Mission Type: Flyby
Launch Vehicle: 8K82K + Blok D (Proton-K no. 237-01)
Launch Site: NIIP-5 / launch site 81L
Spacecraft Mass: about 5,375 kg
Spacecraft Instruments: unknown
Deep Space Chronicle: A Chronology of Deep Space and Planetary Probes 1958-2000, Monographs in Aerospace History No. 24, by Asif A. Siddiqi
This was the sixth attempt at a robotic circumlunar flight in support of the L1 piloted lunar program and the first after the resounding success of the American Apollo 8 in December 1968.
The Proton launch vehicle lifted off on time, and first-stage operation was nominal. However, during second-stage firing, one of the four engines of the stage mysteriously switched off at T+313.66 seconds. The other engines continued firing, but subsequently, the primary third-stage engine also switched off during its firing sequence at T+500.03 seconds due to a breakdown in the main pipeline feeding fuel to the fuel gas generator. After a near-ballistic flight, the L1 payload landed near the border between the USSR and Mongolia.