Deep-space probes run on the heat of decaying plutonium
Past Jupiter, sunlight is too dim to power solar panels, so robotic spacecraft rely on Radioisotope Thermoelectric Generators (RTGs). These space batteries contain ceramic pellets of Plutonium-238. As the radioactive plutonium naturally decays, it produces intense heat, which thermocouples convert directly into electricity with zero moving parts, reliably powering legendary missions like Voyager, New Horizons, and Curiosity for decades.
The Fading Sun in the Outer Solar System
Photovoltaic panels have carried humanity's robotic scouts across the inner solar system, converting solar photons into the electrical current needed to run sensors, transmitters, and onboard computers. However, solar power is governed by the inverse-square law: double the distance from the Sun, and the available light drops to one-quarter of its original strength. By the time a spacecraft reaches Jupiter, sunlight is roughly twenty-five times fainter than it is in Earth orbit. At Saturn, it is nearly a hundred times dimmer, and at the frozen rim of Pluto and the Kuiper Belt, the Sun appears as little more than an intensely bright pinpoint in a velvet sky.
To gather adequate electrical energy at such vast distances using solar power alone, a probe would require massive, heavy photovoltaic wings spanning dozens of square meters. Such arrays add prohibitive mass, create mechanical vulnerabilities, and make spacecraft difficult to orient and stabilize. Furthermore, solar panels are useless in environments with prolonged darkness, such as during the two-week lunar night, in permanently shadowed polar craters, or on planetary surfaces prone to severe dust storms like Mars. For exploration of deep space and harsh extraterrestrial environments, spacecraft require a compact, self-contained, and completely independent power source.
The technological answer developed during the mid-twentieth century is the Radioisotope Thermoelectric Generator, or RTG. Instead of capturing energy arriving from the outside world, an RTG acts as an unyielding thermal engine powered from within. By tapping the steady, predictable natural heat generated by decaying radioactive matter, these systems provide a continuous stream of electrical power that can operate undisturbed across decades of interplanetary travel.