The first three missions were based on the model 3MV planetary probe, intended to explore Venus and Mars. After two failures, Zond 3 was sent on a test mission, becoming the second spacecraft to photograph the far side of the Moon. It then continued out to the orbit of Mars in order to test telemetry and spacecraft systems.
The missions 4 through 8 were test flights under for the Soviet Moonshot during the Moon race. The Soyuz 7K-L1 (also mentioned just as L1) spacecraft was used for the Moon-aimed missions, stripped down to make it possible to launch around the Moon from the Earth. They were launched on the Proton rocket which was just powerful enough to send the Zond on a free return trajectory around the Moon without going into lunar orbit (the type path that Apollo 13 flew in its emergency abort). With minor modification, Zond was capable of carrying two cosmonauts.
In the beginning there were serious reliability problems with both the new Proton rocket and the similarly new Soyuz spacecraft, but the test flights pressed ahead with some glitches. The majority of test flights from 1967–1970 (Zond 4 to Zond 8) showed problems during re-entry.
The Zond spacecraft made only unmanned automatic flights. Four of these suffered malfunctions that would have injured or killed any crew. Instrumentation flown on these missions gathered data on micrometeor flux, solar and cosmic rays, magnetic fields, radio emissions, and solar wind. Many photographs were taken and biological payloads were also flown.
A biological payload of two Russian tortoises, wine flies, meal worms, plants, seeds, bacteria, and other living matter was included in the flight, and were the first Earth lifeforms to travel around the moon and return safely.
The first spacecraft to circle the Moon and return to land on Earth.