Space tether missions
A number of space tethers have been deployed in space missions.
The missions have met with a varying degrees of success; a few have been highly successful.
- 1 Description
- 2 Tether flights on Human Space missions
- 3 Tethers on Satellite Missions
- 4 Space debris removal missions
- 5 Sounding Rocket Flights
- 6 Proposed and Future Missions
- 7 References
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 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
In 1966, Gemini 11 deployed a 30m (100 foot) tether which was stabilized by a rotation which gave 0.00015 g.
Shuttle 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 during STS-46 aboard the Space Shuttle Atlantis from July 31 to August 8, 1992.
The purpose of the TSS-1 mission was to verify the tether concept of gravity gradient stabilization, as well as to provide a research facility for investigating space physics and plasma electrodynamics. This mission discovered a lot about the dynamics of the tethered system, although the satellite was deployed only 260 meters (853 ft) of the 20 km proposed amount due to mechanical problems. 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 conclusively proved 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 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 in 1996 as TSS-1R.
Shuttle TSS-1R mission
Four years later, as a follow-up mission to TSS-1, the TSS-1R satellite was released in February 1996 from the Space Shuttle on the STS-75 mission. The TSS-1R mission objective was to deploy the tether 20.7 km above the space station and remain there collecting data. Scientific objectives for the TSS-1R mission were 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 a motional EMF that would drive a current through the tether system.
TSS-1R was deployed to 19.7 km, but this was still 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 particularly significant finding used in this thesis 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..
Other scientific advancements have resulted from this mission. Improvements have been made in modeling the electron charging of the shuttle and how it effects current collection. In addition, much was learned concerning the interaction of bodies with surrounding plasma, as well as the production of electrical power. For further discussion and analysis of this mission see the referenced documents.
Tethers on Satellite Missions
Short tether systems are commonly used on satellites and robotic space probes. Most notably, tethers are used in the "yo-yo despin" 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 spining satellite. When the cables are cut, much or all of the angular momentum of the spin is transferred to the discarded weights.
Longer tether systems have also been tested on satellite missions.
In 1993 and 1994, NASA launched two "Small Expendable Deployer System" experiments (SEDS-I and SEDS-II), which deployed 20 km tethers attached to a spent Delta-II second stage. 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 1 orbit after the start of deployment. 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.
SEDS-2 was launched on a Delta (along with a GPS Block 2 satellite) on March 9, 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 torques 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 7.2 km length at the Delta end survived with no further cuts until re-entry on May 7, 1994. The tether was an easy naked eye object when lit by the sun and viewed against a dark sky.
In these experiments, not only were tether models verified, the tests successfully showed that a reentry vehicle can be downwardly deployed into a reentry orbit using tethers.
The objectives of the Plasma Motor Generator (PMG) mission were to test the ability of a Hollow Cathode Assembly (HCA) to provide a low impedance bipolar electrical 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 500m length of insulated 18 gauge copper wire. The mission was launched on June 26, 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 operating in power generator and orbit boosting modes. The hollow cathode was able to provide a low power way of connecting the electrons to and from the ambient plasma. This means that the HC demonstrated its electron collection and emission capabilities.
The Tether Physics and Survivability Experiment (TiPS) was launched in 1996 as a project of the US Naval Research Laboratory. The tether was four kilometers long. The two tethered objects were called "Ralph" and "Norton". TiPS was visible from the ground with large 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.
Young Engineers' Satellite (YES)
In 1997, the European Space Agency launched the 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 swinging deployment of the tether system. The orbit as achieved was not as initially planned during the design of the tether experiment and, for safety considerations, the tether was not deployed. The YES was switched on however to perform a number of secondary technology demonstration experiments.
10 years after YES, the successor, the Young Engineers' Satellite 2 (YES2) was flown. The YES2 was a 36 kg student-built tether satellite part of ESA's Foton-M3 microgravity mission. The YES2 satellite employed a 32 km long tether to deorbit a small re-entry capsule "Fotino."
The YES2 satellite was launched September 14, 2007 from Baikonur. The communications system on the capsule failed, and the capsule was lost, but deployment telemetry indicated that the tether deployed to full length and that the capsule presumably deorbited as planned. It has been calculated that Fotino was inserted into a trajectory towards a landing site in Kazakhstan, but no signal was received. The capsule was not recovered.
The Multi-Application Survivable Tether (MAST) launched three 1-kg cubesat modules with a 1-km tether. Two of the cubesat modules ("Ted" and "Ralph") were intended as end-masses on the deployed tether, while the third ("Gadget") served as a climber that could move up and down the tether. The experiment used a multi-line "Hoytether" designed to be resistant to damage. The objectives of the MAST experiment were to obtain on-orbit data on the survivability of space tethers in the micrometeorite/debris orbital environment, to study the dynamics of tethered formations of spacecraft and rotating tether systems, and to demonstrate momentum-exchange tether concepts. The experiment hardware was designed under a NASA Small Business Technology Transfer (STTR) collaboration between Tethers Unlimited, Inc. and Stanford University, with TUI developing the tether, tether deployer, tether inspection subsystem, satellite avionics, and software, and Stanford students developing the satellite structures and assisting with the avionics design, as a part of the University CubeSat program
In April 2007 it was launched as a secondary payload on a Dnepr rocket into a 98°, 647 x 782 km orbit. The experimenter team made contact with the "Gadget" picosatellite, but not with "Ted", the tether-deployer picosatellite. While the system was designed so that the satellites would separate even if communications were not established to the tether deployer, the system did not fully deploy. Radar measurements show the tether deployed just 1 meter.
The Space Tethered Autonomous Robotic Satellite (STARS) mission, developed by the Kagawa Satellite Development Project at Kagawa University, Japan, was launched 23 January 2009 as a secondary payload aboard H-IIA flight 15, which also launched GOSAT. Like MAST, the satellite was based on a "cubesat" platform. After launch, the satellite was named KUKAI, and consisted of two subsatellites, "Ku" and "Kai," to be linked by a 5-meter tether. It was successfully separated from the rocket and transferred into the planned orbit, but the tether deployed only to a length of several centimeters, "due to the launch lock trouble of the tether reel mechanism."
Space debris removal missions
Electrodynamic tethers carry an electrical current and can generate thrust or drag from a planetary magnetic field. The generated thrust may be used to decelerate space debris and thus bring about an earlier reentry into Earth's atmosphere of a piece of space debris.
The Japan Aerospace Exploration Agency (JAXA) will test in early 2014 a 300 meters (980 ft) electrodynamic tether by attaching it to a dead satellite, generating current as it rotates, and decelerating the piece of space junk to bring it into a successively lower orbit until in reenters the atmosphere. As of January 2014[update], it is scheduled to launch on 28 Feb 2014 as a secondary payload aboard an H-2A rocket.
Sounding Rocket Flights
The Cooperative High Altitude Rocket Gun Experiment (CHARGE) 2 was jointly developed by Japan and NASA, to observe the current collection along with many other phenomena. The major objective was to measure the payload charging and return currents during periods of electron emission. Secondary objectives were related to plasma processes associated with direct current and pulsed firings of a low-power electron beam source. On December 14, 1985, the CHARGE mission was launched at White Sands Missile Range, New Mexico. The results indicated that it is, in fact, possible to enhance the electron current collection capability of positively charged vehicles by means of deliberate neutral gas releases into an undisturbed space plasma. In addition, it was observed that the release of neutral gas or argon gas into the undisturbed plasma region surrounding a positively biased platform has been found to cause enhancements to electron current collection. This was due to the fact that a fraction of the gas was ionized, which increased the local plasma density, and therefore the level of return current.
OEDIPUS Tethered Sounding Rocket Missions
OEDIPUS ("Observations of Electric-field Distribution in the Ionospheric Plasma - a Unique Strategy" consisted of two sounding rocket experiments that used spinning, conductive tethers as a double probe for measurements of weak electric fields in the aurora. They were launched using Black Brant 3-stage sounding rockets. OEDIPUS A launched on January 30, 1989 from Andøya in Norway. The tethered payload consisted of two spinning subpayloads with a mass of 84 and 131 kg, connected by a spinning tether. The flight established a record for the length of an electrodynamic tether in space at that time: 958 m. The tether was a teflon coated stranded tin-copper wire of 0.85 mm diameter and it was deployed from a spool-type reel located on the forward subpayload,
OEDIPUS C was launched on November 6, 1995 from the Poker Flat Research Range north of Fairbanks, Alaska on a Black Brant XII sounding rocket. The flight reached an apogee of 843 km and deployed a tether of the same type used in the OEDIPUS-A to a length of 1174 m. It included a Tether Dynamics Experiment to derive theory and develop simulation and animation software for analyses of multi- body dynamics and control of the spinning tether configuration, provide dynamics and control expertise for the suborbital tethered vehicle and for the science investigations, develop an attitude stabilization scheme for the payloads and support OEDIPUS C payload development, and acquire dynamics data during flight to compare with pre-flight simulation.
On August 31, 2010, a Japanese space tether experiment called "Tether Technologies Rocket Experiment (T-REX)," sponsored by the Japanese Aerospace Exploration Agency (ISAS/JAXA), was launched on sounding rocket S-520-25 from Uchinoura Space Center, Japan, reaching a maximum altitude of 309 km. T-Rex was developed by an international team led by the Kanagawa Institute of Technology/Nihon University to test a new type of electrodynamic tether (EDT) that may lead to a generation of propellantless propulsion systems for LEO spacecraft. The 300-m-long tape tether deployed as scheduled and a video of deployment was transmitted to the ground. Successful tether deployment was verified, as was the fast ignition of a hollow cathode in the space environment.
The experiment demonstrated a "Foldaway Flat Tether Deployment System". The educational experiment featured the first bare tape tether deployment (i.e. without insulation, the tether itself acts as anode and collects electrons). 130 m of the total of 300 m of tether was deployed fire-hose style, purely driven by inertia and limited by friction, following a powerful, spring-initiated ejection. Accurate differential GPS data of the deployment was recorded, and video taken from the endmasses.
Proposed and Future Missions
The use of a bare section of a space-borne electrodynamic tether for an electron-collection device has been suggested as a promising alternative to end-body electron collectors for certain applications, provided that electrons are collected in a quasi-orbital-motion-limited (OML) regime (the OML regime is discussed further down this page). For a given V - Vp, plasma probe theory predicts that the collected electron current per unit area (not total current) is maximized in the orbital-motion-limited regime, which is only valid with sufficiently thin wires (explained in Section 2.1.1). NASA’s Propulsive Small Expendable Deployer System (ProSEDS) would deploy 5-km of tether to collect up to 1 – 2 A of current from the ionosphere. The current interacting with the Earth’s magnetic field would produce an electrodynamic drag thrust and reduce the de-orbit time by more than 5-km / day compared to the atmospheric drag. The bare tether concept was to be tested first during this ProSEDS mission. While the mission was canceled after NASA’s space shuttle Columbia accident, the concept could potentially be undertaken in the future. Present bare tether designs, such as the one developed for the ProSEDS mission, use a small, closely packed cross-section of wires or even a single wire as the anode. In future designs, concerns for survivability to collisions with micro-meteoroids and space debris will need to be considered. This will require the use of distributed or sparse tether cross-section geometries, which could span tens of Debye lengths depending on plasma density and temperature. One such technology that has been developed is the Hoytether. For further discussion and analysis see the following referenced documents. 
Cubesat Tether Missions
Tether Electrodynamic Propulsion CubeSat Experiment (TEPCE) is a Naval Research Laboratory electrodynamic tether experiment based on a "triple cubesat" configuration, currently planned for launch as a secondary payload. TEPCE uses two nearly identical endmasses with a stacer spring between them. The spring will separate the endmasses and start deployment of a 1 km long braided-tape conducting tether. Passive braking will be used to reduce speed and hence recoil at the end of deployment. The satellite is intended to be able to drive an electrodynamic current in either direction. It is intended to be able to raise or lower the orbit by several kilometers per day, change libration state, change orbit plane, and actively maneuver. The satellite underwent a successful deployment test in May 2010.
In support of TEPCE, the U.S. Naval Academy is developing TetherSat, a satellite system with a 1-km-long tether, to test the TEPCE tether deployment hardware in LEO and to analyze the dynamics during and after deployment. Twin end masses are 1.5U CubeSats that will contain GPS and other sensors to accurately measure tether libration and orbital motion data. Although the tether is conductive, it will not be used to generate electrodynamic forces.
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