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Americium RTGs: A Century-Scale Fuel for Deep Space Missions
Voyager’s endurance comes from its nuclear batteries, RTGs powered by plutonium-238. As solar panels lose effectiveness far from the Sun, RTGs provide steady power for decades. The video explores why americium-241, a byproduct of plutonium production with a longer half-life, could power spacecraft for centuries. It also introduces Stirling engines as a path to higher efficiency and reliability, and discusses how Europe’s independence from US fuel supplies could shape future missions such as the Interstellar Probe. These ideas connect how fuel choice influences the scale and duration of human exploration beyond our solar system.
Overview
The video from Interesting Engineering surveys how space missions have relied on nuclear power rather than solar panels once they venture far enough from the Sun. It highlights Voyager’s RTGs, explains the basic physics of how these devices convert radioactive heat into electricity, and introduces americium-241 as a potential successor to plutonium-238 for extremely long duration missions. The discussion frames a broader vision of future spacecraft that could operate for centuries, driven by a fuel source that outlives civilizations and a conversion technology that improves reliability and efficiency.
RTGs in Space: How They Work
Radioisotope thermoelectric generators (RTGs) power spacecraft by using heat from radioactive decay to generate electricity with thermocouples. Voyager carries three RTGs containing plutonium-238, each weighing about 5 kilograms of Pu-238. At launch in 1977 these RTGs produced about 470 watts, and after nearly half a century they continue to supply well over 200 watts. Crucially, RTGs have no moving parts, require minimal maintenance, and run on heat rather than sunlight, making them ideal for the frigid, dim outskirts of the solar system.
Pu-238 vs Americium-241
The video explains why plutonium-238 has been the isotope of choice for deep space power due to its high power density and reliable heat output, despite its scarcity. Pu-238 has a half-life of 88 years and a heat density of roughly 0.5 watts per gram. Americium-241, by contrast, has a much longer half-life of 432 years and a lower heat density around 0.1 watts per gram, which means bulkier RTGs would be required for the same power. Americium is abundant as a byproduct of plutonium production and can be sourced by recycling nuclear waste, offering a potential path to energy independence for space programs outside the United States.
Stirling Engines and Higher Conversion Efficiency
The talk introduces Stirling converters as a promising alternative to thermoelectrics. Stirling engines can, in principle, push conversion efficiency above 25 percent, enabling more power from the same isotope or achieving the same power with less fuel. The Leicester research group has demonstrated multi-converter configurations to maintain power even if one unit fails, addressing reliability concerns that have so far kept Stirling RTGs from flying. The potential to use Americium with Stirling converters is presented as a key research frontier for long-duration missions.
Applications, Independence, and the Interstellar Vision
Europe, through the UK and ESA, is exploring Americium RTGs as a means to reduce dependence on US plutonium. The Rosalind Franklin Mars rover case is cited as an example of geopolitics reshaping mission power choices. Looking further ahead, NASA’s Interstellar Probe concept aims to travel far beyond Voyager, necessitating power sources that endure not just decades but centuries. Americium could enable such missions, particularly for tiny instruments or slow-changing measurements where power demands are modest but longevity is paramount. The video also notes that human missions to Mars will require a combination of heating and power, potentially leveraging both Pu-238 and Americium in different roles.
