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The application of a recent concept, a thermionic bare tether (that is, a long conductor coated with a thermionic material), to a practical engineering problem (deorbit space debris from geostationary transfer orbit) is presented. Lorentz drag on a thermionic bare tether, during each pass through an arc close to the perigee, lowers the apogee progressively and produces the object reentry. The performance of a spacecraft equipped with a thermionic bare tether is studied at two different levels, using models that couple thermal and electrical effects. In first place, a simple formula for the eccentricity decrement produced during each perigee pass as a function of TBT properties is derived and used to select TBT optimal dimensions. For a given tether mass, the formula shows that long tethers with small cross-section areas, but large enough to accomplish mechanical constraints, yield the best performance. Second, full numerical simulations of the deorbit maneuver including Lorentz force, air drag, and J2 perturbations are carried out. A spinning thermionic bare tether with a mass of about 16kg, as well as a length, width, and thickness equal to 6 km, 2 cm, and 50 mu m, respectively, passively deorbits a half-ton spacecraft (with a natural deorbit time of about 50 years) in less than six months, without using propellant, expellant, or power supply. The important roles played by the eclipses and the Earth's oblateness on thermionic bare tether performance are highlighted.