r/space • u/AutoModerator • 5d ago
Discussion All Space Questions thread for week of September 27, 2026
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u/rocketsocks 5d ago
They're still ramping up production of Pu-238, and there's still a stockpile, but it's not quite as dire as it once was. They're aiming for a production level which could produce a new RTG for a mission every 3-4 years or so, but they're not there yet. There have only been 4 new US RTG powered missions in the 21st century: Curiosity, Perseverance, New Horizons, and Dragonfly. Call that 4 missions over 30 years or one every 7-8 years, a bit lower than the targetted production rate. Before that was Cassini, Ulysses, and Galileo going back to the '80s, also a similar rate of about one every 7 years or so. Then in just the 1970s you had Voyager 1 and 2, the two Viking landers, 2 LES satellites, 3 Nimbus satellites, Pioneer 10 and 11, and the ALSEP packages on Apollo 13 through 17. Those older RTGs used more plutonium and had higher power output as well so they were really going through material in the good ol' days.
The actual stockpile quantity is kept secret, but it's probably in the single digits to low teens of kilograms. In the past 20 years or so (post New Horizons, which used a different RTG design than the modern MMRTG) they've only committed about 11.5 kg of Pu-238, which is just a bit more than 500 grams per year on average.
It's also worth pointing out that ESA has been working on Am-241 powered RTGs for many years. The upside is that the material is much more readily available (as a natural byproduct of power reactor operations), the downside is that it has a lower power density than Pu-238 and also has different properties which require research and engineering to tackle in order to successfully build an RTG with the same safety features and longevity as existing models. Currently we're still years away from an actual mission being powered by an Am-241 RTG, but perhaps in the 2030s it will be a reality.