The Jupiter Icy Moons Orbiter, or Jimo, would spend month-long stints circling the moons Callisto, Europa and Ganymede, which are believed to have vast oceans tucked beneath thick covers of ice.
The unmanned craft, far larger and more powerful than any other sent to explore the outer solar system, would spend years studying the moons' makeup, geologic history and potential for sustaining life, as well as Jupiter itself.
Besides water, the moons appear to contain two other ingredients necessary for life: energy and the right chemicals. Along with Mars, they are considered the most likely places to have extraterrestrial life within our solar system.
"We don't know if life is there. But this mission will allow to ask that question with some pretty sound tools," said Christopher McKay of the National Aeronautics and Space Administration's Ames Research Center.
Jimo won't launch until at least 2011. On Monday, scientists at the fall meeting of the American Geophysical Union briefed reporters on the mission's progress.
The spacecraft would be the first in a series of robotic NASA probes that rely on uranium-fueled fission reactors to generate large amounts of electricity. While probes such as Galileo and Cassini have made do with hundreds of watts of electricity, Jimo might have thousands of watts to power its thrusters and instruments, said Torrence Johnson of NASA's Jet Propulsion Laboratory.
The reactor conceivably could produce enough electricity to power several U.S. homes. That could provide Jimo a hundredfold boost over previous missions in the amount of data it would be able to beam back to Earth.
Jimo would carry high-resolution cameras and other instruments, including radar and lasers to map the thickness and elevation of the ice that envelops each moon.
Scientists are keen to study the Jovian system because of its complexity. The planet and its stable of moons represent, in many ways, a miniature solar system.
"These are worlds in their own right," said Ron Greeley, of Arizona State University, Tempe.
The spacecraft is envisioned as being 60 to 100 feet in length. Early conceptions place its nuclear reactor at the end of a boom to shield the scientific instruments from radiation.
Jimo also would bristle with fins to dissipate the intense heat from its reactor.
NASA is expected to begin stepping up use of nuclear power in its exploration of the solar system, including Mars. There, it could power rovers capable of roaming the planet for years at a time.