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.