Before dawn on September 20, 1977, people in Petrozavodsk, in Soviet Karelia, saw a large glowing object hang over the city for about 10 to 12 minutes. Witnesses compared it to a jellyfish and said it sent down a shower of fine rays of light. Reports of strange lights that morning came from Copenhagen and Helsinki in the west to Vladivostok in the east.
Northern European governments wrote to the Soviet Academy of Sciences asking whether it was a weapons test. A preliminary report for the Academy gathered eyewitness accounts, radar data and measurements and concluded that the phenomenon could not be satisfactorily explained.
Most researchers now link it to the launch of the military satellite Kosmos-955 from the Plesetsk cosmodrome, which was secret at the time and whose rocket exhaust would have lit up the dawn sky. The event led the USSR to set up Setka, its own research programme on anomalous atmospheric phenomena.