From Johns Hopkins University Applied Physics Lab (US) : “Reviving a Legacy Technology-for Spacecraft Exploration” 

The Johns Hopkins University Applied Physics Lab

From Johns Hopkins University Applied Physics Lab (US)

Jeremy Rehm

More than 20 years ago, production of a material technology that enabled our deepest space missions halted, and the expertise to make it was lost. But a team led by Johns Hopkins APL has paved a way for this hardy technology to be used once again.

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.

Creator: Craig Weiman
Copyright: Copyright 2021 JHU/APL. All rights reserved

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.

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: The National Aeronautics and Space Agency (US)/ Department of Energy (US).

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.

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.

National Aeronautics and Space Administration(US) Galileo 1989-2003

National Aeronautics and Space Administration(US)/European Space Agency [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/ASI Italian Space Agency [Agenzia Spaziale Italiana](IT) Cassini Spacecraft.

“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.

National Aeronautics Space Agency(USA) New Horizons(US) spacecraft

“Without that RTG, there would have been no exploration of Pluto and the Kuiper Belt.”
Kuiper Belt. Minor Planet Center.

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.

NASA The Dragonfly mission to 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.

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 (US), Clemson University (US) and Alfred University (US) 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 the 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 (US)

“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.

Power to Explore

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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JHUAPL campus

Founded on March 10, 1942—just three months after the United States entered World War II— The Johns Hopkins University Applied Physics Lab (US) -was created as part of a federal government effort to mobilize scientific resources to address wartime challenges.

The Applied Physics Lab was assigned the task of finding a more effective way for ships to defend themselves against enemy air attacks. The Laboratory designed, built, and tested a radar proximity fuze (known as the VT fuze) that significantly increased the effectiveness of anti-aircraft shells in the Pacific—and, later, ground artillery during the invasion of Europe. The product of the Laboratory’s intense development effort was later judged to be, along with the atomic bomb and radar, one of the three most valuable technology developments of the war.

On the basis of that successful collaboration, the government, The Johns Hopkins University, and APL made a commitment to continue their strategic relationship. The Laboratory rapidly became a major contributor to advances in guided missiles and submarine technologies. Today, more than seven decades later, the Laboratory’s numerous and diverse achievements continue to strengthen our nation.

The Applied Physics Lab continues to relentlessly pursue the mission it has followed since its first day: to make critical contributions to critical challenges for our nation.

Johns Hopkins University campus

Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

The Johns Hopkins University (US) is a private research university in Baltimore, Maryland. Founded in 1876, the university was named for its first benefactor, the American entrepreneur and philanthropist Johns Hopkins. His $7 million bequest (approximately $147.5 million in today’s currency)—of which half financed the establishment of the Johns Hopkins Hospital—was the largest philanthropic gift in the history of the United States up to that time. Daniel Coit Gilman, who was inaugurated as the institution’s first president on February 22, 1876, led the university to revolutionize higher education in the U.S. by integrating teaching and research. Adopting the concept of a graduate school from Germany’s historic Ruprecht Karl University of Heidelberg, [Ruprecht-Karls-Universität Heidelberg] (DE), Johns Hopkins University is considered the first research university in the United States. Over the course of several decades, the university has led all U.S. universities in annual research and development expenditures. In fiscal year 2016, Johns Hopkins spent nearly $2.5 billion on research. The university has graduate campuses in Italy, China, and Washington, D.C., in addition to its main campus in Baltimore.

Johns Hopkins is organized into 10 divisions on campuses in Maryland and Washington, D.C., with international centers in Italy and China. The two undergraduate divisions, the Zanvyl Krieger School of Arts and Sciences and the Whiting School of Engineering, are located on the Homewood campus in Baltimore’s Charles Village neighborhood. The medical school, nursing school, and Bloomberg School of Public Health, and Johns Hopkins Children’s Center are located on the Medical Institutions campus in East Baltimore. The university also consists of the Peabody Institute, Applied Physics Laboratory, Paul H. Nitze School of Advanced International Studies, School of Education, Carey Business School, and various other facilities.

Johns Hopkins was a founding member of the American Association of Universities (US). As of October 2019, 39 Nobel laureates and 1 Fields Medalist have been affiliated with Johns Hopkins. Founded in 1883, the Blue Jays men’s lacrosse team has captured 44 national titles and plays in the Big Ten Conference as an affiliate member as of 2014.


The opportunity to participate in important research is one of the distinguishing characteristics of Hopkins’ undergraduate education. About 80 percent of undergraduates perform independent research, often alongside top researchers. In FY 2013, Johns Hopkins received $2.2 billion in federal research grants—more than any other U.S. university for the 35th consecutive year. Johns Hopkins has had seventy-seven members of the Institute of Medicine, forty-three Howard Hughes Medical Institute Investigators, seventeen members of the National Academy of Engineering, and sixty-two members of the National Academy of Sciences. As of October 2019, 39 Nobel Prize winners have been affiliated with the university as alumni, faculty members or researchers, with the most recent winners being Gregg Semenza and William G. Kaelin.

Between 1999 and 2009, Johns Hopkins was among the most cited institutions in the world. It attracted nearly 1,222,166 citations and produced 54,022 papers under its name, ranking No. 3 globally [after Harvard University (US) and the Max Planck Society (DE) in the number of total citations published in Thomson Reuters-indexed journals over 22 fields in America.

In FY 2000, Johns Hopkins received $95.4 million in research grants from the National Aeronautics and Space Administration (US), making it the leading recipient of NASA research and development funding. In FY 2002, Hopkins became the first university to cross the $1 billion threshold on either list, recording $1.14 billion in total research and $1.023 billion in federally sponsored research. In FY 2008, Johns Hopkins University performed $1.68 billion in science, medical and engineering research, making it the leading U.S. academic institution in total R&D spending for the 30th year in a row, according to a National Science Foundation (US) ranking. These totals include grants and expenditures of JHU’s Applied Physics Laboratory in Laurel, Maryland.

The Johns Hopkins University also offers the “Center for Talented Youth” program—a nonprofit organization dedicated to identifying and developing the talents of the most promising K-12 grade students worldwide. As part of the Johns Hopkins University, the “Center for Talented Youth” or CTY helps fulfill the university’s mission of preparing students to make significant future contributions to the world. The Johns Hopkins Digital Media Center (DMC) is a multimedia lab space as well as an equipment, technology and knowledge resource for students interested in exploring creative uses of emerging media and use of technology.

In 2013, the Bloomberg Distinguished Professorships program was established by a $250 million gift from Michael Bloomberg. This program enables the university to recruit fifty researchers from around the world to joint appointments throughout the nine divisions and research centers. Each professor must be a leader in interdisciplinary research and be active in undergraduate education. Directed by Vice Provost for Research Denis Wirtz, there are currently thirty two Bloomberg Distinguished Professors at the university, including three Nobel Laureates, eight fellows of the American Association for the Advancement of Science (US), ten members of the American Academy of Arts and Sciences, and thirteen members of the National Academies.