G. Vannaroni

satellite de-orbiting by means of electrodynamic tethers part i: general concepts and requirements

Abstract It has been recently suggested that conductive tethers, besides other applications, may be effective for the de-orbiting of satellites at the end of their operational life. In this paper we give a preliminary evaluation of this capability with respect to a well-defined tether system and using the expertise and results from the recently flown TSS-1 project. The system considered is a long conducting tether, covered by an insulator, with a passive electron collector at the positive termination and a hollow-cathode electron emitter at the opposite end. In analyzing this system, we point out the crucial importance of the contact impedances associated to the coupling between the tether terminations and the ionospheric plasma. We give a first evaluation of the de-orbiting time for a typical case and conclude about feasibility of a tether system to de-orbit satellites.

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.

SATELLITE DE-ORBITING BY MEANS OF ELECTRODYNAMIC TETHERS PART II: SYSTEM CONFIGURATION AND PERFORMANCE☆

Abstract This paper aims to assess the efficiency of a de-orbiting system based upon conductive tethers under realistic assumptions for its interaction with the ionospheric environment. We analyze the configuration made up of a 2.5– 10 km tether, a passive inflatable collector of 2.5– 10 m radius at the positive termination and a hollow cathode at the negative one. Voltages and current in the system are computed from the equation of the equivalent circuit, making use of the IRI-90 ionospheric model. The resulting electromagnetic drag forces have been used to compute the evolution of the orbital elements (especially the semi-major axis) and the re-entry times. Our results indicate that a typical satellite of 500 kg mass at 1300 km altitude can de-orbit in 20–100 days, for a broad range of orbital inclinations and solar activity. The validity of the concept is further strengthened by the comparison with alternative propulsion systems.

Characterization of the interaction between a hollow cathode source and an ambient plasma

The laboratory characterization of the interaction between a hollow cathode plasma source and an ambient plasma of ionospheric type is reported. The investigation was carried out by means of Langmuir probes for various positive polarization levels of the source with respect to the ambient plasma in absence of magnetic field. Measurements put into evidence the formation of a double layer that dominates the current collection process on the hollow cathode source. The electron energy distribution function, derived along the axis of the current path, indicates the presence of an anomalous collisional mechanism due to the bump‐in‐tail instability in the region between the double layer and the source. The experimental data were also compared with the results of a one‐dimensional theoretical model of the interaction; taking into account the approximation of the model, such a comparison indicates that both the position and the voltage drop associated to the double layer match rather well the theoretical predictions.

Plasma waves in the sheath of the TSS‐1R satellite

The current flow in the Tethered Satellite System (TSS) induces a strong excitation of electric and magnetic fluctuations in the potential sheath surrounding the subsatellite. The wave sensors of the experiment RETE (Research on Electrodynamic Tether Effects) have measured power spectra of electromagnetic fields from 180 Hz to 12 MHz, providing information on the physical processes taking place around highly charged bodies in the ionosphere and, in particular, the role of wave-particle interactions and anomalous collisionality. We report on the observations during three events of almost steady current flow at 50, 190 and 55 mA, taking place at positive satellite potentials of about 9, 200 and 2 V. The largest power spectral density occurs at frequencies between 2 and 4 kHz, close to the lower hybrid frequency, where electric field fluctuations up to 12 V/m have been observed. In this frequency band the fields are electrostatic and radially polarized, with a marked ram-wake signature.

Deorbiting with electrodynamic tethers: comparison between different tether configurations

Electrodynamic tethers have been recently proposed for satellite and rocket upper stage deorbiting to mitigate the debris problem at Low Earth Orbits (LEOs). The deorbiting performance of several electrodynamic tethers, where the electron collection from the ionosphere is obtained with either simple bare wires or bare wires terminated with conducting spherical collectors, was analyzed and compared. Our results indicate that the use of the spherical collectors at the positive termination of the system significantly enhances the deorbiting capabilities of the electrodynamic bare tethers.

Deorbiting of LEO satellites with electrodynamic tethers

We analyze and compare to each other the deorbiting performance of several tether systems where electron collection from ionosphere is obtained with either simple bare wires or bare wires terminated with conducting balloons. Besides enhancing the current, the balloon flattens the current profile along the tether and both the effects cause a significant increase of the electrodynamic drag. The first part of paper is focused in evaluating the deorbiting times of a 5km long tether, equipped with balloons of different radii (up to 2.5m) attached to a satellite of 5OOkg mass. This analysis is performed for the simplest case of equatorial orbits, vertically oriented tethers and constant values of magnetic field and orbital velocity. In a second part of paper, referring to a bare tether with a balloon of 2.5m radius, the study is extended to conditions more realistic for practical applications. In particular we consider inclined orbits, a dipole model of the Earth’s magnetic field, the variation of orbital velocity with altitude and, in addition, the possible tether deviation from a vertical alignment under the combined effects of gravity gradient and electrodynamic force. Our resuits -indicate that the use of systems with terminating balloons significantly enhances the deorbiting capabilities of the electrodynamic bare tethers.

Laboratory simulation of the itneraction between a tethered satellite system and the ionosphere

SummaryThe authors report on the measurements performed in the IFSI/CNR plasma chamber at Frascati related to the laboratory investigation of the interaction between a plasma source and an ambient plasma of ionospheric type. Such an interaction is of relevant interest for the possibility of using electrodynamic tethered satellite systems, orbiting at ionospheric altitude, for generating electric power or propulsion in space. The interaction region was analysed at various conditions of ambient magnetic field ((0÷0.5) G) and at different polarization levels of the plasma source ((0÷40) V). The plasma measurements were carried out with a diagnostic system using an array of Langmuir probes movable in the chamber so that a map of the plasma parameters could be obtained at the different experimental conditions.

Interaction of a hollow-cathode source with an ionospheric plasma

Abstract Hollow cathodes to be used as plasma contractors on Rocket-or-Shuttle-based experiments are under development at our Institute jointly with NASA-JSC. Here we report on first results achieved using a large plasma chamber which permits the simulation of an ionospheric plasma with magnetic field compensation and variation. The interaction between a hollow cathode (H.C.) plasma plume ( n e ⋍ 10 14 m 3 ) with an ionospheric plasma beam ( n e ⋍ 10 12 m 3 ) was investigated by means of Langmuir probes at different distances from the H.C. source.

Large plasma density gradients in the low‐altitude low‐latitude ionosphere observed during the TSS‐1R mission

We present results of in situ ionospheric density measurements performed during the TSS-1R tether mission. The almost equatorial orbit of TSS-1R was such that the descending node was syncronous with the day-to-night terminator and the measurements refer to 4 consecutive magnetic equator crossings. The most important feature of the observations is given by the indication of very deep density minima, centered at the magnetic equator, with density values up to a factor 100 smaller than those predicted by the IRI ionospheric model. We will show that a plausible explanation of these pronounced minima can be found in terms of the prereversal enhancement of the E × B drift which is one of the dominant features of the equatorial anomaly. Our data indicate therefore that, near the magnetic equator, the IRI model needs to be updated to include more accurately the sunset strengthening of the plasma drift.