[3] The mission was a precursor to a crewed circumlunar flight which the Soviets hoped could occur in December 1968, thus beating the American Apollo 8.

[4] During the craft's return from the Moon, concerns arose about the falling temperatures of the hydrogen peroxide tanks.

[5] Zond 6 used a relatively uncommon technique called "skip reentry" to shed velocity upon returning to Earth.

A few hours before reentry, on 17 November 1968, the cabin largely depressurised, killing all the animal test subjects aboard.

[5][6] A State Commission investigating the crash later determined that the coronal discharge effect which caused the parachute to jettison would only occur at the 25 mmHg (3.3 kPa) capsule pressure.