Kosmos 1900 is a US-A or Controlled Active Satellite used for RORSAT missions. Launched in 1987 by the Soviet Union, the satellite was plagued from the beginning and never quite reached the cruising orbit for which it was designed.
After several rocket boosts to try to correct its orbit, the satellite continued to lose altitude. Moreover, the nuclear reactor did not make it into its storage orbit. At some point prior to 1995, NASA determined that a cloud of liquid radioactive material had originated from the Kosmos 1900 satellite. NASA claimed that the leak was likely due to a collision with another satellite.