At 4:00 AM on September 20, 1977, residents of Petrozavodsk, capital of the Karelian ASSR of the Soviet Union, were awakened by a silent brilliant light in the pre-dawn sky. Observers across the city and in neighbouring Leningrad Oblast and Finnish Lapland described a slow-moving 'jellyfish-like' object that hovered over Petrozavodsk for approximately twelve minutes, radiating rays of light toward the ground. The object was observed simultaneously from six Soviet oblasts and from northern Finland, including by Helsinki observatory astronomers and by the Soviet Polar Air Defense forces. Reports of unusual holes — clean round 'punctures' in the sand, asphalt, and plate-glass of the city's upper storeys — were filed the following morning.
The Soviet Academy of Sciences opened a formal investigation, the first such institutional Soviet response to a mass UFO event. A 1978 classified report by G. S. Narykov, chief of the Meteorological Institute of Karelia, documented the path, duration, and corroborating observations of the object. The report noted that the object's 1:05:20 AM–1:15:00 AM trajectory coincided with — but was not explained by — the launch of a Kosmos-955 military reconnaissance satellite from Plesetsk cosmodrome at 12:57:51 AM Moscow time. The Kosmos-955 launch was heavily classified at the time; its public announcement occurred only days later. The Plesetsk launch's exhaust plume, ionized by the upper atmosphere, is now the generally-accepted explanation for the luminous 'jellyfish' effect.
The Petrozavodsk phenomenon's significance lies less in its ultimate explanation than in its unprecedented Soviet-government response. The September 1977 report led directly to the 1978 establishment of a Soviet Ministry of Defense formal UAP investigation program called 'Setka' ('Grid'), which operated as the Soviet counterpart to the United States Project Blue Book for thirteen years until 1991. Setka investigated more than 3,000 Soviet UAP reports between 1978 and 1991, and its archives — partially declassified in 1995 — remain the most important state-generated UFO investigation dataset in world history outside the Blue Book files.