NASA is reviewing a list of fission-powered missions that could pre-empt the Jupiter Icy Moons Orbiter (JIMO) effort now being eyed for space travel no sooner than 2015.

A special study team has identified six potential candidate missions that could be done sooner, have shorter mission durations, and would be far less difficult to implement.

JIMO has been touted as the flagship mission for Project Prometheus. JIMO would be the space agency's first mission using nuclear electric propulsion. In September, NASA selected Northrop Grumman Space Technology as the contractor for the proposed Prometheus JIMO spacecraft.

The Prometheus JIMO mission has been billed as part of an ambitious mission to orbit and explore three planet-sized moons -- Callisto, Ganymede and Europa -- of Jupiter. The moons may have vast oceans beneath their icy surfaces. A nuclear reactor would enable the mission.

JIMO would orbit each icy world to perform extensive investigations of their composition, history, and potential for sustaining life.

However, an analysis of alternative mission ideas was completed last month at the Jet Propulsion Laboratory in Pasadena, California. The six ideas are:

         Technology Demonstration Mission to test fission power system in deep space with no specific science goal or destination.

         Lunar Geophysical Orbiter that in extended mission mode could serve as a telecom asset for future lunar missions.

         Next Generation Mars Telecommunications Station.

         Near Earth Object (NEO) Asteroid Mission that would involve stopovers at multiple objects, perhaps landing hardware on a NEO to assess the ability to modify the trajectory of a celestial body.

         Venus Orbiter, more like a Magellan II spacecraft that would carry out low altitude runs over the cloudy planet with state-of-the-art radar.

         Astrophysics Mission that would use high power levels from a fission power source, likely sending collected science information at very high data rates.

In addition to these missions, a Europa Orbiter mission for a 2012 launch, using chemical propulsion, would have the spacecraft energized by radioisotope power system (RPS) technology.

Further work on fleshing out these candidate ideas will be undertaken by the JPL-led study group, looking at variants and options for each mission.