After three failures, the Soviets tried again to accomplish a robotic circumlunar mission.
Besides a complement of sensors to monitor its basic systems and cameras, the spacecraft also carried a large biological payload: two Steppe tortoises (Testudo horsfieldi or the Horsfield’s tortoise), each with a mass of about 0.75 to 0.9 pounds (0.34 to 0.40 kilograms), hundreds of Drosophila eggs of the Domodedovo-32 line; air-dried cells of wheat, barley, peas, carrots, and tomatoes; a flowering plant of Tradescantia paludosa, three different strains of chlorella, and a culture of lysogenic bacteria.
The tortoises, each 6 to 7 years old and weighing about 12 to 14 ounces (340 to 400 grams), were part of a group of eight, with the other six serving as controls on the ground.
The two flight tortoises were placed in the spacecraft on September 2, 12 days before launch.
From that moment on, they (and two of the control group) were deprived of food to ensure that the only effect on them was due to the space mission.
The ascent to orbit was perfect: The 7K-L1 spacecraft + Blok D combination successfully entered a 119 x 136 mile (191 × 219 kilometer) Earth orbit.
About 67 minutes after launch, the Blok D fired again for lunar injection, after which the Soviet press announced the mission as “Zond 5.”
Once again, the 100K star sensor of the spacecraft’s attitude control system failed (due to contamination of its exposed surface). Controllers, however, managed to carry out a mid-course correction at 03:11 UT on Sept. 17, 1968 using the less accurate solar and Earth-directed sensors.
At the time, Zond 5 was 201,946 miles (325,000 kilometers) from Earth. The spacecraft successfully circled around the far side of the Moon at a range of 1,212 miles (1,950 kilometers) on Sept. 18, 1968, taking spectacular high-resolution photos of the Moon and Earth.
On the return leg of the flight, a second attitude control sensor (the 101K that used Earth for attitude control reference) failed and the spacecraft’s three-axis stabilization platform switched off the guided reentry system.
As a result, controllers were forced to maintain reentry attitude using the one remaining sensor (the 99K that used the Sun). They alternately fired two attitude control jets on each side of the vehicle to swing the spacecraft into the proper reentry corridor for a direct ballistic reentry into the backup target area in the Indian Ocean.
Zond 5’s descent module successfully splashed down in the Indian Ocean at 32 degrees 38 minutes south latitude and 65 degrees 33 minutes east longitude, about 65 miles (105 kilometers) from the nearest Soviet tracking ship.
Landing time was 16:08 UT on Sept. 21, 1968, making it a mission lasting 6 days, 18 hours, and 24 minutes. It was the first successful circumlunar mission carried out by any nation.
The tortoises survived the trip and arrived back in Moscow on Oct. 7, 1968. The results of dissection, performed on Oct. 11, 1968 after “a 39-day fast,” showed that “the main structural changes in the tortoises were caused by starvation” rather than flight to lunar distance or the subsequent travel back from the Indian Ocean to Moscow.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.