While many spacecraft get by just fine on solar power, it won't work efficiently past about Jupiter. Most deep space spacecraft rely on a radioisotope thermoelectric generator, an internal nuclear power source. But for a few decades, NASA has been unable to produce new plutonium-238, an element essential in RTGs that keeps Cassini, the Voyager probes, and New Horizons going.
Now, for the first time in 30 years, new plutonium-238 has been enriched at the Oak Ridge National Labs. The 50 grams produced by the lab need to be verified, but if the process is found to work efficiently, the lab can move up to building more plutonium-238. The plutonium isotope is efficient for spacecraft power systems both because of its relatively low radiation for handlers, its ability to produce plenty of heat to keep instruments warm, and an 87.7 year half-life that can power spacecraft for several decades but not provide lasting damage in case of an accident. .
If ORNL is unable to produce more RTG fuel, NASA only has two to three missions worth of viable plutonium left, barring exploration of the outer solar system, whether a return mission to Pluto, the upcoming Europa mission, any potential mission to Enceladus to search for life, or any possible future voyages to Uranus or Neptune. However, the fuel is also used on many Mars missions, including Curiosity, as well as on the Apollo missions.
Plutonium-238 was swept up in a wave of nuclear disarmament around 1988, when the last batch was produced. This is despite the unsuitability of the fuel for nuclear weapons. Since then, NASA has relied on its own stockpile as well as purchasing some from Russia. With production of plutonium-238 again, NASA can begin to look into the farther reaches of our solar system for exploration opportunities again.