[free fall]], such as nausea, muscular weakness, reduced sense of taste, or [[leaching (chemistry)|leaching]] of calcium from their bones.<ref>{{Cite journal |last1=Wolfe |first1=J. W. |last2=Rummel |first2=J. D. |date=1992 |title=Long-term effects of microgravity and possible countermeasures |url=https://pubmed.ncbi.nlm.nih.gov/11536970/ |journal=Advances in Space Research |volume=12 |issue=1 |pages=281–284 |doi=10.1016/0273-1177(92)90296-a |issn=0273-1177 |pmid=11536970|bibcode=1992AdSpR..12a.281W }}</ref><ref>{{Cite journal |date=December 22, 2022 |title=Human Health during Space Travel: State-of-the-Art Review |pmc=9818606 |last1=Krittanawong |first1=C. |last2=Singh |first2=N. K. |last3=Scheuring |first3=R. A. |last4=Urquieta |first4=E. |last5=Bershad |first5=E. M. |last6=MacAulay |first6=T. R. |last7=Kaplin |first7=S. |last8=Dunn |first8=C. |last9=Kry |first9=S. F. |last10=Russomano |first10=T. |last11=Shepanek |first11=M. |last12=Stowe |first12=R. P. |last13=Kirkpatrick |first13=A. W. |last14=Broderick |first14=T. J. |last15=Sibonga |first15=J. D. |last16=Lee |first16=A. G. |last17=Crucian |first17=B. E. |journal=Cells |volume=12 |issue=1 |page=40 |doi=10.3390/cells12010040 |doi-access=free |pmid=36611835 }}</ref>

==Theory==
The [[Tsiolkovsky rocket equation]] shows, using the law of [[conservation of momentum]], that for a [[rocket engine]] propulsion method to change the momentum of a spacecraft, it must change the momentum of something else in the opposite direction. In other words, the rocket must exhaust mass opposite the spacecraft's acceleration direction, with such exhausted mass called [[propellant]] or [[reaction mass]].<ref>{{Cite book |last=Turner |first=Martin J. L. |title=Rocket and spacecraft propulsion: principles, practice and new developments |date=2009 |publisher=Praxis Publ |isbn=978-3-540-69202-7 |edition=3rd |series=Springer-Praxis books in astronautical engineering |location=Chichester, UK}}</ref>{{Rp|location=Sec 1.2.1}}<ref name="ReactiveFlyingMachines">{{Cite web |last=Tsiolkovsky |first=K. |title=Reactive Flying Machines |url=http://epizodsspace.airbase.ru/bibl/inostr-yazyki/tsiolkovskii/tsiolkovskii-nhedy-t2-1954.pdf}}</ref> For this to happen, both reaction mass and energy are needed. The impulse provided by launching a particle of reaction mass with mass ''m'' at velocity ''v'' is ''mv''. But this particle has kinetic energy ''mv''²/2, which must come from somewhere. In a conventional [[solid rocket|solid]], [[liquid-propellant rocket|liquid]], or [[hybrid rocket]], fuel is burned, providing the energy, and the reaction products are allowed to flow out of the [[Rocket engine nozzle|engine nozzle]], providing the reaction mass. In an [[ion thruster]], electricity is used to accelerate ions behind the spacecraft. Here other sources must provide the electrical energy (e.g. a [[Photovoltaic module|solar panel]] or a [[nuclear reactor]]), whereas the ions provide the reaction mass.<ref name="beginners_guide" />

The rate of change of [[velocity]] is called [[acceleration]] and the rate of change of [[momentum]] is called [[force]].<ref>{{Cite web |title=Momentum |url=https://pages.uoregon.edu/jschombe/glossary/momentum.html |access-date=2024-04-19 |website=pages.uoregon.edu}}</ref> To reach a given velocity, one can apply a small acceleration over a long period of time, or a large acceleration over a short time; similarly, one can achieve a given impulse with a large force over a short time or a small force over a long time. This means that for maneuvering in space, a propulsion method that produces tiny accelerations for a long time can often produce the same impulse as another which produces large accelerations for a short time.<ref name=":2">{{Cite web |title=Impulsive Maneuvers — Orbital Mechanics & Astrodynamics |url=https://orbital-mechanics.space/orbital-maneuvers/impulsive-maneuvers.html |access-date=2024-05-18 |website=orbital-mechanics.space}}</ref> However, when launching from a planet, tiny accelerations cannot overcome the planet's gravitational pull and so cannot be used.<ref>{{cite web|publisher=National Air and Space Museum| url=https://howthingsfly.si.edu/ask-an-explainer/how-much-force-rocket-launch | date=January 14, 2014 |access-date=September 7, 2024| title=Ask an Explainer: How much force is in a rocket launch?}}</ref>

Some designs however, operate [[#Without internal reaction mass|without internal reaction mass]] by taking advantage of magnetic fields or light pressure to change the spacecraft's momentum.

=== Efficiency ===
When discussing the efficiency of a propulsion system, designers often focus on the effective use of the reaction mass, which must be carried along with the rocket and is irretrievably consumed when used.<ref name=embry>{{cite web| publisher=Embry Riddle Aeronautical University|url=https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/rocket-engines/ | title=Introduction to Aerospace Flight Vehicles|date=January 2023 |access-date=September 7, 2024|quote=The shape and length of the combustion chamber and exit nozzle are essential design parameters for a rocket engine. The combustion chamber must be long enough for complete propellant combustion before the hot gases enter the nozzle, ensuring efficient combustion and maximizing thrust production. |last1=Leishman |first1=J. Gordon }}</ref> Spacecraft performance can be quantified in ''amount of change in momentum per unit of propellant consumed,'' also called [[specific impulse]]. This is a measure of the amount of [[Impulse (physics)|impulse]] that can be obtained from a fixed amount of reaction mass. The higher the specific impulse, the better the efficiency. [[Ion thruster|Ion propulsion engines]] have high specific impulse (~3000 s) and low thrust<ref>{{Cite web |title=Xenon Ion Propulsion System (XIPS) Thrusters |url=https://www2.l3t.com/edd/pdfs/datasheets/EP_Thrusters-XIPS_PPU%20Overview%20datasheet.pdf |archive-url=https://web.archive.org/web/20180417143504/http://www2.l3t.com/edd/pdfs/datasheets/EP_Thrusters-XIPS_PPU%20Overview%20datasheet.pdf |archive-date=17 April 2018 |access-date=16 March 2019 |website=L3 Technologies}}</ref> whereas chemical rockets like [[Monopropellant rocket|monopropellant]] or [[bipropellant]] rocket engines have a low specific impulse (~300 s) but high thrust.<ref>{{Cite web |title=Chemical Bipropellant thruster family |url=http://www.space-propulsion.com/brochures/bipropellant-thrusters/bipropellant-thrusters.pdf |access-date=16 March 2019 |website=Ariane Group}}</ref>

The impulse per unit weight-on-Earth (typically designated by <math>I_\text{sp}</math>) has units of seconds.<ref name=":2" /> Because the weight on Earth of the reaction mass is often unimportant when discussing vehicles in space, specific impulse can also be discussed in terms of impulse per unit mass, with the same units as velocity (e.g., meters per second).<ref>{{Cite web |title=Specific Impulse |url=https://www.grc.nasa.gov/www/k-12/airplane/specimp.html |access-date=May 18, 2024 |website=[[NASA]]}}</ref> This measure is equivalent to the [[effective exhaust velocity]] of the engine, and is typically designated <math>v_{e}</math>.<ref>{{Cite web |title=Chapter 3: Gravity & Mechanics – NASA Science |url=https://science.nasa.gov/learn/basics-of-space-flight/chapter3-2/ |access-date=2024-04-19 |website=science.nasa.gov |date=20 July 2023 |language=en-US}}</ref> Either the change in momentum per unit of propellant used by a spacecraft, or the velocity of the propellant exiting the spacecraft, can be used to measure its "specific impulse." The two values differ by a factor of the [[standard gravity|standard acceleration due to gravity, ''g''<sub>n</sub>]], 9.80665&nbsp;m/s² (<math>I_\text{sp} g_\mathrm{n} = v_{e}</math>).<ref name=":4">{{Cite web |title=III.4.2.1: Rockets and Launch Vehicles |url=https://www.faa.gov/sites/faa.gov/files/about/office_org/headquarters_offices/avs/III.4.2.1_Rockets_and_Launch_Vehicles.pdf |access-date=May 18, 2024 |website=www.faa.gov}}</ref>

In contrast to chemical rockets, electrodynamic rockets use
electric or magnetic fields to accelerate a charged propellant. The benefit of this method is that it can achieve exhaust velocities, and therefore <math>I_\text{sp}</math>, more than 10 times greater than those of a chemical engine, producing steady thrust with far less fuel. With a conventional chemical propulsion system, 2% of a rocket's total mass might make it to the destination, with the other 98% having been consumed as fuel. With an electric propulsion system, 70% of what's aboard in low Earth orbit can make it to a deep-space destination.<ref>{{Cite web|last=Boyle|first=Alan|date=2017-06-29|title=MSNW's plasma thruster just might fire up Congress at hearing on space propulsion|url=https://www.geekwire.com/2017/msnws-plasma-thruster-just-might-fire-congress-hearing-space-propulsion/|access-date=2021-08-15|website=GeekWire|language=en-US}}</ref>

However, there is a trade-off. Chemical rockets transform  propellants into most of the energy needed to propel them, but their electromagnetic equivalents must carry or produce the power required to create and accelerate propellants. Because there are currently practical limits on the amount of power available on a spacecraft, these engines are not suitable for launch vehicles or when a spacecraft needs a quick, large
impulse, such as when it brakes to enter a capture orbit. Even so, because
electrodynamic rockets offer very high <math>I_\text{sp}</math>, mission planners are
increasingly willing to sacrifice power and thrust (and the extra time it will
take to get a spacecraft where it needs to go) in order to save large amounts
of propellant mass.<ref name=" :4" />

==Operating domains==
Spacecraft operate in many areas of space. These include orbital maneuvering, interplanetary travel, and interstellar travel.

===Orbital===
{{Main|Orbital mechanics}}
Artificial satellites are first [[Rocket launch|launched]] into the desired altitude by conventional liquid/solid propelled rockets, after which the satellite may use onboard propulsion systems for orbital stationkeeping. Once in the desired orbit, they often need some form of [[Spacecraft attitude control|attitude control]] so that they are correctly pointed with respect to the [[Earth]], the [[Sun]], and possibly some [[astronomy|astronomical]] object of interest.<ref>{{cite news |author1=Hess, M. |author2=Martin, K. K. |author3=Rachul, L. J. | title=Thrusters Precisely Guide EO-1 Satellite in Space First | publisher=NASA | date=February 7, 2002 | url=http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm | access-date=2007-07-30 |archive-url = https://web.archive.org/web/20071206154134/http://www.gsfc.nasa.gov/news-release/releases/2002/02-020.htm <!-- Bot retrieved archive --> |archive-date = 2007-12-06}}</ref> They are also subject to [[Atmospheric drag|drag]] from the thin [[Earth's atmosphere|atmosphere]], so that to stay in orbit for a long period of time some form of propulsion is occasionally necessary to make small corrections ([[orbital stationkeeping|orbital station-keeping]]).<ref>{{cite web  |last=Phillips  |first=Tony  |date=May 30, 2000  |url=https://science.nasa.gov/headlines/y2000/ast30may_1m.htm  |archive-url=https://web.archive.org/web/20000619105529/https://science.nasa.gov/headlines/y2000/ast30may_1m.htm  |archive-date=June 19, 2000  |title=Solar S'Mores  |publisher=NASA  |access-date=2007-07-30 
}}</ref> Many satellites need to be moved from one orbit to another from time to time, and this also requires propulsion.<ref>{{cite web | last=Olsen | first=Carrie | date=September 21, 1995 | url=http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html | title=Hohmann Transfer & Plane Changes | publisher=NASA | access-date=2007-07-30 |archive-url = https://web.archive.org/web/20070715042552/http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/hohmann.html <!-- Bot retrieved archive --> |archive-date = 2007-07-15}}</ref> A satellite's useful life is usually over once it has exhausted its ability to adjust its orbit.<ref>{{Cite web |title=Satellite communication – Orbit, Signals, Relay {{!}} Britannica |url=https://www.britannica.com/technology/satellite-communication/How-satellites-work |access-date=2024-04-20 |website=www.britannica.com |language=en}}</ref>

===Interplanetary===
{{Main|Interplanetary spaceflight}}
For [[interplanetary travel]], a spacecraft can use its engines to leave Earth's orbit. It is not explicitly necessary as the initial boost given by the rocket, gravity slingshot, monopropellant/bipropellent attitude control propulsion system are enough for the exploration of the solar system (see [[New Horizons]]). Once it has done so, it must make its way to its destination. Current interplanetary spacecraft do this with a series of short-term trajectory adjustments.<ref>{{cite web|author=Staff |date=April 24, 2007 |url=http://mars.jpl.nasa.gov/odyssey/mission/cruise.html |title=Interplanetary Cruise |publisher=NASA |work=2001 Mars Odyssey |access-date=2007-07-30 |archive-url=https://web.archive.org/web/20070802071234/http://mars.jpl.nasa.gov/odyssey/mission/cruise.html |archive-date=August 2, 2007 }}</ref> In between these adjustments, the spacecraft typically moves along its trajectory without accelerating. The most fuel-efficient means to move from one circular orbit to another is with a [[Hohmann transfer orbit]]: the spacecraft begins in a roughly circular orbit around the Sun. A short period of [[thrust]] in the direction of motion accelerates or decelerates the spacecraft into an elliptical orbit around the Sun which is tangential to its previous orbit and also to the orbit of its destination. The spacecraft falls freely along this elliptical orbit until it reaches its destination, where another short period of thrust accelerates or decelerates it to match the orbit of its destination.<ref>{{cite news | first=Dave | last=Doody | title=Chapter 4. Interplanetary Trajectories | work=Basics of Space Flight | publisher=NASA JPL | date=February 7, 2002 | url=http://www2.jpl.nasa.gov/basics/bsf4-1.html | access-date=2007-07-30 | archive-url=https://web.archive.org/web/20070717143018/http://www2.jpl.nasa.gov/basics/bsf4-1.html | archive-date=July 17, 2007 }}</ref> Special methods such as [[aerobraking]] or aerocapture are sometimes used for this final orbital adjustment.<ref>{{cite conference |last=Hoffman |first=S. |date=August 20–22, 1984 |title=A comparison of aerobraking and aerocapture vehicles for interplanetary missions |url=http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 |conference= |location=Seattle, Washington |publisher=American Institute of Aeronautics and Astronautics |pages= |archive-url=https://web.archive.org/web/20070927230504/http://www.aiaa.org/content.cfm?pageid=406&gTable=mtgpaper&gID=44030 |archive-date=September 27, 2007 |access-date=2007-07-31 |book-title=AIAA and AAS, Astrodynamics Conference}}</ref>

[[File:Ssunsail.jpg|thumb|right|Artist's concept of a solar sail]]
Some spacecraft propulsion methods such as [[solar sail]]s provide very low but inexhaustible thrust;<ref>{{cite web|author=Anonymous |year=2007 |url=http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html |title=Basic Facts on Cosmos 1 and Solar Sailing |publisher=The Planetary Society |access-date=2007-07-26 |archive-url=https://web.archive.org/web/20070703052531/http://www.planetary.org/programs/projects/innovative_technologies/solar_sailing/facts.html |archive-date=July 3, 2007 }}</ref> an interplanetary vehicle using one of these methods would follow a rather different trajectory, either constantly thrusting against its direction of motion in order to decrease its distance from the Sun, or constantly thrusting along its direction of motion to increase its distance from the Sun.{{citation needed|date = July 2023}} The concept has been successfully tested by the Japanese [[IKAROS]] solar sail spacecraft.<ref name="d545">{{cite web | last=Malik | first=Tariq | title=Japanese solar sail successfully rides sunlight | website=NBC News | date=2010-07-13 | url=https://www.nbcnews.com/id/wbna38222268 | access-date=2024-09-27}}</ref>

===Interstellar===
{{Main|Interstellar travel}}
Because interstellar distances are great, a tremendous velocity is needed to get a spacecraft to its destination in a reasonable amount of time. Acquiring such a velocity on launch and getting rid of it on arrival remains a formidable challenge for spacecraft designers.<ref>{{cite web | last=Rahls | first=Chuck | date=December 7, 2005 | url=http://www.physorg.com/news8817.html | title=Interstellar Spaceflight: Is It Possible? | publisher=Physorg.com | access-date=2007-07-31 }}</ref> No spacecraft capable of short duration (compared to human lifetime) [[interstellar travel]] has yet been built, but many hypothetical designs have been discussed. 

==Propulsion technology==
Spacecraft propulsion technology can be of several types, such as chemical, electric or nuclear. They are distinguished based on the physics of the propulsion system and how thrust is generated. Other experimental and more theoretical types are also included, depending on their technical maturity. Additionally, there may be credible meritorious in-space propulsion concepts not foreseen or reviewed at the time of publication, and which may be shown to be beneficial to future mission applications.{{Sfn|Meyer|2012|p=10}}

Almost all types are [[reaction engine]]s, which produce [[thrust]] by expelling [[reaction mass]], in accordance with [[Newton's third law of motion]].<ref>{{Cite web |title=AMT Handbook |url=https://www.faa.gov/sites/faa.gov/files/03_amtp_ch1.pdf |access-date=April 20, 2024 |website=www.faa.gov}}</ref><ref>{{Cite web |title=Rocket Principles |url=https://www.grc.nasa.gov/www/k-12/rocket/TRCRocket/rocket_principles.html |access-date=April 20, 2024 |website=[[NASA]]}}</ref><ref><!--THIS IS UNNECESSARY, IN THIS DITOR'S VIEW:-->This law of motion is most commonly paraphrased as: "For every action force there is an equal, but opposite, reaction force."{{citation needed|date = July 2023}}</ref> Examples include [[jet engine]]s, [[rocket engine]]s, [[pump-jet]], and more uncommon variations such as [[Hall-effect thruster|Hall–effect thrusters]], [[ion drive]]s, [[mass drivers]], and [[nuclear pulse propulsion]].<ref>{{Cite web |title=Chapter 11: Onboard Systems – NASA Science |url=https://science.nasa.gov/learn/basics-of-space-flight/chapter11-4/ |access-date=2024-04-19 |website=science.nasa.gov |date=20 July 2023 |language=en-US}}</ref>

===Chemical propulsion===
{{Main|Rocket engine}}

[[File:SpaceX engine test fire.jpg|thumb|right|[[SpaceX]]'s [[Kestrel (rocket engine)|Kestrel engine]] is tested.]]

A large fraction of [[rocket engine]]s in use today are [[Rocket engine#Chemically powered|chemical rockets]]; that is, they obtain the energy needed to generate thrust by [[chemical reactions]] to create a hot gas that is expanded to produce [[thrust]].<ref>{{Cite web |title=Chapter 14: Launch – NASA Science |url=https://science.nasa.gov/learn/basics-of-space-flight/chapter14-1/ |access-date=2024-04-19 |website=science.nasa.gov |date=20 July 2023 |language=en-US}}</ref> Many different propellant combinations are used to obtain these chemical reactions, including, for example, [[hydrazine]], [[liquid oxygen]], [[liquid hydrogen]], [[nitrous oxide]], and [[hydrogen peroxide]].<ref>{{Cite web |title=4.0 In-Space Propulsion – NASA |url=https://www.nasa.gov/smallsat-institute/sst-soa/in-space_propulsion/ |access-date=2024-04-25 |language=en-US}}</ref> They can be used as a [[Monopropellant rocket|monopropellant]] or in [[Bipropellant rocket|bi-propellant]] configurations.<ref>{{Cite web |title=4.0 In-Space Propulsion – NASA |url=https://www.nasa.gov/smallsat-institute/sst-soa/in-space_propulsion/ |access-date=2024-04-20 |language=en-US}}</ref>

Rocket engines provide essentially the highest specific powers and high specific thrusts of any engine used for spacecraft propulsion.<ref name=":4" /> Most rocket engines are [[internal combustion engine|internal combustion]] [[heat engines]] (although non-combusting forms exist).<ref name="Leishman">{{Cite book |last=Leishman |first=J. Gordon |date=2023-01-01 |title=Introduction to Aerospace Flight Vehicles - Rocket Engines |url=https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/rocket-engines/ |language=en|publisher=Embry Riddle Aeronautical University}}</ref> Rocket engines generally produce a high-temperature reaction mass, as a hot gas, which is achieved by combusting a solid, liquid or gaseous fuel with an oxidiser within a combustion chamber.<ref>{{Cite web |title=Rocket Propulsion |url=https://www.grc.nasa.gov/www/k-12/airplane/rocket.html |access-date=April 21, 2024 |website=[[NASA]]}}</ref> The extremely hot gas is then allowed to escape through a high-expansion ratio bell-shaped [[de Laval nozzle|nozzle]], a feature that gives a rocket engine its characteristic shape.<ref name="Leishman"/> The effect of the nozzle is to accelerate the mass, converting most of the thermal energy into kinetic energy,<ref name="Leishman"/> where exhaust speeds reaching as high as 10 times the speed of sound at sea level are common.{{citation needed|date = July 2023}}

====Green chemical propulsion====
The dominant form of chemical propulsion for [[satellite]]s has historically been [[hydrazine]], however, this fuel is highly toxic and at risk of being banned across Europe.<ref>{{Cite web |date=2017-10-25 |title=Hydrazine ban could cost Europe's space industry billions |url=https://spacenews.com/hydrazine-ban-could-cost-europes-space-industry-billions/ |access-date=2022-08-19 |website=SpaceNews |language=en-US}}</ref> Non-toxic 'green' alternatives are now being developed to replace hydrazine. [[Nitrous oxide]]-based alternatives are garnering traction and government support,<ref>{{Cite web |last=Urban |first=Viktoria |date=2022-07-15 |title=Dawn Aerospace granted €1.4 million by EU for green propulsion technology |url=https://spacewatch.global/2022/07/dawn-aerospace-granted-e1-4-million-by-eu-for-green-propulsion-technology/ |access-date=2022-08-19 |website=SpaceWatch.Global |language=en-US}}</ref><ref>{{Cite web |title=International research projects {{!}} Ministry of Business, Innovation & Employment |url=https://www.mbie.govt.nz/science-and-technology/space/nzspacetalk/international-research-projects/ |access-date=2022-08-19 |website=www.mbie.govt.nz}}</ref> with development being led by commercial companies Dawn Aerospace, Impulse Space,<ref>{{Cite web |last=Berger |first=Eric |date=2022-07-19 |title=Two companies join SpaceX in the race to Mars, with a launch possible in 2024 |url=https://arstechnica.com/science/2022/07/relativity-and-impulse-space-say-theyre-flying-to-mars-in-late-2024/ |access-date=2022-08-19 |website=Ars Technica |language=en-us}}</ref> and Launcher.<ref>{{Cite web |date=2021-06-15 |title=Launcher to develop orbital transfer vehicle |url=https://spacenews.com/launcher-to-develop-orbital-transfer-vehicle/ |access-date=2022-08-19 |website=SpaceNews |language=en-US}}</ref> The first nitrous oxide-based system flown in space was by D-Orbit onboard their ION Satellite Carrier ([[space tug]]) in 2021, using six [[Dawn Aerospace]] B20 thrusters, launched upon a [[SpaceX]] [[Falcon 9]] rocket.<ref>{{Cite web |title=Dawn Aerospace validates B20 Thrusters in space – Bits&Chips |date=6 May 2021 |url=https://bits-chips.nl/artikel/dawn-aerospace-validates-b20-thrusters-in-space/ |access-date=2022-08-19 |language=en-US}}</ref><ref>{{Cite web |title=Dawn B20 Thrusters Proven In Space |url=https://www.dawnaerospace.com/latest-news/b20-thrusters-proven-in-space |access-date=2022-08-19 |website=Dawn Aerospace |language=en-US}}</ref>

===Electric propulsion===
[[File:Ion Engine Test Firing - GPN-2000-000482.jpg|thumb|right|upright=1.2|NASA's 2.3 kW NSTAR [[ion thruster]] for the [[Deep Space&nbsp;1]] spacecraft during a hot fire test at the Jet Propulsion Laboratory]]{{Main|Spacecraft electric propulsion}}

[[File:Xenon hall thruster.jpg|thumb|6 kW Hall thruster in operation at the [[NASA]] [[Jet Propulsion Laboratory]] ]]

Rather than relying on high temperature and [[fluid dynamics]] to accelerate the reaction mass to high speeds, there are a variety of methods that use electrostatic or [[electromagnetism|electromagnetic]] forces to accelerate the reaction mass directly, where the reaction mass is usually a stream of [[ion]]s.{{citation needed|date = July 2023}}

Ion propulsion rockets typically heat a plasma or charged gas inside a [[magnetic bottle]] and release it via a [[magnetic nozzle]] so that no solid matter needs to come in contact with the plasma.<ref>{{Cite web |title=NASA Facts - Ion Propulsion |url=https://www.nasa.gov/wp-content/uploads/2015/08/ionpropfact_sheet_ps-01628.pdf |access-date=May 18, 2024 |website=[[NASA]]}}</ref> Such an engine uses electric power, first to ionize atoms, and then to create a voltage gradient to accelerate the ions to high exhaust velocities.<ref>{{Cite web |title=Ion Propulsion – NASA Science |url=https://science.nasa.gov/mission/dawn/technology/ion-propulsion/ |access-date=2024-04-25 |website=science.nasa.gov |date=23 October 2018 |language=en-US}}</ref> For these drives, at the highest exhaust speeds, energetic efficiency and thrust are all inversely proportional to exhaust velocity.{{citation needed|date = July 2023}} Their very high exhaust velocity means they require huge amounts of energy and thus with practical power sources provide low thrust, but use hardly any fuel.{{citation needed|date = July 2023}}

[[electrically powered spacecraft propulsion|Electric propulsion]] is commonly used for station keeping on commercial [[communications satellites]] and for prime propulsion on some [[space exploration|scientific space missions]] because of their high specific impulse.<ref>{{Cite web |title=Space Power Chapter 7: Electric Rockets – Opening the Solar System – NSS |date=3 August 2017 |url=https://nss.org/space-power-chapter-7-electric-rockets-opening-the-solar-system/ |access-date=2024-04-28 |language=en-US}}</ref> However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.{{Sfn|Meyer|2012|p=5}}<ref name="tomsik">Tomsik, Thomas M. "[http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%20Propulsion/NASA-TM-2000-209941,%20Advances%20in%20Cryo%20Propellant%20Densification%20Technology.pdf Recent advances and applications in cryogenic propellant densification technology] {{Webarchive|url=https://web.archive.org/web/20141129035753/http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%20Propulsion/NASA-TM-2000-209941,%20Advances%20in%20Cryo%20Propellant%20Densification%20Technology.pdf|date=2014-11-29}}." NASA TM 209941 (2000).</ref><ref name="oleson">Oleson, S., and Sankovic, J. "[http://adsabs.harvard.edu/full/2000ESASP.465..717O Advanced Hall electric propulsion for future in-space transportation]." Spacecraft Propulsion. Vol. 465. 2000.</ref><ref>Dunning, John W., Scott Benson, and Steven Oleson. "NASA's electric propulsion program." 27th International Electric Propulsion Conference, Pasadena, California, IEPC-01-002. 2001.</ref>

The idea of electric propulsion dates to 1906, when [[Robert Goddard (scientist)|Robert Goddard]] considered the possibility in his personal notebook.<ref name="choueiri">{{cite journal | last = Choueiri | first = Edgar Y. | year = 2004 | title = A Critical History of Electric Propulsion: The First 50 Years (1906–1956) | journal = Journal of Propulsion and Power | volume = 20 | issue = 2 | pages = 193–203 | url = http://alfven.princeton.edu/publications/choueiri-jpp-2004 | doi = 10.2514/1.9245 | citeseerx = 10.1.1.573.8519 | access-date = 2016-10-18 | archive-date = 2019-04-28 | archive-url = https://web.archive.org/web/20190428155604/https://alfven.princeton.edu/publications/choueiri-jpp-2004 }}</ref> [[Konstantin Tsiolkovsky]] published the idea in 1911.<ref>{{Cite journal |last=Choueiri |first=Edgar |date=2004-06-26 |title=A Critical History of Electric Propulsion: The First Fifty Years (1906-1956) |url=http://dx.doi.org/10.2514/6.2004-3334 |journal=40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit |location=Reston, Virginia |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2004-3334|isbn=978-1-62410-037-6 }}</ref>

Electric propulsion methods include:<ref>{{Cite web |title=4.0 In-Space Propulsion – NASA |url=https://www.nasa.gov/smallsat-institute/sst-soa/in-space_propulsion/ |access-date=2024-04-28 |language=en-US}}</ref>
* [[Ion thrusters]], which accelerate ions first and later neutralize the ion beam with an electron stream emitted from a cathode called a neutralizer;<ref>{{Cite web |title=Deep Space 1: Advanced Technologies: Solar Electric Propulsion FAQ |url=https://www.jpl.nasa.gov/nmp/ds1/tech/ionpropfaq.html |access-date=2024-04-28 |website=www.jpl.nasa.gov}}</ref>
**[[Electrostatic ion thruster]]s
**[[Field-emission electric propulsion]]
**[[MagBeam]] thrusters
**[[Hall-effect thruster]]s
**[[Colloid thruster]]s
* Electrothermal thrusters, wherein electromagnetic fields are used to generate a plasma to increase the [[heat]] of the bulk propellant, the thermal energy imparted to the propellant gas is then converted into kinetic energy by a [[nozzle]] of either physical material construction or by magnetic means;{{citation needed|date = July 2023}}
**[[Arcjet]]s using DC current or microwaves
**[[Helicon double-layer thruster]]s
**[[Resistojet]]s
* Electromagnetic thrusters, wherein ions are accelerated either by the [[Lorentz Force]] or by the effect of electromagnetic fields where the electric field is not in the direction of the acceleration;{{citation needed|date = July 2023}}
**[[Plasma propulsion engine]]s
**[[Magnetoplasmadynamic thruster]]s
**[[Electrodeless plasma thruster]]s
**[[Pulsed inductive thruster]]s
**[[Pulsed plasma thruster]]s
**[[Variable specific impulse magnetoplasma rocket]]s (VASIMR)
**[[Vacuum arc thruster]]s
*[[Mass driver]]s designed for propulsion.{{citation needed|date = July 2023}}

====Power sources====
For some missions, particularly reasonably close to the Sun, [[solar energy]] may be sufficient, and has often been used, but for others further out or at higher power, nuclear energy is necessary; engines drawing their power from a nuclear source are called [[nuclear electric rocket]]s.<ref>{{Cite web |title=Space Nuclear Propulsion – NASA |url=https://www.nasa.gov/tdm/space-nuclear-propulsion/ |access-date=2024-04-28 |language=en-US}}</ref>

Current nuclear power generators are approximately half the weight of solar panels per watt of energy supplied, at terrestrial distances from the Sun.{{citation needed|date = July 2023}} Chemical power generators are not used due to the far lower total available energy.<ref>{{Cite web |last=Luckenbaugh |first=Josh |date=July 31, 2023 |title=Government, Industry Explore Nuclear, Solar Space Engines |url=https://www.nationaldefensemagazine.org/articles/2023/7/31/government-industry-explore-nuclear-solar-space-engines |access-date=2024-04-28 |website=www.nationaldefensemagazine.org}}</ref> Beamed power to the spacecraft is considered to have potential, according to NASA and the [[University of Colorado Boulder]].<ref>{{Cite web |date=2021 |title=Beamed Laser Power for UAVs |url=https://www.nasa.gov/wp-content/uploads/2021/09/120329main_fs-087-dfrc.pdf |access-date=April 24, 2024 |website=[[NASA]]}}</ref><ref>{{Cite web |last=Beam Propulsion |first=Chuck |date=November 28, 2007 |title=Beam Propulsion |url=https://www.colorado.edu/faculty/kantha/sites/default/files/attached-files/final_vaughan.pdf#:~:text=If%20efficient%20magnetic%20nozzles%20can,as%20well%20as%20interplanetary%20missions. |access-date=April 24, 2024 |website=[[University of Colorado Boulder]]}}</ref>

With any current source of electrical power, chemical, nuclear or solar, the maximum amount of power that can be generated limits the amount of thrust that can be produced to a small value.{{citation needed|date = July 2023}} Power generation adds significant mass to the spacecraft, and ultimately the weight of the power source limits the performance of the vehicle.<ref>{{Cite web |title=3.0 Power – NASA |url=https://www.nasa.gov/smallsat-institute/sst-soa/power-subsystems/ |access-date=2024-04-28 |language=en-US}}</ref>

===Nuclear propulsion===
{{Main|Nuclear propulsion#Spacecraft}}
[[File:Plasma propulsion engine.webp|thumb|3D sketch of an [[electromagnetic propulsion]] [[Plasma propulsion engine|fusion plasma]] thruster]]
[[Nuclear fuels]] typically have very high [[specific energy]], much higher than chemical fuels, which means that they can generate large amounts of energy per unit mass. This makes them valuable in spaceflight, as it can enable high [[specific impulse]]s, sometimes even at high thrusts. The machinery to do this is complex, but research has developed methods for their use in propulsion systems, and some have been tested in a laboratory.<ref>{{Cite web |date=2018-05-25 |title=Nuclear Thermal Propulsion: Game Changing Technology for Deep Space Exploration – NASA |url=https://www.nasa.gov/directorates/stmd/tech-demo-missions-program/nuclear-thermal-propulsion-game-changing-technology-for-deep-space-exploration/ |access-date=2024-04-25 |language=en-US}}</ref>

Here, nuclear propulsion moreso refers to the source of propulsion being nuclear, instead of a [[nuclear electric rocket]] where a [[nuclear reactor]] would provide power (instead of solar panels) for other types of electrical propulsion.

Nuclear propulsion methods include:
*[[Fission-fragment rocket]]s
*[[Fission sail]]s
*[[Fusion rocket]]s
*[[Nuclear thermal rocket]]s (NTR)
*[[Nuclear pulse propulsion]]
*[[Nuclear salt-water rocket]]s
*[[Radioisotope rocket]]s

=== Without internal reaction mass ===
There are several different space drives that need little or no reaction mass to function.

==== Reaction wheels ====
Many spacecraft use [[reaction wheels]] or [[Control Moment Gyroscope|control moment gyroscopes]] to control orientation in space.<ref>{{cite journal |last=Tsiotras |first=P. |author2=Shen, H. |author3=Hall, C. D. |year=2001 |title=Satellite attitude control and power tracking with energy/momentum wheels |url=http://www.ae.gatech.edu/people/tsiotras/Papers/jgcd99.pdf |journal=Journal of Guidance, Control, and Dynamics |volume=43 |issue=1 |pages=23–34 |bibcode=2001JGCD...24...23T |citeseerx=10.1.1.486.3386 |doi=10.2514/2.4705 |issn=0731-5090}}
</ref> A satellite or other space vehicle is subject to the [[law of conservation of angular momentum]], which constrains a body from a [[net force|net change]] in [[angular velocity]]. Thus, for a vehicle to change its [[relative orientation]] without expending reaction mass, another part of the vehicle may rotate in the opposite direction. Non-conservative external forces, primarily gravitational and atmospheric, can contribute up to several degrees per day to angular momentum,<ref>{{cite book |last=King-Hele |first=Desmond |url=https://books.google.com/books?id=HSffDG356TkC&pg=PA6 |title=Satellite orbits in an atmosphere: Theory and application |publisher=Springer |year=1987 |isbn=978-0-216-92252-5 |page=6}}</ref> so such systems are designed to "bleed off" undesired rotational energies built up over time. 

====EM wave-based propulsion====
{{original research | section | date=January 2017}}
The [[Conservation law (physics)|law of conservation]] of [[momentum]] is usually taken to imply that any engine which uses no reaction mass cannot accelerate the center of mass of a spaceship (changing orientation, on the other hand, is possible).{{citation needed|date = July 2023}} But space is not empty, especially space inside the Solar System; there are gravitation fields, [[magnetic field]]s, [[electromagnetic waves]], [[solar wind]] and solar radiation.<ref>{{Cite web |title=What keeps space empty? |url=https://wtamu.edu/~cbaird/sq/2012/12/20/what-keeps-space-empty/ |access-date=2024-04-28 |website=Science Questions with Surprising Answers |language=en-US}}</ref> Electromagnetic waves in particular are known to contain momentum, despite being massless; specifically the momentum flux density '''P''' of an EM wave is quantitatively 1/c<sup>2</sup> times the [[Poynting vector]] '''S''', i.e. '''P''' = '''S'''/c<sup>2</sup>, where c is the velocity of light.{{citation needed|date = July 2023}} [[Field propulsion]] methods which do not rely on reaction mass thus must try to take advantage of this fact by coupling to a momentum-bearing field such as an EM wave that exists in the vicinity of the craft; however, because many of these phenomena are diffuse in nature, corresponding propulsion structures must be proportionately large.{{citation needed|date = July 2023}}

====Solar and magnetic sails====
[[File:Solarsail msfc.jpg|right|thumb|NASA study of a solar sail. The sail would be half a kilometer wide.]]
The concept of [[solar sail]]s rely on [[radiation pressure]] from electromagnetic energy, but they require a large collection surface to function effectively.<ref>{{Cite web |title=NASA-Supported Solar Sail Could Take Science to New Heights – NASA |url=https://www.nasa.gov/news-release/nasa-supported-solar-sail-could-take-science-to-new-heights/ |access-date=2024-04-28 |language=en-US}}</ref> [[E-sail]]s propose to use very thin and lightweight wires holding an electric charge to deflect particles, which may have more controllable directionality.{{citation needed|date = July 2023}}

[[Magnetic sail]]s deflect charged particles from the [[solar wind]] with a magnetic field, thereby imparting momentum to the spacecraft.<ref>{{Cite journal |last=Djojodihardjo |first=Harijono |date=November 2018 |title=Review of Solar Magnetic Sailing Configurations for Space Travel |url=http://link.springer.com/10.1007/s42423-018-0022-4 |journal=Advances in Astronautics Science and Technology |language=en |volume=1 |issue=2 |pages=207–219 |doi=10.1007/s42423-018-0022-4 |bibcode=2018AAnST...1..207D |issn=2524-5252}}</ref> For instance, the so-called [[Magnetic sail#Magsail (MS)|Magsail]] is a large superconducting loop proposed for acceleration/deceleration in the [[solar wind]] and deceleration in the [[Interstellar medium]].<ref>{{Cite journal |last1=Zubrin |first1=Robert M. |last2=Andrews |first2=Dana G. |date=March 1991 |title=Magnetic sails and interplanetary travel |url=https://arc.aiaa.org/doi/10.2514/3.26230 |journal=Journal of Spacecraft and Rockets |language=en |volume=28 |issue=2 |pages=197–203 |doi=10.2514/3.26230 |bibcode=1991JSpRo..28..197Z |issn=0022-4650}}</ref> A variant is the [[mini-magnetospheric plasma propulsion]] system<ref>{{Cite journal |last1=Winglee |first1=R. M. |last2=Slough |first2=J. |last3=Ziemba |first3=T. |last4=Goodson |first4=A. |date=September 2000 |title=Mini-Magnetospheric Plasma Propulsion: Tapping the energy of the solar wind for spacecraft propulsion |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/1999JA000334 |journal=Journal of Geophysical Research: Space Physics |language=en |volume=105 |issue=A9 |pages=21067–21077 |doi=10.1029/1999JA000334 |bibcode=2000JGR...10521067W |issn=0148-0227}}</ref> and its successor, the [[Magnetic sail#Magnetoplasma sail (MPS)|magnetoplasma sail]],<ref>{{Cite journal |last1=Funaki |first1=Ikkoh |last2=Asahi |first2=Ryusuke |last3=Fujita |first3=Kazuhisa |last4=Yamakawa |first4=Hiroshi |last5=Ogawa |first5=Hiroyuki |last6=Otsu |first6=Hirotaka |last7=Nonaka |first7=Satoshi |last8=Sawai |first8=Shujiro |last9=Kuninaka |first9=Hitoshi |date=2003-06-23 |title=Thrust Production Mechanism of a Magnetoplasma Sail |url=https://arc.aiaa.org/doi/10.2514/6.2003-4292 |journal=34th AIAA Plasmadynamics and Laser Conference |language=en |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2003-4292 |isbn=978-1-62410-096-3}}</ref> which inject plasma at a low rate to enhance the magnetic field to more effectively deflect charged particles in a plasma wind.

Japan launched a solar sail-powered spacecraft, [[IKAROS]] in May 2010, which successfully demonstrated propulsion and guidance (and is still active as of this date).{{when|date = July 2023}}{{citation needed|date = July 2023}} As further proof of the [[solar sail]] concept, [[NanoSail-D]] became the first such powered satellite to orbit [[Earth]].<ref>{{Cite web |url= http://www.nasa.gov/mission_pages/tdm/solarsail|title=Solar Sail Demonstrator|work=NASA |date=19 September 2016 |last1=Harbaugh |first1=Jennifer }}</ref> As of August 2017, NASA confirmed the Sunjammer solar sail project was concluded in 2014 with lessons learned for future space sail projects.<ref>{{Cite web|url=https://www.nasa.gov/mission_pages/tdm/solarsail/index.html|title=Solar Sail Demonstrator|date=19 September 2016}}</ref> The U.K. [[CubeSail|Cubesail]] programme will be the first mission to demonstrate solar sailing in low Earth orbit, and the first mission to demonstrate full three-axis attitude control of a solar sail.<ref>{{cite web|title=Space Vehicle Control|url=http://www.surrey.ac.uk/ssc/research/space_vehicle_control/cubesail/news/index.htm|website=University of Surrey|access-date=8 August 2015|archive-date=7 May 2016|archive-url=https://web.archive.org/web/20160507025010/http://www.surrey.ac.uk/ssc/research/space_vehicle_control/cubesail/news/index.htm}}</ref>

====Other propulsion types====

The concept of a [[gravitational slingshot]] is a form of propulsion to carry a [[space probe]] onward to other destinations without the expense of reaction mass; harnessing the gravitational energy of other celestial objects allows the spacecraft to gain kinetic energy.<ref>{{Cite journal | year = 2004 | pages = 619–000 | doi = 10.1119/1.1621032| last2 =   Cacioppo| last3 =   Gangopadhyaya| first1 = J. J. | volume = 72| last1 = Dykla | first2 = R.| first3 = A. | journal = American Journal of Physics | title = Gravitational slingshot| issue = 5|bibcode = 2004AmJPh..72..619D | url = http://ecommons.luc.edu/cgi/viewcontent.cgi?article=1007&context=physics_facpubs}}</ref> However, more energy can be obtained from the gravity assist if rockets are used via the [[Oberth effect]].

A [[tether propulsion]] system employs a long cable with a high tensile strength to change a spacecraft's orbit, such as by interaction with a planet's magnetic field or through momentum exchange with another object.<ref>{{cite news|first=Dave |last=Drachlis |title=NASA calls on industry, academia for in-space propulsion innovations |publisher=NASA |date=October 24, 2002 |url=http://www.msfc.nasa.gov/news/news/releases/2002/02-269.html |access-date=2007-07-26 |archive-url=https://web.archive.org/web/20071206095134/http://www.msfc.nasa.gov/news/news/releases/2002/02-269.html |archive-date=December 6, 2007 }}</ref>

[[Beam-powered propulsion]] is another method of propulsion without reaction mass, and includes sails pushed by [[Laser propulsion|laser]], microwave, or particle beams.<ref>{{Cite web |date=2023-01-09 |title=Pellet-Beam Propulsion for Breakthrough Space Exploration – NASA |url=https://www.nasa.gov/general/pellet-beam-propulsion-for-breakthrough-space-exploration/ |access-date=2024-04-24 |language=en-US}}</ref>

===Advanced propulsion technology===
Advanced, and in some cases theoretical, propulsion technologies may use chemical or nonchemical physics to produce thrust but are generally considered to be of lower technical maturity with challenges that have not been overcome.{{Sfn|Meyer|2012|p=20}} For both human and robotic exploration, traversing the solar system is a struggle against time and distance. The most distant planets are 4.5–6 billion kilometers from the Sun and to reach them in any reasonable time requires much more capable propulsion systems than conventional chemical rockets. Rapid inner solar system missions with flexible launch dates are difficult, requiring propulsion systems that are beyond today's current state of the art. The logistics, and therefore the total system mass required to support sustained human exploration beyond Earth to destinations such as the Moon, Mars, or [[near-Earth object]]s, are daunting unless more efficient in-space propulsion technologies are developed and fielded.{{Sfn|Meyer|2012|p=6}}<ref name=huntsb>
{{cite journal
|doi=10.1061/40476(299)45
|citeseerx = 10.1.1.83.3242
|title=Robotics Challenges for Robotic and Human Mars Exploration
|journal=Robotics 2000
|year=2000
|last1=Huntsberger
|first1=Terry
|last2=Rodriguez
|first2=Guillermo
|last3=Schenker
|first3=Paul S.
|isbn=978-0-7844-0476-8
|pages=340–346}}</ref>

A variety of hypothetical propulsion techniques have been considered that require a deeper understanding of the properties of space, particularly [[Inertial frame of reference|inertial frames]] and the [[vacuum state]]. Such methods are highly speculative and include:{{citation needed|date = July 2023}}{{colbegin}}
*[[Black hole starship]]
*[[Breakthrough Propulsion Physics Program#Quantum vacuum energy experiments|Differential sail]]
*[[Gravitational shielding]]
*[[Field propulsion]]
**Diametric drive
**Disjunction drive
**Pitch drive
**Bias drive
*[[Photon rocket]]
*[[Quantum vacuum thruster]]
*[[Nano electrokinetic thruster]]
*[[Reactionless drive]]
**[[Abraham–Minkowski controversy|Abraham—Minkowski drive]]
**[[Alcubierre drive]]
**[[Dean drive]]
**[[EmDrive]]
**[[Heim theory]]
**[[Woodward effect]]

{{colend}}

A NASA assessment of its [[Breakthrough Propulsion Physics Program]] divides such proposals into those that are non-viable for propulsion purposes, those that are of uncertain potential, and those that are not impossible according to current theories.<ref>{{cite conference
 | first =Marc
 | last =Millis
 | title =Assessing Potential Propulsion Breakthroughs
 | book-title =New Trends in Astrodynamics and Applications II
 | date =June 3–5, 2005
 | location =Princeton, NJ
 | url =https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060000022.pdf
 }}</ref>

===Table of methods===
{{more citations needed section|date = July 2023}}
Below is a summary of some of the more popular, proven technologies, followed by increasingly speculative methods. Four numbers are shown. The first is the [[specific impulse|effective exhaust velocity]]: the equivalent speed which the propellant leaves the vehicle. This is not necessarily the most important characteristic of the propulsion method; thrust and power consumption and other factors can be. However,
* if the delta-v is much more than the exhaust velocity, then exorbitant amounts of fuel are necessary (see the section on calculations, above),{{according to whom|date = July 2023}} and
* if it is much more than the delta-v, then, proportionally more energy is needed; if the power is limited, as with solar energy, this means that the journey takes a proportionally longer time.{{according to whom|date = July 2023}}
The second and third are the typical amounts of thrust and the typical burn times of the method; outside a gravitational potential, small amounts of thrust applied over a long period will give the same effect as large amounts of thrust over a short period, if the object is not significantly influenced by gravity.{{citation needed|date = July 2023}} The fourth is the maximum delta-v the technique can give without staging. For rocket-like propulsion systems, this is a function of mass fraction and exhaust velocity; mass fraction for rocket-like systems is usually limited by propulsion system weight and tankage weight.{{citation needed|date = July 2023}} For a system to achieve this limit, the payload may need to be a negligible percentage of the vehicle, and so the practical limit on some systems can be much lower.{{citation needed|date = July 2023}}

{| class="wikitable sortable" style="text-align:center;"
|+ Propulsion methods
! style="text-align:left;" | Method
! [[Specific impulse|Effective exhaust <br />velocity]] (km/s)
! [[Thrust]] (N)
! Firing <br />duration
! Maximum <br />[[delta-v]] (km/s)
! style="text-align:left;" | [[Technology readiness level#NASA definitions|Technology <br />readiness level]]
|-
| style="text-align:left;" | [[Solid-fuel rocket]] || {{ntsh|2.5}}<2.5 || <10<sup>7</sup> || {{Ntsh|60}}Minutes || {{Nts|7}}
| style="text-align:left;" | {{Nts|9}}: Flight proven
|-
| style="text-align:left;" | [[Hybrid rocket]] || <4 || || {{Ntsh|60}}Minutes || {{Ntsh|3}}>3
| style="text-align:left;" | {{Nts|9}}: Flight proven
|-
| style="text-align:left;" | [[Monopropellant rocket]] || {{Ntsh|2}}1–3<ref name=":0">{{Cite web|url=http://www.space-propulsion.com/brochures/hydrazine-thrusters/hydrazine-thrusters.pdf|title=Chemical monopropellant thruster family|website=Ariane Group|access-date=16 March 2019}}</ref>||{{Ntsh|{{#expr:10^.5}}}}0.1–400<ref name=":0" />||{{Ntsh|1}}Milliseconds–minutes || {{Nts|3}}
| style="text-align:left;" | {{Nts|9}}: Flight proven
|-
| style="text-align:left;" | [[Liquid-fuel rocket]] || {{ntsh|4.4}}<4.4 || <10<sup>7</sup> || {{Ntsh|60}}Minutes || {{Nts|9}}
| style="text-align:left;" | {{Nts|9}}: Flight proven
|-
| style="text-align:left;" | [[Electrostatic ion thruster]] || {{Ntsh|112.5}}15–210<ref>{{cite web| title = ESA Portal – ESA and ANU make space propulsion breakthrough|url=https://cordis.europa.eu/article/id/25070-esa-and-australian-team-develop-breakthrough-in-space-propulsion|date=18 January 2006|publisher=European Union}}</ref> || || {{Ntsh|{{#expr:3600*24*365.25*12^-.5}}}}Months–years || {{Ntsh|100}}>100
| style="text-align:left;" | {{Nts|9}}: Flight proven
|-
| style="text-align:left;" | [[Hall-effect thruster]] (HET) || {{Ntsh|29}}up to 50<ref>{{Cite web |url=https://www.grc.nasa.gov/WWW/hall/overview/overview.htm |title=Overview of Hall thrusters |access-date=2020-05-29 |archive-date=2020-05-23 |archive-url=https://web.archive.org/web/20200523031148/https://www.grc.nasa.gov/WWW/hall/overview/overview.htm }}</ref> || || {{Ntsh|{{#expr:3600*24*365.25*12^-.5}}}}Months–years || {{Ntsh|100}}>100
| style="text-align:left;" | {{Nts|9}}: Flight proven<ref>Hall-effect thrusters have been used on Russian and antecedent Soviet bloc satellites for decades.{{Original research inline|date = July 2023}}{{citation needed|date = July 2022}}</ref>
|-
| style="text-align:left;" | [[Resistojet rocket]] || {{Ntsh|4}}2–6 || {{Ntsh|{{#expr:10^-.5}}}}10<sup>−2</sup>–10 || {{Ntsh|60}}Minutes || {{dunno}}
| style="text-align:left;" | {{Nts|8}}: Flight qualified<ref>[https://www.webcitation.org/5sq1FWURx?url=http://pdf.aiaa.org/preview/CDReadyMJPC2005_1177/PV2005_4260.pdf ''A Xenon Resistojet Propulsion System for Microsatellites''] (Surrey Space Centre, University of Surrey, Guildford, Surrey)</ref>
|-
| style="text-align:left;" | [[Arcjet rocket]] || {{Ntsh|10}}4–16 || {{Ntsh|{{#expr:10^-.5}}}}10<sup>−2</sup>–10 || {{Ntsh|60}}Minutes || {{dunno}}
| style="text-align:left;" | {{Nts|8}}: Flight qualified{{Citation needed|date=February 2010}}
|-
| style="text-align:left;" | [[Field-emission electric propulsion|Field-emission<br />electric propulsion]] (FEEP) || {{Ntsh|115}}100<ref name="feep">{{cite web| url = http://www.alta-space.com/index.php?page=feep| archive-url = https://web.archive.org/web/20110707120124/http://www.alta-space.com/index.php?page=feep| archive-date = 2011-07-07| title = Alta - Space Propulsion, Systems and Services - Field Emission Electric Propulsion<!-- Bot generated title -->}}</ref>–130 || {{Ntsh|{{#expr:10^-4.5}}}}10<sup>−6</sup>–10<sup>−3</sup><ref name="feep" /> || {{Ntsh|{{#expr:3600*24*365.25*12^-.5}}}}Months–years || {{dunno}}
| style="text-align:left;" | {{Nts|8}}: Flight qualified<ref name="feep" />
|-
| style="text-align:left;" | [[Pulsed plasma thruster]] (PPT) || {{Nts|20}} || {{Nts|0.1}} || {{Ntsh|{{#expr:3600000*10^((1+ln2/ln10)/2)}}}}80–400 days || {{dunno}}
| style="text-align:left;" | {{Nts|7}}: Prototype demonstrated in space
|-
| style="text-align:left;" | [[Dual mode propulsion rocket|Dual-mode propulsion rocket]] || {{Ntsh|2.85}}1–4.7 || {{Ntsh|1000}}0.1–10<sup>7</sup> || {{Ntsh|1}}Milliseconds–minutes || {{Ntsh|6}}3–9
| style="text-align:left;" | {{Nts|7}}: Prototype demonstrated in space
|-
| style="text-align:left;" | [[Solar sail]]s
| [[Radiation pressure|{{Nts|299792.458}}]], [[Speed of light]]
| {{Ntsh|9}} 9.08/km<sup>2</sup> at 1&nbsp;[[astronomical unit|AU]]<br />908/km<sup>2</sup> at 0.1&nbsp;AU<br />10<sup>−10</sup>/km<sup>2</sup> at 4 [[Light-year|ly]]
| Indefinite
|| {{Ntsh|40}}>40
| {{Ntsh|6.9}}{{unbulleted list
 | 9: Light pressure attitude-control flight proven
 | 6: Model, 196 m<sup>2</sup> 1.12 mN 400 m/s delta-v demonstrated in interplanetary space<ref>{{cite web |url=http://www.isas.jaxa.jp/home/IKAROS-blog/?itemid=1017|title=今日の IKAROS(8/29) – Daily Report – Aug 29, 2013 | publisher=Japan Aerospace Exploration Agency (JAXA) |date=29 August 2013 | access-date=8 June 2014 |language=ja}}</ref>
 }}
|-
| style="text-align:left;" | [[Tripropellant rocket]] || {{Ntsh|3.9}}2.5–5.3{{Citation needed|date=February 2011}} || {{Ntsh|1000}}0.1–10<sup>7</sup>{{Citation needed|date=February 2011}} || {{Ntsh|60}}Minutes || {{Nts|9}}
| style="text-align:left;" | {{Nts|6}}: Prototype demonstrated on ground<ref>[http://www.astronautix.com/engines/rd701.htm RD-701<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20100210203238/http://www.astronautix.com/engines/rd701.htm |date=2010-02-10 }}</ref>
|-
| style="text-align:left;" | [[Magnetoplasmadynamic thruster|Magnetoplasmadynamic<br />thruster]] (MPD) || {{Ntsh|60}}20–100 || {{Nts|100}} || {{Ntsh|{{#expr:3600*24*7}}}}Weeks || {{dunno}}
| style="text-align:left;" | {{Nts|6}}: Model, 1&nbsp;kW demonstrated in space<ref>{{cite web| url = https://translate.google.com/translate?hl=en&sl=ja&u=http://www.isas.jaxa.jp/ISASnews/No.190/labo-5.html| title = Google Translate<!-- Bot generated title -->}}</ref>
|-
| style="text-align:left;" | [[Nuclear thermal rocket|Nuclear–thermal rocket]] || {{Nts|9}}<ref name="rd0410">[http://www.astronautix.com/engines/rd0410.htm RD-0410<!-- Bot generated title -->] {{webarchive|url=https://web.archive.org/web/20090408122011/http://www.astronautix.com/engines/rd0410.htm |date=2009-04-08 }}</ref> || {{Ntsh|10000000}}10<sup>7</sup><ref name="rd0410" /> || {{Ntsh|60}}Minutes<ref name="rd0410" /> || {{Ntsh|20}}>20
| style="text-align:left;" | {{Nts|6}}: Prototype demonstrated on ground
|-
| style="text-align:left;" | Propulsive [[mass driver]]s || {{Ntsh|15}}0–30 || {{Ntsh|1000000}}10<sup>4</sup>–10<sup>8</sup> || {{Ntsh|{{#expr:3600*24*31}}}}Months || {{dunno}}
| style="text-align:left;" | {{Nts|6}}: Model, 32&nbsp;MJ demonstrated on ground
|-
| style="text-align:left;" | [[Tether propulsion]] || {{n/a}} || {{Ntsh|1000000}}1–10<sup>12</sup> || {{Ntsh|60}}Minutes || {{Ntsh|7}}7
| style="text-align:left;" | {{Nts|6}}: Model, 31.7&nbsp;km demonstrated in space<ref name="Tether">[http://www.yes2.info/ Young Engineers' Satellite 2<!-- Bot generated title -->]  {{webarchive|url=https://web.archive.org/web/20030210014335/http://www.yes2.info/ |date=2003-02-10 }}</ref>
|-
| style="text-align:left;" | [[Air-augmented rocket]] || {{Ntsh|5.5}}5–6 || {{Ntsh|1000}}0.1–10<sup>7</sup> || {{Ntsh|{{#expr:60^.5}}}}Seconds–minutes || {{Ntsh|7}}>7?
| style="text-align:left;" | {{Nts|6}}: Prototype demonstrated on ground<ref>[http://astronautix.com/lvs/gnom.htm Gnom] {{webarchive|url=https://web.archive.org/web/20100102204131/http://astronautix.com/lvs/gnom.htm |date=2010-01-02 }}</ref><ref>[http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html NASA GTX] {{webarchive |url=https://web.archive.org/web/20081122140310/http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html |date=November 22, 2008 }}</ref>
|-
| style="text-align:left;" | [[Liquid air cycle engine|Liquid-air-cycle engine]] || {{Nts|4.5}} || {{Ntsh|100000}}10<sup>3</sup>–10<sup>7</sup> || {{Ntsh|{{#expr:60^.5}}}}Seconds–minutes || {{dunno}}
| style="text-align:left;" | {{Nts|6}}: Prototype demonstrated on ground
|-
| style="text-align:left;" | [[Pulsed inductive thruster|Pulsed-inductive thruster]] (PIT) || {{Ntsh|45}}10–80<ref name="PIT">{{cite web| url = https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930023164_1993023164.pdf| title = The PIT MkV pulsed inductive thruster}}</ref> || {{Nts|20}} || {{Ntsh|{{#expr:3600*24*31}}}}Months || {{dunno}}
| style="text-align:left;" | {{Nts|5}}: Component validated in vacuum<ref name="PIT" />
|-
| style="text-align:left;" | [[Variable specific impulse magnetoplasma rocket|Variable-specific-impulse<br />magnetoplasma rocket]]<br />(VASIMR) || {{Ntsh|155}}10–300{{Citation needed|date=February 2011}} || {{Ntsh|620}}40–1,200{{Citation needed|date=February 2011}} || {{Ntsh|{{#expr:3600*24*31^.5}}}}Days–months || {{Ntsh|100}}>100
| style="text-align:left;" | {{Nts|5}}: Component, 200&nbsp;kW validated in vacuum
|-
| style="text-align:left;" | [[Magnetic field oscillating amplified thruster|Magnetic-field oscillating<br />amplified thruster]] (MOA) || {{Ntsh|70}}10–390<ref name="MOA">{{cite web| url = http://scidoc.org/articlepdfs/IJASAR/IJASAR-2470-4415-10-102.pdf| title = Thermal velocities in the plasma of a MOA Device, M.Hettmer, Int J Aeronautics Aerospace Res. 2023;10(1):297-300}}</ref> || {{Ntsh|{{#expr:10^-.5}}}}0.1–1 || {{Ntsh|{{#expr:3600*24*31^.5}}}}Days–months || {{Ntsh|100}}>100
| style="text-align:left;" | {{Nts|5}}: Component validated in vacuum
|-
| style="text-align:left;" | [[Solar thermal rocket|Solar–thermal rocket]] || {{Ntsh|9.5}}7–12 || {{Ntsh|10}}1–100 || {{Ntsh|{{#expr:3600*24*7}}}}Weeks || {{Ntsh|20}}>20
| style="text-align:left;" | {{Nts|4}}: Component validated in lab<ref>{{cite news|url=https://spaceref.com/press-release/pratt-whitney-rocketdyne-wins-22-million-contract-option-for-solar-thermal-propulsion-rocket-engine/|title= Pratt & Whitney Rocketdyne Wins $2.2 Million Contract Option for Solar Thermal Propulsion Rocket Engine|date= June 25, 2008|publisher=Pratt & Whitney [[Rocketdyne]])}}</ref>
|-
| style="text-align:left;" | [[Radioisotope rocket]]/[[World Is Not Enough (spacecraft propulsion)|Steam thruster]] || {{Ntsh|7.5}}7–8{{citation needed|date=January 2011}} || {{Ntsh|1.4}}1.3–1.5 || {{Ntsh|{{#expr:3600*24*31}}}}Months || {{dunno}}
| style="text-align:left;" | {{Nts|4}}: Component validated in lab
|-
| style="text-align:left;" | [[Nuclear electric rocket|Nuclear–electric rocket]]
| colspan=4 {{n/a|As electric propulsion method used}}
| style="text-align:left;" | {{Nts|4}}: Component, [[Safe Affordable Fission Engine|400&nbsp;kW validated in lab]]
|-
| style="text-align:left;" | [[Project Orion (nuclear propulsion)|Orion Project]] (near-term<br />nuclear pulse propulsion) || {{Ntsh|60}}20–100 || {{Ntsh|{{#expr:10^10.5}}}}10<sup>9</sup>–10<sup>12</sup><!--not a misprint--> || {{Ntsh|{{#expr:3600*24*7}}}}Days || {{Ntsh|45}}30–60
| style="text-align:left;" | {{Nts|3}}: Validated, 900&nbsp;kg proof-of-concept<ref>{{cite web| url=http://nuclearweaponarchive.org/Usa/Tests/Plumbob.html#PascalB| title=Operation Plumbbob|date=July 2003| access-date=2006-07-31}}</ref><ref>{{cite web| url=http://nuclearweaponarchive.org/Usa/Tests/Brownlee.html| title=Learning to Contain Underground Nuclear Explosions| first=Robert R.| last=Brownlee|date=June 2002| access-date=2006-07-31}}</ref>
|-
| style="text-align:left;" | [[Space elevator]] || {{n/a}} || {{n/a}} || Indefinite || {{Ntsh|12}}>12
| style="text-align:left;" | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;" | [[Reaction Engines SABRE]]<ref name="SABRE" /> || {{Ntsh|17.25}}30/4.5 || {{Ntsh|1000}}0.1 – 10<sup>7</sup> || {{Ntsh|60}}Minutes || {{Nts|9.4}}
| style="text-align:left;" | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;" | [[Electric sail]]s
| [[Solar wind#Emission|145–750, solar wind]]
| {{dunno}} || Indefinite || {{Ntsh|40}}>40
| style="text-align:left;" | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;" | [[Magnetic sail#Magsail (MS)|Magsail]] in [[Solar wind]]|| {{n/a}} || {{Ntsh|644}}{{Nts|644}}<ref name=":AndrewsZubrin90">{{Cite journal |last1=Andrews |first1=Dana |last2=Zubrin |first2=Robert |date=1990 |title=MAGNETIC SAILS AND INTERSTELLAR TRAVEL |url=https://www.academia.edu/78476000 |journal=Journal of the British Interplanetary Society |volume=43 |pages=265–272 |via=JBIS}}</ref>{{Efn|Divided by 3.1 correction factor.<ref name=":Freeland2015" />}} || Indefinite || {{Ntsh|250}}250–750
| style="text-align:left;" | {{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;" | [[Magnetic sail#Magnetoplasma sail (MPS)|Magnetoplasma sail]] in [[Solar wind]]<ref name=":222">{{Cite journal |last1=Funaki |first1=Ikkoh |last2=Kajimura |first2=Yoshihiro |last3=Ashida |first3=Yasumasa |last4=Yamakawa |first4=Hiroshi |last5=Nishida |first5=Hiroyuki |last6=Oshio |first6=Yuya |last7=Ueno |first7=Kazuma |last8=Shinohara |first8=Iku |last9=Yamamura |first9=Haruhito |last10=Yamagiwa |first10=Yoshiki |date=2013-07-14 |title=Magnetoplasma Sail with Equatorial Ring-current |url=https://arc.aiaa.org/doi/10.2514/6.2013-3878 |journal=49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference |series=Joint Propulsion Conferences |language=en |location=San Jose, CA |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2013-3878 |isbn=978-1-62410-222-6}}</ref>|| 278 || {{Ntsh|700}}700 || {{Ntsh|{{#expr:3600*24*31}}}}Months–Years || {{Ntsh|250}}250–750
| style="text-align:left;" | {{Nts|4}}: Component validated in lab<ref name=":3">{{Citation |last1=Funaki |first1=Ikkoh |title=Solar Wind Sails |date=2012-03-21 |url=http://www.intechopen.com/books/exploring-the-solar-wind/solar-wind-sails |work=Exploring the Solar Wind |editor-last=Lazar |editor-first=Marian |publisher=InTech |language=en |bibcode=2012esw..book..439F |doi=10.5772/35673 |isbn=978-953-51-0339-4 |access-date=2022-06-13 |last2=Yamakaw |first2=Hiroshi|s2cid=55922338 |doi-access=free }}</ref>
|-
| style="text-align:left;" | [[Magnetic sail#Magsail (MS)|Magsail]] in [[Interstellar medium]]<ref name=":Freeland2015">{{Cite journal |last=Freeland |first=R.M. |date=2015 |title=Mathematics of Magsail |url=https://bis-space.com/shop/product/mathematics-of-magsails/ |journal=Journal of the British Interplanetary Society |volume=68 |pages=306–323 |via=bis-space.com}}</ref>
|{{n/a}}
|88,000 initially
|{{Ntsh|{{#expr:3600*24*365*10}}}}Decades
|{{Ntsh|15000}}15,000
| style="text-align:left;" |{{Nts|3}}: Validated proof-of-concept
|-
| style="text-align:left;" | [[Beam-powered propulsion|Beam-powered]]/[[laser propulsion|laser]]
| colspan=4 {{n/a|As propulsion method powered by beam}}
| style="text-align:left;" | {{Nts|3}}: Validated, 71&nbsp;m proof-of-concept
|-
| style="text-align:left;" | [[Launch loop]]/[[orbital ring]] || {{n/a}} || {{Ntsh|10000}}10<sup>4</sup> || {{Ntsh|60}}Minutes || {{Ntsh|20.5}}11–30
| style="text-align:left;" | {{Nts|2}}: [[Maglev (transport)|Technology]] concept formulated
|-
| style="text-align:left;" | [[Nuclear pulse propulsion]]<br />([[Project Daedalus]]' drive) || {{Ntsh|510}}20–1,000 || {{Ntsh|{{#expr:10^10.5}}}}10<sup>9</sup>–10<sup>12</sup><!--not a misprint--> || {{Ntsh|{{#expr:3600*24*365.25}}}}Years || {{Ntsh|15000}}15,000
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Gas core reactor rocket|Gas-core reactor rocket]] || {{Ntsh|15}}10 – 20 || {{Ntsh|{{#expr:10^4.5}}}}10<sup>3</sup>–10<sup>6</sup> || {{dunno}} || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Nuclear salt-water rocket]] || {{Nts|100}} || {{Ntsh|100000}}10<sup>3</sup>–10<sup>7</sup> || {{Ntsh|1800}}Half-hour || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Fission sail]] || {{dunno}} || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Fission-fragment rocket]] || {{Nts|15000}} || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Nuclear photonic rocket|Nuclear–photonic rocket]]/[[Photon rocket]] || [[Radiation pressure|{{Nts|299792.458}}]], [[Speed of light]] || {{Ntsh|{{#expr:10^-2.5}}}}10<sup>−5</sup>–1 || {{Ntsh|{{#expr:3600*24*365.25*10^.5}}}}Years–decades || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Fusion rocket]] || {{Ntsh|550}}100–1,000{{citation needed|date=January 2011}} || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Antimatter-catalyzed nuclear pulse propulsion|Antimatter-catalyzed<br />nuclear pulse propulsion]] || {{Ntsh|2100}}200–4,000 || {{dunno}} || {{Ntsh|{{#expr:3600*24*7^.5}}}}Days–weeks || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Antimatter rocket]] || {{Ntsh|55000}}10,000–100,000{{citation needed|date=January 2011}}  || {{dunno}} || {{dunno}} || {{dunno}}
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
| style="text-align:left;" | [[Bussard ramjet]] || {{Ntsh|10001.1}}2.2–20,000 || {{dunno}} || Indefinite || {{Ntsh|30000}}30,000
| style="text-align:left;" | {{Nts|2}}: Technology concept formulated
|-
! style="text-align:left;" | Method
! [[Specific impulse|Effective exhaust <br />velocity]] (km/s)
! [[Thrust]] (N)
! Firing <br />duration
! Maximum <br />[[delta-v]] (km/s)
! style="text-align:left;" | [[Technology readiness level#NASA definitions|Technology <br />readiness level]]
|}
'''Table Notes'''{{Notelist}}

==Planetary and atmospheric propulsion==
[[File:Lightcraft.jpg|thumb|A successful proof of concept [[Lightcraft]] test, a subset of [[beam-powered propulsion]]]]

===Launch-assist mechanisms===
{{Main|Space launch}}

There have been many ideas proposed for launch-assist mechanisms that have the potential of substantially reducing the cost of getting to orbit. Proposed [[non-rocket spacelaunch]] launch-assist mechanisms include:<ref>{{Cite web |date=1970-01-01 |title=Can We Get Into Space Without Big Rockets? |url=https://science.howstuffworks.com/can-get-into-space-without-big-rocket.htm |access-date=2024-04-28 |website=HowStuffWorks |language=en-us}}</ref><ref>{{Cite web |last=Bolonkin |first=Alexander |date=January 2011 |title=Review of new ideas, innovations of non- rocket propulsion systems for Space Launch and Flight (Part 2) |url=https://www.researchgate.net/publication/268426650 |access-date=April 28, 2024 |website=www.researchgate.net}}</ref>

*[[Skyhook (structure)|Skyhook]] (requires reusable suborbital launch vehicle, not feasible using presently available materials)
*[[Space elevator]] (tether from Earth's surface to geostationary orbit, cannot be built with existing materials)
*[[Launch loop]] (a very fast enclosed rotating loop about 80&nbsp;km tall)
*[[Space fountain]] (a very tall building held up by a stream of masses fired from its base)
*[[Orbital ring]] (a ring around Earth with spokes hanging down off bearings)
*[[Mass driver|Electromagnetic catapult]] ([[railgun]], [[coilgun]]) (an electric gun)
*[[Rocket sled launch]]
*[[Space gun]] ([[Project HARP]], [[ram accelerator]]) (a chemically powered gun)
*[[Beam-powered propulsion]] rockets and jets powered from the ground via a beam
*[[High-altitude platform]]s to assist initial stage

===Air-breathing engines===
{{Main|Jet engine|Air-breathing electric propulsion}}

{{more citations needed section | date = July 2023}}
Studies generally show that conventional air-breathing engines, such as [[ramjets]] or [[turbojets]] are basically too heavy (have too low a thrust/weight ratio) to give significant performance improvement when installed on a launch vehicle.{{Citation needed|date=April 2024}} However, launch vehicles can be [[air launch]]ed from separate lift vehicles (e.g. [[B-29 Superfortress|B-29]], [[Pegasus rocket|Pegasus Rocket]] and [[Scaled Composites White Knight|White Knight]]) which do use such propulsion systems. Jet engines mounted on a launch rail could also be so used.{{Citation needed|date=April 2024}}

On the other hand, very lightweight or very high-speed engines have been proposed that take advantage of the air during ascent:
* [[Reaction Engines SABRE|SABRE]] – a lightweight hydrogen fuelled turbojet with precooler<ref name="SABRE">{{cite web
 |author=Anonymous 
 |year=2006 
 |url=http://www.reactionengines.co.uk/sabre.html 
 |archive-url=https://web.archive.org/web/20070222125903/http://www.reactionengines.co.uk/sabre.html 
 |archive-date=2007-02-22 
 |title=The Sabre Engine 
 |publisher=Reaction Engines Ltd. 
 |access-date=2007-07-26 
}}</ref>
* [[ATREX]] – a lightweight hydrogen fuelled turbojet with precooler<ref>{{cite journal
 |author1=Harada, K. |author2=Tanatsugu, N. |author3=Sato, T. | title=Development Study on ATREX Engine
 | journal=Acta Astronautica
 | year=1997 | volume=41 | issue=12 | pages=851–862
 | doi=10.1016/S0094-5765(97)00176-8 |bibcode=1997AcAau..41..851T}}</ref>
* [[Liquid air cycle engine]] – a hydrogen-fuelled jet engine that liquifies the air before burning it in a rocket engine
* [[Scramjet]] – jet engines that use supersonic combustion
* [[Shcramjet]] – similar to a scramjet engine, however it takes advantage of shockwaves produced from the aircraft in the combustion chamber to assist in increasing overall efficiency.

Normal rocket launch vehicles fly almost vertically before rolling over at an altitude of some tens of kilometers before burning sideways for orbit; this initial vertical climb wastes propellant but is optimal as it greatly reduces airdrag. Airbreathing engines burn propellant much more efficiently and this would permit a far flatter launch trajectory. The vehicles would typically fly approximately tangentially to Earth's surface until leaving the atmosphere then perform a rocket burn to bridge the final [[delta-v]] to orbital velocity.

For spacecraft already in very low-orbit, [[air-breathing electric propulsion]] could use residual gases in the upper atmosphere as a propellant. Air-breathing electric propulsion could make a new class of long-lived, low-orbiting missions feasible on Earth, [[Mars]] or [[Venus]].<ref>{{cite news|title=World-first firing of air-breathing electric thruster|url=http://www.esa.int/Our_Activities/Space_Engineering_Technology/World-first_firing_of_air-breathing_electric_thruster|access-date=7 March 2018|work=Space Engineering & Technology|publisher=[[European Space Agency]]|date=5 March 2018}}</ref><ref>[http://erps.spacegrant.org/uploads/images/2015Presentations/IEPC-2015-271_ISTS-2015-b-271.pdf Conceptual design of an air-breathing electric propulsion system] {{Webarchive|url=https://web.archive.org/web/20170404043702/http://erps.spacegrant.org/uploads/images/2015Presentations/IEPC-2015-271_ISTS-2015-b-271.pdf |date=2017-04-04 }}. (PDF). 30th International Symposium on Space Technology and Science. 34th International Electric Propulsion Conference and 6th Nano-satellite Symposium. Hyogo-Kobe, Japan July 4, 2015.</ref>

===Planetary arrival and landing===
{{Main|Atmospheric entry}}
{{More sources needed|section|date=April 2024}}
[[File:Pathfinder Air Bags - GPN-2000-000484.jpg|thumb|right|A test version of the [[Mars Pathfinder]] airbag system]]
When a vehicle is to enter orbit around its destination planet, or when it is to land, it must adjust its velocity.<ref>{{Cite web |title=Chapter 4: Trajectories – NASA Science |url=https://science.nasa.gov/learn/basics-of-space-flight/chapter4-1/ |access-date=2024-04-24 |website=science.nasa.gov |date=20 July 2023 |language=en-US}}</ref> This can be done using any of the methods listed above (provided they can generate a high enough thrust), but there are methods that can take advantage of planetary atmospheres and/or surfaces.

* [[Aerobraking]] allows a spacecraft to reduce the high point of an elliptical orbit by repeated brushes with the atmosphere<ref>{{Cite web |title=Definition of AEROBRAKE |url=https://www.merriam-webster.com/dictionary/aerobrake |access-date=2024-04-24 |website=www.merriam-webster.com |language=en}}</ref> at the low point of the orbit. This can save a considerable amount of fuel because it takes much less delta-V to enter an elliptical orbit compared to a low circular orbit. Because the braking is done over the course of many orbits, heating is comparatively minor, and a heat shield is not required. This has been done on several Mars missions such as ''[[Mars Global Surveyor]]'', ''[[2001 Mars Odyssey]]'', and ''[[Mars Reconnaissance Orbiter]]'', and at least one Venus mission, ''[[Magellan (spacecraft)|Magellan]]''.
* [[Aerocapture]] is a much more aggressive manoeuver, converting an incoming hyperbolic orbit to an elliptical orbit in one pass. This requires a heat shield and more controlled navigation because it must be completed in one pass through the atmosphere, and unlike aerobraking no preview of the atmosphere is possible. If the intent is to remain in orbit, then at least one more propulsive maneuver is required after aerocapture—otherwise the low point of the resulting orbit will remain in the atmosphere, resulting in eventual re-entry. Aerocapture has not yet been tried on a planetary mission, but the [[Skip reentry|re-entry skip]] by [[Zond 6]] and [[Zond 7]] upon lunar return were aerocapture maneuvers, because they turned a hyperbolic orbit into an elliptical orbit. On these missions, because there was no attempt to raise the perigee after the aerocapture, the resulting orbit still intersected the atmosphere, and re-entry occurred at the next perigee.
* A [[ballute]] is an inflatable drag device.<ref>{{Cite web |title=Definition of BALLUTE |url=https://www.merriam-webster.com/dictionary/ballute |access-date=2024-04-26 |website=www.merriam-webster.com |language=en}}</ref>
* [[Parachute]]s can land a probe on a planet or moon with an atmosphere, usually after the atmosphere has scrubbed off most of the velocity, using a [[Atmospheric reentry|heat shield]].
* [[Airbag]]s can soften the final landing.
* [[Lithobraking]], or stopping by impacting the surface, is usually done by accident. However, it may be done deliberately with the probe expected to survive (see, for example, the [[Deep Impact (spacecraft)|Deep Impact spacecraft]]), in which case very sturdy probes are required.

==Research==

Development of technologies will result in technical solutions that improve thrust levels, [[specific impulse]], power, [[specific mass]], (or [[Power-to-weight ratio|specific power]]), volume, system mass, system complexity, operational complexity, commonality with other spacecraft systems, manufacturability, durability, and cost. These types of improvements will yield decreased transit times, increased payload mass, safer spacecraft, and decreased costs. In some instances, the development of technologies within this technology area will result in mission-enabling breakthroughs that will revolutionize space exploration. There is no single propulsion technology that will benefit all missions or mission types; the requirements for in-space propulsion vary widely according to their intended application.{{Sfn|Meyer|2012|p=5}}<ref name=leone/>

One institution focused on developing primary propulsion technologies aimed at benefitting near and mid-term science missions by reducing cost, mass, and/or travel times is the [[Glenn Research Center]] (GRC).{{citation needed|date = July 2023}} [[Electric propulsion]] architectures are of particular interest to the GRC, including [[Ion thruster|ion]] and [[Hall thruster]]s.{{citation needed|date = July 2023}} One system combines [[solar sail]]s, a form of propellantless propulsion which relies on naturally-occurring starlight for propulsion energy, and Hall thrusters. Other propulsion technologies being developed include advanced chemical propulsion and aerocapture.<ref name="leone" /><ref name="grcspace">[https://www1.grc.nasa.gov/space/sep/ Solar Electric Propulsion (SEP)]. Glenn Research Center. NASA. 2019</ref><ref name="glenion">[http://www.grc.nasa.gov/WWW/ion/ Ion propulsion system research] {{Webarchive|url=https://web.archive.org/web/20060901214224/http://www.grc.nasa.gov/WWW/ion/|date=2006-09-01}}. Glenn Research Center. NASA. 2013</ref>

===Defining technologies===
The term "mission pull" defines a technology or a performance characteristic necessary to meet a planned NASA mission requirement. Any other relationship between a technology and a mission (an alternate propulsion system, for example) is categorized as "technology push." Also, a space demonstration refers to the spaceflight of a scaled version of a particular technology or of a critical technology subsystem. On the other hand, a space validation would serve as a qualification flight for future mission implementation. A successful validation flight would not require any additional space testing of a particular technology before it can be adopted for a science or exploration mission.{{Sfn|Meyer|2012|p=5}}

===Testing===
Spacecraft propulsion systems are often first statically tested on Earth's surface, within the atmosphere but many systems require a vacuum chamber to test fully.<ref>{{Cite journal |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 |date=November 2021 |title=In-orbit demonstration of an iodine electric propulsion system |journal=Nature |language=en |volume=599 |issue=7885 |pages=411–415 |doi=10.1038/s41586-021-04015-y |pmid=34789903 |issn=1476-4687|pmc=8599014 |bibcode=2021Natur.599..411R }}</ref> Rockets are usually tested at a [[rocket engine test facility]] well away from habitation and other buildings for safety reasons. [[Ion drive]]s are far less dangerous and require much less stringent safety, usually only a moderately large vacuum chamber is needed.{{Citation needed|date=April 2024}} Static firing of engines are done at [[Rocket engine test facility#Rocket ground test facilities|ground test facilities]], and systems which cannot be adequately tested on the ground and require launches may be employed at a [[rocket launch site|launch site]].

==In fiction==
{{Main|Space travel in science fiction#Methods of travel}}
[[File:Wormhole travel as envisioned by Les Bossinas for NASA.jpg|thumb|right|Artist's conception of a warp drive design]]
In science fiction, space ships use various means to travel, some of them scientifically plausible (like solar sails or ramjets), others, mostly or entirely fictitious (like [[anti-gravity]], [[warp drive]], [[Spindizzy (Cities in Flight)|spindizzy]] or [[hyperspace travel]]).<ref name="visual">{{Cite book|last=Ash|first=Brian|url=https://books.google.com/books?id=-SUYAAAAIAAJ&q=Visual+Encyclopedia+of+Science+Fiction|title=The Visual Encyclopedia of Science Fiction|date=1977|publisher=Harmony Books|isbn=978-0-517-53174-7|language=en}}</ref>{{Rp|8, 69–77}}<ref name=":Prucher">{{Cite book|last=Prucher|first=Jeff|url=https://books.google.com/books?id=lJCS0reqmFUC&q=Earthling+%22science+fiction%22&pg=PP2|title=Brave New Words: The Oxford Dictionary of Science Fiction|date=2007-05-07|publisher=Oxford University Press|isbn=978-0-19-988552-7|pages=|language=en}}</ref>{{Rp|142}}

==Further reading==
{{more science citations needed|section|date=July 2023}}
* {{cite book |author=Heister, Stephen D. |author2=Anderson, William E. |author3=Pourpoint, Timothée L. |author4=Cassady, R. Joseph |date=2019 |title=Rocket Propulsion |edition= |volume=47 |series=Cambridge Aerospace Series |location=Cambridge England |publisher=Cambridge University Press |isbn=978-1-108-39506-9 |url=https://books.google.com/books?id=zZCFDwAAQBAJ |access-date=22 July 2023}}
* {{cite book |author=Sutton |first1=George P. |last2=Biblarz |first2=Oscar |date=2016 |title=Rocket Propulsion Elements |edition=9th |location=New York, New York |publisher=John Wiley & Sons |isbn=978-1-118-75365-1 |url=https://books.google.com/books?id=XwSRDQAAQBAJ |access-date=22 July 2023}}
* {{cite journal |author=Taploo, A |author2=Lin, Li |author3=Keidar, Michael | date = 1 September 2021 | title = Analysis of Ionization in Air-Breathing Plasma Thruster | journal = Physics of Plasmas  | volume = 28 | issue = 9 | page = 093505 | doi = 10.1063/5.0059896 | bibcode = 2021PhPl...28i3505T | s2cid = 240531647}}{{primary source inline|date = July 2023}} See also: {{cite journal |author=Taploo, A |author2=Lin, Li |author3=Keidar, Michael | date = 2022 | title = Air Ionization in Self-Neutralizing Air-Breathing Plasma Thruster | journal = J. Electr. Propuls.  | volume = 1 | issue =  1| page = 25 | doi = 10.1007/s44205-022-00022-x | bibcode = 2022JElP....1...25T | s2cid = 253556114| doi-access = free }}{{primary source inline|date = July 2023}}
* {{cite journal |vauthors=Taploo A, Soni V, Solomon H, McCraw M, Lin L, Spinelli J, Shepard S, Solares S, Keidar M |date=12 October 2023 |title=Characterization of a circular arc electron source for a self-neutralizing air-breathing plasma thruster |journal= Journal of Electric Propulsion |volume=2 |issue=21 |doi=10.1007/s44205-023-00058-7 |doi-access=free |bibcode=2023JElP....2...21T }}
{{Portal| Spaceflight }}
==See also==
{{Div col}}
* [[Anti-gravity]]
* [[Artificial gravity]]
* [[Atmospheric entry]]
* [[Breakthrough Propulsion Physics Program]]
* [[Flight dynamics (spacecraft)]]
* [[Index of aerospace engineering articles]]
* [[Interplanetary Transport Network]]
* [[Interplanetary travel]]
* [[List of aerospace engineering topics]]
* [[Lists of rockets]]
* [[Orbital maneuver]]
* [[Orbital mechanics]]
* [[Pulse detonation engine]]
* [[Rocket]]
* [[Rocket engine nozzles]]
* [[Satellite]]
* [[Spaceflight]]
* [[Space launch]]
* [[Space travel using constant acceleration]]
* [[Specific impulse]]
* [[Tsiolkovsky rocket equation]]

{{Div col end}}

==References==
{{reflist|2}}

==External links==
* [https://web.archive.org/web/20040402092917/http://www.grc.nasa.gov/WWW/bpp/ NASA Breakthrough Propulsion Physics project]
* [http://www.projectrho.com/rocket/rocket3c2.html Different Rockets] {{Webarchive|url=https://web.archive.org/web/20100529014413/http://www.projectrho.com/rocket/rocket3c2.html |date=2010-05-29 }}
* [http://www.islandone.org/LEOBiblio/ Earth-to-Orbit Transportation Bibliography] {{Webarchive|url=https://web.archive.org/web/20160615200556/http://www.islandone.org/LEOBiblio/ |date=2016-06-15 }}
* [http://vc.airvectors.net/tarokt.html Spaceflight Propulsion] – a detailed survey by Greg Goebel, in the public domain
* [http://www.cpia.jhu.edu/ Johns Hopkins University, Chemical Propulsion Information Analysis Center]
* [http://software.lpre.de/ Tool for Liquid Rocket Engine Thermodynamic Analysis]
* [http://howthingsfly.si.edu/ Smithsonian National Air and Space Museum's How Things Fly website]
* Fullerton, Richard K. "[http://spacecraft.ssl.umd.edu/design_lib/ICES01-2200.EVA_roadmaps.pdf Advanced EVA Roadmaps and Requirements]." Proceedings of the 31st International Conference on Environmental Systems. 2001.
* [http://www.projectrho.com/public_html/rocket/engineintro.php ''Atomic Rocket'' – Engines]: A site listing and detailing real, theoretical and fantasy space engines.
{{Spaceflight}}
{{Spacecraft propulsion}}

{{DEFAULTSORT:Spacecraft Propulsion}}
[[Category:Spacecraft propulsion| ]]
[[Category:Spacecraft components]]
[[Category:Spaceflight technology]]
[[Category:NASA programs]]
[[Category:Glenn Research Center]]
[[Category:Discovery and exploration of the Solar System]]