Charging of mirror surfaces in space

Abstract

Spacecraft often charge to negative potentials of several kilovolts in eclipse at geosynchronous altitudes. We suggest that optical mirrors at geosynchronous altitudes will charge in sunlight as if in eclipse. Modern mirrors can attain very high reflectance, the reflected light being nearly as intense as the incoming light. With high reflectance, the sunlight photon energy imparted to mirror surfaces is greatly reduced, resulting in little or no photoemission. As a result, mirrors will charge as if they would in eclipse, the equilibrium potential being governed by the balance of currents without photoelectrons. When the plasma electron temperature is high, the equilibrium potential may reach several kilovolts negative, despite sunlight. This occurs often in the morning hours and in severe space weather. We stress that in general, the finite reflectance and Sun angle should be included in calculations of spacecraft charging in sunlight. As an important application for mirror charging, we bring to attention recent news, the Boeing 702 model geosynchronous satellite fleet, featuring two long solar panels on each side. Each solar panel is equipped with two mirrors flanking both sides for sunlight enhancement on the solar cells. The entire satellite fleet has suffered a similar fate, namely, gradual, permanent, and sometimes stepwise degradation. While the true cause of the Boeing solar panels deterioration may never be known, we suggest that sudden development of differential charging between the solar panels and the mirrors on their sides could be the culprit. Differential charging of mirrored solar panels may develop rapidly when the satellite is coming out of eclipse. Indeed, the sudden 25% degradation of PanAmSat PAS‐7, a Boeing 702 model satellite, did occur shortly after eclipse exit in the morning of 6 September 2001. Finally, we suggest a simple mitigation method for solving the problem.