Zond 4, part of the Soviet Zond program and an uncrewed version of Soyuz 7K-L1 crewed Moon-flyby spacecraft, was one of the first Soviet experiments towards crewed circumlunar spaceflight. It was launched to test the spaceworthiness of the new capsule and to gather data about flights in circumterrestrial space. It was the first Soviet spacecraft to possess a computer, the 34 kg Argon 11.^[1]

Zond 4

Names	Soyuz 7K-L1 s/n 6
Mission type	Lunar flyby Spacecraft test
Operator	OKB-1
COSPAR ID	1968-013A
SATCAT no.	03134
Spacecraft properties
Bus	Soyuz 7K-L1
Manufacturer	OKB-1
Launch mass	4,425 kilograms (9,755 lb)
Dimensions	4.5 m x 2.2 m x 2.72 m
Start of mission
Launch date	2 March 1968, 18:29:23 UTC
Rocket	Proton-K/D
Launch site	Baikonur 81/6
End of mission
Disposal	deorbited/destroyed
Decay date	9 March 1968 (1968-03-09)
Orbital parameters
Reference system	Geocentric
Regime	Elliptical orbit
Perigee altitude	200 kilometres (120 mi)
Apogee altitude	400,000 kilometres (250,000 mi)
Inclination	51.53°
Zond ← Zond 1967B Zond 1968A →

The spacecraft was successfully launched by a Proton D-1e on 2 March 1968 into a 400,000 km apogee orbit 180 degrees away from the Moon.^[2] It was launched away from the Moon probably to avoid trajectory complications with lunar gravity. However, on re-entry the L1's guidance system failed. It hit the atmosphere precisely at the calculated time, but was not guided to generate lift and fly out of the atmosphere again. A ballistic re-entry would mean no recovery on Soviet soil, so the APO destruct system automatically blew up the capsule at 10 to 15 km altitude, 180–200 km off the African coast at Guinea.^[1]

The Zond 4 flight was in some ways similar to the American Apollo 4 and 6 missions as they were uncrewed test flights in highly elliptical Earth orbits.^[3]