In 1987, the Soviet Union launched Kosmos 1818, which was powered by a TOPAZ 1 (or thermionic) nuclear reactor. The purpose of Kosmos 1818 was as a naval surveillance satellite, or RORSAT (Radar Ocean Reconnaissance Satellite). Unfortunately, the nuclear reactor on Kosmos 1818 operated for only five months before shutting down.
In 1978, a similar satellite reentered the atmosphere and crashed into Earth, spreading radioactive material over Canada. Kosmos 1818 was placed into high orbit to avoid a similar catastrophe. However, its high orbit also means that it has a high collision probability.
Any collision might accelerate the descent of possibly contaminated materials to Earth. Some of the objects and liquid released from the spacecraft are thought to be radioactive and are still in orbit.
