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Electrospray (ES) thrusters are unique in their ability to emit both positive and negative beams, each contributing similarly to thrust. Spacecraft charging could be prevented without a dedicated neutralizer if currents of simultaneously emitted opposing polarity beams were matched; despite a lack of implicit coupling between them. We discuss and evaluate experimentally both an active current-balance control circuit and a passive self-balancing configuration. Our highly ionic ES source includes a pair of machined porous glass ES emitter arrays in a single holder. In the passive configuration, unbalanced charge collection on a common floating extractor yields rapid charging towards artificially imposed 200 V limits without means for charge suppression, showing the unsuitability of that architecture. Active balancing of emitted currents was fully effective at charge neutralization during stable periods of emission, yet insufficient during potential alternations. We present methods to improve this approach, including demonstrating beam current modulation at up to 450 Hz. Direct thrust measurements when simultaneously emitting opposing polarity beams of the ionic liquid 1-ethyl-3-methylimidazolium tetrauoroborate are presented. Direct thrust measurements up to 35 µN were in excellent agreement with commanded levels and computations based on measured currents (up to 190 µA) and voltages (up to 2100 V).

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<p>The objective of ESA’s Next Generation Gravity Mission (NGGM) is long-term monitoring of the temporal variations of Earth’s gravity field at high temporal (down to 3 days) and spatial (100 km) resolution. 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 Change with otherwise unavailable data. The observable is the variation of the distance between two satellites measured by a laser interferometer; ultra-precise accelerometers measure the non-gravitational accelerations to correct the gravity signal in the data processing. The optimal satellite system comprises two pairs of satellites on low (between 396 and 488 km) circular orbits, at 220 km separation, one pair quasi-polar and the other around 65°-70° inclination. The satellite-to-satellite tracking technique for detecting the temporal variations of gravity was established by GRACE (300-400 km spatial resolution at monthly intervals) using tracking in the microwave band. Today, GRACE is being continued by GRACE-Follow-On, with similar objectives, where the laser interferometry has improved the measurement resolution by a factor of 100 (upper MBW). At 150 km spatial resolution, mass change would become observable in 80% of all significant river basins, against 10% achieved with GRACE. High temporal resolution will reveal large-scale sub-weekly mass variations, with applications in water and emergency management. </p><p>NGGM is a candidate Mission of Opportunity for ESA-NASA cooperation in the framework of MAGIC (MAss Change and Geosciences International Constellation). The MAGIC constellation will build upon the heritage from the GOCE, GRACE and GRACE-FO missions, the ESA NGGM Phase 0 System studies and past technology pre-developments on laser ranging interferometry, and other key technologies, developed over the years in preparation for NGGM and for the LISA mission. MAGIC will be composed of two pairs of satellites. The first Pair (P1) is to be implemented via a DE-USA fast-paced cooperation programme to ensure continuity of observations with GRACE-FO, with some potential ESA in-kind contributions. The second pair (P2) is to be implemented via a Europe-USA cooperation programme with some potential NASA in-kind contributions with a target launch date compatible to maintain at least 4 years of combined operations.</p><p>The presentation focusses on the on-going Phase A system design, giving an overview of the activities at system and technology level for NGGM, as currently running at the European Space Agency.</p>