Far beyond the reach of strong sunlight, some of humanity’s greatest explorers rely on one of the most dependable power sources ever built: Radioisotope Thermoelectric Generators (RTGs). Powering exploration beyond the Sun In the vast expanse of space, where the Sun’s rays barely reach, RTGs are often essential for missions operating far from the Sun or in environments where sunlight is scarce. These devices convert heat from the decay of plutonium‑238 into electricity, using thermocouples powered by the Seebeck effect, a principle that converts temperature differences into electrical energy. See the diagram below for an illustration of RTG thermoelectric conversion. Why RTGs excel in deep space RTGs are widely used where solar panels cannot function effectively for extended periods, such as in the outer solar system, shadowed craters, or dusty planetary surfaces. They operate by relying on a temperature gradient: the decay of plutonium‑238 provides the heat source, while the cold vacu...