US-based L3Harris Technologies said it has finalised the design of a next-generation nuclear-based power source for future NASA deep space missions. The Next-Generation Radioisotope Thermoelectric Generator (Next Gen RTG) has cleared its critical design review (CDR), paving the way for a new era of outer solar system exploration.
NASA deep space probes starting in the early 2030s, including a proposed Uranus orbiter could use two Next Gen RTGs for power and to enable its temperature-sensitive components to operate in the frigid environment of the outer solar system.
RTGs, which have been in use for 60 years, convert heat from the radioactive decay of plutonium-238 into electricity and have been used in probes that are too far from the Sun to rely on solar power. Early versions continue to supply power to NASA’s twin Voyager probes launched in 1977 that are now traveling in interstellar space.
The Next Gen RTG is an evolution of the general-purpose heat source RTGs that supplied power to NASA’s Cassini Saturn orbiter and, more recently, New Horizons probe, which carried out a Pluto flyby in 2015 and is now exploring the icy Kuiper Belt. Unlike the L3Harris-built Multi-Mission RTGs currently powering NASA’s Curiosity and Perseverance Mars rovers, the Next Gen RTGs are optimised for spacecraft operating in the vacuum of space rather than on the surface of a planet.
In 2021, the Department of Energy’s Idaho National Laboratory asked L3Harris to re-establish the key technologies from the heritage system and update the design in response to growing interest in new deep space missions. The contract is expected to end in 2027 with a production readiness review to verify that the next-generation system can be built using the materials and components that have been re-established.
The L3Harris Next Gen RTG differs from heritage systems by modernising obsolete components, and is optimised for the vacuum of deep space, providing a significantly higher power-to-weight ratio than recent planetary rover units. While it leverages the architectural legacy of the General-Purpose Heat Source (GPHS-RTGs) flown on Cassini and New Horizons, it introduces several key advancements to support NASA’s early 2030s missions.
The original GPHS-RTGs used technology conceived in the late 20th century. L3Harris reconstructed incomplete historical blueprints and sourced modern engineering equivalents for materials and electrical parts that are no longer manufactured. These include updated thermoelectric couples produced by Teledyne Energy Systems. The Silicon-Germanium (SiGe) unicouples have been re-engineered to maximise beginning-of-life efficiency (roughly 6%) while keeping standard 30-volt load configurations. In addition, BAE Space and Mission Systems developed upgraded insulation to preserve thermal energy more effectively than older configurations, forcing more heat to travel through the power-generating thermocouples.
The units currently powering the Mars Curiosity and Perseverance rovers are Multi-Mission RTGs (MMRTGs). which must be pressurised to prevent the Martian atmosphere from leaking into the system. By contrast, the Next Gen RTG is structurally built specifically for the vacuum of the outer solar system. The envelope lacks atmospheric pressure constraints, enabling the passive cooling fins to reject waste heat into space far more efficiently.
Compared to a standard MMRTG, the Next Gen unit delivers roughly double the power output within a similar mass. The system is engineered to generate approximately 245 watts of electricity at launch (converting 4,000 watts of thermal energy from 9.6 kg of Plutonium-238) and maintain at least 177 watts after 17 years in deep space. This extended lifespan includes an allowance for three years of fuelled storage on Earth prior to launch.
“Passing the CDR is an important milestone because it validates that our design meets all the technical requirements and can be manufactured,” said Bill Sack, General Manager, RocketWorks and Power Systems at L3Harris. “It also demonstrates we’ve successfully re-established this critical capability after years of limited production…. The Next Gen RTG represents a significant leap forward in efficiency. We’re delivering more power in the same mass envelope, which is critical when every kilogram matters for deep space missions.”
The availability of Next Gen RTGs makes possible a range of ambitious missions that NASA is aiming for. Beyond the Uranus orbiter, these power systems could enable extended missions to Neptune and its moon, Triton; Kuiper Belt object explorers beyond the range of the New Horizons spacecraft; long-duration missions to the outer planets’ moons; and interstellar precursor missions that push even farther than Voyager 1 and Voyager 2.
