A tether satellite is a satellite connected to another by a thin cable called a tether. The "space tether" idea had its origin in the late 1800s. The idea became more popular in the 1960s, and subsequently NASA examined the feasibility of the idea and gave direction to the study of tethered systems, especially tethered satellites. Some concepts that were brought up during the 1970s were:
- Orbiting antennas
- Shuttle-borne tethered satellites
- Electrodynamic-powered tethers
- Space station tether systems.
NASA currently has a single short-term goal for tether satellite researchers—to achieve 100 km deployment with the creation of new-materials, control laws, and all the supporting subsystems. Tethered satellites are broken up into three parts. There is 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 shuttle, space station, or moon. The tether is what keeps the two satellites connected. The tether is generally a complicated composite that is made up of primarily a copper core and kevlar. The sub-satellite is released from the base-satellite towards an attracting body.
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Analysis
There are, in general, three dynamic phases of a tethered satellite system: the deployment phase, the station-keeping phase, and the retracting phase. This deployment phase is by its nature stable and needs very little control. When the sub-satellite is released it is attracted to the Earth, or other primary body, at a certain rate, initially slow, but growing exponentially with time. The only way to speed up the rate of deployment is with some sort of (initial) propulsion. 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 problems 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 space stations center of mass as well as controlled variations of the offset
- Transverse vibrations of the tether
- External forces
Assumptions need to be made when analysis is done. The control laws that are discussed in literature are based on early knowledge of nonlinear control systems. Previous literature talks about and compares various types of control laws types including tension, thruster, and offset control.
TSS-1 mission
Tethered Satellite System-1 (TSS-1) was flown during STS-46, aboard the Space Shuttle Atlantis, from July 31 to August 8, 1992. The TSS-1 mission discovered a lot about the dynamics of the tethered system, although the satellite was deployed only 260 meters (853 ft). It was far enough, though, to show that it could be deployed, controlled, and retrieved, and that the TSS is easy to control and even more stable than predicted. The voltage and current reached using a shorter tether 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 learned about on the electrons that carry the "return-tether" current. The mission was reflown as TSS-1R.
TSS-1R mission
This was the follow-up mission to TSS-1. It was released in February 1996 from STS-75. Over 19 kilometers of the tether were deployed before the tether broke. It remained in orbit for a number of weeks and was easily visible from the ground, appearing something like a small but surprisingly bright fluorescent light traveling through the sky.
TiPS
The Tether Physics and Survivability Experiment (TiPS) is the only tethered satellite system currently in orbit other than the MAST experimental satellite. It was launched in 1996 as a project of the US Naval Research Laboratory. The tether is four kilometers long. The two tethered objects are called "Ralph" and "Norton". TiPS can be visible from the ground with large binoculars or a telescope and is occasionally accidentally spotted by amateur astronomers.
JAXA initiatives
A "Foldaway Flat Tether Deployment System" will fly into space, although it won't reach orbit, as part of a mission sponsored by the Japanese Aerospace Exploration Agency (ISAS/JAXA). [1]
European initiatives
In 1997, ESA launched the first Young Engineers' Satellite (YES) of about 200 kg into GTO with a 35 km double-strand tether, and planned to deorbit a probe at near-interplanetary speed by rotation of the tether system. This complex dynamics experiment was cancelled due to late changes in the launcher's orbit combined with an increased collision risk with satellites in Low Earth Orbit. The European Space Agency (ESA) currently is preparing the launch of a 31.6 km tether (of which 30 km are to be deployed) aboard the second YES, YES2, this time a 36 kg satellite. While aimed at accurate re-entry and soft landing of a 6 kg re-entry capsule, Fotino, tether deployment stopped at 8.5 km, forcing an early release of the capsule which has not been recovered since. The YES satellites are entirely built and qualified by students and young engineers with experts' support. The tether technology in use is inspired by the successful SEDS missions of the 1990's. The (American) SEDS missions hold the current length record in space—at 20 km—and were based on a simple spool deployer with friction brake, combined with a thin Dyneema (polyethylene) tether.
MAST
The MAST tether experiment was launched 17 April 2007 aboard a Dnepr rocket. This 1 km multistrand, interconnected tether (Hoytether) is being used to test and prove the long-term survivability for tethers in space. Unfortunately the tether failed to deploy. 
