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An interstellar "precursor" mission has been under discussion in the scientific community for over 25 years. Fundamental scientific questions about the interaction of the Sun with the interstellar medium can only be answered with in situ measurements that such a mission could provide. The Innovative Interstellar Explorer is a funded NASA Vision Mission Study that investigates the use of Radioisotope Electric Propulsion (REP) to enable such a mission. The problem is the development of a probe that can provide the required measurements and can reach a heliocentric distance of at least 200 astronomical units (AU) in a reasonable mission time. The required flyout speed in the direction of the inflowing interstellar medium is provided by a high-energy launch, followed by long-term, low-thrust, continuous acceleration. Trades from also using gravity assists have been studied along with trades between advanced Multi-mission radioisotope thermoelectric generators (MMRTGs) and Stirling radioisotope generators (SRGs), both powered by Pu-238. While subject to mass and power limitations for the instruments on board, such an approach relies on known General Purpose Heat Source (GPHS), Pu-238 technology and current launch vehicles for

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The proposed Prometheus 1 mission is an ambitious plan to orbit and explore the Jovian moons of Callisto, Ganymede, and Europa. Such an ambitious mission is enabled by the first interplanetary nuclear electric propulsion (EP) system. This EP system pushes the state of the art in power (> 100 kW), specific impulse (> 600 s), and lifetime (> 70,000 hours). A team from the Jet Propulsion Laboratory, Glenn Research Center, Marshall Space Flight Center, and Northrop Grumman Space Technology has collaborated to complete the preliminary system design concept. The EP system consists of three different thruster subsystems and a common high pressure feed system. Primary propulsion for the cruise phase of the mission is provided by six primary and two redundant 30 kW ion thrusters. A high thrust subsystem, consisting of six 20 kW hall thrusters, is provided to allow stable trajectories during the Jovian moon transfer phases. Six primary and six redundant sub kilowatt hall thrusters are provided for attitude control. A common high pressure feed system consisting of a single 12,000 kg capacity tank, an isolation and pressure regulation module, and a xenon recovery system provide xenon propellant to the individual thruster subsystems. Planning and risk analysis activities have been performed to provide confidence in the feasibility of delivering this system in the proposed mission timeframe. I.