The US space agency Nasa has awarded military and space contractor Northrop Grumman a $400m contract to co-design its Jimo space probe to Jupiter.
Jimo is part of the ambitious Prometheus propulsion technologies programme
The Jupiter Icy Moons Orbiter (Jimo) is part of an ambitious mission to explore the three planet-sized Jovian moons Callisto, Ganymede and Europa.
Designing the spacecraft poses a significant engineering challenge given the long distances involved.
Controversially, the probe is to be powered by a nuclear propulsion system.
Callisto, Ganymede and Europa are thought to harbour oceans of probable water beneath an icy crust.
Jimo would be the first Nasa mission using nuclear electric propulsion.
It would enable the spacecraft to orbit each icy world in turn, to perform extensive investigations of their composition, history, and potential for sustaining life.
Under the contract, Northrop Grumman will work with a US government team to complete a preliminary design for the spacecraft.
The probe's nuclear reactor-based propulsion system will be designed by the US Department of Energy's Office of Naval Reactors. Nasa will supply the mission launch vehicle.
The work includes developing hardware, software and test activities for the design of the non-nuclear portion of the spacecraft.
It also includes developing the interfaces for the spacecraft, space reactor, and science instruments. They will also undertake the assembly, integration, and testing of the hardware.
Data from the Galileo probe suggests Callisto may harbour a salty ocean beneath its surface
Northrop beat Boeing and Lockheed Martin to take the contract, which will cover work over the next four years.
In January, a Jimo science definition team delivered a report to Nasa which strongly advocated the mission carry a Europa lander to search for signs of past or present life on the Jovian moon.
Launch is officially scheduled for 2011, but there are signs this could slip to 2015.
In the past few years, Nasa's embrace of nuclear propulsion technologies has sparked opposition and active protests from anti-nuclear lobbyists.