Project Prometheus

NASA said the name Prometheus indicates its hopes of establishing a new tool for understanding nature and expanding capabilities for the exploration of the Solar System.

Due to their distance from the Sun, spacecraft exploring the outer planets are severely limited in that they cannot use solar power as a source of electrical energy for onboard instrumentation or for ion propulsion systems.

Previous missions to the outer planets such as Voyager and Galileo probe have relied on radioisotope thermoelectric generators (RTGs) as their primary power source.

Unlike RTGs which rely on heat produced by the natural decay of radioactive isotopes, Project Prometheus called for the use of a small nuclear reactor as the primary power source.

[5] In September 2004, JPL chose Northrop Grumman Space Technology to co-design the projected Prometheus spacecraft.

Nuclear reactors could be used to power ion engines such as this one used on Deep Space 1 .
Prometheus I (Jupiter Icy Moons Orbiter)