What Are Radioisotope Thermoelectric Generators?

Radioisotope thermoelectric generators (RTGs) are power systems that convert the heat produced by the natural radioactive decay of a nuclear fuel into electricity through the Seebeck thermoelectric effect. Unlike solar panels or chemical batteries, RTGs require no sunlight and contain no moving parts, making them uniquely suited for long-duration missions in environments where other power sources are impractical or unavailable. They have powered some of the most distant spacecraft ever built, including Voyager 1 and 2, which were still transmitting data decades after launch.

The technology draws on nuclear physics for the selection and processing of the radioisotope heat source, and on solid-state physics for the thermoelectric materials that perform the energy conversion. Plutonium-238 (Pu-238) dioxide is the preferred fuel for space RTGs because its half-life of approximately 87.7 years provides a stable long-term power output, its decay releases primarily alpha particles that are easily shielded, and its heat-to-mass ratio is high relative to other radioisotopes considered for similar applications.

Thermoelectric Conversion

RTGs produce electricity through thermocouples, pairs of dissimilar semiconducting or metallic materials joined at two junctions, one held at the elevated temperature of the heat source and the other exposed to a cold sink. The Seebeck coefficient of each thermocouple pair determines how much voltage is generated per degree of temperature difference across the junction. As NASA explains in its description of how RTGs use the Seebeck effect, the hot side of a space RTG can exceed 500 degrees Celsius while the cold side faces the near-absolute-zero environment of deep space, creating the temperature gradient that drives electrical generation. Arrays of thermocouples connected in series and parallel accumulate the individual voltages and currents into a usable power output. Lead telluride (PbTe) and silicon-germanium (SiGe) alloys have been the principal thermoelectric materials in flight-proven RTGs, chosen for their efficiency across the operating temperature range and their long-term stability.

Radioisotope Heat Sources

The heat source assembly encapsulates the fuel in multiple layers of protective material to ensure that the radioisotope remains contained in the event of a launch accident or re-entry. In the General Purpose Heat Source (GPHS) module design used in NASA systems, Pu-238 dioxide pellets are housed in iridium-alloy cladding, then placed within graphite impact shells and aeroshell blocks that can withstand the thermal and mechanical loads of atmospheric re-entry. The Department of Energy article on Multi-Mission Radioisotope Thermoelectric Generators (MMRTGs) describes how the MMRTG design used on the Curiosity and Perseverance Mars rovers stacks several GPHS modules to produce approximately 2,000 watts of heat, yielding around 110 watts of electrical power at the start of the mission, with output declining gradually as the fuel decays.

Performance and System Design

The efficiency of RTGs, defined as the ratio of electrical output to total thermal power, is governed by the thermoelectric figure of merit (ZT) of the converter materials and the operating temperature ratio. Space RTGs typically achieve efficiencies in the range of 6 to 8 percent. Although this is low compared with dynamic power conversion cycles, the simplicity and reliability of solid-state conversion outweigh the efficiency penalty for long missions. NASA's program overview for radioisotope power systems outlines the history of RTG development, from the SNAP-3 unit that flew on the Transit navigation satellite in 1961 through the MMRTG currently operating on Mars.

Applications

Radioisotope thermoelectric generators have applications across a range of fields, including:

• Deep space exploration missions beyond the inner solar system, where sunlight is too dim for solar power
• Mars surface operations, for rovers requiring continuous power through dust storms and night periods
• Lunar surface equipment requiring reliable power through the two-week lunar night
• Remote terrestrial monitoring stations in polar regions and other locations inaccessible for regular servicing
• Cardiac pacemakers, in which small RTGs powered implants in thousands of patients in the 1970s before lithium batteries became standard