Definitions

Tether satellite

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:

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.

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).

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) has launched a 31.7 km tether (of which 30 km were to be deployed) aboard the second YES, YES2, this time a 36 kg satellite. The system was designed to re-enter and soft-land a 6 kg re-entry capsule, Fotino. Tether deployment completed in two controlled stages mostly as planned, but due to an electrical failure occurring near the end of deployment, the tether overdeployed to its full length. From the mission data it could be demonstrated that the capsule was nevertheless released into a near-nominal re-entry trajectory. No signal from the capsule was received after landing, possibly due to a water landing, harsh impact or scorch of the re-entry. 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 (American) SEDS missions of the 1990s, that were also based on a simple spool deployer with friction brake, combined with a thin Dyneema (polyethylene) tether. With the 32 km tether deployment, YES2 broke the world-record previously held by SEDS (20 km).

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.

Tethered Formation Flying

Spacecraft formation flight is becoming a key research area, where distributed computation and decentralized control schemes, as well as information flows between elements, are explored. One such example includes stellar interferometers in which multiple apertures in controlled formation collect light for coherent interferometric beam combination, thereby achieving a fine angular resolution comparable to a large monolithic aperture telescope. The possible architectures of spaceborne interferometers include a structurally connected interferometer (SCI)Space Interferometry Mission, which allows for very limited baseline changes, and a separated spacecraft interferometer (SSI) Terrestrial Planet Finder where the usage of propellant can be prohibitively expensive. A tethered formation flight interferometer represents a balance between SCI and SSI. Such a system is currently being considered for NASA’s Submillimeter Probe of the Evolution of Cosmic Structure (SPECS) mission The dynamics of SSI are coupled by the definition of relative attitude whereas tethered formation spacecraft exhibit inherently coupled nonlinear dynamics.

The MIT Space Systems Laboratory conducted ground experiments that tested a fully decentralized nonlinear control law, which eliminates the need for inter-satellite communications Contraction theory was used to prove that a nonlinear control law stabilizing a single-tethered spacecraft can also stabilize arbitrarily large circular arrays of tethered spacecraft, as well as a three-spacecraft inline configuration. In order to validate the effectiveness of the decentralized control and estimation framework, a new suite of hardware has been designed and added to the SPHERES (Synchronize Position Hold Engage and Reorient Experimental Satellite) testbed. This work introduced a novel relative attitude estimator, in which a series of Kalman filters incorporate the gyro, force-torque sensor, and relative distance measurements. The closed-loop control experiments can be viewed at The MIT team also reported the first propellant-free underactuated control results for tethered formation flight. This is motivated by a controllability analysis that indicates that both array resizing and spin-up are fully controllable by the reaction wheels and the tether motor.