Atlas-Agena-D lifts off with SNAPSHOT satellite.
Fifty years ago on April 3, 1965, NASA launched an Atlas rocket from Vandenberg Air Force Base in California that lifted the SNAPSHOT nuclear reactor test satellite into orbit. The Agena booster carried the satellite into a polar orbit, where four hours later the reactor was remotely triggered to life. Within a half-day of starting, the reactor was producing almost 600 watts of electricity.
NASA illustration of the SNAPSHOT satellite still attached to the Agena booster stage.
It was planned to have the reactor generate electricity for one year, but in 43 days the voltage regulator failed, causing the need for ground engineers to shut down the experiment. Scientists expected that the radioactive elements in the reactor would decay to a safe level within 100 years, but the orbit of the craft was so solid that engineers calculated it would remain in orbit for 3000-4000 years!
SNAPSHOT during ground testing.
It is thought that in 1979, the craft may have collided with space debris in its orbit. Ground radars have tracked pieces of the satellite that would account for 50 pounds of the satellite's mass. It's not certain if any radioactive elements have been released, but certainly none of that would survive re-entry onto the ground.