An Account of the Tethered Satellite System Program

by Dr. Nobie Stone

(See biographical sketch)   

https://www.marshallretirees.org/orgiginal-memorial-database/person-detail/?pdb=45973

In 1985, Dr Stone was asked to serve as Project Scientist for the development of the joint US-Italian Tethered Satellite System (TSS).  He then served as Mission Scientist for the flight of the TSS on Space Shuttle missions STS-46 in 1993 and STS-75 in 1996.  The following account is based on his observations of the TSS program over its decade-long development and two space missions.  The TSS missions involved deploying a specially designed satellite on a 12-mile long tether, which was about the diameter of a boot lace. The tether contained a bundle of copper wires so that the Space Shuttle Orbiter and the satel­lite, which was deployed up­ward from the Shuttle payload bay, would be electrically connected. 

As the 12-mile long conducting tether moved through the magnetic field of the earth at an orbital speed of 18,000 miles per hour, the TSS would become a giant electric gener­ator—employing the exact same principle on which an electric generator operates here on earth—and would create an electric potential (or voltage) between the satellite and the Shuttle Orbiter.¹  The magnitude of the voltage that would be generated was easily calculated from basic principles—the amount of current that would flow in the tether, however, would be a complex function of the electrical conductivity of the tether, solar radiation intensity, the characteris­tics of the ionospheric plasma, parameters of the Space Shuttle’s orbit, and the TSS design characteristics.  

The problems encountered during the TSS-1 and TSS-1R missions are well documented—less so what was learned of complex orbital dynamics, spacecraft charging, current collection by highly biased satellites in high altitude orbits, and new advanced in-space propulsion technologies. Herein, we will focus on what was learned from the Tethered Satellite program.   

The TSS carried a dozen experiments (see Table 1) that were selected from US  and Italian institu­tions, and was supported by a team of more than a hundred scientist from half a dozen countries. The various measurements were designed to work together in con­cert as one unified “experiment,”—much in the manner that an experiment is con­ducted in the labor­a­tory, where a number of different factors must be measured with various instru­ments and the results combined to understand the phenome­non being studied.¹  This required the TSS Science Investigator team led by Dr Stone to coordi­nate the design and operations of the various instruments—together with the Space Shuttle operations—to ensure that all meas­ure­ments necessary to study the various phenomena associated with the electrody­namic tether were obtained.  The TSS Program also included a coordinated one-year post-mission data analysis effort.    

Table 1:  TSS-1 and -1R Science Experiments

 Experiment                                           Principal Investigator              Institution  

Core Equipment (CORE)                          Carlo Bonifazi                  Agenzia Spaziale Italiana  

Research on Orbital Plasma                     Nobie Stone*                   NASA/Marshall Space Flight Ctr

   Electrodynamics (ROPE)

Research on Electrodynamic                    Marino Dobrowolny^ǂ         Consiglio Nazionale delle 

   Tether Effects (RETE)                                                                  Ricerche/Instituto Fisica Spazio 

Magnetic Field Experiment                        Franco Mariani                 Second University of Rome

   for TSS Missions (TMAG)

Shuttle Electrodynamic                             Peter Banks¹/                   University of Michigan

   Tether System (SETS)                          Bryan Gilchrist^1R

Shuttle Potential and Return                     Marylin Oerhardt¹/            Dept. of the Air Force, Phillips 

   Electron Experiment (SPREE)                David Hardy^1R                  Laboratory

Tether Optical Phenomena                       Stephen Mende               Lockheed

   Experiment (TOP)

Investigation of Electromagnetic                Robert Estes                    Smithsonian Astrophysical 

   Emissions for Electrodynamic                                                       Observatory (SAO)

   Tether (EMET) 

Observations at Earth’s Surface                Giorgio Tacconi                University of Genoa

   of Electromagnetic Emissions

   by TSS (OESEE)

The Investigation and Meas-                     Gordon Gullahorn             Smithsonian Astrophysical

   urement of Dynamic Noise                                                            Observatory (SAO)

   in the TSS (IMDN)

Theoretical and Experi-                             Silvio Bergamaschi           Institute of Applied Mechanics

   mental Investigations of 

   TSS Dynamics (TEID)

Theory and Modeling in Support                Adam Drobot                   Science Applications 

   of Tether Satellite Applications                                                      International Corp. (SAIC)

    (TMST) 

*NASA Mission Scientist;  ^ǂASI Mission Scientist

Electric Generator Mode Operations

On the TSS-1R mission, flown in 1996, during “generator mode” operations, the motion of the TSS through the earth’s magnetic field created an electric potential between the tethered satellite and the Space Shuttle Orbiter greater than 1,000 volts.  This caused an electrical current of more than one Ampere to flow through the tether, which generated more than 1,000 Watts of electrical power.²  

The electrodynamic tether had converted the kinetic energy of the Space Shuttle’s orbital motion directly into electrical power.  This demonstrated that spacecraft orbital momen­tum can be used to provide the electrical power required to operate space missions.  The power would be generated at the expense of the spacecraft’s orbital velocity, but the lost speed could easily be regained with a short thruster burn.  

These results also confirmed earlier suggestions that the loss of orbital velocity, or drag created by  an electrody­namic tether can be used to de-orbit a space­craft, thereby eliminating the need to carry and store extra propellant to enable controlled re-entry at the end of a mission.  The mass savings could then go to increase the payload.   

Electric Motor Mode Operations

The results further showed that a high-voltage power supply can be used to drive a current through the tether in the opposite direction. In this mode, instead of generating  electrical power, the tether system behaves like an electric motor, creating a force on the tether in the direction of orbital motion. The result is an accelerating thrust on the spacecraft that is produced without burning pro­pel­lant, as would be required by a rocket motor. Further, if solar cells are used to run the power supply that drives the current in the tether, the energy required for accel­er­ation is ob­tained directly from solar radiation and the tether “elec­tric motor mode” thruster requires virtually no fuel. (A small amount of gas may need to be injected into the satellite’s environment to increase electrical con­tact and allow the required current to flow between the satellite and the ionosphere.³)  

Note that both, the electric generator and the electric motor operational modes of an electrodynamic tether can be used to control the orbit of a spacecraft at any planet that possesses a magnetosphere—in other words, any planet that has a planetary magnetic field and an ionized layer of gas in its upper atmosphere.  

High-Voltage Charging Effects

The TSS experiments led to an improved understanding of the “spacecraft charg­ing problem” that had long plagued high-altitude satellites in syn­chro­nous orbits, causing them to behave erratically.  The electrical bias on the TSS satellite could be swept, allow­ing the satellite-ionospheric interaction to be studied at high voltages—well beyond the capabilities of the laboratory simulation studies.²  This led to the realization that a the­o­retical model that had been relied upon for decades to describe the way a space­craft charges up when exposed to the space environment that exists 10 to 20 thousand miles above the earth—was incorrect.⁴  

In-Space Propulsion Applications: The Electric Sail  

Space tether technology can enable more rapid interplanetary travel, where the tether system serves as an “Electric Sail.”  The idea of a sail is that it “catches” the wind, transferring its momentum to the ship.  In a similar way, an “Electric Sail” would use a spinning array of radially de­ployed wires several kilometers in length (geometrically similar to the spokes of a bicycle wheel) to intercept the solar wind—a  hypersonic stream of ionized gas blown out into space by the Sun.  The radial wires would be kept taught by the centrifugal force produced by the spinning motion of the system.  Since the solar wind particles are electrically charged, they would be de­flected by the electric field surrounding the charged wires, resulting in a loss of momen­tum by the deflected particles that would be transferred to the “sail and, therefore, the spacecraft.”   

Although the array would be more than 98 percent transparent (thereby minimizing its mass), the electric field cre­ated by an electric potential applied to the wires would extend out radially, increasing the effective diameter of each wire (and, therefore, the area over which the solar wind particles will be de­flected) by a factor of 1,000 to 10,000, depend­ing on the applied voltage. As a result, the number of charged particles deflected and, there­fore, the thrust produced, would be increase by the same factor. This also means that the level of thrust can be controlled by varying the volt­age applied to the array. 

Since the solar wind speed can be as high as 700 kilometers per second (420 miles per second—or about 1.5 million miles per hour) and extends out beyond all of the planets, even though the thrust force devel­oped is very small (in the range of mille-pounds), when ap­plied over a number of months, the “top end speed” speed reached by a spacecraft using such a propul­sion system would be enormous compared to that of a chemical rocket.  

Calculations based on earlier theoretical models of space­craft charging and current collection predicted a rather modest benefit from using an electric sail.  How­ever, as mentioned above, a more detailed understanding of charging effects (first obtained from early laboratory simulations and, later, corroborated in space by measurements made during Spacelab-2 and TSS) showed this model to significantly underestimate the effects of charging.    

Using the results of the laboratory and in-space measurements, the calculated thrust produced by an electric sail was found to be significantly greater and provided propor­tion­ally faster transfer times.  For example, the Voyager-1 spacecraft, using chemical rocket propulsion, took thirteen years to reach the orbit of Pluto, and about 30 years to reach the heliopause (the boundary of the solar system where the influence of our Sun is matched by that of the interstellar medium).  However, a satellite using Electric Sail Propulsion could leave earth and pass through the heliopause in only seven years—an important difference if you are an investigator waiting  for data return!  

Electrodynamic tethered satellites have tremendous potential for technological applications that will be “enabling” for  some types of space missions.  

References

1.     Dobrowolny, M. and N. H. Stone, A Technical Overview of TSS-1:The First Tethered-Satellite System Mission, Il Nuovo Cimento 17C, 1, (1994). 

2.     Stone, N.H. and C. Bonifazi, The TSS-1R Mission: Overview and Scientific Context, Geophys. Res. Letters 25,409, Feb. (1998).

        Stone, N. H., The Plasma Wake of Mesosonic Conducting Bodies: Part-1, An Experimental Parametric Study of Ion Focusing by the Plasma Sheath, J. Plasma Phys. 25, 351 (1981).

        Stone, N. H., The Plasma Wake of Mesosonic Conducting Bodies: Part-2, An Experimental Parametric Study of the Mid-Wake Ion Density Peak, J. Plasma Phys. 26, 385 (1981). 

        Stone, N.H. and U. Samir, Bodies in Flowing Plasmas: Laboratory Studies, Adv. Space Res. 1, 361 (1981). 

3.     Stone, N. H., Development of a Solid-Expellant Plasma Contactor (SOLEX):  Harnessing the TSS-1R Tether-Break Physics, Paper SPS VIII-04, JANNAF, 2005.  

        Stone, N. H., Electrodynamic Tether Operations Beyond the Ionosphere in the Low-Density Magnetosphere, presented JANNAF Propulsion Meeting, 2006.

4.     Thompson, D.C., C. Bonifazi, B.E. Gilchrist, S.D. Williams, W.J. Raitt, J.-P Lebreton, W.J. Burke, N.H. Stone, and K. H. Wright, Jr., The Current-Voltage Characteristic of a Lage Probe  in Low Earth Orbit: TSS-1R Results, Geophys. Res. Letters 25,413 (1998). 

        Wright, K.H., N. H. Stone and U. Samir, A Study of Plasma Expansion Phenomena in Laboratory Generated Plasma Wakes: Preliminary Results, J. Plasma Phys.33, 71 (1985). 

        Stone, N. H., K. H. Wright, Jr., U. Samir, and K. S. Hwang, On the expansion of ionospheric plasma into the near-wake of the Space Shuttle Orbiter, Geophys. Res. Lett. 15, 1169,1988.