NASA plans to dispatch a hulking nuclear-powered spacecraft to determine whether three of Jupiter's icy, planet-sized moons have the potential to harbor life. The Jupiter Icy Moons Orbiter, or Jimo, would spend monthlong 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.


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