Ever wonder how a spacecraft millions or even billions of miles from the Sun can keep its lights on? After all, solar panels become less efficient the farther you travel into the outer solar system and storing huge batteries for years on end would be impractical.
The answer, it turns out, is nuclear technology – specifically, radioisotope thermoelectric generators, or RTGs. While the name might conjure images of a mini power plant, RTGs are power-efficient nuclear batteries that have fueled legendary missions such as the Apollo Moon landings, the Voyager probes, and the Perseverance rover on Mars.
A radioisotope thermoelectric generator (RTG) is a small, long-lasting power source at its core. Instead of chemical reactions like in everyday lithium-ion batteries, RTGs generate electricity by harnessing the heat from radioactive decay. The primary isotope used in modern spacecraft RTGs is plutonium-238 (Pu-238).
Unlike the material used in fission-based nuclear reactors on Earth, Pu-238 decays independently, emitting alpha particles that produce a steady heat flow. That constant heat, in turn, is converted into electricity via the Seebeck effect. When two different conductive materials are joined together, and each side of that joint is exposed to different temperatures, an electric current naturally forms.
An RTG exploits this effect by keeping one side near the decaying plutonium hot while exposing the other side to the cold of outer space. The temperature difference can often reach hundreds of degrees Fahrenheit, allowing the device to produce a stable, reliable trickle of electrical current that can power a spacecraft for decades.
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