Nov. 10, 1968: Launch
Nov. 14, 1968: Lunar Flyby
Nov. 17, 1968: Return to Earth
Zond 6 was the second spacecraft that the Soviets sent around the Moon as part of the human circumlunar program.
Soon after translunar injection, which happened at 20:18:30 UT on launch day, ground controllers discovered that the vehicle’s high-gain antenna had failed to deploy. Given that the main attitude control sensor was installed on the antenna boom, controllers had to make plans to use a backup sensor for further attitude control.
After a midcourse correction at 05:41 UT on Nov. 12, 1968 (at a distance of 152,858 miles or 246,000 kilometers), the spacecraft circled the far side of the Moon at a closest range of 1,504 miles (2,420 kilometers), taking high resolution black-and-white photographs of the Moon at a range of 6,835 to 2,051 miles (11,000 to 3,300 kilometers).
During the return flight, temperatures in a hydrogen peroxide tank for the attitude control thrusters dropped far below acceptable levels. 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.
Despite the failures, Zond 6 conducted two mid-course corrections (on Nov. 16, 1968 at 06:40 UT and Nov. 17, 1968 at 05:36 UT), and then successfully carried out a fully automated guided reentry, requiring two successive “dips” into the atmosphere, each reducing velocity significantly (the first from 7 miles per second to 4.7 miles per second (11.2 kilometers per second to 7.6 kilometers per second), and the second down to just 656 feet per second (200 meters per second) and headed for the primary landing zone in Kazakhstan.
Each of the “dips” was automatically and expertly controlled by attitude control jets to vary roll control so as to provide lift and reduce g-loads. The Zond 6 descent module experienced a maximum of only 4 to 7 g’s.
After the successful reentry, a gamma-ray altimeter, detecting the now practically depressurized spacecraft (with pressure down to only 25 millibars), issued a command to jettison the main parachute at an altitude of about 3.3 miles (5.3 kilometers) instead of much lower.
As a result, the spacecraft plummeted down to the ground and was destroyed, with impact at 14:10 UT, about 10 miles (16 kilometers) from where the spacecraft had been launched 6 days and 19 hours previously.
Although the main biological payload -- unspecified by the Soviets -- was killed, rescuers salvaged film from the cameras and even managed to scavenge seedlings carried on board.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.