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Powering Missions to Deep Space in a New Way
June 5, 2012

In the lab with the Advanced Stirling ConvertorThis Advanced Stirling Convertor is considered the workhorse component of a new generation of radioisotope power systems. Image credit: NASA How do you get scientific missions into deep space or shady environments and keep them operating safely and successfully for many years? It takes reliable, efficient power systems that last a long time.

A new generation of electrical power systems for potential missions to some of the darkest, coldest locations in the solar system and beyond is under development by Sunpower, Inc., an Ohio-based company working with NASA Glenn Research Center, the Department of Energy (DOE) and Lockheed Martin.

Sunpower recently demonstrated the newest version of its Advanced Stirling Convertor (ASC). The ASC is the critical "workhorse" component of this new generation of radioisotope power systems, which are known as Advanced Stirling Radioisotope Generators (ASRGs). The ASRG incorporates two identical ASC units each independently heated by radioisotope sources.

The new Stirling convertors operate with an internal gas-supported reciprocating piston system that is driven by the heat from the radioisotope plutonium-238. This gas bearing system ensures the pistons operate with no contact and no wear, so the expected lifetime of the ASC is very long; the ASRG is being designed for a life of at least 17 years.

This demonstration by Sunpower of the third-generation ASC engineering unit, known as E-3, is one of the final steps in the preparation to build flight-quality systems for possible use on a NASA mission as soon as 2016. NASA is considering missions to a comet and to Saturn's moon Titan that could use ASRGs, along with a Mars lander that would use solar power; one of the three will be selected for development and flight in mid-summer 2012.

scientist in lab with the Advanced Stirling Radioisotope GeneratorsThe Advanced Stirling Radioisotope Generators incorporate two identical convertors and are designed to last at least 17 years. Image credit: NASA An engineering unit of the ASRG with earlier versions of the ASCs has operated continuously for over 25,000 hours in the laboratory. Additionally, multiple ASC convertors are undergoing extended operation and four of them have also exceeded 25,000 operating hours.

"The new power system is three to four times more efficient than more traditional radioisotope generators, so it can use four times less plutonium-238 fuel than the current technology to generate a comparable amount of electrical power," says John Hamley, program manager for NASA's Radioisotope Power Systems Program.

The key technologies in the ASC that contribute to high efficiency are its hydrostatic gas bearings and high-frequency operations, a moving-magnet linear alternator and the use of high-temperature materials.

Sunpower's ASC development, initially funded by NASA Glenn's Small Business Innovative Research Program, began in 2001 to address the power conversion needs of future radioisotope power systems.

"This Stirling power system will make missions possible where solar or chemical power sources, including batteries or fuel cells, are not feasible or are not adequate to meet mission requirements," says Wayne Wong, Advanced Stirling Convertor project manager at Glenn. "The convertors manufactured by Sunpower have completed early tests and will continue to be tested to demonstrate key aspects of the expected operation of the flight units."

For more information about NASA's use of radioisotope power systems, visit

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Page Last Updated: July 28th, 2013
Page Editor: NASA Administrator