Active Spacecraft Potential Control for the Magnetospheric Multiscale Mission

The NASA Magnetospheric Multiscale (MMS) mission to be launched in 2014 is going to study events of magnetic reconnection in the Earth's magnetosphere by means of four observatories in close formation on near-equatorial orbits. The required highly accurate and undisturbed plasma and electric field measurements will be supported by an actively controlled spacecraft potential provided by Active Spacecraft Potential Control (ASPOC) instruments. ASPOC uses capillary-type liquid metal ion sources with indium as the source material to generate ion beams. Two instruments per spacecraft will generate beams in antiparallel direction. The instruments for MMS will be based on heritage from the ESA Cluster and Double Star missions, but several significant changes and improvements to accommodate the requirements of the new mission have been implemented.