Linear stability of electrodynamic tethers

We investigate the dynamic stability of electrodynamic tethers of the type envisaged for future applications like deorbiting spacecraft and/or propulsion. In suc tethers, electrodynamic forces couple the tether natural oscillations in the orbital plane with those out of the orbital plane. The investigation is done with reference to a model where the time variation of the tether current is assumed to be given. In addition, we address both the case of a bare tether and that of an insulated conducting tether collecting current through a terminating balloon. In the linear approximation, we find possible instabilities of the tether libration modes, at moderate current values, for both the case of a bare tether and that of an insulated conducting tether. Instabilities are also found for the lateral oscillations (string modes) and the case of a bare tether. The equations obtained allow a discussion of the instability mechanisms.