Half a kilo of cosmic nuclear fuel reignites NASA's deep space dreams

There ain't no party like a Pu-238 party

NASA has celebrated a shipment of half a kilo of plutonium oxide by the US Department of Energy, the largest since US production of plutonium-238 was restarted just over a decade ago.

Plutonium-238 (Pu-238) is essential for NASA missions using Radioisotope Power Systems (RPS). RPS makes use of the natural decay of Pu-238 to provide heat and electricity through systems such as the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG). As such, it's essential for missions where solar power is not an option.

NASA's recent Mars rovers use the fuel, and Perseverance uses an MMRTG to provide the robot with continuous heat and approximately 110 watts of electricity. Other missions, such as Cassini, also used the isotope, and Pu-238 power has kept the Voyager probes running decades after the launch.

According to NASA: "Three dozen missions have explored space for decades using the reliable electricity and heat provided by RPS."

However, supplies have been limited. Pu-238 is a byproduct of the process used to make nuclear weapons. As the need for new nuclear weapons has ebbed over the years, stockpiles of the fuel have dwindled. In 2015, Popular Science estimated that approximately 35 kg remained available to NASA, and of that, only 17 kg was good enough to use.

Hence the need to restart production. In 2010, the US Department of Energy submitted a report [PDF] to Congress with the goal of achieving an average production rate of 1.5 kg per year by 2015. According to NASA, that target won't be hit until at least 2026.

That said, small quantities of the fuel have been delivered in recent years. Some was used in the Perseverance Mars rover.

RTGs are the only option for deep space missions as far as NASA is concerned. Two technologies that could have made more efficient use of Pu-238 – the Advanced Stirling Radioisotope Generator (ASRG) and the Enhanced Multi-Mission Radioisotope Thermoelectric Generator (eMMRTG) – were axed by the agency due to technical, schedule, and cost issues.

NASA is having another crack with its Next Generation RTGs, which it claims will produce more than twice the power of an MMRTG with a significantly lower power degradation over time. ®

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