The Soviet Union's Zond (Russian for "probe") spacecraft series was designed to carry two cosmonauts around the Moon—that is, to conduct a circumlunar flight. Zond, also known as L-1, was a stripped-down Soyuz spacecraft. Modifications to the Soyuz design were designed primarily to reduce weight and included removal of various components, such as the third cosmonaut couch, a backup engine, and a backup parachute. Weight reduction was necessary so that Zond's chosen booster, a two-stage Proton rocket with a Block D third stage, could launch it around the Moon. In addition, Zond included a large radio antenna for communication across the 380,000 kilometers (235,600 miles) separating Earth and the Moon.

The Soviet Union conducted fourteen unpiloted Zond launches in three phases. The first four Zond tests aimed to prepare the vehicle for a piloted circumlunar flight to mark the fiftieth anniversary of the Communist Revolution in October 1967. The next...