This third-generation deep space probe had originally been slated for a Mars flyby in late 1964 but could not be prepared on time. Instead, Soviet designers diverted the mission for a simple lunar flyby in 1965 to test its basic systems and to photograph the far side of the Moon.
After a successful trans-lunar injection burn, Zond 3 approached the Moon after only a 33-hour flight. Its imaging mission began at 01:24 hours on July 20, 1965 at a range of 7,210 miles (11,600 kilometers) from the near side of the Moon and was completed 68 minutes later.
Zond 3 carried an imaging system somewhat similar to the one carried on the Automatic Interplanetary Station (Luna 3), with onboard exposure, development, fixing, and drying prior to scanning for transmission to Earth. The new system, known as 15P52, weighed 14 pounds (6.5 kilograms), was developed by the Moscow-based NII-885 (as opposed to VNII-380, which developed the Luna 3 system).
In total, the spacecraft took 25 visual and three ultraviolet images during its flyby. Closest approach was to 5,730 miles (9,220 kilometers). These pictures were successfully transmitted back to Earth on July 29, 1965, nine days after its lunar encounter when it was 1.4 million miles (2.2 million kilometers) from Earth.
Further communications sessions occurred on October 23, 1965 (involving photo transmissions) when Zond 3 was 19.6 million miles (31.5 million kilometers) from Earth. Last contact was sometime in early March 1966 when the spacecraft was 95.4 million miles (153.5 million kilometers) away. During the mission, it photographed the unseen 30 percent of the far side of the Moon.
Zond 3 also demonstrated successful course corrections using both solar and stellar orientation, a first for a Soviet spacecraft.
Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.