RPS offer the key advantage of operating continuously over long-duration space missions, largely independent of changes in sunlight, temperature, charged particle radiation, or surface conditions like thick clouds or dust. In addition, some of the excess heat produced by some RPS can be used to enable spacecraft instruments and onboard systems to continue to operate effectively in extremely cold environments.
A uniquely capable source of power is the radioisotope thermoelectric generator (RTG), a nuclear battery that reliably converts heat into electricity. RTGs work by converting heat from the natural decay of radioisotope materials into electricity. RTGs consist of two major elements: a heat source that contains plutonium-238 dioxide, and a set of solid-state thermocouples that converts the plutonium’s heat energy to electricity. The thermocouples in RTGs use heat from the natural radioactive decay of plutonium-238 to heat the hot junction of the thermocouple, and use the cold of outer space to produce a low temperature at the cold junction of the thermocouple.
RTGs have enabled NASA to explore the solar system for many years. Apollo missions to the Moon, Viking missions to Mars, and the Pioneer, Voyager, Galileo, and Cassini missions all used RTGs.
NASA and the Department of Energy developed a new generation of power systems that can be used on space missions. The new RTG, called the Multi- Mission Radioisotope Thermoelectric Generator (MMRTG), was designed to operate on planetary bodies with atmospheres such as that on Mars, as well as in the vacuum of space. The MMRTG is a more flexible, modular design capable of meeting the needs of a wider variety of missions, since it generates electrical power in smaller increments — slightly above 100 Watts. The design goals for the MMRTG include ensuring safety, optimizing power levels over a minimum lifetime of 14 years, and minimizing weight.
The MMRTG is designed to use a heat source composed of eight General Purpose Heat Source (GPHS) modules. The MMRTG contains a total of 10.6 pounds of plutonium dioxide that initially provides approximately 2000 Watts of thermal power and 120 Watts of electrical power. The thermoelectric materials have demonstrated extended lifetime and performance, and are the same as those used for the two Viking spacecraft that landed on Mars in 1976.
The MMRTG also powered the Mars Science Laboratory (Curiosity) mission to Mars, and will power the next generation of Mars rovers, the Mars 2020 rover. The power system does not require sunlight, permitting spacecraft to land at more diverse locations regardless of season, time of day, or latitude. Because of the number of moving parts, rovers and other mobile explorers require more power than landers. If rovers are solar-powered, they must land and operate within a fairly narrow latitude band near the equator where enough sunlight shines to provide adequate electricity.
MMRTGs will enable an operating lifespan on Mars’ surface of a full Martian year (687 Earth days; a little less than two Earth years) over a wide latitude range. That means it opens up more regions of Mars to exploration, giving mission planners more choices in selecting landing sites that have characteristics related to Mars’ potential as a habitat for life. The MMRTG is also crucial for Curiosity’s thermal stability. Waste heat from the unit is circulated throughout the rover system to keep instruments, computers, mechanical devices, and communications systems within their operating temperature ranges. This system-wide thermal control does not draw on the rover’s electrical power, and precludes the need for radioisotope heater units for spot heating.