A space tug is a type of spacecraft used to transfer 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 lunar transfer, or an escape trajectory.

Reusable, modular 1969 NASA vision Space Tug (canceled)
The Jupiter bound Galileo spacecraft and its attached Inertial Upper Stage (IUS) being deployed after being launched by the 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.

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[1] or the proposed Russian Parom, and has sometimes been used to refer to expendable upper stages,[1] such as Fregat,[2] Spaceflight Industries Sherpa, and the Inertial Upper Stage, when such stages are optional.

Background edit

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 novel published in 1953 as the sequel to Space Platform, another novel about such a space station.[3]

Existing space tugs edit

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 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 edit

Mission Extension Vehicle edit

In 2011 ViviSat a joint project between U.S. Space and ATK proposed the Mission Extension Vehicle. In 2016 ViviSat was dissolved when U.S. Space declared bankruptcy and ATK merged with Orbital Science Corporation to form Orbital ATK. In 2017 Orbital ATK got the go ahead from the 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 30 on an Ariane 5 to perform a similar maneuver with Intelsat-1002.[4][5][6]

Shijian-21 edit

In December 2021 - January 2022, China's 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.[7]

Smaller tugs and dispensers edit


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 and hosted payloads. 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.[8][9]

ION Satellite Carrier edit

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, 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.[10]

Long Duration Propulsive ESPA (LDPE) edit

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 smallsats. 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 kg (4,230 lb). The system is also able to provide 400 meters per second of delta-V via a Hydrazine propulsion module.[11]

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

Momentus Space edit

Momentus Space develops different space tug versions focusing on large velocity changes over 1 km/s. Two demonstration missions of their Vigoride platform took place on May 25, 2022, and January 3, 2023[12] with key tests occurring through 2022.[13] 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.[14]

Epic Aerospace edit

Epic Aerospace's Chimera LEO 1 launched on January 3, 2023.[12]

Launcher edit

Reports surfaced circa June 15, 2021, of Launcher's Orbiter space tug.[15] 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.[16] Orbiter SN1 launched on January 3, 2023.[12]

Impulse Space edit

Impulse Space successfully launched Mira, a 300 kg (660 lb) 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.[17][18]

Early Concepts - NASA Space Transportation System edit

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.[1] 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.[19]

Space Shuttle era edit

Expendable upper stages edit

The Shuttle program filled the role of high-energy orbital transfer by the development[when?] of a solid-fueled single-stage Payload Assist Module and two-stage Inertial Upper Stage.[citation needed]

A more powerful liquid hydrogen fueled Centaur-G stage was developed for use on the Shuttle, but was cancelled as too dangerous after the Challenger disaster.[20]

Orbital Maneuvering Vehicle edit

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, and bring them to Freedom for repair or retrieval, or to service uncrewed orbital platforms.[21][22] In 1984, the Orbital Maneuvering Vehicle (OMV) preliminary design studies were initiated through a competitive award process with systems studies conducted by TRW, Martin Marietta Aerospace, and LTV Corporation.[23]

Twenty-first century proposals edit

Parom edit

The Russian RKK Energia corporation proposed a space tug named Parom in 2005[24] which could be used to ferry both the proposed Kliper crew vehicle or uncrewed cargo and fuel resupply modules to ISS.[25] Keeping the tug in space would have allowed for a less massive Kliper, enabling launch on a smaller booster than the original Kliper design.


The 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.[26][needs update]


Indian Space Research Organisation has built an upper stage called PAM-G (Payload Assist Module for GSLV) capable of pushing payloads directly to MEO or GEO orbits from low Earth orbits.[27][28] PAM-G is powered by hypergolic liquid motor with restart capability, derived from PSLV's fourth stage. As of 2013, ISRO has realized the structure, control systems, and motors of PAM-G and has conducted hot tests.[29][30][31] PAM-G would form the fourth stage of GSLV Mk2C launch vehicle,[32] sitting on top of GSLV's cryogenic third stage.

Jupiter edit

Lockheed Martin made a concept proposal to NASA in 2015 for a design called the Jupiter space tug, to be based on the designs of two earlier Lockheed Martin spacecraft—Mars Atmosphere and Volatile Evolution Mission and the Juno—as well as a robotic arm from 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 4.4 m (14 ft)-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.[33][34][35] 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 edit

One of NASA's Artemis Program's proposed 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'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. NASA chose to select a different approach in April 2021.[36]

Moon Cruiser edit

Designed by Airbus, the Moon Cruiser is a conceptual lunar logistics vehicle based on the ATV and 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[37]: 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] to deliver the European 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.[38][39][37]

Skyrora Space Tug edit

British launch vehicle manufacturer Skyrora shared details of their Space Tug[40] 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 edit

Exotrail unveils the April 12, 2022, of Orbital Transfer Vehicle, SpaceVan.[41] 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.[42][43]

Impulse Space Helios edit

In addition to their currently flying Mira vehicle, Impulse Space is developing a far larger vehicle called Helios designed to carry 4,000 kg (8,800 lb) to 5,000 kg (11,000 lb) payloads directly to geosynchronous orbit. A first launch is planned for 2026.[18]

Atomos Space edit

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.[44]

Firefly Aerospace edit

Firefly Aerospace is developing an OTV called the Elytra that will fly on its Alpha rocket in 2024.[45]

Space Machine's Optimus edit

In October 2022 Space Machines announced a deal with Arianespace to produce Optimus-1 a 270 kg space tug aiming to launch on SpaceX Falcon 9 in Q2 2023.[46]

Exolaunch's Reliant tugs edit

Exolaunch Reliant tugs have standard and pro versions. Testing and flight qualification was planned to begin in 2022 on SpaceX's rideshare missions.[47][48]

Astroscale's Lexi edit

Astroscale is developing Life Extension In-orbit (LEXI).[49][50]

Orbit Fab edit

Orbit fab is attempting to develop an in-space propellant supply chain aiming to provide 'Gas Stations in Space'.[51] On January 11, 2022, it was announced they had reached an agreement to refuel Astroscale's LEXI.[52]

ULA Common Centaur as a SpaceTug edit

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 [53]

See also edit

Other sources edit

References edit

  1. ^ a b c "Space Tug". Astronautix. Archived from the original on October 11, 2011. Retrieved July 25, 2014.
  2. ^ Zak, Anatoly. Chabot, Alain (ed.). "Fregat space tug". RussianSpaceWeb.com. Archived from the original on January 9, 2024. Retrieved July 25, 2014.
  3. ^ Leinster, Murray (1953). Space Tug. Shasta Publishers. OCLC 6570191.
  4. ^ Henry, Caleb (April 17, 2020). "Intelsat-901 satellite, with MEV-1 servicer attached, resumes service". SpaceNews.com. Archived from the original on January 9, 2024. Retrieved May 20, 2020.
  5. ^ Cox, Vicki; Macdonald, Meghan (April 17, 2020). "Intelsat 901 Satellite Returns to Service Using Northrop Grumman's Mission Extension Vehicle". Northrop Grumman Newsroom (Press release). Northrop Grumman. Archived from the original on August 12, 2023. Retrieved May 20, 2020.
  6. ^ Howell, Elizabeth (August 15, 2020). "Ariane 5 rocket launches robotic space tug into orbit alongside 2 communications satellites". Space.com. Archived from the original on September 21, 2023. Retrieved August 20, 2020.
  7. ^ "China's Shijian-21 towed dead satellite to a high graveyard orbit". January 27, 2022. Archived from the original on January 27, 2022.
  8. ^ Wattles, Jackie (December 4, 2018). "SpaceX launched 64 satellites in record-breaking mission". CNN. Archived from the original on April 19, 2023.
  9. ^ Sorensen, Jodi (August 6, 2018). "Spaceflight prepares historic launch of more than 70 spacecraft aboard SpaceX Falcon9" (Press release). Spaceflight Industries. Archived from the original on August 7, 2018. Retrieved August 6, 2018.
  10. ^ "D-Orbit Launches its Sixth ION Satellite Carrier Mission" (Press release). Fino Mornasco, Italy: D-Orbit. May 25, 2022. Archived from the original on April 23, 2023. Retrieved August 19, 2022 – via GlobeNewswire News Room.
  11. ^ Kordina, Florian (October 30, 2022). "USSF-44 - Falcon Heavy". Everyday Astronaut. Archived from the original on June 5, 2023. Retrieved October 30, 2022.
  12. ^ a b c Clark, Stephen (January 3, 2023). "Live coverage: SpaceX counting down to first launch of 2023". Spaceflight Now. Archived from the original on June 6, 2023.
  13. ^ Wall, Mike (September 10, 2020). "Space tug to test out robotic arm on 2022 demonstration mission". Space.com. Archived from the original on April 19, 2023. Retrieved November 27, 2020.
  14. ^ "Momentus to Become Public Through Merger with Stable Road Acquisition Corp". SEC. Archived from the original on December 21, 2022. Retrieved October 14, 2021.
  15. ^ Foust, Jeff (June 15, 2021). "Launcher to develop orbital transfer vehicle". SpaceNews. Archived from the original on January 9, 2024. Retrieved November 20, 2021.
  16. ^ "Launcher Orbiter". Archived from the original on December 15, 2021. Retrieved November 20, 2021.
  17. ^ Berger, Eric (November 13, 2023). "SpaceX founding employee successfully moves from rockets to in-space propulsion". Ars Technica. Retrieved January 19, 2024.
  18. ^ a b Berger, Eric (January 17, 2024). "Meet Helios, a new class of space tug with some real muscle". Ars Technica. Retrieved January 19, 2024.
  19. ^ "The Space Shuttle Decision: NASA's Search for a Reusable Space Vehicle". nasa.gov. Retrieved July 25, 2014. 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.
  20. ^ Mangels, John (December 11, 2011). "Long-forgotten Shuttle/Centaur boosted Cleveland's NASA center into manned space program and controversy". Cleveland.com. Archived from the original on April 28, 2023. Retrieved July 25, 2014.
  21. ^ "NASA's New Launch Systems May Include the Return of the Space Tug". SpaceRef. August 7, 2005. Archived from the original on February 2, 2013. Retrieved July 25, 2014.
  22. ^ Portree, David S. F. (December 2013). "Linking Space Station & Mars". WIRED. Archived from the original on May 18, 2023. Retrieved July 25, 2014.
  23. ^ Department of Defense appropriations for 1986, pt. 1, p. 242.
  24. ^ Zak, Anatoly (February 9, 2010). "Parom orbital tug". RussianSpaceWeb. Archived from the original on December 2, 2023. Retrieved July 26, 2014.
  25. ^ Coppinger, Rob (November 1, 2005). "Lighter Kliper could make towed trip to ISS". Flight Global. Archived from the original on January 9, 2024. Retrieved July 26, 2014.
  26. ^ "VASMIR". Ad Astra Rocket Company. Retrieved July 24, 2014.
  27. ^ Somanath, S. "ISRO's Current Launch Capabilities & Commercial Opportunities" (PDF). Archived from the original (PDF) on September 3, 2013. Retrieved July 8, 2014.
  28. ^ Raj, N. Gopal (October 1, 2014). "Upgrading Indian rockets for future Mars missions". The Hindu. Thehindu.com. Archived from the original on April 28, 2023. Retrieved March 17, 2015.
  29. ^ "Annual Report" (PDF). Archived from the original (PDF) on February 25, 2014. Retrieved July 8, 2014.
  30. ^ "Outcome Budget 2010-2011" (PDF). Archived from the original (PDF) on October 13, 2011. Retrieved July 8, 2014.
  31. ^ "Outcome Budget of the Department of Space Government of India 2009-2010" (PDF). Archived from the original (PDF) on November 23, 2010. Retrieved July 8, 2014.
  32. ^ "GSLV". Space.skyrocket.de. Retrieved March 17, 2015.
  33. ^ "'Jupiter' Space Tug Could Deliver Cargo To The Moon". March 12, 2015. Retrieved March 17, 2015.
  34. ^ Jeff Foust (March 13, 2015). "Lockheed Martin Pitches Reusable Tug for Space Station Resupply". Space News.
  35. ^ Avery, Greg (March 12, 2015). "Lockheed Martin proposes building ISS cargo ship for NASA". Denver Business Journal. Retrieved March 13, 2015.
  36. ^ "Human Landing System, Option A Source Selection Statement" (PDF). NASA. Retrieved May 12, 2021.
  37. ^ a b Airbus Moon Cruiser Concept, Airbus video, via YouTube, September 2019, accessed May 20, 2020.
  38. ^ Barensky, Stefan (July 24, 2019). "Airbus propose un remorqueur translunaire". Aerospatium (in French). Retrieved January 10, 2020.
  39. ^ "Fly me to the Moon… with Airbus". Airbus. Retrieved January 10, 2020.
  40. ^ Skyrora Space Tug, May 7, 2021
  41. ^ Exotrail Unveils Orbital Transfer Vehicle, SpaceVan, April 12, 2022, retrieved June 26, 2022
  42. ^ "SpaceVan - Fast and flexible constellation deployment services". Retrieved June 26, 2022.
  43. ^ "ExoTrail wins contract to demonstrate orbital transfer for French agencies". October 11, 2022.
  44. ^ "Atomos Space secures funding to develop space tug business". January 12, 2022.
  45. ^ "Elytra".
  46. ^ "Arianespace partners with Australian space tug startup". October 31, 2022.
  47. ^ "Exolaunch Introduces Eco Space Tug Program". April 12, 2021. Retrieved January 13, 2023.
  48. ^ "Reliant". Retrieved January 13, 2023.
  49. ^ "Key Capabilities of our Life Extension In-orbit (LEXI™) Servicer". October 5, 2021. Retrieved January 14, 2023.
  50. ^ "Space Tugs as a Service: In-orbit service providers are bracing for consolidation". SpaceNews.com. July 6, 2021.
  51. ^ "Gas Stations in Space™". Retrieved January 14, 2023.
  52. ^ "Space Tug Operator Astroscale Prebuys Fuel Fill-Up From Orbit Fab". January 11, 2022.
  53. ^ Evans, Michael E.; Graham, Lee D. (December 1, 2020). [/ "A Flexible Lunar Architecture for Exploration (FLARE) supporting NASA's Artemis Program"]. Acta Astronautica. 177: 351–372. Bibcode:2020AcAau.177..351E. doi:10.1016/j.actaastro.2020.07.032. PMC 7385728. {{cite journal}}: Check |url= value (help)

Bibliography edit

  • Wade, Mark. "Space Tug". Encyclopedia Astronautica. Archived from the original on July 2, 2002. Retrieved June 15, 2011.

External links edit