Necessity and feasibility of spacecraft potential control in the Hermean magnetosphere

During its passages through the magnetosphere of Mercury the electron detector on board the Mariner 10 spacecraft provided first evidence of significant positive charging of the spacecraft of the order of 100V positive. At the same time the disadvantages of lacking spacecraft potential control became evident through the loss of information on the low energy part of the plasma distribution functions. This experience and model calculations lead to the conclusion that provisions to keep the potential at a low level are highly desirable, if not mandatory, for satisfactory measurements in the plasma environment of Mercury. The required charge for spacecraft potential control can be provided by emitters of positive ions, as demonstrated by an instrument on the Geotail spacecraft. This paper formulates the scientific and technical requirements on a spacecraft potential control system in the space environment of Mercury. The results of a feasibility study for liquid metal ion emitters in a design which represents a further development of the instruments used on Geotail and in preparation for the Cluster and Interball spacecraft are also presented. It is demonstrated that the large ion currents required for a mission to Mercury can be supplied by moderate mass and power resources.