NASA marks 60 years of nuclear power in space

Transit IV-A's SNAP-3B radioisotope generator produced 2.7 W of electrical power - which according to NASA's Glenn Research Center is "about enough to power an LED lightbulb". Nevertheless, the satellite broke a mission-duration record - according to NASA's annual chronology for 1966 the satellite had travelled over 25,000 times around the Earth by the time it became the oldest operating US satellite in May 1964 - and also confirmed that the Earth's equator is elliptical.

Radioisotope power systems use thermocouples to convert heat from the decay of plutonium-238 into electricity and are one of only two practical ways to provide long-term electrical power in space. Solar panels are another option, but solar power becomes less efficient as spacecraft travel farther from the Sun, and may also be limited by local environmental conditions such as a planet's local weather and seasons.

Radioisotope power systems are reliable and efficient, according to June Zakrajsek, manager for NASA's Radioisotope Power Systems Program office at the Glenn Research Center.

"They operate continuously over long-duration space missions regardless of sunlight, temperature, charged particle radiation, or surface conditions like thick clouds or dust. They’ve allowed us to explore from the Sun to Pluto and beyond," she said.

The plutonium-238 fuel used in NASA's radioactive power systems is provided through a partnership with the US Department of Energy (DOE). The isotope is made by irradiating neptunium-237, and is currently produced by Oak Ridge National Laboratory in partnership with Idaho and Los Alamos National Laboratories.

Radioactive power systems powered the Lunar Surface Experiment Package - a collection of geophysical instruments designed to continue to monitor the environment of each Apollo landing site for a period of at least a year after the astronauts had departed - as well as the Pioneer, Viking, Voyager, Galileo, Ulysses, Cassini and New Horizons space missions. NASA's Perseverance rover, which landed on Mars in February, is powered by a Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) using a Pu-238 radioisotope power system.

The next NASA mission with plans to use an MMRTG is Dragonfly, which is set to launch in 2027, according to an article by Jan Wittry of the Glenn Research Center. Dragonfly - part of the New Frontiers programme - is an octocopter which will explore and collect samples on Saturn's moon Titan. The MMRTG will provide power output to charge the lander's battery, as well as heat to keep its instruments and electronics warm.

"Flight is a very high-power activity," Zibi Turtle, principal investigator for the Dragonfly mission, said. "We'll use a battery for flight and science activities and recharge the battery using the MMRTG. The waste heat from the power system is a key aspect of our thermal design. The surface of Titan is very cold, but we can keep the interior of the lander warm and cozy using the heat from the MMRTG."

NASA is also working with DOE and industry to develop fission-based power systems to enable a human presence on the surface of the Moon and, eventually, human missions to Mars.