Cosmos 1402 Russia

RORSAT, meaning Radar Ocean Reconnaissance Satellite, is the western term used to describe a series of Soviet Union satellites.
 These satellites were launched between 1967 and 1988, to monitor NATO and merchant vessels using radar.
 They were referred to as Cosmos satellites and carried type BES-5 nuclear reactors fueled by uranium-235.

 For the radar to work efficiently the satellites were placed into low Earth orbit. The plan was for the spacecraft to jettison the reactor into high Earth orbit when the satellites effective life had ended.

However, there were several failures.One such failure was Cosmos 1402. At the end of the satellites intended operational period, the reactor did not separate into high Earth orbit as planned.

 When the satellite reentered the atmosphere on February 7, 1983, the reactor was the last piece to come home to Earth. It landed somewhere in the South Atlantic Ocean.