Zond (Russian: Зонд, lit. 'probe') was the name given to two distinct series of Soviet robotic spacecraft launched between 1964 and 1970. The first series, based on the 3MV planetary probe, was intended to gather information about nearby planets.
The second series of test spacecraft was intended as a precursor to remote-controlled robotic circumlunar loop flights, using a stripped-down variant of Soyuz spacecraft, consisting of the service and descent modules, but lacking the orbital module.
Two tortoises and other lifeforms aboard Zond 5 were the first terrestrial organisms to travel around the Moon and return to Earth.
Main article: 3MV planetary probe
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 (after Luna 3). It then continued out to the orbit of Mars in order to test telemetry and spacecraft systems.
The missions Zond 4 through Zond 8 were test flights 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 same kind of path flown by Apollo 13 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 to 1970 (Zond 4 to Zond 8) showed problems during re-entry.
The Zond spacecraft made only uncrewed 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.
Circumlunar Zond spacecraft en route to the Moon. Artist's impression