Spin Decay, Spin-Precession Damping, and Spin-Axis Drift of the Telstar Satellite

For spin stabilization of a communications satellite, it is required that the satellite be statically and dynamically balanced so as to make the principal axis of maximum moment of inertia coincide with the axis of symmetry of the antenna pattern, about which the satellite is given an initial spin. This principal axis, referred to henceforth as the spin axis, is in line with the invariant angular momentum vector and is thus fixed in direction, as desired, in an inertial space, provided there are no external torques acting on the spinning satellite. However, as the satellite is spinning and traveling in the geomagnetic field, eddy-current and magnetic torques continuously act on the satellite so that the angular momentum changes its magnitude and direction, as characterized by spin decay and spin precession. As a consequence of spin decay, the satellite becomes less stable for the same external disturbing torques, and a tumbling motion may eventually result. The precession of the spin axis about the instantaneous angular momentum vector will cause wobbling of the antenna 21G9