Spacecraft missions require reliable energy storage solutions to handle the variable demands of orbital operations, including eclipse periods and high-power payload activities. Hybrid energy storage systems, combining batteries with radioisotope power sources, provide a robust solution for missions where solar energy is intermittent or insufficient. These systems ensure continuous power delivery, thermal management, and mission longevity, particularly in deep space or extreme environments. NASA and ESA have implemented such hybrid systems in several missions, demonstrating their effectiveness in real-world scenarios.

In a hybrid storage system, batteries typically handle short-term, high-power demands, while radioisotope systems provide long-term, steady power. Batteries excel at delivering rapid bursts of energy for instruments, communications, or propulsion, whereas radioisotope thermoelectric generators (RTGs) or radioisotope heater units (RHUs) supply consistent baseline power and thermal stability. This combination is particularly useful during eclipse periods, when solar panels cannot generate electricity, or in missions far from the Sun where sunlight is too weak to be practical.

One of the primary challenges in spacecraft design is managing eclipse periods, which can last from minutes to hours depending on the orbit. During these periods, batteries alone would deplete quickly if relied upon for all systems. Hybrid systems mitigate this by using RTGs to provide continuous low-level power, reducing the drain on batteries. For example, NASA’s Cassini mission to Saturn employed RTGs alongside batteries to ensure uninterrupted operation during its lengthy eclipses behind the planet. The RTGs supplied steady power for critical systems, while the batteries handled peak loads from scientific instruments and data transmission.

Payload demands further complicate power management. Scientific instruments, propulsion systems, and communication equipment often require high power for short durations. Batteries can meet these spikes efficiently, but without a hybrid system, the spacecraft would need oversized batteries, adding mass and complexity. The Mars Science Laboratory (Curiosity rover) uses a multi-mission radioisotope thermoelectric generator (MMRTG) alongside lithium-ion batteries. The MMRTG provides a constant 110 watts of electrical power, while the batteries supply additional energy during high-demand activities like drilling or sample analysis. This setup allows the rover to operate regardless of dust covering its solar panels or during Martian nights.

NASA’s Europa Clipper mission, planned for Jupiter’s icy moon Europa, will also use a hybrid approach. The spacecraft will rely on solar panels for primary power but will incorporate batteries to manage load variations. The extreme radiation environment and long eclipses in Jupiter’s shadow necessitate a system that can handle prolonged periods without sunlight. The batteries will store excess solar energy during illuminated phases and discharge during eclipses, while the RTGs ensure essential systems remain powered.

ESA’s Rosetta mission to comet 67P/Churyumov–Gerasimenko faced similar challenges. The spacecraft used a combination of solar panels and batteries, but its journey included periods far from the Sun where solar power was minimal. The batteries provided critical power during close encounters with the comet, while the solar panels were optimized for energy collection during the mission’s sunlit phases. Although Rosetta did not use RTGs, its power management strategies highlight the importance of hybrid-like systems in deep-space missions where energy availability fluctuates.

Thermal management is another critical aspect of hybrid storage systems. RTGs not only generate electricity but also produce heat, which can be used to keep spacecraft components within operational temperatures. Batteries, particularly lithium-ion types, are sensitive to extreme cold and may require heating to function correctly. In NASA’s New Horizons mission to Pluto, RTGs provided both power and thermal stability, ensuring the spacecraft’s instruments and batteries remained functional in the frigid outer solar system. The batteries supported high-energy events like the Pluto flyby, while the RTGs maintained baseline operations.

The design of hybrid systems involves careful balancing of mass, power output, and longevity. RTGs are heavy and have fixed power outputs that decay over time due to the half-life of their radioactive fuel. Batteries, while lighter, have limited cycle life and energy density. Engineers must optimize the ratio of battery capacity to RTG output to ensure the spacecraft meets its mission duration and power requirements. For instance, the Voyager probes, which rely solely on RTGs, have operated for decades but were designed with minimal high-power payload demands. In contrast, modern missions with more sophisticated instruments benefit from the flexibility of hybrid systems.

Future missions to the Moon, Mars, and beyond will likely continue adopting hybrid storage solutions. Lunar night periods last approximately 14 Earth days, making solar power alone impractical for long-term surface missions. NASA’s Artemis program and ESA’s lunar ambitions will require batteries paired with other technologies, possibly including small-scale RTGs or fuel cells, to ensure continuous operation. Similarly, Mars missions must contend with dust storms that can block sunlight for weeks, necessitating reliable backup power sources.

Hybrid energy storage systems represent a pragmatic solution for spacecraft facing variable power demands and environmental extremes. By combining the high-energy density of batteries with the reliability of radioisotope systems, missions can achieve greater flexibility and endurance. NASA and ESA’s experiences with Cassini, Curiosity, Rosetta, and New Horizons demonstrate the effectiveness of these systems in some of the most challenging environments explored by humanity. As space exploration advances, hybrid storage will remain a cornerstone of mission design, enabling longer, more ambitious journeys into the cosmos.