In the present paper we first give a short account of the mission TSS-1 flown on the Shuttle sts-46 in August 1992 and its basic electrical configurations. We then show some results obtained from the experiment RETE on board the satellite which are relevant for the issue of satellite charging

Dobrowolny, M.

Guidoni, U.

Lebreton, J. P.

Melchioni, E.

Vannaroni, G.

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IL NUOVO CIMENTO Novembre-Dicembre 1997VOL. 20 C, N. 6The TSS-1 mission: Results on satellite charging (*)M. DOBROWOLNY (1), E. MELCHIONI (1), G. VANNARONI (1)U. GUIDONI (2) and J. P. LEBRETON (3)(1) Istituto di Fisica dello Spazio Interplanetario, CNR - Frascati, Italy(2) Agenzia Spaziale Italiana - Roma, Italy(3) Space Science Department, ESA/ESTEC - Noordwijk, The Netherlands(ricevuto il 5 Dicembre 1996; approvato il 27 Febbraio 1997)Summary. — In the present paper we first give a short account of the mission TSS-1flown on the Shuttle sts-46 in August 1992 and its basic electrical configurations. Wethen show some results obtained from the experiment RETE on board the satellitewhich are relevant for the issue of satellite charging.PACS 94.20 – Physics of the ionosphere.PACS 01.30.Cc – Conference proceedings.1. – IntroductionIn this paper we give a short overview of the first TSS mission (TSS-1) which flewon the Shuttle Atlantis in August 1992. We will also show that, although the missionwas not nominal, some of the data obtained are quite interesting with respect to theissue of satellite charging that was one of the primary objectives of the mission.The project TSS was developed to provide the capability of deploying satellites onlong, gravity-gradient–stabilized tethers from the Space Shuttle. It was proposed toNASA and the Italian Space Agency in the early 70’s by M. Grossi of the SmithsonianAstrophysical Observatory and G. Colombo of the University of Padova. It became areality in 1984 when NASA and ASI signed a Memorandum of Understanding in whichNASA agreed to develop a deployer system and tether and ASI to develop the satellite.The goals of TSS-1 were to demonstrate the feasibility of deploying and controllinglong tethers in space and to demonstrate some of the unique applications of this systemas a tool for research in space plasma physics.During TSS-1 the satellite was supposed to be deployed 20 km outward into spaceabove the Shuttle on a conducting tether. It is the use of the conducting tether whichmakes TSS a unique active experiment in space plasma physics.The motional e.m.f. induced by the motion of the system through the Earth’smagnetic field and the consequent electrodynamic interaction of the system with theionosphere opens up, in fact, several areas of investigation on basic plasma processes.(*) Paper presented at the VII Cosmic Physics National Conference, Rimini, October 26-28, 1994.G Società Italiana di Fisica 851M. DOBROWOLNY, E. MELCHIONI, G. VANNARONI, ETC.852TABLE I. – TSS-1 science investigations.Acronym Title PrincipalInvestigatorOrganizationCORESPREESETSROPERETETEMAGEMETTEIDIMDNTMSTOESEETOPCore EquipmentShuttle Potential & Return ElectronExperimentSpace Electrodynamic Tether SystemResearch on Orbital Plasma ElectrodynamicsResearch on Electrodynamic Tehater EffectsMagnetic Field Experiment for TSS MissionsInvestigation of EM Emissions by theElectrodynamic TetherTheoretical & Experimental Investigationon TSS DynamicsInvestigation and Measurement of DynamicNoiseTheory and Modelling in Support of TetherObservations at the Earth’s Surfaceof EM Emission by TSSTether Optical PhenomenaC. BonifaziD. HardyP. BanksN. StoneM. DobrowolnyF. MarianiR. EstesS. BergamaschiD. GullahornA. DrobotG. TacconiS. MendeASIAFGLUniv of StanfordMSFCCNRUniv. of RomeSAOUniv. of PadovaSAOSAICUniv. of GenovaLockheedTable I gives a list of the investigations selected for TSS-1 with the correspondingPrincipal Investigators. Two investigations refer to tether dynamics (PIs Gullahornand Bergamaschi). A third one (PI Drobot) refers to theoretical electrodynamicstudies. The remaining investigations all involve instrumentation which is on theOrbiter, on the satellite and within ground-based facilities.The present paper, after a short discussion on the electrical configuration of TSS-1and a general account of the first mission will concentrate on data results on satellitecharging obtained mainly with the experiment RETE. For a detailed discussion on theremaining TSS-1 instruments as well as a discussion of the science issues of this projectwe refer to ref. [1].2. – TSS-1 electrical configurationsFigure 1 shows the basic electrical configurations of TSS-1. This diagram, whichrefers to the instrumentation on the Orbiter, has its two main elements in two electronguns called EGA (Electron Gun Assembly) and FPEG (Fast Pulsed Electron Gun).EGA is an electron gun capable of giving 0.7 A at 5 kVolt, whose main feature is thatof having the cathode connected to the tether, whereas the anode is grounded to theOrbiter [2]. In this way, the gun is powered by the tether electromotive force. This typeof connection was undoubtedly the basic electrical configuration of TSS-1 from thepoint of view of electrodynamic science. With EGA, one had the additional possibility ofrequiring a certain current value, which was reached and maintained through afeedback loop on the temperature of the filament emitting electrons. When this isactually achieved, the satellite voltage with respect to the ionosphere is also controlled.By varying the gun current (and when the required values can be actually obtained)one obtains, in fact, the current-voltage characteristics of the satellite and of the entiretether system as well.THE TSS-1 MISSION: RESULTS ON SATELLITE CHARGING 853Fig. 1. – TSS-1 electrical configuration.Notice, however, that this capability of current control may not always be possible. Inparticular, when requiring high currents from the EGA gun, the ionosphere at the satel-lite end may not be able to supply the required amount of electrons to sustain the com-manded current. This is especially true at night time, when the electron density is minimum.Coming back to fig. 1, the second electron gun FPEG, which is part of theexperiment SETS, is a pulsed electron gun with its own power supply and thereforeindependent of the tether electromotive force. A second important electrical configura-tion was the one where the tether was connected (through a variable resistor) to theOrbiter ground and the FPEG operating and ejecting an electron beam in theionosphere. The maximum current capability of FPEG is 100 mA.It must be stressed that the two configurations (with either the EGA or the FPEGinserted into the circuit) are basically different. The FPEG gives first of all thepossibility of performing electron beam experiments in the ionospheric plasma.However, when the tether is grounded to the Orbiter, it also induces some current inthe tether in a way which depends on the interaction of the beam with the surroundingplasma and the return currents to the Shuttle. In addition, when both electron guns areoperating, the current emission from FPEG allows to avoid Orbiter charging related toleakage currents from the EGA gun.Most of the TSS-1 nominal mission was configured, from an operational point ofview, as a sequence of cycles where either one or the other of the above electricalconfigurations (each with several variations) were adopted. The actual cycles accordingto which the mission was planned are described in detail in ref. [3].M. DOBROWOLNY, E. MELCHIONI, G. VANNARONI, ETC.8543. – Description of the August 1992 missionSeveral problems in the Deployer mechanism caused the TSS-1 mission to be farfrom nominal and, in fact, the satellite ended up to be deployed at a maximum distanceof only 256 m (instead of the nominal 20 km). When, after many attempts, the satellitewas unlocked from this position, there was general uncertainty as to the emergedproblems. As a consequence, there was significant concern that further attempts todeploy migth, on the one hand, not lead to a sufficient distance to fulfill primaryelectrodynamic objectives but, on the other hand, could probably result in a loss of thesatellite. This led to a unanimous consensus among the science and engineering teamsto start a satellite retrieval which was then successfully accomplished. In all, thesatellite stayed about 20 hours at the maximum distance with all the experiments (bothon the satellite and on the Orbiter) operating during most of the mission duration.We will give here some comments on the significance of what has been achieved inthe first TSS-1 mission. A main goal of TSS-1, although of an engineering nature, wasobviously that of proving the dynamics of the system. Although the deploymentobtained was clearly insufficient to prove dynamics fully, there were about 20 hours ofstable deployment in the near vicinity of the Shuttle and the operations at suchdistances, which were supposed to be critical for the mission, were fully tested withcomplete control of the tether and satellite dynamics. This experience will certainly beof great significance to the future use of long tethers in space because it showed theTSS to be safe, stable and easy to control.Apart from the dynamic goals, TSS-1 was supposed to constitute a unique activeexperiment in the ionospheric plasma, the most interesting phenomena being related tothe V3B QL voltages associated with the long tether.Turning then to electrodynamics, with the maximum deployed tether length of256 m, the e.m.f. maximum voltage was around 50 V, far below the high-voltage rangecorresponding to the primary electrodynamic objectives of the mission. No substantialFig. 2. – Location of the Italian experiments on the TSS satellite.THE TSS-1 MISSION: RESULTS ON SATELLITE CHARGING 855Fig. 3. – From top to bottom: electron temperature, DCBP potential, tether current and plasmadensity.current was reached on the tether. More precisely, the current was around 20 mA withthe tether connected to the Orbiter ground and the satellite was shown to be either notcharged or charged to at most 10 V most of the time. Although it is still of interest toinvestigate I-V characteristics in the available range of current and voltages, it must beclearly stated that the primary objectives of the mission were not reached. There were,however, some other science objectives, which were considered as secondary in that notdirectly related to having long tether deployment, but still scientifically interesting.Examples include the investigation of electron beam dynamics (through the use ofFPEG), Orbiter charging characteristics and Shuttle glow. These type of objectiveswere successfully accomplished. In fact, the type of experiments which were conductedwere somewhat similar to those on the CHARGE 2 rocket with the difference, however,of a much longer measurement time and of a rather complete diagnostic instru-mentation working both on the Orbiter and on the satellite side.M. DOBROWOLNY, E. MELCHIONI, G. VANNARONI, ETC.8564. – Data results on satellite chargingIn this section we will show some first results relevant to a primary objective ofTSS-1, namely the issue of satellite charging. All of the data shown are from theexperiment RETE and an ammeter on the satellite [3] giving the tether current.The experiment RETE (Research on Electrodynamic Tether Effects) [4] had itssensors on two cylindrical canisters attached to two deployable booms (DRBs) asshown in fig. 2. One of the canisters, called ACBP (AC boom package), was containingelectric dipoles, on three axes, and search coil antennas (on two axes). These allowedmeasurements of electric and magnetic fluctuations in the frequency range160 Hz–12 MHz, which include all relevant characteristic frequencies of theionospheric plasma. A second canister, called DCBP (DC boom package), on the otherboom, was devoted to DC measurements and contained three Langmuir probes. Two ofthese were used as a dipole to obtain wave form electric fields from 0 to 175 Hz. Thethird was operated as a Langmuir probe properly to determine local potential, localelectron density and temperature. In a nominal mission, with the DRBs moving, theFig. 4. – Satellite charging event during TSS-1. From top to bottom: DCBP potential, tethercurrent and plasma density.THE TSS-1 MISSION: RESULTS ON SATELLITE CHARGING 857Fig. 5. – Sequence of Langmuir probe characteristics during the charging event of fig. 4.RETE measurements were supposed to be taken at different radial distances from thesatellite skin (and at different azimuths due to the satellite rotation), so as to clarify thestructure of the space charge region around the satellite and the possible wavephenomena occurring there.Figure 3 shows some interesting data in the context of satellite charging and, ingeneral, I-V characteristics.In the two middle panels we find the potential of the DCBP canister with respect tothe spacecraft, as measured by RETE and the tether current as measured by anammeter on the satellite [2]. The remaining two panels are electron temperature andplasma density as derived from Langmuir probe characteristics. Notice that there is aclear correspondence between current spikes and satellite charging. In fact thecurrent spikes correspond to operations of the electron gun FPEG on the Orbiter side.When the gun fires electrons (up to 100 mA), the satellite, in order to collect thatM. DOBROWOLNY, E. MELCHIONI, G. VANNARONI, ETC.858amount of current, has to charge. This type of data is now being used to derive currentvoltage characteristics of the satellite, although in a very modest current and voltagerange with respect to that expected in a nominal mission.As one can see, the maximum charging in the period shown in fig. 3 amounts to atmost 10 V. Only once during the mission, corresponding to an exceptionally lowionospheric density, we measured a higher level of charging. This is shown in fig. 4where the satellite potential is shown in the upper graph and tether current in themiddle graph. The maximum charging obtained is in this case of about 80 V.Figure 5 shows a sequence of Langmuir probe characteristics obtained along thecharging event of fig. 4. One may notice the change in the appearance of suchcharacteristics. The first three measurements are the typical characteristics of aMaxwellian unperturbed plasma (electrons are in the top part and ions in the lowerpart). Then, upon increase of charging, we see that the characteristics modify. They infact develop an almost linear region which indicates the presence of an electron beam.The interpretation is that, upon increasing the charge of the satellite, the probe findsitself in the space charge region and there electrons from the unperturbed plasma areaccelerated towards the satellite. In particular, the last but one I-V characteristicsshow no current. In this case, ions are completely repelled from the region where ourprobe system is sitting and it can be shown that, in this case, no more current isexpected.Sequences such as these, therefore, indicate the extent of the sheath surroundingthe satellite. It is on the basis of these data, and the density and temperature derivedfrom the Langmuir probe characteristics, that the analysis will proceed.5. – ConclusionsIn conclusion, the capability of deploying at some distance from the Orbiter andthen retrieving a satellite was demonstrated by the first TSS mission. In addition, allthe instrumentation aboard TSS-1 proved to work nominally. Finally, with respect tothe electrodynamic objectives of the mission, we have obtained data which aresignificant for the issue of satellite charging and current-voltage characteristics of thesystem, although, obviously, in a much reduced range (of currents and voltages) withrespect to that of a nominal mission.R E F E R E N C E S[1] DOBROWOLNY M. and MELCHIONI E., J. Geophys. Res. A, 98 (1993) 13, 761, 778.[2] BONIFAZI C., MANARINI G., SABBAGH J., SVELTO F., THOMSON D. C., GILCRIST B. E., BANKS P.M. and DOBROWOLNY M., Nuovo Cimento C, 16 (1993) 515.[3] DOBROWOLNY M. and STONE N., Nuovo Cimento C, 17 (1994) 1.[4] DOBROWOLNY M., MELCHIONI E., GUIDONI U., IESS L., MAGGI M., ORFEI R., DE CONCHY Y.,HARVEY C. C., MANNING R. M., WOUTERS F., LEBRETON G. P., EKHOLM S. and BUTLER A.,Nuovo Cimento C, 17 (1994) 101.

The TSS-1 mission: Results on satellite charging

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The TSS-1 mission: Results on satellite charging

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