The main objective is to initiate the development of a miniaturised Gridded Ion Engine capable of delivering thrust up to 2mN for the lateral drag force compensation and the attitude control of the future NGGM (Next-Generation Gravity Mission), as well as other EO missions requiring fine attitude and orbit control by means of low-noise force actuators, e.g. the possible high-resolution GEO imaging mission

Since 2003 ESA has promoted studies in order to establish scientific requirements, identify the most appropriate measurement techniques, start the associated technology developments, and define the system scenarios for a Next-Generation Gravity Mission (NGGM). Such a mission will make use of the Low-Low Satellite-to-Satellite Tracking (LL-SST) technique to monitor the temporal variations of the Earth gravity field over a long time span but with much higher spatial resolution and with improved temporal resolution. The low-thrust Gridded Ion Engine (GIE) with variable thrust has been identified as one of the key technologies for the realisation of the NGGM. In particular, it allows:-satellite orbit maintenance at its operational altitude;-satellite formation control;-implementation of the drag-free control at level of each satellite;-attitude control of each satellite;-laser beam pointing control.For the broad range of NGGM applications, more than one thruster typology is required, depending also on the satellite configuration and on the type of formation geometry. For the simplest in-line formation, in which the satellites chase each other along the same orbit and experience a main drag force always along the same axis, the need of two types of thrusters has been identified (main thrusters and lateral thrusters). Among the various electric propulsion technologies reviewed for this application, the Gridded Ion Engines (GIE) appears very promising for the NGGM due to its scalability. Indeed, it seems possible to implement the GIE thrusters in the required two typologies:-main GIE thrusters for the along-track drag force compensation, orbit and formation control; -lateral miniaturised GIE thrusters for the lateral drag force compensation and the attitude/laser beam control. Development up to breadboard level of the miniaturised GIE has been initiated by the Agency in 2007 leading to a first breadboard demonstrating capability to deliver thrust in the 50 - 500 microN range. In line with the latest results from the NGGM preparatory studies additional work is required at thruster level to demonstrate capability to fully meet the NGGM specific needs: such as dynamic thrust range increase (50 microN to 2 milliN) to cope with the large variation of the drag forces encountered in a long duration mission, minimisation of the specific power, specific impulse increase. The activity aims at further investigating the miniaturised GIE thruster and breadboarding a model covering up to 2 milliN thrust. The thruster shall demonstrate compliance to the major NGGM performance requirements via testing. Particular attention is required for the thrust noise density requirement in the frequency region from 1mHz to 0.1Hz for which direct measurements shall be performed.The technology will be applicable to many other missions, for EO but also for other objectives, which require precise attitude and orbit control by means of low-noise force actuators that are widely configurable. Examples include the high-resolution geostationary (or geosynchronous) imaging mission concept under definition by the Agency and the formations of new missions with EO operational missions (e.g., EO convoy concept).