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 pot ential control became evident through the loss of information on the l ow energy part of the plasma distribution functions. This experience a nd model calculations lead to the conclusion that provisions to keep t he potential at a low level are highly desirable, if not mandatory, fo r satisfactory measurements in the plasma environment of Mercury. The required charge for spacecraft potential control can be provided by em itters of positive ions, as demonstrated by an instrument on the Geota il spacecraft. This paper formulates the scientific and technical requ irements on a spacecraft potential control system in the space environ ment of Mercury. The results of a feasibility study for liquid metal i on emitters in a design which represents a further development of the instruments used on Geotail and in preparation for the Cluster and Int erball spacecraft are also presented. It is demonstrated that the larg e ion currents required for a mission to Mercury can be supplied by mo derate mass and power resources. (C) 1997 Elsevier Science Ltd.