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:
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 30 m (98 ft) 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 256 meters (840 ft) before sticking again, where the effort finally ended (the total proposed length was 20,000 meters (66,000 ft)). A protruding bolt 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.
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. The TSS-1R mission objective was to deploy the tether 20.7 km (12.9 mi) 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 19.7 km (12.2 mi) when the tether broke. The break was attributed to an electrical discharge through a broken place in the insulation. 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, but was never flown.
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. 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 1.44 kg (3.2 lb) each deployed on 12-meter (39 ft) cables.
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