The reactor wasn’t very powerful, but it was NASA’s first nuclear-powered satellite…
On June 29 of 1961, NASA launched the Transit 4-A satellite into orbit. It was equipped with tools to measure solar x-rays and the flux of charged particles associated with the aurora. According to official reports, the satellite weighed 175 pounds and was the first to contain a small but effective 2.7-watt onboard radioactive power source launched into space.
While not a fission reactor like those in commercial nuclear power plants, it did use decay heat as its power source:
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”…
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.From World Nuclear News, “NASA marks 60 years of nuclear power in space”
The Transit 4-A used solar cells as its primary source of power. Later, the Transit 5BN-1, launched in 1963, would be the first to use a nuclear source for all primary power.
One of the satellite’s major achievements is that it confirmed the equator was elliptical, measuring a difference of 250 feet in the long and short axes.
The Transit 4-A and other Transit satellites were part of a network called NAVSAT that served as the predecessor to the Navstar Global Positioning System. According to Wikipedia:
The radio navigation system was primarily used by the U.S. Navy to provide accurate location information to its Polaris ballistic missile submarines, and it was also used as a navigation system by the Navy’s surface ships, as well as for hydrographic survey and geodetic surveying.
NASA has plans to use a nuclear source on its upcoming Dragonfly vehicle, which is intended to explore Saturn’s moon, Titan. It is scheduled to launch in 2027.