Spacecraft designed for Soviet human circumlunar missions. It was unrelated to Zond spacecraft designed for manned circumlunar missions (Soyuz 7K-L1). A number of studies investigated sending a Gemini spacecraft onto a circumlunar trajectory. The first spacecraft to fly a circumlunar trajectory was Luna 3. Several modifications reduced vehicle mass and increased circumlunar capability. In 1968, Leonov was selected to be commander of a circumlunar Soyuz 7K-L1 flight. The Gemini-Centaur proposal was predicted to have been able to achieve a 72-hour circumlunar flight. He also proposed adapting an existing spacecraft design for the circumlunar mission, the single-cosmonaut LK-1. Zond 4 was launched on March 2, 1968, and successfully made a circumlunar flight.