Space tug
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 to a higher-energy orbit like a geostationary transfer orbit, a 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 or the proposed Russian Parom, and has sometimes been used to refer to expendable upper stages, such as Fregat, Spaceflight Industries 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 novel published in 1953 as the sequel to Space Platform, another novel about such a space station.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 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
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.Shijian-21
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.Smaller tugs and dispensers
SHERPA
developed SHERPA, which builds upon the capabilities of the Spaceflight Secondary Payload System 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.ION Satellite Carrier
, 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.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 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. The system is also able to provide 400 meters per second of delta-V via a Hydrazine propulsion module.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.
Vigoride by Momentus Space
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 with key tests occurring through 2022. 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.Epic Aerospace
's Chimera LEO 1 launched on January 3, 2023.Launcher
Reports surfaced circa June 15, 2021, of Launcher's Orbiter space tug. 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. Orbiter SN1 launched on January 3, 2023.After Launcher was acquired by VAST, the Orbiter program was cancelled.
Impulse Space
successfully launched Mira, a 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.Early Concepts - NASA Space Transportation System
A reusable space tug was studied by NASA in the late 60s and early 70s as part of a reusable Space Transportation System. 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. 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.Space Shuttle era
Expendable upper stages
The Shuttle program filled the role of high-energy orbital transfer by the development of a solid-fueled single-stage Payload Assist Module and two-stage Inertial Upper Stage.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.