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Soyuz 7K-T No.39

Names	Soyuz 18a, Soyuz 18-1, April 5th Anomaly
Mission type	Docking with Salyut 4
Operator	Soviet space program
Mission duration	21 minutes 27 seconds 60 days (planned)
Orbits completed	Failed to orbit
Apogee	192.0 km (sub-orbital spaceflight)
Spacecraft properties
Spacecraft	Soyuz 7K-T No.6
Spacecraft type	Soyuz 7K-T
Manufacturer	OKB-1
Launch mass	6830 kg
Landing mass	1200 kg
Crew
Crew size	2
Members	Vasily Lazarev Oleg Makarov
Callsign	Урал (Ural – "Ural")
Start of mission
Launch date	5 April 1975, 11:04:54 UTC
Rocket	Soyuz
Launch site	Baikonur, Site 1/5[1]
End of mission
Landing date	5 April 1975, 11:26:21 UTC
Landing site	Altai Mountains, Kazakhstan (official) 50°50′N 83°25′E / 50.833°N 83.417°E / 50.833; 83.417
Orbital parameters
Reference system	Geocentric orbit (planned)
Regime	Low Earth orbit
Altitude	192.0 km
Inclination	51.6°
Period	90.0 minutes
Docking with Salyut 4 (planned)
Salyut program insignia Soyuz programme ← Soyuz 17 Soyuz 18 →

Soyuz 7K-T No.39 (also named Soyuz 18a or Soyuz 18-1 by some sources and also known as the April 5 Anomaly)^{[2]: 192–3} was an unsuccessful launch of a crewed Soyuz spacecraft by the Soviet Union in 1975. The mission was expected to dock with the orbiting Salyut 4 space station, but due to a failure of the Soyuz launch vehicle the crew failed to make orbit. The crew consisted of commander Vasily Lazarev, and flight engineer Oleg Makarov, a civilian. Although the mission was aborted and did not accomplish its objective, the craft exceeded common space boundaries and therefore is recognized as a sub-orbital spaceflight, which the crew survived. The crew, who initially feared they had landed in China, were successfully recovered.^[3][4]

The accident was partly disclosed by the normally secretive Soviets as it occurred during preparations for their joint Apollo-Soyuz Test Project with the United States which flew three months later. Lazarev never flew to space again and never fully recovered from the accident; Makarov made two more flights on board a Soyuz (both of which were to the Salyut 6 space station).

Position	Cosmonaut
Commander	Vasily Lazarev Second and last spaceflight
Flight Engineer	Oleg Makarov Second spaceflight

Position	Cosmonaut
Commander	Pyotr Klimuk
Flight Engineer	Vitaly Sevastyanov

Soyuz 7K-T No.39 was intended to be the second mission to take cosmonauts to the Soviet Salyut 4 space station for a 60-day mission.^[5] Both cosmonauts were on their second mission and had flown their first mission together, Soyuz 12, in September 1973 to test a new type of Soyuz spacecraft after the fatal accident of Soyuz 11.

The launch proceeded according to plan until T+288.6 seconds at an altitude of 145 km (90 mi),^[5] when the second and third stages of the booster began separation. Only three of the six locks holding the stages together released and the third stage's engine ignited with the second stage still attached below it. The third stage's thrust broke the remaining locks, throwing the second stage free but putting the booster under unexpected strain that caused it to deviate from the proper trajectory. At T+295 seconds, the deviation was detected by the Soyuz's guidance system, which activated an automatic abort program. As the escape tower was long gone by this point, the abort had to be performed with the Soyuz's own engines. This separated the spacecraft from the third-stage booster and then separated the orbital and service modules of the Soyuz from the re-entry capsule. At the time, when the safety system initiated separation the spacecraft was already pointed downward toward Earth, which accelerated its descent significantly. Instead of the expected acceleration in such an emergency situation of 15 g (147 m/s²), the cosmonauts experienced up to 21.3 g (209 m/s²).^[2] Despite very high overloading, the capsule's parachutes opened properly and slowed the craft to a successful landing after a flight of only 21 minutes.

The capsule landed southwest of Gorno-Altaysk at a point 829 km (515 mi) north of the Chinese border.^[5] The capsule landed on a snow-covered slope and began rolling downhill towards a 152 m (499 ft) sheer drop before it was stopped by the parachutes becoming snagged on vegetation.^[2] Having landed in chest-deep powder snow and a local temperature of −7 °C (19 °F), the cosmonauts donned their cold-weather survival clothing. The cosmonauts were uncertain if they had landed in China, at a time when Sino-Soviet relations were extremely hostile, so they quickly destroyed documents relating to a military experiment planned for the flight.^[2] They reportedly exited the spacecraft shortly after landing and built a fire. Soon, the crew was in radio contact with a rescue team in an approaching helicopter, who confirmed they were in the Soviet Union, near the town of Aleysk. The deep snow, the high altitude, and the terrain meant the rescuers had great difficulty in making contact with the cosmonauts. It was the next day before they were safely airlifted out.^[2] The crew were returned to Star City; the capsule was recovered some time later.

Initial Soviet reports stated the men had suffered no ill effects from their flight. However, subsequent reports claimed that Lazarev was injured by the high acceleration of re-entry. Makarov went on to take part in Soyuz 26, Soyuz 27, and Soyuz T-3 missions.

In Brezhnev's time, it was rare to disclose anything about Soviet failures, and so the first (Soviet) publication about the realities of the flight was made only a month later (8 May 1975).^[6] The Americans were informed on 7 April 1975 after the crew had been rescued. However, as the failure occurred during preparations for the joint Apollo-Soyuz Test Project, the United States requested that a more detailed report of the accident be provided. (There was even a United States congressional inquiry regarding this failure and several others.^[7]) In the report the Soviets made to the Americans, the abort was referred to as the "April 5th anomaly", and as this was the only term the Soviets ever used for the incident, that became the "official" designation for years afterwards. It was also disclosed that the booster used in the launch was an older model and not the same model that would be used for Soyuz 19.^[8] The mission is referred to in some literature as Soyuz 18a or Soyuz 18–1, since the following Soyuz mission in May 1975 received the name Soyuz 18. (The Soviets only gave numbers to successful launches.)

The exact landing site of the capsule was a subject of debate amongst space historians in subsequent years. A Russian source^[9] quoted by James Oberg has stated that the landing occurred in Mongolia.^[10]

The failed Soyuz mission was the only case of a crewed booster accident at high altitude until the accident with Soyuz MS-10 on 11 October 2018.

• Mass: 6,830 kg (15,060 lb)
• Apogee: 192.0 km (119.3 mi)

• Soyuz 7K-ST No. 16L, 1983 Soyuz launch failure
• Soyuz MS-10, 2018 in-flight failure
• List of space accidents