Transit IV-A was powered both by solar cells and a radioisotope thermoelectric generator (RTG) using Plutonium 238 for fuel. More from on the same subject (written by the same reporter) on
Built for the U.S. Navy by the Applied Physics Laboratory of Johns Hopkins University, Transit IV-A was lofted into orbit on June 29, 1961. The spacecraft's primary duty was to act as one of four operational navigational satellites for use by ships and aircraft - a forerunner of today's GPS constellation.
Subsequent to this launch, the use of radioisotope power has extended our reach across the solar system, enabling spacecraft to fly by and observe Jupiter, Saturn, Uranus and Neptune; to orbit and study the Sun, Jupiter and Saturn in greater detail; and to land and collect local surface science data on the Moon and Mars.