It is unlikely that the United States will be in any position, after Bush is done with it, to do any real planetary space science with robot craft. We are doomed.
Still these folks under thinking about what a mission would have looked like, had the citizens of the United States retained some of their collective sanity:
""It's all part of the history of our solar system," said Andrew Ingersoll, a study leader and planetary scientist at the California Institute of Technology (CalTech), in a telephone interview. "Neptune and Uranus are ice giants, and made mostly of heavier stuff than Jupiter or Saturn."
Ingersoll and his colleagues envision a Cassini-like mission that could use conventional rocket propulsion and gravity assists to reach Neptune.
Meanwhile, another version of the Neptune mission, which features the use of a nuclear fission reactor and ion propulsion to reach the ice giant and a timescale that spans two decades, is also under scrutiny.
"What makes Neptune unique is Triton," explained David Atkinson, a University of Idaho professor and the science principal investigator for the second study, in an e-mail interview. "It is speculated that Triton is actually a Kuiper Belt Object that was captured by Neptune."
Boeing Satellite Systems' Bernie Bienstock leads the second study with Atkinson...
...Ingersoll's Neptune-bound craft would take a page from many of NASA's far-flung planetary exploration missions and rely on radioisotope thermal generators (RTGs), a long-lasting battery fueled by plutonium, for electric power. The Cassini orbiter currently at Saturn, for example, uses RTGs for power since the vast distance makes solar panels unpractical.
"Yes, we'd need RTGs and yes RTGs carry plutonium," Ingersoll said, adding that the power source can only a danger if it is vaporized over a city, a very unlikely case since most launch scenarios would have them dropping into the ocean in an emergency. "There's been a lot if irrationality about nuclear power and fuels."
Ingersoll's team estimates their spacecraft would take about 12 years to reach Neptune, but stopping once it arrives may be a challenge. His team is studying how to use aerocapture, a maneuver that allows a spacecraft to enter orbit around a planet using the atmosphere and no fuel..."
http://www.space.com/businesstechnology/technology/neptune_orbiter_techwed_041215.html