GPHS Module Expanded
The GPHS module provides steady heat for a radioisotope power system. Image credit: NASA

Radioisotope power systems, or RPS, provide electricity and heat that can enable spacecraft to undertake scientific missions to environments beyond the capabilities of solar power, chemical batteries and fuel cells.

RPS are sometimes referred to as a type of "nuclear battery." While some spacecraft, like Cassini, do run their systems directly off of their RPS, others like the Mars Science Laboratory rover can use the RPS to charge batteries and run their systems and instruments off of stored battery power. In either case, the RPS is attached directly to a spacecraft, much like a power cord being plugged in.

These technologies are capable of producing electricity and heat for decades under the harsh conditions of deep space without refueling. All of these power systems, flown on more than two dozen NASA missions since the 1960s, have functioned for longer than they were originally designed.

The RPS used to power NASA spacecraft are supplied by the U.S. Department of Energy (DOE). NASA and DOE continue to collaborate on maintaining and developing several types of RPS.

The Building Block

This 3D animation shows the main components of the General Purpose Heat Source module, or GPHS—the essential building block for the radioisotope generators used by some NASA spacecraft to explore the solar system. These modules contain and protect the plutonium-238 (or Pu-238) fuel that gives off heat for producing electricity.

The General Purpose Heat Source module, or GPHS, is the essential building block for the radioisotope generators used by NASA. These modules contain and protect the plutonium-238 (or Pu-238) fuel that gives off heat for producing electricity. The fuel is fabricated into ceramic pellets of plutonium-238 dioxide (238PuO2) and encapsulated in a protective casing of iridium, forming a fueled clad. Fueled clads are encased within nested layers of carbon-based material and placed within an aeroshell housing to comprise the complete GPHS module. 

Color image of small metal pellet.
A Pu-238 fuel pellet in its protective iridium cladding. Image Credit: U.S. Dept. of Energy

Each GPHS is a block about four by four by two inches in size, weighing approximately 3.5 pounds (1.5 kilograms). They are nominally designed to produce thermal power at 250 watts at the beginning of a mission, and can be used individually or stacked together.

Modules have been subjected to extreme testing conditions that significantly exceeded the intensity of a wide range of potential accidents. Such tests have included simulating multiple reentries for a single module through Earth's atmosphere, exposure to high temperature rocket propellant fires, and impacts onto solid ground.

The enhanced GPHS modules used in the latest generation of radioisotope power systems incorporate additional rugged, safety-tested features that build upon those used in earlier generations. For example, additional material (20 percent greater in thickness) has been added to the graphite aeroshell and to the two largest faces of the block-like module. These modifications provide even more protection to help to contain the fuel in a wide range of accident conditions, further reducing the potential for release of plutonium-238 that might result.

Types of Radioisotope Power Systems


Radioisotope Thermoelectric Generator, or RTG provides power for spacecraft by converting heat generated by the natural radioactive decay of its fuel source, plutonium dioxide, into electricity using devices called thermocouples. RTGs have no moving parts. The latest RPS to be qualified for flight, the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is powering the Mars Science Laboratory rover, Curiosity, which landed on Mars in August 2012. The MMRTG continues to perform as designed, providing both power and heat for the rover.

The MMRTG is designed to be used in the vacuum of space as well as within the atmosphere of Mars.

Radioisotope Heaters

Radioisotope Heater Unit, or RHU, employs a small, pencil eraser-sized pellet of plutonium dioxide to generate heat for spacecraft structures, systems, and instruments, enabling their successful operation throughout a mission. Some missions employ just a few RHUs for extra heat, while others have dozens. NASA has also studied the potential for using the same small fuel pellet in an RHU to power a compact system that could provide a few dozen milliwatts of electrical power.

Additional Nuclear Technologies for Space Exploration

NASA and DOE have explored other types of nuclear power technology over the years, including space nuclear reactors and nuclear propulsion technologies. Continued research and development of these and other related technologies might one day enable space missions to deliver more payloads on cargo missions, achieve faster trip times on piloted missions, or even provide power for crew stations on the surface of Mars or the moon.

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