Power-generation and heating units using radioactive decay as their primary energy source have been successfully used in space and on Earth for over 60 years.

RHU principles and technology are similar in many ways to those of the RTG, but with important differences in objective and implementation.

Q: We’ve looked at the use of radioactive decay for power, but what about heat alone?
A: That’s the role of the radioactive thermal unit or RTU. These are much smaller than an RPG and deliver the modest amounts of localized heat needed to keep spacecraft subsystems operational; more than 300 RHUs have been launched on a wide variety of historic scientific instruments and spacecraft systems, including Apollo 11 in 1969, through the long-lived Mars Exploration Rovers Spirit and Opportunity that landed on January 2004. They are also on the probes sent into the atmospheres of Jupiter and Titan (and their ‘parent’ spacecraft, Galileo and Cassini), plus nine RHUs were aboard the Voyager 1 and 2 deep-space probes, each of which continues outward more than 12 billion miles from Earth – and these RTUs, like the RTGs onboard, are still functioning.

Q: What is in the RHU?
A: An RHU contains a Pu-238 fuel pellet about the size of a pencil eraser and produces about 1 W of heat (the entire RHU is about the size of a C-cell battery.)

Q: Are there more “construction” details?
A: An RHU is very compact, about 1.3 inches (3.2 centimeters) long by one inch (2.5 centimeters) in diameter, with a total mass of about 1.4 ounces (40 grams) (Figure 1). Their nominal one-watt thermal power decreases by about one-hundredth of a watt per year, due to the natural decay of the Pu-238 radioisotope fuel.

Q: One watt doesn’t seem like much power or heat; is it actually useful?
A: Due to the small size of the RHU compared to the size of the spacecraft, it is easier from a design standpoint to use more than one RHU and locate them where needed. Some missions employ just a few for extra heat, while others have dozens. By using RHUs, the spacecraft designer can allocate scarce electrical power to operate the spacecraft systems and instruments, rather than heating.

As non-electrical heaters, RHUs also provide the added benefit of reducing potential (electromagnetic) interference with instruments or electronics that might be generated by electrical heating systems.

Q: What are some actual RTU usage numbers?
A: Here are some NASA missions enabled by radioisotope heater units:

• Apollo 11 EASEP Lunar Radioisotope Heater: contained two 15W RHUs
• Pioneer 10 & 11: 12 RHUs each
• Voyager 1 & 2: 9 RHUs each
• Galileo: 120 RHUs (103 on orbiter, 17 on atmospheric probe)
• Mars Pathfinder Sojourner Rover: 3 RHUs
• Cassini: 117 RHUs (82 on orbiter, 35 on Huygens Titan probe)
• Mars Spirit & Opportunity Rovers: 8 RHUs each

Q: Is the construction of the RTU the same as the RTG?
A: They are similar, but as the RTU does not need the thermocouple assembly or wiring, they are also different. The outermost aeroshell is made from extremely tough material called fine-weave pierced fabric, plus a graphite-based insulator to protect the fuel in a RHU from potential physical impacts, fires, and atmospheric reentry conditions. The fuel is encapsulated in a high-strength, platinum-rhodium metal shell (or “clad”) that further protects and helps contain the fuel during a potential accident (Figure 2).

Q: What about RHU design and construction for safety?
A: The design of an RHU provides a rugged, layered containment system to prevent or minimize the release of plutonium dioxide fuel even when subjected to severe accident conditions. Containment is assured through multiple layers that are resistant to the heat and impact that might be encountered during a spacecraft accident.

Summary
RTGs and RHUs have enabled terrestrial, orbital, and deep-space missions (both voyages and landings) that would have been very difficult or impossible using solar cells and conventional chemical batteries or batteries alone. Their proven track record of success and safety is a little-known story of advanced physics and nuclear technology, combined with the “ancient” Seebeck effect, innovative design, materials, and production techniques.

 

Related EE World Content
Thermal electric generators, Part 1: Principles and implementation
Thermal Electric Generators, Part 2: Applications
Gravity-assist “Slingshot,” Part 1: Background and principle
Gravity-assist “Slingshot,” Part 2: Application
Making sense of thermocouples and interfaces (Part 1)

 

External References (with some comments)
NASA, JPL, Dept. of Energy

• NASA/JPL, “Radioisotope power systems reference book for mission designers and planners” (Detailed, fact-packed 91-page book!)
• NASA/JPL. “A Look at the Next Generation of Radioisotope Thermoelectric Generators” (15 slides, excellent)
• NASA, “Cassini Radioisotope Thermoelectric Generators (RTGs)”
• NASA, “After 60 Years, Nuclear Power for Spaceflight is Still Tried and True”
• NASA, “RHU Pull-Apart Animation”
• NASA, “FAQ About RPS” (very useful)
• NASA, “A Step Forward in Reestablishing the Radioisotope Power Systems Supply Chain” (plutonium production issues)
• NASA, “Power Systems” (summary with video of RTGs)
• NASA, “Thermal Systems” (summary with video of RTUs)
• NASA, “NASA’s RHU-Heated and RTG-Powered Spacecraft” (broad overview)
• NASA, “So What’s an RTG and Are They Safe?
• NASA, “General Purpose Heat Source – Light-Weight Radioisotope Heater Unit”
• NASA, “RHU Fact Sheet”
• S. Department of Energy, “What is a Radioisotope Power System?”
• S. Department of Energy, “The History of Nuclear Power in Space”
• S. Department of Energy, “Overview of Light Weight Radioisotope Heater Unit (LWRHU) User’s Guide”

Other sources

• American Nuclear Society, Science and Technology Policy Institute,” Current Status and Future of Space Nuclear Power” (provides various technical and policy perspectives)
• Stanford University, “An Overview of Radioisotope Thermoelectric Generators”
• Wikipedia, “Radioisotope heater unit”
• Wikipedia, “Radioisotope thermoelectric generator”

Earth’s heat and radioactivity

• University of Notre Dame Nuclear Science Laboratory “Radioactivity, Lecture 11: The radioactive Earth”
• Physics World, “Radioactive decay accounts for half of Earth’s heat”
• Scientific American, “Do transuranic elements such as plutonium ever occur naturally?”
• Lawrence Berkeley National Laboratory “What Keeps the Earth Cooking?”