This was the last of three test vehicles launched as part of the third generation (“3MV”) Soviet interplanetary probes, and the first intended toward Mars. These were designed to test out key technologies during deep space missions.
Originally intended to fly in the April-May 1964 time period, this launch was delayed and then ultimately timed for the late 1964 Mars opportunity. Besides carrying out long-distance communications tests and imaging Earth on the way out into deep space, this vehicle’s trajectory was designed to allow it to intercept Mars (on approximately Aug. 6, 1965) and to become the first probe to enter its atmosphere and impact on its surface.
After successfully entering a trans-Mars trajectory, ground controllers discovered that the probe’s solar panels had not completely unfurled, depriving the vehicle of full power. Later investigation indicated that a tug cord designed to pull the panels free at the moment of separation from the Block L upper stage had broken off.
Controllers were able to fully open the panel only on Dec. 15, 1964, after “carrying out a number of dynamic operations on the station” according to the official institutional history, but by then the time for the first mid-course correction to fly by Mars had already passed. (Other reports suggest that even after the panels opened, they were partially obscured by radiators installed at the end of the solar panels which had not properly deployed).
Additionally, there had been a failure in the onboard programmed timer immediately after trans-interplanetary injection that led to inappropriate thermal conditions for the spacecraft between communications sessions.
Before loss of contact, Zond 2 successfully fired six plasma electric rocket engines (twice) at a distance of 3.34 million miles (5.37 million kilometers) from Earth. They were left on for 70 minutes and successfully maintained orientation of the spacecraft. These were technology demonstrators for future deep space missions. (These were actually carried on an earlier 3MV-1A model launched on Nov. 11, 1963, but that spacecraft failed to leave Earth orbit and was named Kosmos 21.)
The silent probe passed by Mars at a range of 404,000 miles (650, 000 kilometers) on Aug. 6, 1965 and entered heliocentric orbit. The spacecraft returned usable data on cosmic radio emissions far from Earth.