The SMART-1 Spacecraft Potential Investigations

The SMART-1 mission was successfully completed on September 3, 2006, with a controlled crash on the visible side of the Moon. The primary electric-propulsion (EP) system included a nonredundant SNECMA PPS-1350-G Hall effect thruster. The thruster discharge was electrically floating with respect to the spacecraft ground. The cathode floating potential (U _CRP) is an important thruster parameter, giving information on cathode performance and life expectancy. During ground testing and steady-state operation, U _CRP was approximately constant and negative. However, in space, it was observed that the U _CRP was slightly positive and was periodically varying in time by a few volts. The capability to compare to ground-test data and check the cathode good health and life potential was largely impaired. Fortunately, SMART-1 included a plasma diagnostic package (electric-propulsion diagnostic package) with a Langmuir probe and a retarding-potential analyzer, both positioned on the same plane as the plasma thruster and provided data on the plasma potential as well as the energy of the charge-exchange ions. The short- and long-term correlations of these data with the cathode reference potential (CRP) were performed. It was concluded that the U _CRP is evolving with respect to the plume potential near the cathode, and this potential difference is shown to be constant over the mission. Plume-potential distribution is adapting itself relative to the local plasma potential. The spacecraft potential with respect to the plasma potential is defined in steady state when the electron and ion currents entering and leaving the spacecraft are balanced. This balance depends primarily on the solar-array electrical configuration and orientation and thruster operating conditions. When the local plasma is chosen as a reference, the absolute cathode potential is found to be the same as the one on the ground. This paper confirms - - the good health of the cathodes. Contrary to a spacecraft without EP, the SMART-1 spacecraft potential varies within a limited range and is comparatively close to zero during electric-thruster operation. In addition, U _CRP can be used to derive the spacecraft potential throughout the mission. All the various cyclic and long-term effects influencing the spacecraft potential on SMART-1 have been identified and characterized across the mission lifetime. In particular, a strong correlation between the solar-array rotation and the variation of the spacecraft potential has been established.