Busek proposes to continue development of an engineering model radio frequency discharge, gridded micro ion thruster that produces sub-mN to mN thrust precisely adjustable over a wide dynamic thrust range. Phase I testing proved the feasibility of the concept and estimated performance was exceptional. Measuring beam current and voltage, a thrust up to 1.6mN, thrust efficiencies up to 40% and propellant utilization up to 80% were estimated. Specific impulse ranged from 500-3000seconds. Beam power maximum was 38 watts and the rf power at this maximum was about 30watts total and approximately 20 watts rf deposited into the plasma. The rf discharge ionizer eliminated any need for an internal cathode. In Phase II, Busek will measure thrust directly, characterize the thruster over a wide range of operating conditions and predict lifetime. MIT will advance a numerical simulation of the rf thruster to be used for scaling and design purposes. For missions with cryogenic sensors, a thruster utilizing non-condensable propellant(s) will be evaluated. In addition, a beam neutralizer will be investigated. An rf cathode or carbon nanotube field emission cathode are the two most promising neutralizers.