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 the 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.
The JIMO spacecraft 
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 system via either the HiPEP 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 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 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 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.
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.
Preliminary design specifications 
- Science payload mass: 1,500 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 7,000 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
Mission profile 
Three launches were planned for May of 2015 to LEO in order to assemble the two transfer stages and the probe. Transfer stages were designed to launch the probe on its trajectory to Jupiter during the launch window extending from late October 2015 to mid-January 2016. During the first month of flight, the probe's main structures would be deployed, the nuclear reactor activated, and the thrusters tested. Interplanetary flight would have lasted until April 2021 (the ion engines were supposed to work two third of the time). Once the probe was in the influence area of Jupiter, the navigation would become more complex and difficult. The probe would have to use gravity assist maneuvers to enter orbit. The probe would have studied Callisto, then Ganymede, for 3 months each and finally Europa for 1 month (a study of Io was also planned when the orbital conditions would have been favorable). At the end of the mission in September 2025, the vehicle would have been parked in a stable orbit around Europa.
Due to a shift in priorities at NASA that favored manned space missions, the project lost funding in 2005, effectively cancelling the JIMO mission. Among other issues, the proposed nuclear technology was deemed too ambitious, as was the multiple-launch and in-orbit assembly mission architecture. Engineers at the Jet Propulsion Laboratory with JIMO were laid off or reassigned during the spring and summer of 2005.
As a result of the budget changes, 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 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.
After JIMO, NASA and ESA planned a joint mission to Jupiter's moons, the Europa Jupiter System Mission. This joined mission was also cancelled in 2011.
ESA has since then continued separately on the design and on 2 May, 2012 selected the Jupiter Icy Moon Explorer mission over two other ESA missions. The JUICE mission will study the Jupiter moons Europa, Callisto and Ganymede and be launched as an ESA L-class mission in 2022 on an Ariane 5 carrier rocket.
- Berger, Brian (7 February 2005). "NASA 2006 Budget Presented: Hubble, Nuclear Initiative Suffer". Space.com. Retrieved 2007-06-06.
|Wikimedia Commons has media related to: JIMO|
- Project Prometheus final report (2005)
Project Prometheus webpage at NASA(defunct)
- Animation outlining the mission