A number of space tethers have been deployed in space missions. Tether satellites can be used for various purposes including research into tether propulsion, tidal stabilisation and orbital plasma dynamics.

The missions have met with varying degrees of success; a few have been highly successful.

Description Tethered satellites are composed of three parts: the base-satellite; tether; and sub-satellite. The base-satellite contains the sub-satellite and tether until deployment. Sometimes the base-satellite is another basic satellite, other times it could be a spacecraft, space station, or the Moon. The tether is what keeps the two satellites connected. The sub-satellite is released from the base assisted by a spring ejection system, centrifugal force or gravity gradient effects.

Tethers can be deployed for a range of applications, including electrodynamic propulsion, momentum exchange, artificial gravity, deployment of sensors or antennas etc. Tether deployment may be followed by a station-keeping phase (in particular if the target state is a vertical system orientation), and, sometimes, if the deployment system allows, a retraction.

The station-keeping phase and retraction phase need active control for stability, especially when atmospheric effects are taken into account. When there are no simplifying assumptions, the dynamics become overly difficult because they are then governed by a set of ordinary and partial nonlinear, non-autonomous and coupled differential equations. These conditions create a list of dynamical issues to consider:NASA, Tethers In Space Handbook, edited by M.L. Cosmo and E.C. Lorenzini, Third Edition December 1997 (accessed 20 October 2010); see also version at NASA MSFC ; available on scribd

Three-dimensional rigid body dynamics (librational motion) of the station and subsatellite Swinging in-plane and out-of-plane motions of the tether of finite mass Offset of the tether attachment point from the base-satellite center of mass as well as controlled variations of the offset Transverse vibrations of the tether External forces

Tether flights on human space missions

Gemini 11

In 1966, Gemini 11 deployed a tether which was stabilized by a rotation which gave 0.00015 g.

Shuttle TSS missions

TSS-1 mission

Tethered Satellite System-1 (TSS-1) was proposed by NASA and the Italian Space Agency (ASI) in the early 1970s by Mario Grossi, of the Smithsonian Astrophysical Observatory, and Giuseppe Colombo, of Padua University. It was a joint NASA-Italian Space Agency project, was flown in 1992, during STS-46 aboard the Space Shuttle Atlantis from 31 July to 8 August.

The purposes of the TSS-1 mission were to verify the tether concept of gravity gradient stabilization, and to provide a research facility for investigating space physics and plasma electrodynamics. This mission uncovered several aspects about the dynamics of the tethered system, although the satellite did not fully deploy. It stuck at 78 meters; after that snag was resolved its deployment continued to a length of before sticking again, where the effort finally endedHeart Breaker, Air & Space/Smithsonian, June/July 1996, pp. 18-23 (the total proposed length was ). A protruding boltNASA Science Missions page TSS Tethered Satellite System (accessed 10 October 2010) due to a late-stage modification of the deployment reel system, jammed the deployment mechanism and prevented deployment to full extension. Despite this issue, the results showed that the basic concept of long gravity-gradient stabilized tethers was sound. It also settled several short deployment dynamics issues, reduced safety concerns, and clearly demonstrated the feasibility of deploying the satellite to long distances.

The voltage and current reached using the short tether length were too low for most of the experiments to be run. However, low-voltage measurements were made, along with recording the variations of tether-induced forces and currents. New information was gathered on the "return-tether" current. The mission was reflown in 1996 as TSS-1R.The Space Tether Experiment

TSS-1R mission Four years later, as a follow-up mission to TSS-1, the TSS-1R satellite was released in latter February 1996 from the Space Shuttle Columbia on the STS-75 mission.Stone, N.H., and Bonifazi, C. (February 15, 1998). "The TSS-1R mission: Overview and scientific context", Geophysical. Res. Lett., 25, No. 4, 409-412. (Retrieved 9 Sept. 2026). The TSS-1R mission objective was to deploy the tether above the orbiter and remain there collecting data. The TSS-1R mission was to conduct exploratory experiments in space plasma physics. Projections indicated that the motion of the long conducting tether through the Earth's magnetic field would produce an EMF that would drive a current through the tether system.

TSS-1R was deployed (over a period of five hours) to when the tether broke.Ben Evans, "The Second Flight of the Tethered Satellite": 'Rock Solid' (Part 1) and 'The Tether Is Broken' (Part 2), AmericaSpace, Feb. 2014 (retrieved 8 June 2016). The break was attributed to an electrical discharge through a broken place in the insulation.Szalai, Kenneth J., et al. (May 31, 1996). "TSS-1R Mission Failure Investigation Board", NASA Technical Memorandum TM-112426. (Retrieved 9 Sept. 2026).

Despite the termination of the tether deployment before full extension, the extension achieved was long enough to verify numerous scientific speculations. These findings included the measurements of the motional EMF, the satellite potential, the orbiter potential, the current in the tether, the changing resistance in the tether, the charged particle distributions around a highly charged spherical satellite, and the ambient electric field. In addition, a significant finding concerns the current collection at different potentials on a spherical endmass. Measured currents on the tether far exceeded predictions of previous numerical models by up to a factor of three. A more descriptive explanation of these results can be found in Thompson, et al. Improvements have been made in modeling the electron charging of the shuttle and how it affects current collection, and in the interaction of bodies with surrounding plasma, as well as the production of electrical power.

A second mission, TSS-2, had been proposed to use the tether concept for upper atmospheric experimentation,Space Studies Board, Division on Engineering and Physical Sciences, An Implementation Plan for Priorities in Solar-System Space Physics, Chapter 9, "Detailed Mission Plans-- Upper Atmospheric Physics," pp. 42-43 and 54-55, National Academies Press, 15 Jan 1985. but was never flown.Anderson, J. L., "Tethered Satellite System-2 - A proposed program". AIAA PAPER 89-1561, 3rd International Conference on Tethers in Space - Toward Flight; 17–19 May 1989; San Francisco, CA; (accessed 7 July 2016)

Tethers on satellite missions Longer tether systems have also been used on satellite missions, both operationally (as yo-yo despin systems) and in missions designed to test tether concepts and dynamics.

Yo-yo despin

Short tether systems are commonly used on satellites and robotic space probes. Most notably, tethers are used in the "yo-yo de-spin" mechanism, often used in systems where a probe set spinning during a solid rocket injection motor firing, but needs the spin removed during flight.Kenneth S. Bush, "The Yo-Yo Despin Mechanism," presented at the Second Aerospace Mechanisms Symposium, San Francisco CA, 4–5 May 1967; NASA TM-X-60068 (pdf version. Retrieved 16 February 2012) In this mechanism, weights on the end of long cables are deployed away from the body of the spinning satellite. When the cables are cut, much or all of the angular momentum of the spin is transferred to the discarded weights. As an example, the third stage of NASA's Dawn mission utilized two weights with each deployed on cables.Dawn Journal, 12 September 2007

NASA Small Expendable Deployer System experiments In 1993 and 1994, NASA launched three missions using the "Small Expendable Deployer System" (SEDS), which deployed (SEDS-1 and SEDS-2) and (PMG) tethers attached to a spent Delta-II second stage. The three experiments were the first successful flights of long tethers in orbit, and demonstrated both mechanical and electrodynamic tether operation.

SEDS-1 The first fully successful orbital flight test of a long tether system was SEDS-1, which tested the simple deploy-only Small Expendable Deployer System. The tether swung to the vertical and was cut after one orbit. This slung the payload and tether from Guam onto a reentry trajectory off the coast of Mexico. The reentry was accurate enough that a pre-positioned observer was able to videotape the payload re-entry and burnup.Joseph A. Carroll and John C. Oldson, "Tethers for Small Satellite Applications", presented at the 1995 AIAA/USU Small Satellite Conference in Logan, Utah (accessed 20 October 2010)

SEDS-2 SEDS-2 was launched on a Delta (along with a GPS Block 2 satellite) on 9 March 1994. A feedback braking limited the swing after deployment to 4°. The payload returned data for 8 hours until its battery died; during this time tether torque spun it up to 4 rpm. The tether suffered a cut 3.7 days after deployment. The payload reentered (as expected) within hours, but the length at the Delta end survived with no further cuts until re-entry on 7 May 1994. The tether was an easy naked-eye object when lit by the sun and viewed against a dark sky.

In these experiments, tether models were verified, and the tests demonstrated that a reentry vehicle can be downwardly deployed into a reentry orbit using tethers.

PMG A follow-on experiment, the Plasma Motor Generator (PMG), used the SEDS deployer to deploy a 500-m tether to demonstrate electrodynamic tether operation.David Darling, Internet Encyclopedia of Science, SEDS (accessed 20 October 2010)

The PMG was planned to test the ability of a Hollow Cathode Assembly (HCA) to provide a low–impedance bipolar electric current between a spacecraft and the ionosphere. In addition, other expectations were to show that the mission configuration could function as an orbit-boosting motor as well as a generator, by converting orbital energy into electricity. The tether was a 500 m length of insulated 18 gauge copper wire.

The mission was launched on 26 June 1993, as the secondary payload on a Delta II rocket. The total experiment lasted approximately seven hours. In that time, the results demonstrated that current is fully reversible, and therefore was capable of generating power and orbit boosting modes. The hollow cathode was able to provide a low–power way of connecting the current to and from the ambient plasma. This means that the HC demonstrated its electron collection and emission capabilities.Grossi, Mario D., Plasma Motor Generator (PMG) electrodynamic tether experiment, Report NASA-CR-199523, 1 Jun 1995 (papers presented at Fourth International Space Conference on Tethers in Space, Washington, DC, April 1995). Retrieved 8 June 2016.

NRL, TiPS, and ATEx experiments

TiPS The Tether Physics and Survivability Experiment (TiPS) was launched in 1996 as a project of the US Naval Research Laboratory; it incorporated a 4,000 meter tether. The two tethered objects were called "Ralph" and "Norton". TiPS was visible from the ground with binoculars or a telescope and was occasionally accidentally spotted by amateur astronomers. The tether broke in July 2006. This long-term statistical data point is in line with debris models published by J. Carroll after the SEDS-2 mission, and ground tests by D. Sabath from TU Muenchen. Predictions of a maximum of two years survivability for TiPS based on some other ground tests have shown to be overly pessimistic (e.g. McBride/Taylor, Penson). The early cut of the SEDS-2 therewith must be considered an anomaly possibly related to the impact of upper stage debris.

ATEx