Editors Note
This blog was originally posted during Energy.gov’s #SpaceWeek in 2015.
Nuclear Power Systems in Space Travel
You may not associate space travel with the Energy Department, but you should. Nuclear power systems developed here have enabled amazing interplanetary missions.
The Energy Department’s Office of Space and Defense Power Systems and its predecessors, alongside National Labs and private industry partners, have developed radioisotope power systems for NASA. These systems have powered long-term missions from Voyagers 1 and 2 to the Mars rovers, providing essential fuel and warmth to spacecraft in the cold, dark reaches of space.
Despite their simplicity, these systems have been vital for some of the most inspiring missions in U.S. space history. Explore our interactive timeline of these missions and learn more about the “space batteries” that made them possible.
Power from Plutonium
Only two practical ways supply electrical power for multi-year space missions: solar energy or heat from radioactive decay. Radioisotope power systems directly convert heat from plutonium-238 decay into electricity, essential for missions to distant parts of the solar system.
Plutonium-238 withstands high temperatures, produces significant heat in small amounts, and emits low radiation levels, making it ideal for space power. It’s processed into a ceramic form, much like coffee mugs. In case of an accident, it fractures into larger chunks, preventing environmental contamination.
Early Nuclear Space Power Systems
In 1961, the U.S. Navy’s Transit 4A satellite was the first U.S. spacecraft powered by nuclear energy, utilizing a radioisotope thermoelectric generator (RTG). Eight generations of these systems have since been developed for use by NASA, the U.S. Navy, and the U.S. Air Force.
RTGs, with no moving parts, convert plutonium-238 decay heat into electricity using thermocouples. For example, Transit 4A generated 2.7 watts and traveled nearly 2 billion miles.
NASA launched the RTG-powered Nimbus III in 1969, first to measure air pressure and solar UV radiation while relying on solar cells and RTGs.
Space Batteries on the Moon
The Apollo missions included experimental packages known as the Apollo Lunar Surface Experiment Package (ALSEP). The first package was solar-powered but used RTGs to keep instruments warm. Later packages used 70-watt SNAP-27 RTGs, significantly contributing to lunar knowledge before being shut down in 1977.
Exploring the Solar System
RTGs powered various planetary missions, including Galileo, which orbited Jupiter in 1989, and Ulysses, which studied the heliosphere. The Cassini mission, launched in 1997, continues to explore Saturn with multiple RTGs.
Launched in 2006, the New Horizons spacecraft studied Pluto and the Kuiper Belt. It successfully passed Jupiter and returned detailed images of Pluto.
Reaching for Interstellar Space
Pioneer 10 and 11, launched in the 1970s, were precursors to the Voyager missions. They were designed to travel far and lasted well beyond their expected lifespans, transmitting valuable data.
The Voyager missions, launched in the late 1970s, enabled pivotal discoveries about the solar system, with both spacecraft still operational today, sending data back to Earth.
Missions to Mars
NASA’s Viking missions in 1975 aimed to gather surface data from Mars, achieving far more than expected. Both used RTGs and returned over 55,000 images.
In 1996, the Mars Pathfinder mission extended its expected mission duration significantly while utilizing solar panels and RTGs. The twin rovers Spirit and Opportunity launched in 2003, further exploring Mars.
NASA’s 2011 Curiosity rover employs a multi-mission radioisotope thermoelectric generator for power and heat. This rover is tasked with studying Martian geology and climate.
Future Missions
NASA plans a similar Mars rover mission in 2020, carrying advanced instruments for further exploration. The RTG for this mission is currently under construction and testing by various national laboratories.
