Chief Designer Sergei Korolev had originally envisioned a crewed lunar spacecraft launched in pieces by R-7 boosters and assembled in Earth orbit.

However, Chelomei also proposed his own, competing for lunar spacecraft, the LK-1, and Soviet premier Nikita Khrushchev gave his approval in August 1964.

At the end of the year, Korolev revived his proposal for the Soyuz spacecraft but concealed his true intentions by billing it as an Earth orbital vehicle for testing rendezvous and docking maneuvers.

In October 1965, a mere three months before his death, Korolev was granted official approval for developing a crewed lunar spacecraft, which would be a modified Soyuz.

Korolev originally intended to piece together the lunar Soyuz in Earth orbit because he did not believe the UR-500 was powerful enough to launch the full vehicle or that it wouldn't be safe for the crew.

One of those also performed an unsafe (for humans) descent of up to 20 Gs of deceleration, the other suffered main parachute failure, and only one flight - Zond 7 - would have been safe for cosmonauts.

Zond 5 was the first spacecraft to carry a group of terrestrial creatures (tortoises being the most complex) on a circumlunar flight and return them relatively safely to Earth.

Although unsafe for humans, these high Gs apparently didn't affect the tortoises' health, and they were reportedly able to breed afterward.