The PIROG 5 Incident: when a Su-15 Interceptor shot down a Swedish high-altitude research balloon that floated into Soviet Airspace

The PIROG 5 Incident: when a Su-15 Interceptor shot down a Swedish high-altitude research balloon that floated into Soviet Airspace

By Dario Leone
Dec 9 2023
Sponsored by: Helion & Company
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Combating aerostats that floated into Soviet Airspace

Arguably, the vast majority of professional military aviation researchers, as well as amateur enthusiasts, would most likely associate balloon busting with the First World War. Yet if numbers are anything to go, by it was also an important part of the Cold War. As far as it could be established, no totals were ever published in Russian or other post-Soviet sources, but combating aerostats that floated into Soviet Airspace was an important task of the Air Defence Force (PVO) fighter aviation.


In order to deal with foreign aerostats several types of interceptors were used: MiG-19, MiG-21, Yak-28, Su-15 and even Tu-128.

Su-15, the balloon buster

As explained by Krzysztof Dabrowski in his book Defending Rodinu: Volume 2 – Development and Operational History of the Soviet Air Defence Force, 1961-1991, among the PVO’s interceptors, the Su-15 proved to be a real balloon buster. The Flagon in fact either shot down or incapacitated numerous aerostats.

PIROG 5 incident

A particularly noteworthy incident of this kind took place on Sep. 3, 1990. However, before relating those events, it is fitting to devote a few words to a seemingly different subject, namely the use of balloons for the purpose of scientific observation. During the 1980s, a new astronomical observation system was developed known under the acronym PIROG which stood for Pointed Infra-Red Observation Gondola. The system compromised a high-altitude hydrogen filled balloon with a gondola containing observation equipment attached underneath. Due to the ability to reach high altitudes, astronomical observations of the interstellar medium in the infra-red spectral range could be performed above the lower layers of the atmosphere by the PIROG. Once a mission was accomplished, the flight was terminated by cutting the balloon line via ground command with the gondola descending to earth by means of parachutes.

Four flights were successfully performed, and nothing indicated that the fifth one appropriately, named PIROG 5, which was launched on Aug. 27, 1990 from the European Space Range site in Kiruna, Sweden would be any different. However, for reasons not known, the separation system by radio command failed. Thus, the out-of-control aerostat with the gondola attached, drifted over the Arctic Sea floating into Soviet airspace in the vicinity of Murmansk by Sep. 2, 1990.

Su-15 downing a Swedish research balloon

The balloon was tracked by Soviet AD radars and it was decided to deal with it as with numerous other foreign aerostats which had entered Soviet airspace in times past, that is to shoot it down. However, the decision was also made not to down it with surface-to-air missiles, even though this could have been done with ease. There were a number of reasons for this: first of all, after firing the SAMs and hitting the target, their falling debris could pose a hazard; in addition, it seemed advisable to visually inspect the balloon and its payload. Finally, it was prudent to ensure that the balloon was not manned, even if the possibility was remote.

The PIROG 5 Incident: when a Su-15 Interceptor shot down a Swedish high-altitude research balloon that floated into Soviet Airspace

Thus, the aerostat floated unhindered in the vicinity of Murmansk giving the inhabitants of the city and surroundings, many visual thrills for as the lighting conditions, changed so did its colour. Finally, the balloon drifted out of sight in the southern direction. Once it was over sparsely inhabited areas, its fate was sealed. A Su-15TM of the 431st Fighter Regiment took off from Afrikanda Air Base with Captain Igor Zdatchenko at the controls, tasked with intercepting and downing the aerostat. The actual shoot-down took place some 50km north-west of the town Kovdor at an altitude of ca 14 000m, with the date and time being Sep. 3, 1991 and 08:08 respectively.

Two R 60 air-to-air missiles were launched by the Soviet fighter, one of which struck the balloon’s envelope. The fact it had enough of a thermal signature for the IR guided missile to home on to, is in itself, noteworthy. The missile’s warhead detonated, shredding the aerostat’s envelope and sending its gondola plunging earthwards. However, soon the parachutes deployed and thus the gondola floated down, coming to rest intact on the ground.

Incident aftermath

The incident had an interesting aftermath. Not surprisingly, the Swedes requested the gondola to be returned but the Soviets’ official reply was, that the item came down in a swamp and sunk into it. Meanwhile however, Soviet scientists had written to their Swedish colleagues about the coming down of the scientific balloon’s gondola and enclosed a photograph of it with the letter. Once ‘armed’ with ad oculus evidence of its survival, the Swedes took the matter to the Soviet authorities for a second time. Since the evidence was irrefutable, the Soviets could not deny the possession of the gondola and eventually returned it but according to the Swedes, removed a number of high-tech items first. It should also be added that the incident described did not derail the PIROG programme which subsequently continued with more balloon launches.

Defending Rodinu: Volume 2 – Development and Operational History of the Soviet Air Defence Force, 1961-1991 is published by Helion & Company and is available to order here.

Photo credit: U.S. Air Force, U.S. Navy and

The PIROG 5 Incident

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Dario Leone

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.
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