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Robotic exploration of the outer solar system has increasingly relied on radioisotope thermoelectric generators, or RTGs, rather than solar panels, because sunlight intensity diminishes sharply with distance from the sun, rendering conventional photovoltaic arrays impractical beyond the orbit of Jupiter. RTGs convert the heat released by the natural radioactive decay of isotopes, most commonly plutonium-238, into electricity through thermocouples, providing a steady power source that can last for decades without moving parts or the need for direct sunlight. This reliability has made RTGs indispensable for long-duration missions to Saturn, Uranus, Neptune, and beyond, where a spacecraft may operate for twenty years or more before reaching its final target and continuing to transmit data afterward. However, plutonium-238 is not a byproduct of standard nuclear reactor operation and must be deliberately manufactured through a specialized production process, which several countries, including the one that pioneered its use in deep-space missions, largely suspended for decades due to cost and proliferation concerns. This created a global shortage that constrained the planning of new missions, since without a guaranteed supply of the isotope, engineers could not confidently design power systems for spacecraft with multi-decade lifespans. In recent years, renewed production efforts have partially alleviated the shortage, though output remains limited relative to the ambitions of proposed missions to icy moons and distant dwarf planets. Some engineers have proposed hybrid power architectures that combine smaller RTGs with advanced battery storage and highly efficient electronics to reduce the total plutonium required per mission, extending the available supply across a larger number of spacecraft. Whether these efficiency gains can keep pace with the growing scientific appetite for outer solar system exploration remains an open question that will shape mission planning for the coming decades.