C. Bonifazi

The current-voltage characteristics of a large probe in low Earth orbit: TSS-1R results

Measurements of the current collected by the Tethered Satellite System (TSS) satellite as a function of voltage and ambient plasma parameters are presented. The satellite current is found to vary approximately with the square root of the potential from below 10 to nearly 1200 V. The collected current exceeded premission expectations, based on the Parker and Murphy [1967] collection model, by factors of two to three. Possible reasons for discrepancies between the measurements and model are briefly discussed.

The TSS‐1R Mission: Overview and scientific context

The Tethered Satellite System (TSS) program is a binational collaboration between NASA and the Italian Space Agency (ASI) with NASA providing the Shuttle-based deployer and tether and ASI providing a satellite especially designed for tethered deployment. Twelve science investigations (see Table 1) were supported by NASA, ASI, or the Air Force Philips Laboratory. The goals of the TSS-IR mission, which was the second flight of the TSS hardware, were to provide unique opportunities to explore (1) certain space plasma-electrodynamic processes—particularly those involved in the generation of ionospheric currents, and (2) the orbital mechanics of a gravity-gradient stabilized system of two satellites linked by a long conducting tether. TSS-IR was launched February 22, 1996 on STS-75 into a 300-km, circular orbit at 28.5° inclination. Satellite flyaway occurred at MET 3/00:27 and a unique data set was obtained over the next 5 hours as the tether was deployed to a length of 19,695 m. At MET 3/05:11, during a day pass, the tether suddenly broke near the top of the deployer boom. The break resulted from a flaw in the tether insulation which allowed the ignition of a strong electrical discharge that melted the tether. The operations that had begun at satellite flyaway, however, allowed significant science to be accomplished.

Current-voltage characteristic of the TSS-1R satellite: Comparison with isotropic and anisotropic models

In this paper we present an analysis of the satellite current-voltage (I-V) characteristics observed during the TSS-IR mission. The satellite I-V curves were compared with the predictions by two theoretical models: the first one [Parker and Murphy, 1967] considers the channeling effect of the terrestrial magnetic field as the dominating process in the current collection, whereas the second one [Alpert et al., 1965] neglects the magnetic field and assumes an isotropic spherical geometry. Even though neither of the two theories reproduces rigorously the TSS experimental situation, as they neglect both the motion of the body and possible anomalous collisions, we find that both the models, in certain conditions, could reproduce theoretical curves in agreement with the data. However we suggest that, when the effect of the plasma turbulence is considered, the data could be better interpreted on the basis of the Alpert formulation.

Enhanced electrodynamic tether currents due to electron emission from a neutral gas discharge: Results from the TSS‐1R Mission

During the reflight of the first electrodynamic Tethered Satellite System (TSS-1R) mission, the unplanned separation of the tether at the Orbiter end resulted in the highest tether current during the mission. In the moments just prior to the tether separation with 19.7 km of tether deployed and a generated electromotive force (EMF) of 3482 V, currents reaching approximately 0.97 A were shunted through the tether to the Orbiter electrical ground, which was in contact with the ionosphere primarily through its main engine surfaces. This current level was nearly twice as large as observed during any nominal operating period. As the failure point of the tether entered into the ambient plasma, the current increased to 1.1 A and maintained this level even after the break for approximately 75 s. The principal surprise in these results was that the broken end of the tether, with only a few short strands of copper wire, could support higher currents than the much larger Orbiter conducting surface areas. Analysis of possible current enhancement mechanisms revealed that only a gas-enhanced electrical discharge, providing an electron emission source, was plausible. Ground plasma chamber tests confirmed this analysis. The TSS-1R results thus represent the highest electron current emission from a neutral plasma source yet demonstrated in a space plasma. This is of interest for current collection processes in general and plasma contactor development in particular.

TSS core equipment. I: Electrodynamic package and rationale for system electrodynamic analysis

SummaryThe first Tethered-Satellite System (TSS-1) Electrodynamic mission has been launched aboard the Space Shuttle STS-46 on July 31, 1992, as a joint mission between the United States and Italy. A 500 kg, spherical Satellite (1.6 m diameter), attached to the Orbiter by a thin (0.24 cm), conducting, insulated wire (Tether), has been reeled upwards from the Orbiter payload bay to a distance of 256 m, rather than the expected 20 km, when the Shuttle was at a projected altitude of 300 km. ASI, the Italian Space Agency, had the responsibility for developing the reusable Satellite, while NASA had the responsibility for developing the Deployer system and the Tether, integrating the payload and providing transportation into space. One of the main scientific goals of this first mission is to demonstrate the possibility of energy conversion from mechanical to electrical by using a long Tether orbiting through the Earths magnetic field. ASI designed and developed an active experiment, referred to as Core Equipment, in order to carry out this demonstration, and to support all the scientific investigations related to the study of the TSS electrodynamic interactions with the Earths ionosphere. The experiment uses two Electron Generator Assemblies (EGAs), located on the Orbiter, to re-emit into the ionosphere, as an electron beam, the electrons collected on the Satellite from the ionosphere. Other instruments provide current, voltage, and ambient pressure measurements, and allow, via a series of switches, different electrical configurations of the TSS. The Core Equipment was innovative for space experiments in general and Shuttle experiments in particular. In fact, it was the first flight in which the Shuttle has been used as an integral part of the experiment and not only as an observing platform. It was the first mission with an integrated approach to science, will all the instrumentation and their operative modes selected to characterize the electric properties of the TSS.

Observations of ionosphere heating in the TSS‐1 subsatellite presheath

The first flight of the Tethered Satellite System (TSS-1) was to investigate the mechanical and electrical dynamics of a conducting satellite deployed from the orbiter via a tether whose core was a conducting wire [Dobrowolny and Melchioni, 1993; Dobrowolny, 1987; Dobrowolny and Stone, 1994). In the TSS-1 system the satellite deployed from the orbiter radially away from the Earth. The relative motion between the tether and Earths magnetic field generated an electromotive force (EMF) that is the product of orbiter velocity, Earths magnetic field, and the length of the deployed tether. This EMF drove a current through the tether. Electrons were collected on the satellites electrically conductive skin and traveled through the tether to the orbiter, where they either went to orbiter structural ground or were emitted into the ionosphere via active electron emission. During TSS-1 this electron emission was accomplished mainly by the 100 mA, 1-keV fast pulsed electron gun (FPEG) of the shuttle electrodynamic tether system (SETS) (Williamson et al., 1988; Banks et al., 1994; V. M. Aguero et al., manuscript in preparation, 1994]. The FPEG electron emission was much higher than either ambient ion collection at the orbiter end or electron collection at the satellite. Potentials of the orbiter with respect to the ambient plasma were obtained from measurements from the Shuttle Potential and Return Electron Experiment (SPREE) (Oberhardt et al., 1993a, b, 1994), the SETS tether current voltage monitor (Thompson et al., 1993), and the Agenzia Spaziale Italiana deployer and satellite core equipment (Bonifazi et al., 1988, 1994). Despite the limited tether deployment length of 268 m the TSS-1 system proved capable, during certain events, of generating satellite potentials sufficient to illuminate a previously unexplored aspect of plasma physics: that of an ion repelling, electron attracting, moving probe in a magnetoplasma. During such events the satellite boom-mounted Langmuir probe flown as part of the Research on Electrodynamic Tethers experiment (Dobrowolny et al., 1994) measured an increase in the electron plasma temperature in the quasi-neutral ionospheric region beyond the satellite sheath. This observed heating of the presheath electrons was distinctly different from the acceleration of the electrons in the sheath, which was also observed when the sheath expanded such that the probe was completely in the sheath. We show that the observed elevated electron temperatures are consistent with the formation of a Bohm stable electron collecting sheath.

Negative shuttle charging during TSS 1R

We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the tether to shuttle ground. Ion spectral peaks detected by the Shuttle Potential and Return Electron Experiment indicate that the shuttle consistently charged negatively with respect to the local plasma. With the 15 Ω shunt in the circuit, shuttle potential, Φs, decreased from −17 to −245 V as tether length, L, increased to 2.6 km. Current in the circuit depended strongly on ionospheric density. With the 25 kΩ resistor in place, Φs ≈ −300 V in the low density, nightside ionosphere with L = 5.1 km. Near local noon Φs ≈ −80 V with L = 17.2 km. The shuttle charged to ∼−600 V during two dawn terminator crossings, one with and one without thruster firings. As on TSS 1, firings of two aft vernier thrusters significantly increased |Φs|. In the case without thruster firings, simultaneous variations of Φs, tether current, and the inferred satellite potential are consistent with strong azimuthal and vertical ionospheric density gradients. These are the first known direct measurements of strong negative shuttle charging.

Suprathermal electrons observed on the TSS-1R satellite

Particle measurements up to 27,000 eV were made on the TSS-1R (Tethered Satellite System) satellite. The TSS satellite developed a positive bias due to the Lorentz force. It was the intent that electron measurements on the TSS satellite could be used to track the spacecraft potential and collected current. What was observed was quite different. Accelerated ionospheric electrons were observed to only ∼70 eV even though larger spacecraft potentials were observed by other diagnostics on the TSS satellite. When observed they agreed with these independent measurements of the potential. In addition to the anticipated accelerated thermals, a suprathermal population of electrons was observed to be centered around 200 eV. This population exhibited a 4 orders-of-magnitude increase in intensity as the spacecraft potential exceeded the O+ ram energy. The disappearance of the accelerated thermals is explained by the observation that the suprathermal flux becomes larger in magnitude, thus hiding the thermals. However the suprathermals cannot be the dominant current carriers if they are the result of a DC process as their calculated current magnitude exceeds that observed. These results are best explained if one assumes an AC acceleration of the suprathermal electrons whose free energy is derived from the differential drift between electrons and ions.

Positive spacecraft charging as measured by the Shuttle potential and Return Electron experiment

The authors report on obsevations of positive charging of the Orbiter during the deployed phase of the TSS-1 (Tethered Satellite System 1). The charging was observed to occur when the Orbiter was in darkness, during periods of low ionospheric density and while the SETS FPEG (Shuttle Electrodynamic Tether System Fast Pulsed Electron Gun) was emitting a 1-keV, 100-mA electron beam. The charging occurred when the ambient plasma density was too low to provide a current to match the FPEG emission. In the cases where the ambient plasma temperature was approximately 0.1 eV, and assuming a conducting area of the Orbiter of approximately 25 m/sup 2/, the positive charging occurred for densities below 6*10/sup 5/ electrons/cm/sup 3/. The charging is seen as an acceleration of the relatively hot electron population in the energy range above 10 eV that had already been greatly enhanced by the operation of the FPEG. Intense fluxes of electrons were observed at low energy for both the charged and uncharged Orbiter. This low energy component tended to be isotropic with densities as high as 4.6*10/sup 3/ electrons/cm/sup 3/. The return flux at the charging peak was anisotropic, with the anisotropy varying with the level of charging and the pitch angle. >

Current collection at the shuttle orbiter during the tethered satellite system tether break

We present measurements of currents, orbiter potentials, and plasma spectra during high-voltage arcing coincident with the tether break event on the Tethered Satellite System reflight. In addition to the unexpectedly high tether currents observed, plasma spectra indicate the presence of ion and electron populations with broad energy ranges. These data were used in combination with satellite and orbiter current collection models to investigate the circuit behavior of the systems components. We find that arcing at the lower end of the tether supported the current flow in the tether during each phase of the break event, but with different mechanisms dominant depending on the location of the break point. With the break point inside deployer control structures, current arced to the orbiter ground, charging it to high negative potentials and allowing secondary ionization of neutral molecules near orbiter conducting surfaces. The most likely source of these neutrals is air trapped inside the tether at 1 atm of pressure that escaped through the hole in the tether insulation. When the break point was exposed to the exterior environment the tether current arced directly to the plasma. As long as the break point remained near the orbiter, the collection of electrons by conducting surfaces caused it to float at a low level of negative charging. The source of the energetic electrons detected in the payload bay remains uncertain. However, they can only have come from a region within the sheath that was more negatively charged than the orbiters conducting surfaces.