Satellite electricity on orbital time scales

Abstract

The theory of electricity on gravity gradient gyrostats is generalized from a static and average charge with the Lorentz torque of previous efforts to the instantaneous and dynamic one here with the introduction of the Faraday torque due to a current along the metallic gravity gradient boom. There are then two sources for roll torque driven by orthogonal components of the geomagnetic field. The mass properties and wheel angular momentum lead to gyroscopic resonances near three times per orbit which greatly enhances the response to tiny charges and currents. The roll and yaw data streams are converted to charge and current by use of the solutions of the forced gyroscopic equations. Data from gyrostats at the (3, 2) and (3, 0) resonances are analyzed in detail to determine the time scales for the electrical flow. Onboard electrical activity with five minute and daily time scales must be due to relays and telemetry. Thus, it is the practice of the gounding of boxes to the bus which is the cause of the librations seen here. The spacecraft current loop is closed by the ambient photo-electric plasma, and it is the slow dissipation of this charge to the environment that limits the librations.