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Artists's Conception of Jupiter Icy Moons Orbiter

The Jupiter Icy Moons Orbiter (JIMO) was a proposed spacecraft designed to explore the icy moons of Jupiter. The main target was Europa, the suspected ocean of which is one of the places where simple alien life is a possibility in our solar system. Ganymede and Callisto, which are now thought to have liquid, salty oceans beneath their icy surfaces, were also targets of interest for the probe.

Due to a shift in priorities at NASA that favored manned space missions, the project lost funding in 2005, effectively cancelling the JIMO mission. Source. Engineers at the Jet Propulsion Laboratory connected with JIMO were laid off or reassigned during the spring and summer of that year.

The US president's 2006 budget request to Congress essentially cut funding for JIMO. Among other issues, the proposed nuclear technology was deemed too ambitious, as was the multiple-launch and in-orbit assembly mission architecture. NASA is instead considering a demonstration mission to a target closer to Earth to test out the reactor and heat rejection systems. The spacecraft would possibly be scaled down from its original size as well.

When it was cancelled, the JIMO mission was in its 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 viable means of spacecraft propulsion.

JIMO was to have a large number of revolutionary features. Throughout its main voyage to the Jupiter moons, it was to be propelled by an ion propulsion called HiPEP, 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 RTG based power system, 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 Jupiter's moons, creating more thorough observation and mapping windows than exist for the 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 must be launched into orbit is minimized. This configuration is thought to be less risky than the radioisotope thermoelectric generators (RTGs) used on previous missions to the outer solar system. RTGs contain large amounts of highly radioactive material which generate heat for the thermoelectric couples.

Northrop Grumman was selected on September 20, 2004 for a $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.

• Science payload mass: 1500 kg
• Electric turboalternators: multiple 104 kW (440 V AC)
• Deployable radiator: 422 m² surface area
• Electric Herakles ion thrusters: multiple 30 kW high efficiency, specific impulse 7000 s (69 kN·s/kg)
• Hall thrusters: high power, higher thrust
• Telecommunications link: 10 Mbit/s (4×250 watt TWTA)
• Deployed size: 58.4 m long × 15.7 m wide
• Stowed size: 19.7 m long × 4.57 m wide
• Mission design life: 20 years
• Launch date: 2017
• Launch Vehicle: Delta 4H.

Source

• Project Prometheus webpage at NASA
• Animation outlining the mission