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Jupiter Icy Moons Orbiter (JIMO) is an ambitious project that aims to visit and collect data on Jupiter's icy moons: Callisto, Ganymede and Europa.

Haven't we done this already?

It is true that we already have quite a lot of data on these moons. Galileo made many flybys over the years. In many ways though this data has just whetted our appetite, instead of flybys, scientists want to put a spacecraft in orbit in order to conduct detailed observations. Galileo's data is strongly in favour of the presence of water below the surface of these moons. Scientists want to verify this and they want to know more: how much water is there, how deep below the surface? And of course the ultimate question: could life be hiding there? JIMO would orbit these moons and have far more sophisticated instruments and onboard computers than Galileo and would therefore be able to returned far more detailed information.

Science goals:

  • Evaluate whether the moons have the potential for sustaining life. This would include verifying the presence of subsurface oceans, searching for signs of organic compounds and determining the thickness of ice layers. Investigation of potential landing sites is also a priority.
  • Investigate the origin and evolution of these moons. This includes determining inner structure, surface features and composition.
  • Determine the radiation environments around these moons. Callisto, Ganymede and Europa are all within the powerful magnetic field generated by Jupiter. How has this affected the moons and their potential for bearing life?
The scientific payload has yet to be determined, but some of the basic characteristics are known. Very likely candidates for instrumentation include : It is expected that the whole of Callisto and Ganymede and roughly half of Europa will be mapped with sufficient resolution to see details as small as a house.


JIMO's mission plan is to first enter an orbit around Jupiter before moving on to orbit 3 of its moons. Entering and leaving orbits requires energy, in other words you have to fire rockets, and rockets use fuel. Chemical propulsion systems require a lot of propellant and there are currently hard limits on the mass of an object we can send to Jupiter. JIMO on the other hand will use an ion drive. Ion drives are very fuel efficient if somewhat low power propulsion devices, however unlike current probes which use solar panels or radioisotope thermoelectric generators, JIMO's power source will be a nuclear reactor which will enable the use of a far more powerful ion drive. After Deep Space 1 became the first spacecraft to use an ion drive as its primary propulsion system, the reliability of the technology is no longer in question.

Nuclear Power

JIMO will be powered by a nuclear fission reactor. While many probes travelling to the outskirts of our solar system have used RTGs, it will be the first time an actual fission reactor is sent this far out into space. A space reactor has a different design from a terrestrial reactor, one of the obvious differences is that they are much much smaller. The second key feature is they are designed to remain inactive (and therefore not at risk from a core meltdown or other types of catastrophic failure that can occur during operation of a nuclear reactor) until specifically signalled to initiate operation. A major concern in the design is a nuclear powered spacecraft is protecting the scientific payload from the heat and radiation of the reactor. Current sketches of JIMO show the reactor pod at the end of a long boom laden with head-shedding radiator panels.

Why nuclear?

Up until now radioisotope power systems have sufficed for NASA's deep space long-term power needs, however it is expected that the shift from surveying planets to exploring them will cause power requirements to exceed the capabilities of radioisotope power systems. Nuclear fission would allow the use of a wider range of instruments, increased data transmission speeds (through the use of a more powerful transmitter) and the use of high powered ion drives which in turn would allow more direct trajectories and more extensive orbital manoeuvres. Current radioistope power sources provided a few hundred watts whereas a fission based system could provide tens to hundreds of kilowatts of power.


This is obviously a key issue as no one wants the fragments of a broken nuclear reactor in their back garden. The ability of a space reactor to remain in an inactive state until the spacecraft has been launched does however limits risks due to a launch failure. NASA's designs will be reviewed for safety by an independent organism and ultimately by the Office of the president. Another concern is the risk of an old, out of control satellite re-entering earth's atmosphere. Although in the case of deep space missions such as JIMO this is highly unlikely, NASA plans to use "safekeeping" orbits designed to let spacecraft float in space indefinitely for missions where this could be a risk.

It is likely that JIMO's use of nuclear power will generate much controversy. There were numerous protests over Cassini's RTG, because it contained substantially larger amounts of plutonium than previous probes and because of the relatively close flyby of Earth it performed on its way to Saturn. The idea of sending a full-blown nuclear reactor into space will probably trigger similar reactions.

Project Prometheus

Project Prometheus is NASA's project for the investigation of radioisotope power systems and nuclear power and propulsion for the exploration of the solar system. NASA aims to prove the a nuclear reactor powered spacecraft is not only possible but safe and reliable in long term deep space missions. Nuclear fission systems would provide spacecraft with lots more power allowing more power hungry instruments and faster data transmission speeds. JIMO would be the first mission to incorporate this new technology. It is not however the first time a nuclear reactor has been put into space, as early as 1965 NASA succesfully launched the SNAP-10 space reactor.


Things are still hazy at this point in time as the project is still very far from being finalised. Launch would probably occur in 2011 or after, onboard some type of classic expendable launch vehicle. Once in high earth orbit the spacecraft would use its ion drive to start the journey to Jupiter. Once in orbit around Jupiter, JIMO would orbit Callisto, Ganymede and finally Europa. Europa is surrounded by intense radiation belts that will be dangerous for the ship's electronics. It is thought that damage from radiation is what will limit the duration of this mission, which is why Europa will be the last moon explored.

JIMO is still on the drawing board and in the past many projects have been cancelled at this stage. However the National Research Council ranked a Europa orbiter proposal as a top priority for a flagship mission because of the discovery of Europa's ocean and its potential for life. This obviously puts JIMO in a very good position.


In the end it doesn't look like JIMO will happen. In its 2006 budget plan, NASA declared that the project was too ambitious.

Sources: http://www.jpl.nasa.gov/jimo/

Update 21/09/2004: Northrop Grumman has been awarded a $400 million contract to co-design the probe.

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