MMRTGs

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is a type of power source used by spacecraft to produce electricity from the heat released by the decay of plutonium-238. It provides reliable, long-lasting power in environments where sunlight is weak or unavailable, enabling missions to distant planets, moons, and shadowed terrain. The MMRTG is designed for multi-mission use and is developed through collaboration between NASA and the U.S. Department of Energy.

In construction, the MMRTG uses several GPHS (General Purpose Heat Source) modules that contain plutonium-238 in ceramic form. Heat from radioactive decay is converted into electricity by thermoelectric generators, which use thermocouples to generate current from a temperature difference. The system is designed to deliver a modest, steady electrical output over many years while dissipating excess heat through radiators. Typical electrical output at the start of a mission is around 100 to 120 watts, gradually declining as the fuel decays, with the unit weighing several tens of kilograms.

Missions and usage: The MMRTG has supported NASA missions such as the Mars Science Laboratory, which powers the Curiosity rover. It is also considered for future expeditions, including the Dragonfly mission to Titan, where solar power would be less reliable. The MMRTG is favored for its ability to operate in darkness and extreme temperatures, providing continuous power for science instruments and mobility.

Safety and development: Pu-238 is highly radiotoxic if released, so MMRTGs are built with robust containment and multiple safety features to withstand launch stresses and accidents. The design emphasizes long-term reliability, limited maintenance, and predictable performance across mission lifetimes.