The Soviets decided to send this next 7K-L1 spacecraft not on a circumlunar flight, but to about 205,000 miles (330,000 kilometers) into deep space in the opposite direction of the Moon in order to test the main spacecraft systems without the perturbing effects of the Moon, much like the Surveyor model test flights in 1965-1966.

After returning from its high apogee, the spacecraft would carry out a high-speed reentry into Earth’s atmosphere and would be recovered and be investigated for the effects of reentry.

After launch into Earth orbit, at T+71 minutes and 56 seconds, the Blok D upper stage fired a second time (for 459 seconds) to send the 7K-L1 on a highly elliptical Earth orbit with an apogee of about 220,000 miles (354,000 kilometers).

The Soviet news agency TASS publicly named the spacecraft “Zond 4,” thus connecting the mission to a series of completely unrelated deep space probes -- a typically obfuscating maneuver from the Soviet media.

Controllers were unable to carry out a mid-course correction at 04:53 UT on March 4, 1968 when a star sensor (the 100K) of the attitude control system failed. A second attempt also failed the following day and the main omni-directional antenna also did not deploy fully, compromising communications. A third attempt at a correction proved successful, by using a special filter on the sensor to read signals accurately.

After the spacecraft separated into its two constituent parts, however, the descent module was unable to maintain a stable and proper attitude for a guided reentry, instead moving into a ballistic reentry trajectory, entering the atmosphere at 18:18:58 UT on March 9, 1968.

A crew on board would have experienced about 20 g’s but probably would have survived. However, because the descent module was falling into an unanticipated area, to prevent “foreign” observers from recovering the wayward spacecraft, an automatic emergency destruct system destroyed the returning capsule at an altitude of about 6 to 9 miles (10–15 kilometers) over the Gulf of Guinea.

For some years, the official Soviet press claimed that Zond 4 had entered heliocentric orbit.

Primary Source

Siddiqi, Asif A. Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016. NASA History Program Office, 2018.

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