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{{Short description|Spacecraft used to transfer cargo from one orbit to another}}
{{Use mdy dates|date=October 2023}}
[[File:Space tug parts.jpg|thumb|Reusable, modular 1969 NASA vision Space Tug (canceled)]]
[[Image:Galileo Deployment (high res).jpg|thumb|250px|The Jupiter bound ''Galileo'' spacecraft and its attached Inertial Upper Stage (IUS) being deployed after being launched by the [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] on the [[STS-34]] mission. The IUS was an optional payload for missions where the [[Space Shuttle]] was used to take a payload beyond [[Low Earth Orbit]]. By contrast, the [[Space Shuttle external tank]] was always included and used whenever a shuttle launch took place.]]

A '''space tug''' is a type of [[spacecraft]] used to transfer [[orbital spaceflight|spaceborne]] cargo from one [[orbit]] to another orbit with different energy characteristics. The term can include expendable upper stages or spacecraft that are not necessarily a part of their launch vehicle. However, it can also refer to a spacecraft that transports payload already in space to another location in outer space, such as in the [[Space Transportation System]] concept. An example would be moving a spacecraft from a [[low Earth orbit]] (LEO) to a higher-energy orbit like a [[geostationary transfer orbit]], a [[Trans-lunar injection|lunar transfer]], or an [[escape trajectory]].

The term is often used to refer to reusable, space-based vehicles. Some previously proposed or built space tugs include the NASA 1970s STS proposal<ref name="anxtug">{{cite web |title=Space Tug |url=http://www.astronautix.com/fam/spacetug.htm |archive-url=https://web.archive.org/web/20111011090207/http://astronautix.com/fam/spacetug.htm |url-status=dead |archive-date=October 11, 2011 |website=Astronautix |access-date=July 25, 2014}}</ref> or the proposed Russian [[Parom]], and has sometimes been used to refer to expendable [[upper stage]]s,<ref name="anxtug"/> such as [[Fregat]],<ref name="rsw201407">{{Cite web |editor-last=Chabot |editor-first=Alain |last=Zak |first=Anatoly |title=Fregat space tug |url=https://www.russianspaceweb.com/fregat.html |url-status=live |archive-url=https://web.archive.org/web/20240109173551/https://www.russianspaceweb.com/fregat.html |archive-date=January 9, 2024 |access-date=July 25, 2014 |website=RussianSpaceWeb.com }}</ref> [[Spaceflight Industries]] [[SHERPA (space tug)|Sherpa]], and the [[Inertial Upper Stage]], when such stages are optional.

== Background ==
The space tug was first envisioned in the post-[[World War II]] era as a support vehicle for a permanent, Earth-orbiting [[space station]]. It was used by science fiction writer [[Murray Leinster]] as the title of a [[Space Tug (novel)|novel published in 1953]] as the sequel to ''[[Space Platform]]'', another novel about such a space station.<ref>{{Cite book |last1=Leinster |first1=Murray |url=https://archive.org/details/spacetug0000unse |title=Space Tug |date=1953 |publisher=[[Shasta Publishers]] |oclc=6570191 }}</ref>

== Existing space tugs ==

Space tugs can be roughly categorised into a few types:
* Large tugs that dock with satellites in orbit which may be able to perform services like refuelling or repairs or enhancements as well as changing the satellites orbit whether that is to extend life of satellite or to deorbit it.
* Optional rocket kick stages used to place some payloads into higher orbits. An example would be [[Rocket Lab#Photon satellite bus|Photon Satellite Bus]] but this might just be considered part of the rocket system rather than a space tug and this article does not really consider these in detail.
* Smaller tugs that are mainly cubesat deployers with some propulsion to deploy the cubesats to different orbits.

=== Large tugs that dock ===
==== Mission Extension Vehicle ====
{{Main|Mission Extension Vehicle}}
In 2011 ViviSat a joint project between U.S. Space and [[Alliant Techsystems|ATK]] proposed the [[Mission Extension Vehicle]]. In 2016 ViviSat was dissolved when U.S. Space declared bankruptcy and ATK merged with [[Orbital Sciences Corporation|Orbital Science Corporation]] to form [[Orbital ATK]]. In 2017 Orbital ATK got the go ahead from the [[Federal Communications Commission|FCC]] to begin development of the spacecraft with new partner [[Northrop Grumman]] who was developing a tug of their own. In June 2018, both companies pooled their resources and merged to form a new company called [[Northrop Grumman Innovation Systems]]. On October 9, 2019, the first of these tugs MEV-1 was launched from [[Baikonur Cosmodrome]] in [[Kazakhstan]] on a [[Proton-M]] rocket. In February 2020, MEV-1 successfully docked with [[Intelsat 901]] and returned it to geosynchronous orbit, allowing it to continue operating 4 years past its service life. MEV-1 will continue to maintain this position for a 5-year period, after which it will move the satellite back into a [[graveyard orbit]] for retirement. MEV-2 was launched August 15, 2020, with [[Galaxy (satellite)|Galaxy 30]] on an [[Ariane 5]] to perform a similar maneuver with [[Intelsat 10-02|Intelsat-1002]].<ref name="spacenews-20200417">{{Cite news |last=Henry |first=Caleb |date=April 17, 2020 |title=Intelsat-901 satellite, with MEV-1 servicer attached, resumes service |url=https://spacenews.com/intelsat-901-satellite-with-mev-1-servicer-attached-resumes-service/ |url-status=live |archive-url=https://archive.today/20240109174005/https://spacenews.com/intelsat-901-satellite-with-mev-1-servicer-attached-resumes-service/ |archive-date=January 9, 2024 |access-date=May 20, 2020 |website=SpaceNews.com }}</ref><ref name="northrup-20200417">{{Cite press release |last1=Cox |first1=Vicki |last2=Macdonald |first2=Meghan |date=April 17, 2020 |title=Intelsat 901 Satellite Returns to Service Using Northrop Grumman's Mission Extension Vehicle |url=https://news.northropgrumman.com/news/releases/intelsat-901-satellite-returns-to-service-using-northrop-grummans-mission-extension-vehicle |url-status=live |archive-url=https://web.archive.org/web/20230812060505/https://news.northropgrumman.com/news/releases/intelsat-901-satellite-returns-to-service-using-northrop-grummans-mission-extension-vehicle |archive-date=August 12, 2023 |access-date=May 20, 2020 |website=Northrop Grumman Newsroom |publisher=[[Northrop Grumman]] }}</ref><ref name="spacecom-20200816">{{Cite news |last=Howell |first=Elizabeth |date=August 15, 2020 |title=Ariane 5 rocket launches robotic space tug into orbit alongside 2 communications satellites |url=https://www.space.com/ariane-5-rocket-mev-2-space-tug-satellite-launch-success.html |url-status=live |archive-url=https://web.archive.org/web/20230921115446/https://www.space.com/ariane-5-rocket-mev-2-space-tug-satellite-launch-success.html |archive-date=September 21, 2023 |access-date=August 20, 2020 |work=[[Space.com]] }}</ref>

==== Shijian-21 ====
In December 2021 - January 2022, China's [[Shijian#Shijian-21|Shijian-21]] space debris mitigation satellite has docked with the defunct Beidou-2 G2 navigation satellite to drastically alter its geostationary orbit, demonstrating capabilities only previously exhibited by the United States.<ref name="spacenews-20220127">{{Cite news |date=January 27, 2022 |title=China's Shijian-21 towed dead satellite to a high graveyard orbit |url=https://spacenews.com/chinas-shijian-21-spacecraft-docked-with-and-towed-a-dead-satellite/ |url-status=live |archive-url=https://archive.today/20220127233802/https://spacenews.com/chinas-shijian-21-spacecraft-docked-with-and-towed-a-dead-satellite/ |archive-date=January 27, 2022 }}</ref>

=== Smaller tugs and dispensers ===

==== SHERPA ====
{{main|SHERPA (space tug)}}
[[Spaceflight Inc.]] developed SHERPA, which builds upon the capabilities of the Spaceflight Secondary Payload System (SSPS) by incorporating propulsion and power generation subsystems, which creates a propulsive tug dedicated to maneuvering to an optimal orbit to place [[Secondary payload|secondary]] and [[hosted payload]]s. The maiden flight of two separate unpropelled variants of the dispenser was in December 2018 on a [[Falcon 9]] rocket. This flight deployed 64 small satellites from 17 countries.<ref name="cnn-20181204">{{Cite news |last=Wattles |first=Jackie |date=December 4, 2018 |title=SpaceX launched 64 satellites in record-breaking mission |url=https://edition.cnn.com/2018/12/03/tech/spacex-record-sso-a-mission/index.html |url-status=live |archive-url=https://web.archive.org/web/20230419042036/https://edition.cnn.com/2018/12/03/tech/spacex-record-sso-a-mission/index.html |archive-date=April 19, 2023 |work=[[CNN]] }}</ref><ref name="Sorensen">{{Cite press release |last=Sorensen |first=Jodi |date=August 6, 2018 |title=Spaceflight prepares historic launch of more than 70 spacecraft aboard SpaceX Falcon9 |url=http://spaceflight.com/spaceflight-prepares-historic-launch-of-more-than-70-spacecraft-aboard-spacex-falcon-9/ |url-status=dead |archive-url=https://web.archive.org/web/20180807002106/http://spaceflight.com/spaceflight-prepares-historic-launch-of-more-than-70-spacecraft-aboard-spacex-falcon-9/ |archive-date=August 7, 2018 |access-date=August 6, 2018 |publisher=[[Spaceflight Industries]] }}</ref>

==== ION Satellite Carrier ====
[[D-Orbit]], an Italian space logistics and transportation company, developed the InOrbit NOW [[ION Satellite Carrier]]. The first launch occurred on September 3, 2020, on a [[Vega (rocket)|Vega]] rocket, but subsequent launches have all been on SpaceX [[Falcon 9]] Transporter missions. On January 3, 2023, the company launched its seventh and eighth vehicles, Second star to the right, aboard the [[SpaceX]] Transporter-6 Mission.<ref name="dorbit-20220525">{{Cite press release |date=May 25, 2022 |title=D-Orbit Launches its Sixth ION Satellite Carrier Mission |url=https://www.globenewswire.com/en/news-release/2022/05/25/2450797/0/en/D-Orbit-Launches-its-Sixth-ION-Satellite-Carrier-Mission.html |url-status=live |archive-url=https://web.archive.org/web/20230423192750/https://www.globenewswire.com/en/news-release/2022/05/25/2450797/0/en/D-Orbit-Launches-its-Sixth-ION-Satellite-Carrier-Mission.html |archive-date=April 23, 2023 |access-date=August 19, 2022 |publisher=D-Orbit |place=Fino Mornasco, Italy |via=GlobeNewswire News Room }}</ref>

==== Long Duration Propulsive ESPA (LDPE) ====
{{main|EELV Secondary Payload Adapter#Long Duration Propulsive ESPA (LDPE)}}
LDPE is based on a [[Northrop Grumman]] payload adapter used to help attach the upper stage to the main satellite in addition to hosting a few slots for other [[smallsat]]s. However, the entire system is powered by the ESPAStar satellite bus, which is in charge of power consumption and distribution as well as propulsion making it a fully operational space tug capable of deploying different payloads at different orbits. ESPAStar has the capability to host 6 smallsat payloads totaling 1,920&nbsp;kg (4,230&nbsp;lb). The system is also able to provide 400 meters per second of delta-V via a Hydrazine propulsion module.<ref name="everyastro-ussf44">{{Cite web |last=Kordina |first=Florian |date=October 30, 2022 |title=USSF-44 - Falcon Heavy |url=https://everydayastronaut.com/ussf-44-falcon-heavy/ |url-status=live |archive-url=https://web.archive.org/web/20230605123526/https://everydayastronaut.com/ussf-44-falcon-heavy/ |archive-date=June 5, 2023 |access-date=October 30, 2022 |website=Everyday Astronaut }}</ref>

The first LDPE was launched on December 7, 2021, on an [[Atlas V]] rocket as part of the [[Space Test Program|STP-3]] mission. The second launch was on November 1, 2022, on a [[Falcon Heavy]] rocket as part of the [[United States Space Force|USSF-44]] mission. A third mission was on January 15, 2023, on [[United States Space Force|USSF-67]] mission.

==== Momentus Space ====
{{Main|Momentus space}}
''[[Momentus Space]]'' develops different space tug versions focusing on large velocity changes over 1&nbsp;km/s. Two demonstration missions of their [[Vigoride]] platform took place on May 25, 2022, and January 3, 2023<ref name="sfn3123">{{Cite news |last=Clark |first=Stephen |date=January 3, 2023 |title=Live coverage: SpaceX counting down to first launch of 2023 |url=https://spaceflightnow.com/2023/01/03/falcon-9-transporter-6-live-coverage/ |url-status=live |archive-url=https://web.archive.org/web/20230606215946/https://spaceflightnow.com/2023/01/03/falcon-9-transporter-6-live-coverage/ |archive-date=June 6, 2023 |work=Spaceflight Now }}</ref> with key tests occurring through 2022.<ref name="spacecom-20200910">{{Cite news |last=Wall |first=Mike |date=September 10, 2020 |title=Space tug to test out robotic arm on 2022 demonstration mission |url=https://www.space.com/momentus-space-tug-robotic-arm-demonstration-mission.html |url-status=live |archive-url=https://web.archive.org/web/20230419042036/https://www.space.com/momentus-space-tug-robotic-arm-demonstration-mission.html |archive-date=April 19, 2023 |access-date=November 27, 2020 |work=[[Space.com]] }}</ref> Momentus Space became widely known in October 2020 when it reached a SPAC investment deal with Stable Road Acquisition Corp valuing the combined entity at over $1 billion.<ref name="sec-momentus">{{Cite web |title=Momentus to Become Public Through Merger with Stable Road Acquisition Corp. |url=https://www.sec.gov/Archives/edgar/data/1781162/000121390020030403/ea127854ex99-1_stableroad.htm |url-status=live |archive-url=https://web.archive.org/web/20221221212629/https://www.sec.gov/Archives/edgar/data/1781162/000121390020030403/ea127854ex99-1_stableroad.htm |archive-date=December 21, 2022 |access-date=October 14, 2021 |website=[[U.S. Securities and Exchange Commission|SEC]] }}</ref>

==== EPIC AEROSPACE ====
EPIC AEROSPACE produces a family of orbit transfer vehicles called "Chimeras" for low-earth orbit, and geosynchronous-orbit applications.

The spacecraft count with delta-vs ranging from 250 m/s, to 900 m/s for LEO, and up to 1800 m/s for GTO to GEO applications. Nominal payload capacity is 200 kg.

[[Epic Aerospace]]'s [[Chimera LEO&nbsp;1]] launched on January 3, 2023.<ref name=sfn3123 />

==== Launcher ====
Reports surfaced circa June 15, 2021, of [[Launcher (company)|Launcher]]'s Orbiter space tug.<ref name="spacenews-20210615">{{Cite news |last=Foust |first=Jeff |date=June 15, 2021 |title=Launcher to develop orbital transfer vehicle |url=https://spacenews.com/launcher-to-develop-orbital-transfer-vehicle/ |url-status=live |archive-url=https://archive.today/20240109180256/https://spacenews.com/launcher-to-develop-orbital-transfer-vehicle/ |archive-date=January 9, 2024 |access-date=November 20, 2021 |work=[[SpaceNews]] }}</ref> Launching on its own rocket as well as SpaceX’s Falcon 9 it provides 150 kilograms of payload, either 90 units of [[CubeSat]] or else larger satellites using standard smallsat separation systems. With a chemical propulsion system using ethylene and nitrous oxide propellants it is capable of 500 meters per second of delta-v, more with additional propellant tanks.<ref name="launcherorbiter">{{Cite web |title=Launcher Orbiter |url=https://launcherspace.com/orbiter |url-status=dead |archive-url=https://web.archive.org/web/20211215012818/https://launcherspace.com/orbiter |archive-date=December 15, 2021 |access-date=November 20, 2021 }}</ref> Orbiter SN1 launched on January 3, 2023.<ref name=sfn3123 />

==== Impulse Space ====
[[Impulse Space]] successfully launched Mira, a {{convert|300|kg|sp=us|abbr=on|adj=on}} space tug, on [[SpaceX]]'s Transporter-9 mission in November 2023, deploying satellites and performing tests of its propulsion system. Future missions are planned for Transporter-11 and Transporter-12.<ref name="mira-launch">{{Cite web |last=Berger |first=Eric |date=November 13, 2023 |title=SpaceX founding employee successfully moves from rockets to in-space propulsion|url=https://arstechnica.com/space/2023/11/impulse-space-appears-to-succeed-with-its-first-spacecraft/ |access-date=January 19, 2024 |website=Ars Technica |language=en-US}}</ref><ref name="helios-unveil-and-mira-update">{{Cite web |last=Berger |first=Eric |date=January 17, 2024 |title=Meet Helios, a new class of space tug with some real muscle|url=https://arstechnica.com/space/2024/01/meet-helios-a-new-class-of-space-tug-with-some-real-muscle/ |access-date=January 19, 2024 |website=Ars Technica |language=en-US}}</ref>

== Early Concepts - NASA Space Transportation System ==
[[File:Space tug module for astronauts.jpg|thumb|Space Tug crew module concept]]
A reusable space tug was studied by [[NASA]] in the late 60s and early 70s as part of a reusable [[Space Transportation System]] (STS). This consisted of a basic propulsion module, to which a crew module or other payload could be attached. Optional legs could be added to land payloads on the surface of the [[Moon]].<ref name=anxtug/> This, along with all other elements of STS except the [[Space Shuttle]], was never funded after cutbacks to NASA's budget during the 1970s in the wake of the [[Apollo program]].<ref>{{cite web |title=The Space Shuttle Decision: NASA's Search for a Reusable Space Vehicle |url=https://history.nasa.gov/SP-4221/ch6.htm#266 |website=nasa.gov |access-date=July 25, 2014 |quote=Because a rising tide lifts all boats, NASA's flight rates during the 1960s had been buoyed powerfully by the agency's generous budgets. The OMB had no intention of granting such largesse during the 1970s.}}</ref>

== Space Shuttle era ==

=== Expendable upper stages ===
The Shuttle program filled the role of high-energy orbital transfer by the development{{when|date=January 2018}} of a [[solid rocket|solid-fueled]] single-stage [[Payload Assist Module]] and two-stage [[Inertial Upper Stage]].{{Citation needed|date=January 2018}}

A more powerful liquid hydrogen fueled [[Centaur (rocket stage)#Shuttle-Centaur (Centaur G and G-Prime)|Centaur-G]] stage was developed for use on the Shuttle, but was cancelled as too dangerous after the [[Challenger disaster]].<ref name="cleveland-20111211">{{Cite web |last=Mangels |first=John |date=December 11, 2011 |title=Long-forgotten Shuttle/Centaur boosted Cleveland's NASA center into manned space program and controversy |url=https://www.cleveland.com/science/2011/12/long-forgotten_shuttlecentaur.html |url-status=live |archive-url=https://web.archive.org/web/20230428201934/https://www.cleveland.com/science/2011/12/long-forgotten_shuttlecentaur.html |archive-date=April 28, 2023 |access-date=July 25, 2014 |website=Cleveland.com }}</ref>

=== Orbital Maneuvering Vehicle ===
{{Main|Orbital Maneuvering Vehicle}}
NASA studied another space tug design, termed the Orbital Maneuvering Vehicle (OMV), along with its plans for [[Space Station Freedom]]. The OMV's role would have been a reusable space vehicle that would retrieve satellites, such as [[Hubble Space Telescope|Hubble]], and bring them to Freedom for repair or retrieval, or to service uncrewed orbital platforms.<ref>{{cite web |url=http://www.spaceref.com/news/viewnews.html?id=1057 |title=NASA's New Launch Systems May Include the Return of the Space Tug |date=August 7, 2005 |access-date=July 25, 2014 |website=SpaceRef |archive-date=February 2, 2013 |archive-url=https://archive.today/20130202223346/http://www.spaceref.com/news/viewnews.html?id=1057 |url-status=dead}}</ref><ref name="wired-20131205">{{Cite magazine |last=Portree |first=David S. F. |date=December 2013 |title=Linking Space Station & Mars |url=https://www.wired.com/2013/12/linking-space-station-mars-the-imuse-strategy-1985/ |url-access=subscription |url-status=live |archive-url=https://web.archive.org/web/20230518191436/https://www.wired.com/2013/12/linking-space-station-mars-the-imuse-strategy-1985/ |archive-date=May 18, 2023 |access-date=July 25, 2014 |magazine=[[Wired (magazine)|WIRED]] }}</ref> In 1984, the Orbital Maneuvering Vehicle (OMV) preliminary design studies were initiated through a competitive award process with systems studies conducted by [[TRW Inc.|TRW]], [[Martin Marietta Aerospace]], and [[LTV Corporation]].<ref>[https://babel.hathitrust.org/cgi/pt?id=uc1.b4293006;view=1up;seq=254 Department of Defense appropriations for 1986], pt. 1, p. 242.</ref>

== Twenty-first century proposals ==

=== Parom ===
{{main|Parom}}
The Russian [[RKK Energia]] corporation proposed a space tug named [[Parom]] in 2005<ref name="russianspaceweb-parom">{{Cite web |last=Zak |first=Anatoly |date=February 9, 2010 |title=Parom orbital tug |url=https://www.russianspaceweb.com/parom.html |url-status=live |archive-url=https://web.archive.org/web/20231202121253/https://russianspaceweb.com/parom.html |archive-date=December 2, 2023 |access-date=July 26, 2014 |website=RussianSpaceWeb }}</ref> which could be used to ferry both the proposed [[Kliper]] crew vehicle or uncrewed cargo and fuel resupply modules to ISS.<ref name="coppinger-20051101">{{Cite web |last=Coppinger |first=Rob |date=November 1, 2005 |title=Lighter Kliper could make towed trip to ISS |url=https://www.flightglobal.com/lighter-kliper-could-make-towed-trip-to-iss/63482.article |url-status=live |archive-url=https://web.archive.org/web/20240109181312/https://www.flightglobal.com/lighter-kliper-could-make-towed-trip-to-iss/63482.article |archive-date=January 9, 2024 |access-date=July 26, 2014 |website=Flight Global }}</ref> Keeping the tug in space would have allowed for a less massive Kliper, enabling launch on a smaller booster than the original Kliper design.

=== VASIMR ===
The [[Vasimir#Use as a space tug or orbital transfer vehicle|VASIMR]] electric plasma rocket could be used to power a high-efficiency space tug, using only 9 tons of Argon propellant to make a round trip to the Moon, delivering 34 tons of cargo from [[Low Earth Orbit]] to low lunar orbit. {{as of|2014}}, [[Ad Astra Rocket Company]] had put forward a concept proposal to utilize the technology to make a space tug.<ref>{{cite web |title=VASMIR |url=http://www.adastrarocket.com/aarc/VASIMR |website=Ad Astra Rocket Company |access-date=July 24, 2014}}</ref>{{update after|2018|1|26}}

=== ISRO PAM-G ===
[[Indian Space Research Organisation]] has built an upper stage called PAM-G (Payload Assist Module for [[Geosynchronous Satellite Launch Vehicle|GSLV]]) capable of pushing payloads directly to [[Medium Earth orbit|MEO]] or [[Geostationary orbit|GEO]] orbits from [[low Earth orbit]]s.<ref>{{cite web |url=http://www.ciidefence.com/world-biz-Presentation/Day%201/Session%20I/SomnathProjectDirector.pdf |last1=Somanath |first1=S |access-date=July 8, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20130903010232/http://www.ciidefence.com/world-biz-Presentation/Day%201/Session%20I/SomnathProjectDirector.pdf |archive-date=September 3, 2013 |title=ISRO's Current Launch Capabilities & Commercial Opportunities}}</ref><ref name="hindu-20141001">{{Cite news |last=Raj |first=N. Gopal |date=October 1, 2014 |title=Upgrading Indian rockets for future Mars missions |url=https://www.thehindu.com/sci-tech/science/Upgrading-Indian-rockets-for-future-Mars-missions/article60390522.ece |url-status=live |archive-url=https://web.archive.org/web/20230428054616/https://www.thehindu.com/sci-tech/science/Upgrading-Indian-rockets-for-future-Mars-missions/article60390522.ece |archive-date=April 28, 2023 |access-date=March 17, 2015 |work=[[The Hindu]] |publisher=Thehindu.com }}</ref> PAM-G is powered by [[Hypergolic propellant|hypergolic]] liquid motor with restart capability, derived from [[Polar Satellite Launch Vehicle|PSLV]]'s fourth stage. As of 2013, ISRO has realized the structure, control systems, and motors of PAM-G and has conducted hot tests.<ref>{{cite web |url=http://www.isro.org/pdf/Annual%20Report%202012-13.pdf |access-date=July 8, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140225085353/http://www.isro.org/pdf/Annual%20Report%202012-13.pdf |archive-date=February 25, 2014 |title=Annual Report}}</ref><ref>{{cite web |title=Outcome Budget 2010-2011 |url=http://www.isro.org/pdf/Outcome%20Budget%202010-2011.pdf |access-date=July 8, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20111013202144/http://isro.org/pdf/Outcome%20Budget%202010-2011.pdf |archive-date=October 13, 2011}}</ref><ref>{{cite web |url=http://www.isro.org/pdf/OutcomeBudget2009-2010.pdf |access-date=July 8, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20101123012524/http://www.isro.org/pdf/OutcomeBudget2009-2010.pdf |archive-date=November 23, 2010 |title=Outcome Budget of the Department of Space Government of India 2009-2010}}</ref> PAM-G would form the fourth stage of [[Geosynchronous Satellite Launch Vehicle|GSLV]] Mk2C launch vehicle,<ref>{{cite web |url=http://space.skyrocket.de/doc_lau/gslv.htm |title=GSLV |publisher=Space.skyrocket.de |access-date=March 17, 2015}}</ref> sitting on top of GSLV's [[CE-7.5|cryogenic third stage]].

=== Jupiter ===
{{main|Jupiter (spacecraft)}}
Lockheed Martin made a concept proposal to [[NASA]] in 2015 for a design called the ''[[Jupiter (spacecraft)|Jupiter]]'' space tug, to be based on the designs of two earlier Lockheed Martin spacecraft—[[MAVEN|Mars Atmosphere and Volatile Evolution Mission]] and the [[Juno (spacecraft)|Juno]]—as well as a [[robotic arm]] from [[MacDonald, Dettwiler and Associates|MDA]] derived from technology used on [[Canadarm]], the robotic arm technology previously used on the [[Space Shuttle]]. In addition to the ''Jupiter'' space tug itself, the Lockheed concept included the use of a new {{convert|4.4|m|sp=us|abbr=on|adj=on}}-diameter cargo transport module called ''[[Exoliner]]'' for carrying cargo to the ISS. ''Exoliner'' is based on the earlier (2000s) [[ESA]]-developed [[Automated Transfer Vehicle]], and was to be jointly developed with [[Thales Alenia Space]].<ref name=aw20150312>{{cite news |url=http://aviationweek.com/space/jupiter-space-tug-could-deliver-cargo-moon |title='Jupiter' Space Tug Could Deliver Cargo To The Moon |date=March 12, 2015 |access-date=March 17, 2015}}</ref><ref name=sn20150313>{{cite news |url=http://spacenews.com/lockheed-martin-pitches-reusable-tug-for-space-station-resupply/ |title=Lockheed Martin Pitches Reusable Tug for Space Station Resupply |date=March 13, 2015 |author=Jeff Foust |publisher=Space News}}</ref><ref name=dbj20150312>{{cite news |last1=Avery |first1=Greg |title=Lockheed Martin proposes building ISS cargo ship for NASA |url=http://www.bizjournals.com/denver/blog/boosters_bits/2015/03/lockheed-martin-proposes-building-iss-cargo-ship.html?page=all |access-date=March 13, 2015 |work=Denver Business Journal |date=March 12, 2015}}</ref> In the event, NASA did not agree to fund the Jupiter development, and Lockheed Martin is not developing the tug with private capital.

=== Artemis Transfer Stages ===
{{Main|Artemis program}}
One of [[NASA|NASA's]] [[Artemis program|Artemis Program's]] proposed [[Artemis program#Human Landing System (HLS)|lunar landers]] is a partially reusable three stage design. One of its main elements is a transfer stage to move the lander from the [[Lunar Gateway|Lunar Gateway's]] orbit to a low lunar orbit. Future versions should be able to return to the Gateway for refueling and reuse with another lander. [[Northrop Grumman]] has proposed building this transfer stage based on its [[Cygnus (spacecraft)|Cygnus spacecraft]]. NASA chose to select a different approach in April 2021.<ref>{{cite web |title=Human Landing System, Option A Source Selection Statement |url=https://www.nasa.gov/sites/default/files/atoms/files/option-a-source-selection-statement-final.pdf |website=NASA |access-date=May 12, 2021}}</ref>

=== Moon Cruiser ===
Designed by [[Airbus]], the Moon Cruiser is a conceptual lunar logistics vehicle based on the [[Automated Transfer Vehicle|ATV]] and [[European Service Module|ESM]] that is proposed to be used to support the international [[Lunar Gateway]]. If funded, it would make up a part of ESA's contribution to the Lunar Gateway program. As of January 2020, it was in the early design process. Planned to be launched on the [[Ariane 6]]—with the capability to also be launched with US heavy launchers<ref name=airbus201909/>{{rp|1:56}}—the vehicle is intended to be able to refuel lunar landers and deliver cargo to the Gateway. It will also be used{{citation needed|date=May 2020}} to deliver the European [[European System Providing Refueling, Infrastructure and Telecommunications|ESPRIT]] module to the Gateway no earlier than 2025. It has also been proposed to turn the vehicle into a transfer stage for a lunar lander. Concepts for a lander variant of the vehicle exist but have not received funding.<ref>{{Cite web |url=https://www.aerospatium.info/airbus-propose-un-remorqueur-translunaire/ |title=Airbus propose un remorqueur translunaire |last=Barensky |first=Stefan |date=July 24, 2019 |website=Aerospatium |language=fr-FR |access-date=January 10, 2020}}</ref><ref name=airbuspr20200110>{{Cite web |url=https://www.airbus.com/newsroom/news/en/2019/07/fly-me-to-the-moon-with-airbus.html |title=Fly me to the Moon… with Airbus |website=Airbus |language=en |access-date=January 10, 2020}}</ref><ref name=airbus201909>[https://www.youtube.com/watch?v=_HmAcDPo4HI Airbus Moon Cruiser Concept], Airbus video, via YouTube, September 2019, accessed May 20, 2020.</ref>

=== Skyrora Space Tug ===
British launch vehicle manufacturer [[Skyrora]] shared details of their Space Tug<ref>{{Citation |title=Skyrora Space Tug |date=May 7, 2021 |url=https://www.skyrora.com/post/skyrora-s-space-tug-gives-space-sustainability-dream-a-lift}}</ref> in 2021, revealing it to be usable as the third stage of their Skyrora XL rocket. The company shared a video of the Space Tug undergoing a live test in January 2021. As well as being able to move a satellite from one orbit to another the Space Tug can perform a number of in-space operations including space debris removal.

=== Exotrail SpaceVan Orbital Transfer Vehicle ===
[[:fr:Exotrail|Exotrail]] unveils the April 12, 2022, of Orbital Transfer Vehicle, SpaceVan.<ref>{{Citation |title=Exotrail Unveils Orbital Transfer Vehicle, SpaceVan |date=April 12, 2022 |url=https://www.satellitetoday.com/in-space-services/2022/04/12/exotrail-unveils-orbital-transfer-vehicle-spacevan/ |access-date=June 26, 2022}}</ref> The debut SpaceVan mission will launch on board a Falcon 9 rideshare mission in October 2023 following a launch service agreement signed between Exotrail and SpaceX. At least three subsequent missions are planned throughout 2024 onboard multiple different launchers.<ref>{{cite web |title=SpaceVan - Fast and flexible constellation deployment services |url=https://exotrail.com/transportation-services/ |access-date=June 26, 2022}}</ref><ref>{{cite web |url=https://spacenews.com/exotrail-wins-contract-to-demonstrate-orbital-transfer-for-french-agencies/ |title=ExoTrail wins contract to demonstrate orbital transfer for French agencies |date=October 11, 2022}}</ref>

=== Impulse Space Helios ===
In addition to their currently flying Mira vehicle, Impulse Space is developing a far larger vehicle called Helios designed to carry {{convert|4000|kg|sp=us|abbr=on|adj=on}} to {{convert|5000|kg|sp=us|abbr=on|adj=on}} payloads directly to geosynchronous orbit. A first launch is planned for 2026.<ref name="helios-unveil-and-mira-update" />

=== Atomos Space ===
In January 2022, [[Atomos Space]] announced it had raised $5 million it had been trying to raise since 2020. Atomos plans to launch two of its Quark reusable orbital transfer vehicle in 2023.<ref>{{cite web |url=https://spacenews.com/atomos-space-secures-funding-to-develop-space-tug-business/ |title=Atomos Space secures funding to develop space tug business |date=January 12, 2022}}</ref>

=== Firefly Aerospace ===
[[Firefly Aerospace#In Space Transportation|Firefly Aerospace]] is developing an OTV called the Elytra that will fly on its Alpha rocket in 2024.<ref>{{cite web |url=https://fireflyspace.com/elytra/ |title=Elytra }}</ref>

=== Space Machine's Optimus ===
In October 2022 [[Space Machines]] announced a deal with [[Arianespace]] to produce [[Optimus-1]] a 270&nbsp;kg space tug aiming to launch on SpaceX [[Falcon 9]] in Q2 2023.<ref>{{cite web |url=https://spacenews.com/arianespace-partners-with-australian-space-tug-startup/ |title=Arianespace partners with Australian space tug startup |date=October 31, 2022}}</ref>

=== Exolaunch's Reliant tugs ===
[[Exolaunch]] Reliant tugs have standard and pro versions. Testing and flight qualification was planned to begin in 2022 on SpaceX's rideshare missions.<ref>{{cite web |url=https://exolaunch.com/news_54 |title=Exolaunch Introduces Eco Space Tug Program |date=April 12, 2021 |access-date=January 13, 2023}}</ref><ref>{{cite web |url=https://www.exolaunch.com/reliant#1 |title=Reliant |access-date=January 13, 2023}}</ref>

=== Astroscale's Lexi ===
[[Astroscale]] is developing [[Life Extension In-orbit]] (LEXI).<ref>{{cite web |url=https://astroscale-us.com/lexi-life-extension-capabilities/ |title=Key Capabilities of our Life Extension In-orbit (LEXI™) Servicer |date=October 5, 2021 |access-date=January 14, 2023}}</ref><ref>{{cite web |url=https://spacenews.com/space-tugs-as-a-service-in-orbit-service-providers-are-bracing-for-consolidation/ |title=Space Tugs as a Service: In-orbit service providers are bracing for consolidation |publisher=SpaceNews.com |date=July 6, 2021}}</ref>

=== Orbit Fab ===
[[Orbit Fab|Orbit fab]] is attempting to develop an in-space propellant supply chain aiming to provide 'Gas Stations in Space'.<ref>{{cite web |url=https://www.orbitfab.com/ |title=Gas Stations in Space™ |access-date=January 14, 2023}}</ref> On January 11, 2022, it was announced they had reached an agreement to refuel Astroscale's LEXI.<ref>{{cite web |url=https://aviationweek.com/aerospace/commercial-space/space-tug-operator-astroscale-prebuys-fuel-fill-orbit-fab |title=Space Tug Operator Astroscale Prebuys Fuel Fill-Up From Orbit Fab |date=January 11, 2022}}</ref>

=== ULA Common Centaur as a SpaceTug ===
The Flexible Lunar Architecture for Exploration (FLARE) is a concept to deliver four crew to the lunar surface for a minimum of seven days and then return them safely to Earth. A key component of FLARE is the modified ULA Common Centaur used as a SpaceTug to deliver an uncrewed human lander to lunar orbit and to assist NASA's Orion capsule returning crew to Earth <ref>{{cite journal |url=/ |title=A Flexible Lunar Architecture for Exploration (FLARE) supporting NASA's Artemis Program |journal=Acta Astronautica |date=December 1, 2020|doi=10.1016/j.actaastro.2020.07.032 |last1=Evans |first1=Michael E. |last2=Graham |first2=Lee D. |volume=177 |pages=351–372 |bibcode=2020AcAau.177..351E |pmc=7385728 }}</ref>

== See also ==
{{Commons category|Space tug|Space Tug}}
* [[Satellite dispenser]]
* [[Service module]]
* [[Multistage rocket]]
** [[Three-stage-to-orbit]]
** [[Two-stage-to-orbit]]
** [[Single-stage-to-orbit]]
* [[Apogee kick motor]]

== Other sources ==
* [https://web.archive.org/web/20060104192245/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810065609_1981065609.pdf NASA Report, Technical Study for the Use of the Saturn 5, INT-21 and Other Saturn 5 Derivatives to Determine an Optimum Fourth Stage (space tug). Volume 1: Technical Volume, Book 1.]

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

== Bibliography ==
*{{cite encyclopedia |last=Wade |first=Mark |title=Space Tug |encyclopedia=Encyclopedia Astronautica |url=http://www.astronautix.com/craft/spacetug.htm |archive-url=https://web.archive.org/web/20020702065612/http://www.astronautix.com/craft/spacetug.htm |url-status=dead |archive-date=July 2, 2002 |access-date=June 15, 2011}}

== External links ==
* [http://libarchstor.uah.edu:8081/repositories/2/resources/61 Bill Huber Collection, The University of Alabama in Huntsville Archives and Special Collections] Files of Bill Huber, project manager on the Orbital Maneuvering Vehicle.

[[Category:Spacecraft propulsion]]
[[Category:Spacecraft]]
[[Category:Space tugs| ]]

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