The Jupiter Icy Moons Orbiter (JIMO) was a proposed NASA spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, where an ocean of liquid water may harbor alien life. Ganymede and Callisto, which are now thought to also have liquid, salty oceans beneath their icy surfaces, were also targets of interest for the probe.

Throughout JIMO's main voyage to the Jupiter moons, it was to be propelled by an ion propulsion system via either the High Power Electric Propulsion or NEXIS engine, and powered by a small fission reactor. A Brayton power conversion system would convert reactor heat into electricity. Providing a thousand times the electrical output of conventional solar- or radioisotope thermoelectric generator (RTG)-based power systems, the reactor was expected to open up opportunities like flying a full scale ice-penetrating radar system and providing a strong, high-bandwidth data transmitter.

Using electric propulsion (8 ion engines, plus Hall thrusters of varying sizes) would make it possible to go into and leave orbits around the moons of Jupiter, creating more thorough observation and mapping windows than exist for current spacecraft, which must make short fly-by maneuvers because of limited fuel for maneuvering.

The design called for the reactor to be positioned in the tip of the spacecraft behind a strong radiation shield protecting sensitive spacecraft equipment. The reactor would only be powered up once the probe was well out of Earth orbit, so that the amount of radionuclides that would be launched into orbit is minimized. This configuration is thought to be less risky than the RTGs used on previous missions to the outer Solar System, because a reactor can be launched with minimally radioactive uranium fuel unlike an RTG whose fuel releases energy from spontaneous decay as opposed to chain-reaction triggered fission. Such a reactor's fuel's radioactivity would only be created by subsequent operation of the reactor.

The Europa Lander Mission proposed to include a small nuclear-powered Europa lander on JIMO. It would travel with the orbiter, which would also function as a communication relay to Earth. It would investigate Europa's habitability and assess its astrobiological potential by confirming the existence of water within and below Europa's icy shell, and determining its characteristics.

Northrop Grumman was selected on September 20, 2004 for a US$400 million preliminary design contract, beating Lockheed Martin and Boeing IDS. The contract was to have run through to 2008. Separate contracts, covering construction and individual instruments, were to be awarded at a later date.

The JIMO mission was to include a nuclear electric propulsion system named HiPEP with power provided by a small 200 kWe fission power system. The nuclear propulsion program was conducted from 2003 to 2005 by the Naval Reactors branch of the DOE.

The proposed system design was a gas-cooled reactor and Brayton power conversion to generate a peak output of 200 kilowatts of power.