Reviving a Legacy Technology for Spacecraft Exploration

Technology rarely makes a comeback after it’s gone or (more often) replaced. But sometimes — because it’s retro, it shows new promise or people just won’t let it go — the tech of the past can breathe life anew.

That’s what’s happening with a material called silicon-germanium. Thanks in part to recent work by a team led by the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, this legacy material is making a comeback with a new twist in NASA’s next-generation nuclear power source for spacecraft. Its resurgence will enable NASA missions to travel farther and longer than current capabilities allow, meeting the demands of a science community with ambitious ideas.


For around 30 years, silicon-germanium, or SiGe, was a key material made for NASA’s radioisotope thermal generators (RTGs), a technology that APL helped develop and that earned NASA and the Department of Energy a Lifetime Achievement Award during the 2021 Nuclear Science Week opening ceremony in Washington, DC, last week. RTGs take heat from the natural decay of plutonium oxide and generate electricity by passing the heat through devices called unicouples. From the 1970s, those unicouples were made of either SiGe or lead-telluride/TAGS (PbTe) alloys. They enabled the exploration of the outer solar system and have powered more than a dozen NASA spacecraft, including the history-making Voyagers 1 and 2, Cassini, New Horizons and the Viking Mars landers.

Image of a tall, cylindrically shaped radioisotope thermoelectric generator
Originally developed in the 1970s for the U.S. Air Force using a nuclear power source and silicon-germanium unicouples, Multi-Hundred-Watt RTGs were used on the Voyager 1 and Voyager 2 spacecraft and are still powering them today. Credit: NASA/U.S. Department of Energy

But by the late 1990s, after a short restart of the RTG program for the development of NASA’s Galileo and Cassini spacecraft, RTG production halted. NASA’s flight program needs were met, the manufacturing costs were deemed too expensive and no contractual agreements were created to sustain production.

“The only reason we flew one on New Horizons [in 2006] was because we used one of the Cassini spares,” said Paul Ostdiek, a program manager at APL. “Without that RTG, there would have been no exploration of Pluto and the Kuiper Belt.”

In the early 2000s, unicouples of PbTe and related materials found new life when NASA started developing its multi-mission RTG (MMRTG), which has powered NASA’s Mars Rovers and is expected to power NASA’s APL-led Dragonfly mission to Saturn’s moon Titan. With a maximum lifespan of 17 years, however, MMRTGs aren’t enough for deep-space missions like New Horizons that require decades of spaceflight. And compared with SiGe unicouples, they can’t operate at as high of a temperature, which affects power production efficiency, and they degrade faster.

It wasn’t until 2018, when NASA moved ahead with developing a Next-Generation RTG for use by 2030, that SiGe entered discussions again. Companies preferred SiGe for unicouple material in the Next-Gen RTG, but because nobody had built or worked with it in over 20 years (and those who had had either retired or died), they were considering newer and riskier materials.

A man wearing a lab coat and purple nitrile gloves holds a small silver-colored disc of silicon-germnaium
APL’s Richard Ung holds a disc of silicon-germanium.
Source: Johns Hopkins APL/Craig Weiman


But in spring 2020, after becoming familiar with research happening in Rama Venkatasubramanian’s thermoelectric labs in APL’s Research and Exploratory Development Department, NASA tasked Venkatasubramanian’s team, among others, with probing the risks and challenges of developing SiGe materials again as well as turning them into devices. They were to mitigate any hazards and develop a unicouple as close to the original design and functionality of those in the 1990s as possible.

The team didn’t disappoint. In just three months, Venkatasubramanian’s APL team and partners from the University of Virginia, Clemson University and Alfred University recreated SiGe and other materials with modern fabrication techniques. They produced functioning unicouples that worked as well as (and potentially better than) those from the past.

“I think what Rama and his team were able to lead and pull off through spring 2020 and into the summer — during the [COVID-19] pandemic, no less — was just amazing,” Ostdiek said.

The team went on to show that it could create operative SiGe unicouples with modern, cost-effective techniques in the labs with various partner institutions. “That partnership helped prove that this technique can be portable and replicable for industry adoption,” Venkatasubramanian said.

The results demonstrated the possibility of resurrecting SiGe technology. And after further investigation, NASA decided to include SiGe unicouples in the Next-Gen RTG design.

“APL’s quick work helped NASA understand the risks industry might face when reestablishing this capability and demonstrated that they were manageable,” said June Zakrajsek, the Radioisotope Power System program manager at NASA Glenn Research Center in Ohio.

“APL’s contributions to the Next-Gen Project’s top risk have been invaluable,” added Next-Gen Project Manager Rob Overy, also of NASA Glenn.

A woman wearing a mask, lab coat and purple nitrile gloves holds a small U-shaped metallic unicouple
APL’s Priya Gajendiran holds a unicouple made of silicon-germanium, a legacy technology whose production was halted in the 1990s. Thanks in part to a team led by APL, this technology will aid in powering space missions again.
Source: Credit: Johns Hopkins APL/Craig Weiman


Beyond reestablishing the capability, the team is excited by the new possibilities for the future.

“We think SiGe is a long-range platform technology that we are developing for the Next-Gen RTG,” Venkatasubramanian said. “Our approach will likely not only meet the current goals of a 2030 mission but could lay the foundation for a long-term, higher-performing RTG converter technology for future missions.”

Among the most conspicuous of future candidate missions is the Interstellar Probe, a conceptual mission led by APL researchers and engineers. The idea would push modern technology to the very edge, propelling a spacecraft out of the solar system faster than any spacecraft before it and returning data for at least 50 years.

“Basically, silicon-germanium RTG technology is an absolute necessity for the Interstellar Probe,” Venkatasubramanian said.

Down the road, the APL team also believes the technology could fit into a modular device architecture like that of MMRTGs, and it could easily make its way into the commercial sector in high-temperature power generation to complement high-temperature energy storage.

“Time will tell,” Venkatasubramanian said. “Our goal for now in the next two to three years is to understand the risks NASA faces, and help transition this technology into future use — both by NASA and other markets.”

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