rewrite this title Are Spacecraft That Work for Centuries Possible?

Next-generation RTG technology featuring Americium-241 is paving the way for deep space vehicles that can operate for centuries. Moreover, Americium-241 can be sourced sustainably.

Behind the missions that have reached the farthest points of space made by humans lies a small but critical technology that has been working silently for years: radioisotope thermoelectric generators (RTGs). These power sources enable spacecraft like Voyager 1 and Voyager 2, launched in 1977, to still send signals to Earth. The Plutonium-238 used at that time continues to provide uninterrupted energy to the spacecraft for nearly half a century. However, the energy produced is now in its final stages. This means the 48-year-old spacecraft, and naturally humanity’s furthest symbols, will fall silent. It is predicted that the spacecraft will be able to produce enough energy to communicate with us until 2036. But scientists are working on a much longer-lasting fuel: Americium. This fuel could power missions not just for decades, but for centuries.

Americium, expected to shape future deep space missions, stands out not only for its promise of longevity but also for providing strategic independence. The production of Plutonium-238 is limited and expensive. Moreover, access to the source outside the US is difficult. In contrast, Americium-241 is an isotope that naturally accumulates in nuclear waste. This makes it easier to find resources for deep and long-lasting space missions. The presence of large quantities of Americium-241 in civilian nuclear waste stockpiles, especially in the United Kingdom, turns this fuel into a strategic opportunity for Europe.

Nuclear Power is an Indispensable Option

Nuclear power is currently an indispensable option for missions outside the Solar System. As is known, solar panels are predominantly used for energy in spacecraft within the Solar System. However, in regions beyond Jupiter, RTGs are essential. The main reason for this is the difficulty of harnessing sunlight at such distances. It’s not impossible, of course, but it is logistically close to impossible. For example, if the Voyager spacecraft had used solar panels, they would have needed massive panels, each larger than a football field. RTGs, however, are about the size of a trash can. These generators operate thanks to Thermocouples, which convert the heat produced by the radioactive fuel into an electric current. Since there are no moving parts, the risk of failure is negligible. Thanks to the 88-year half-life of Plutonium-238, each of Voyager’s RTGs produced about 470 watts in the early years. Today, this power is just over 200 watts. It is sufficient for the vehicle’s critical systems for now, but eventually, the critical systems will also fail to be powered.

On the other hand, Plutonium wasn’t just used in Voyager. It was also used in the satellites and vehicles left on the Moon during the Apollo missions. Later, RTGs were also included in vehicles going to Jupiter, Saturn, and Pluto. For example, Cassini, which orbited Saturn from 2004 to 2017, used 33 kilograms of Plutonium-238. However, there is a critical problem with plutonium. And it is very difficult to solve. This is because Plutonium-238 is not found naturally. It must be produced in nuclear reactors.

The stockpiles produced during the Cold War have been depleted, and it took the US many years to resume new production. Since NASA requires kilograms of plutonium for each deep space mission, current production is not sustainable in the long term. In 2012, Curiosity’s RTG took a significant portion of the last major reserves. The sustainability problem stems from the annual production being limited to a few hundred grams. Considering that kilograms are needed for deep space missions, it is clear that relying on this fuel is no longer logical.

This is precisely where americium comes into play. First produced in 1944 during the Manhattan Project, americium is one of the lesser-known synthetic elements. The half-life of Americium-241, which is attractive for space, is 432 years. This is five times that of Plutonium-238.

This means a new generation of vehicles drifting into interstellar space, continuing to operate for centuries. Furthermore, the fact that new reactors are not required for production reduces costs and offers a significant environmental advantage. This is because Americium-241 forms naturally within nuclear waste, meaning it accumulates over time. Therefore, it is possible to find quite high amounts of Americium-241 on Earth.

Not Everything is Perfect

Plutonium-238 is still indisputably considered the most efficient fuel for high-power missions. This isotope, producing about 0.5 watts of heat per gram, is also chemically extremely stable. In contrast, Americium-241 can only provide about 0.1 watts of heat at the same mass. Because of this difference, an americium-based RTG needs to be heavier and bulkier to produce power equivalent to plutonium. This is a serious disadvantage, considering every gram is critical in space missions. However, the low power output of americium offers an ideal advantage for long-life missions with low energy requirements.

While plutonium will continue to be the fuel for powerful and compact systems, americium stands out as a suitable option for smaller-scale vehicles that operate at lower temperatures and will last for centuries.

The First Spacecraft Are Not Far Off

Europe is already conducting concrete work on americium-based power systems. The University of Leicester, in collaboration with ESA and the UK Space Agency, has been developing both full-scale RTGs and small heater units to protect hardware in extremely cold environments for over a decade. This technology could be used in many low-power missions, from long-term probes studying the internal structure of icy moons to devices drifting for centuries into interstellar space. This fuel is expected to play a role in future projects like the Rosalind Franklin Mars mission. NASA’s conceptual Interstellar Probe project is also among the missions that will require the very long-lasting energy that americium can provide. Even in manned Mars missions, americium is planned to provide support as a stable heat source during the multi-year journey.

The low power density of americium increases the importance of more efficient conversion technologies. At this point, scientists are concentrating on the Stirling engine, which has been known for a long time but could open a new door for space missions. While thermoelectrics used in traditional RTGs offer only about 5% efficiency, Stirling converters can achieve electricity production efficiency of over 25%. This means much more energy can be obtained from the same amount of fuel. Although the presence of moving parts raises reliability concerns, the stable heat output of americium paves the way for systems where multiple Stirling units operate in parallel, distributing the risk of failure.

Although americium-based RTGs have not yet been tested in space, laboratory tests are promising. As this technology matures, deep space exploration may cease to be the monopoly of just a few countries. Plutonium for high power and americium for long-term exploration could become two complementary parts of the same ecosystem. An interesting irony is that this long-lasting space fuel, developed in laboratories, also appears in our daily lives. The small radioactive source used in ionizing smoke detectors is Americium-241 itself.

You Might Also Like;