The Soviet Zond 6 mission (1968)
The Soviet Union successfully launched its Zond 6 spacecraft on November 10, 1968, in a mission to perform a lunar flyby and a subsequent return to Earth. Zond 6 managed to circle the far side of the Moon, but the mission experienced a host of problems, including the failed deployment of its high-gain antenna (a backup sensor was used to regain control of the spacecraft), and temperatures within a thruster tank dropped far below acceptable levels during the return trip. “Engineers attempted to heat the tank by direct sunlight, but as they later discovered, such a procedure affected the weak pressurization seal of the main hatch and led to slow decompression of the main capsule, which would have undoubtedly killed a crew on board,” according to NASA. And as a last insult, the spacecraft failed to properly deploy its parachutes during re-entry, resulting in the complete loss of the vehicle on the plains of Kazakhstan.
