Choosing the Right Electrical Power Supply

Scientists could find clues for answering these and other questions about life’s origins in the ice-covered seas 

that may exist on Jupiter’s icy moon, Europa, or in water deposits hidden beneath the frigid surface of Mars. 

Evidence of the building blocks of life, or chemical 

environments similar to the early Earth, might be found 

in the atmospheres, surfaces or sub-surfaces of various 

planets and moons, or within the intriguing small bodies 

of the solar system, comets and asteroids. 

Creating robotic spacecraft that could thrive in these 

extreme environments demands technical innovation. 

One of the most important components for such missions is their electrical power supply. For most space 

exploration missions where sunlight is abundant, solar 

power has been the preferred choice. But as the environments at chosen destinations grow harsher, and missions evolve to be more demanding, it becomes more 

likely that effective power and heating for a spacecraft 

would require a Radioisotope Power System (RPS). 

An RPS converts the heat generated by the natural 

decay of the radioactive isotope plutonium-238 into 

electricity; this material is not used in weapons and cannot explode like a bomb. A portion of this decay heat 

often has an important secondary use in helping to keep 

spacecraft subsystems warm in cold environments. 

An RPS offers the key advantage of operating continuously, independent of unavoidable variations in sunlight. 

Such systems could provide power for long periods of 

time (significantly longer than chemical batteries), and at 

vast distances from the Sun. Additionally, an RPS has 

little sensitivity to temperature, radiation or other space 

environmental effects. They are ideally suited for missions involving autonomous, long-duration operations in 

the most extreme environments in space and on planetary surfaces.

Developing and Improving RPS Technology

Seven generations of RPS have been flown in space 

by the United States since 1961, powering 26 missions 

that have enabled world-renowned scientific exploration of the Moon, the Sun, Venus, Mars, Jupiter, Saturn, 

Uranus, Neptune, and—soon—Pluto. All of the RPS on 

these historic solar system exploration missions have 

worked beyond their design lifetimes. An eighth RPS 

configuration, called the Multi-Mission Radioisotope 

Thermoelectric Generator (MMRTG), has recently been 

qualified for flight. It is planned for use on the Mars 

Science Laboratory rover, Curiosity.

Radioisotope Power Systems

for Space Exploration

National Aeronautics and Space Administration

NASA is exploring ideas for future missions to send robotic spacecraft to harsh and distant places that hold 

great promise for major new discoveries. Landers, rovers, orbiters and flyby craft could be sent on pioneering 

missions to some of the coldest, hottest, and darkest 

environments imaginable beyond Earth.

The questions that drive these visions for future exploration largely revolve around the origin and evolution of 

life. While the solar system presents a variety of hostile 

environments not favorable to life on first glance, scientists have found lifeforms on Earth thriving in incredibly 

extreme conditions, such as the areas surrounding 

deep, dark, hot water vents on the ocean floor. Such 

observations on Earth and our human curiosity lead us 

to ask whether life could exist, or may have existed, in 

other exotic places in our solar system. 

If life arose elsewhere in the solar system, how did it 

start? How might it be similar or different than life on 

Earth? These profound questions are within our scientific reach for the first time.

NASA is studying a 

concept for a future 

mission to orbit 

Jupiter’s icy moon 

Europa.

Artist’s concept of the Mars Science Laboratory rover Curiosity on Mars. 
400-1432 03/11 NASA Facts

www.nasa.gov

For more information on the development of radioisotope power systems 

for space exploration, see: http://www.ne.doe.gov

The overall science goal of Curiosity is to enable scientists to determine the past and present potential of Mars to support microbial life 

at an intriguing location on the Red Planet. The MMRTG will provide 

the electrical power and onboard heat required for the mission to 

accomplish its ambitious goals over one Martian year (or about two 

Earth years), including the capability to explore latitudes that are too 

high for the rover to efficiently use solar power. 

The MMRTG is based on the proven RPS design used to provide 

electrical power for NASA’s two earlier Viking landers, which operated on the surface of Mars for 40 months and more than six years, 

respectively. Other missions in NASA’s heritage of safe and successful use of such generators for solar system exploration over the 

past 40 years include Voyager 1 and 2. The Voyagers continue to 

operate more than three decades after their launches, seeking the 

boundary of true interstellar space more than nine billion miles from 

the Sun.

As part of an ongoing partnership with the U.S. Department of 

Energy (DOE), NASA is conducting a mission-driven RPS Program 

whose purpose is to develop the next generation of reliable radioisotope power systems, enabling a broad range of science missions 

that could operate more widely and efficiently than their predecessors. This program is developing and validating two basic RPS 

units: the MMRTG and the Advanced Stirling Radioisotope 

Generator (ASRG). 

A cutaway model of a 

Multi-Mission Radioisotope 

Thermoelectric Generator 

(MMRTG). The vertical red blocks 

in the center are individual heat 

source modules and the white 

fins on either side are radiators.

The MMRTG passively converts the heat generated by the natural 

decay of its nuclear fuel into about 110 watts of electricity through 

simple thermocouples, which have no moving parts. Each MMRTG 

carries eight individually shielded general purpose heat source 

modules, compared to 18 modules in the previous RPS generation. 

Unlike some of these previous generations of RPS, the MMRTG can 

operate in the atmosphere of a planet as well as in space. 

Now that the MMRTG is ready for use in flight, the RPS Program is 

turning much of its focus to the ASRG. This new unit also employs 

plutonium-238 as its heat source (producing about 130-140 watts 

of electricity), but would use this heat to drive a moving piston. This 

system can generate electrical current over four times more efficiently than an MMRTG, thus preserving a limited resource of fuel. 

Both the MMRTG and the ASRG are modular, meaning that more 

than one generator could be used when higher power levels would 

be needed.

For descriptions of possible future missions and their power requirements, see:

http://solarsystem.nasa.gov/missions/index.cfm

Possible Future Missions

Any NASA mission that proposes to use an RPS undergoes a 

comprehensive multi-agency environmental review, including public 

meetings and open comment periods during the mission planning 

and decision-making process, as part of NASA’s compliance with 

the National Environmental Policy Act. Additionally, any such mission proposed by NASA would not launch until formal approval for 

the mission’s nuclear launch safety is received from the Office of the 

President.

Radioisotope power systems are used when they enable or significantly enhance missions to destinations where inadequate sunlight, 

harsh environmental conditions, or operational requirements make 

other electrical power systems infeasible.

Beyond Jupiter’s fascinating moon Europa, Saturn’s moon Titan is 

often discussed as a highly desirable destination for science. Titan 

features a thick atmosphere of organic chemicals and a scientifically 

interesting surface where liquid hydrocarbons have carved lakes and 

streambeds. Ideas for future missions to Titan include an orbiter, a 

lander, various mobile rovers, or robots that could traverse the atmosphere or float in a lake. 

Other recent mission studies include probes to study the Sun, missions to various asteroids and comets, and an orbiter for Neptune or 

its large, active moon Triton. Because an RPS is designed to work 

steadily in environmental extremes, it could also be considered for a 

surface rover to a permanently shadowed crater on Mercury or the 

Moon, or an aerobot to explore the atmosphere of Venus, cruising 

high above its lava-covered surface. 

An Advanced Stirling 

Radioisotope Generator 

(ASRG) being prepared for 

vibration testing.

Lockheed Martin

In addition, NASA and DOE are developing improved power conversion technologies that aim to increase the efficiency and capability of 

key RPS subsystems, thus increasing the watts produced per unit of 

fuel. These advanced technologies could also help produce an RPS 

device with lower mass (thus increasing its specific power, or power 

per unit mass), longer life, and use in a wider range of environments.