Zond 6

The Soviet Union launches an unmanned Soyuz 7K-L1 spacecraft, designated Zond 6, on a seven-day flight around the moon and back to Earth. Like Zond 5 before it, Zond 6 carries specimens of Earth animals and plants, but a rupture of the Soyuz pressure vessel results in these specimens being exposed to hard vacuum, resulting in instant death. Furthermore, the spacecraft opens and jettisons its parachutes too early, leaving it to slam into the ground at high speed. With continued glitches plaguing the Zond flights, Soviet mission planners aren’t confident enough in the 7K-L1 capsule to put cosmonauts in it for the next flight.