When NASA sends spacecraft to areas where solar power becomes difficult without very large panels that would be expensive and difficult to use, they usually rely on a simple power source called RTGs (Radioisotope Thermoelectric Generators), which create electricity from heat given off by plutonium decay (Pu-238 specifically, a safe form of plutonium, radiation aside). These generators last for years, but the thermocouples used in them are very inefficient. This combined with a plutonium shortage since the 1990s has led NASA to research a more efficient generator. The ASRG (Advanced Sterling Radioisotope Generator) would combine the heat of the plutonium with a simple piston engine to generate power at four times the efficiency, making the plutonium supplies last longer.
Unfortunately, NASA's Planetary Science division recently announced they are canceling work on the ASRG due to budget constraints. The Department of Energy has recently begun making more plutonium for NASA use, but they are making it at a slower rate compared to the amount used on spacecraft like Cassini and New Horizons, meaning future probes could wait years for a sufficient power source using traditional RTGs. Curiously, NASA has been put in charge of the cost of the plutonium manufacturing and storage rather than the Department of Energy, who's traditionally been responsible for it, placing an extra burden that when combined with sequestration forced NASA to abandon the ASRG program. This despite that it would ultimately serve to alleviate the plutonium shortage problem by using less.
To read more about the ASRG and its cancellation, including a video showing how it works, head here: http://www.planetary.org/blogs/casey-dreier/2013/20131115-nasa-just-cancelled-its-asrg-program.html
This is just the latest in a long line of things NASA has had to abandon due to a diminishing budget despite how easily it could be overcome. 0.48% of the federal budget goes to NASA, 1% would be enough to save this and many other projects. It has been done before (and beyond), it can be done again. Let Congress know what you think: http://www.penny4nasa.org/take-action