Electrical Charging of Shuttle Orbiter

The Space Shuttle Orbiter has the geometrical configuration of an aircraft and nonconducting outer skin that is expected to generate considerable transient charging and local potential differences due to the ambient ionospheric plasma. Emission of high-current electron beams has been proposed for the AMPS Spacelab payload that would induce large potentials and return currents to the orbiter skin. Quantitative estimates are presented for the magnitude and transient behavior for both the passive and active conditions. A comprehensive model of the Orbiter environment has been defined that includes the ambient ionospheric medium at a nominal altitude of 400 km and the neutral gas cloud expelled by the vehicle. The outer skin of the vehicle is covered with ~ 1300 m2 of thermal insulation which is fibrous silicon dioxide, and ~ 60 m2 of metal surfaces on rocket motors and exposed instrumentation on the spacelab pallet. The thermal insulation has the dielectric quality of air and an electrical conductivity that has been extimated by NASA at 10-9 to 10-10 per meter but may be as low as 10-13 mhos/m. Local equilibrium potentials for passive conditions (no electron gun current) have been calculated for various ion, electron, and photon shadow zones. These potentials very from -2.4 volts where only electrons and insulator leakage current are present to +3.9 volts where only ion ram current is collected.