The Next Generation Gravity Mission and the qualification of the indium-fed mN-FEEP thruster

ESA’s Next Generation Gravity Mission (NGGM) is a candidate Mission of Opportunity for ESA-NASA cooperation in the frame of the MAss change and Geosciences International Constellation (MAGIC) . The mission aims at enabling long-term monitoring of the temporal variations of Earth’s gravity field at relatively high temporal (down to 3 days) and increased spatial resolutions (up to 100 km) at longer time. Such variations carry information about mass change induced by the water cycle and the related mass exchange among atmosphere, oceans, cryosphere, and land, and will complete our picture of global and climate change with otherwise partial or unavailable data. Over the last 15 years, numerous system and technology activities have been initiated by the Earth Observation Programmes (EOP) Directorate of the European Space Agency with the aim of advancing the maturity of the NGGM system and the key subsystems: particular attention was devoted to the design of the fine attitude control system, enabled by proportional thruster like variable specific impulse electrostatic thrusters based on Field Emission Electric Propulsion (FEEP), in which a liquid propellant is electrostatically extracted and accelerated to high exhaust velocity. The core element of this propulsion technology is a passively-fed, porous tungsten crown emitter, consisting of 28 sharp needles. This emitter technology has been developed and qualified over more than a decade at FOTEC and the Austrian Institute of Technology under ten ESA/EOP contracts since 2005 targeting the NGGM needs and it has recently been adapted for use as the main propulsion system in commercial nano- and small-satellites. This paper summarizes the development efforts of the last decade and provides an assessment of the performance of this thruster technology, after extensive simulation and testing campaigns.