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.

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.

[2] Northrop Grumman was selected on September 20, 2004 for a US$400 million preliminary design contract, beating Lockheed Martin and Boeing IDS.

During the first month of flight, the probe's main structures would be deployed, the nuclear reactor activated, and the thrusters tested.

Among other issues, the proposed nuclear technology was deemed too ambitious, as was the multiple-launch and in-orbit assembly mission architecture.

[citation needed] When it was cancelled, the JIMO mission was in an early planning stage and launch wasn't expected before 2017.

It was to be the first proposed mission of NASA's Project Prometheus, a program for developing nuclear fission into a means of spacecraft propulsion.