By the beginning of 1963 as the crippled Mars 1 was making its way towards the Red Planet, Chief Designer Sergei Korolev and his team at the OKB-1 design bureau were already making plans for an improved family of interplanetary spacecraft designated Object 3MV that would be sent to Venus and Mars during the next launch opportunities in 1964. These new spacecraft would incorporate a laundry list of incremental improvements and design changes based on the experience with the earlier 2MV series of spacecraft (see “You Can’t Fail Unless You Try: The Soviet Venus & Mars Missions of 1962”). Among these was a set of experimental ion thrusters that could serve as a backup to the primary compressed nitrogen attitude control system that had failed on Mars 1.

Problems with the 8K78 launch vehicle (later to be known as “Molniya” after the series of Soviet communications satellite which would use this rocket), which had doomed eight out of ten planetary probes launched between 1960 and 1962, not to mention the first two out of three E-6 unmanned lunar landers launched during the first quarter of 1963, were also to be addressed with a new version called the 8K78M under development at OKB-1’s Branch No. 3 in Samara under the direction of Dmitri Kozlov (which became an independent organization in 1974 and is today known as RKTs Progress and is responsible for the Soyuz family of launch vehicles). The upgraded 8K78M incorporated its own list of improvements, including upgraded engines and significant modifications to the Blok L escape stage, whose poor performance had stranded a half dozen planetary probes in their temporary parking orbits around the Earth.

The New 3MV

Externally, the 3MV series of spacecraft resembled the earlier 2MV series. The 3.6-meter tall 3MV would was built around a cylindrical orbital compartment with a diameter of 1.1 meters that was about as tall. This compartment contained control systems, power supplies, and communications gear, as well as some instrument electronics with a planetary compartment mounted beneath that was geared towards specific investigations of the target planet. Like the earlier 2MV design, mounted on top of the orbital compartment was a course correction system that employed a KDU-414 engine designed and built at the OKB-2 design bureau led by Aleksei Isayev. The pressure-fed KDU-414 had a 35-kilogram supply of UMDH (unsymmetrical dimethylhydrazine) and nitric acid to generate two kilonewtons of thrust. Depending on the actual mass of the spacecraft, this was sufficient propellant for a delta-v of 80 to 100 meters per second. Normally this engine would be employed twice during a typical mission: once a few days after leaving the Earth to correct the 3MV trajectory for launch errors, and a second time a few days before encountering its target to refine its approach trajectory to meet the mission’s objectives. Also located here was the attitude control system, which used pressurized nitrogen stored in a pair of tanks mounted on the orbital compartment.

Mounted either side of the orbital compartment were a pair of solar panels with a total span of about 4 meters that provided power for the spacecraft. Attached to the ends of the solar panels were hemispherical radiators designed to provide thermal control for the spacecraft’s interior systems. Water pumped through heat exchangers in the interior would circulated out through black- or white-painted sections of the radiators to heat or cool the spacecraft systems as needed to maintain the interior’s temperature between 20°C and 30°C. A two-meter in diameter high-gain directional antenna mounted on the anti-sun side of the orbital compartment was used for long distance communications. Various low gain antennae were also mounted on the exterior of the orbital compartment to provide an omni-directional communications capability. Instrument sensors to measure magnetic fields, various types of radiation and micrometeoroids that were mounted on the exterior rounded out the orbital compartment.

As before, the planetary compartment came in two varieties. The first was a one meter tall compartment with a film camera system, a set of ultraviolet and infrared instruments designed to study the target planet during a close flyby as well as its own transmitter which fed through the orbital compartment’s high gain antenna. The other version was a roughly spherical lander with a diameter of about 0.9 meters designed to detach from the orbital compartment before encounter and touch down on the target planet. Both the orbital and planetary compartments were pressurized to provide a laboratory-like environment for the internal equipment in order to simplify the design and testing of various systems as well as provide easier thermal control.

As with the 2MV, the 3MV design had four design variants. The 3MV-1 would have a launch mass of 948 kilograms and carry a planetary compartment designed to land on Venus. Its sister craft, the 3MV-2, would have a launch mass of 935 kilograms and sport a planetary compartment designed to study Venus during a flyby. The nominal launch window to Venus extended from late March to early April 1964. The 3MV probes would follow fast Type I trajectories to arrive at Venus in the second half of July. Because of the success of NASA’s Mariner 2 mission, in January 1963 the United States cancelled plans to launch a follow on mission using Mariner R-3 (to have been assembled from spare flight hardware) during this window, so these Soviet Venera spacecraft would be flying without competition.

Because the launch window to Mars in November 1964 was so much more favorable than the earlier launch window used by Mars 1 in November 1962, the Mars-bound 3MV spacecraft would be significantly more massive than their earlier 2MV counterparts. The 3MV-3 designed to land on Mars would have a launch mass of 1,042 kilograms while the 3MV-4 flyby craft would weigh in at 1,037 kilograms. These craft would carry a significantly more massive instrument payload as a result. The 3MV-3 and 3MV-4 would be four times more massive and much more capable that the American Mariner-Mars 1964 flyby mission that was also under development at this time.

Engineering Test Flights

Korolev understood that there was a need to flight test the new 3MV design in order to iron out the problems that would inevitably crop up. To this end, Korolev envisioned flying two or three dedicated 3MV engineering missions to be launched into interplanetary space starting in the summer of 1963. It was hoped that any problems uncovered during these test missions could be resolved in time for the 1964 planetary missions thus increasing their chances for success. Since these test flights would not be directed towards Venus or Mars, after launch they would receive the generic name of “Zond,” which means “probe” in Russian.

The first Zond version would be a stripped down model of the Venus lander craft designated 3MV-1A. Originally envisioned to have a launch mass of about 800 kilograms, this spacecraft would carry little scientific instrumentation and a lightweight 275-kilogram entry probe. The 8K78 rocket would be used to launch the 3MV-1A into a nearly circular solar orbit inclined about 5° to the ecliptic. About nine days after launch, a course correction maneuver using the propulsion system employing the KDU-414 engine would be performed. In this solar orbit, the probe would recede to a maximum distance of 12 to 16 million kilometers before returning back to the vicinity of the Earth five to six months after launch. About 10 to 15 days before reaching Earth, the 3MV-1A would make one more course correction to refine its final approach trajectory. Before reaching the Earth, the entry probe would detach from the orbital compartment and reenter the atmosphere at a speed in excess of 11.5 kilometers per second to simulate the conditions of an entry into the Venusian atmosphere. After reentry, the parachute system would be tested and, presumably, the lander would be recovered. In addition to giving Korolev’s engineers a chance to test the 3MV-1 design on a long mission, it would be the first spacecraft ever to return to the Earth from interplanetary space, giving the Soviet Union yet another space first.

The second Zond variant would be a stripped down version of the Mars flyby spacecraft designated 3MV-4A. Originally envisioned to have a launch mass of 996 kilograms, this spacecraft would carry a planetary compartment equipped with a significantly updated miniaturized film-based imaging system and other scientific instruments. The 3MV-4A would use an 8K78 to send it on a simulated trajectory towards the orbit of Mars. At a distance of 40,000 to 200,000 kilometers, the 3MV-4A would turn its camera back towards the receding Earth and acquire a sequence of photographs that would be subsequently developed automatically on board. The spacecraft would then transmit its scanned photographs and other data gathered on the interplanetary environment out to distances as great as 200 to 300 million kilometers as part of a long distance communications test. The 3MV-4A mission, if successful, would provide the first images of the Earth taken from deep space and give the Soviets another long distance communication record in addition to a much needed engineering test of the 3MV-4 design.

On March 21, 1963, the Soviet government officially approved the 3MV program. It would consist of one or two 3MV-1A flights and a single 3MV-4A flight to be launched in 1963 as well as a total of six operational 3MV spacecraft to be launched to Venus and Mars in 1964. As had happened all too frequently in the past with other projects, however, design and construction of the new 3MV took longer than expected. However, by July of 1963 Korolev had set the launch schedule for the Zond tests. A 3MV-1A would launch sometime between September 1 and October 15 (with the optimum launch date being October 12) on an Earth-return mission. This launch date would allow the 3MV-1A to be visible high in the sky as viewed from Soviet tracking stations during most of its mission and would result in a return just as the launch window to Venus was opening. While this would be too late to make any major changes to the design of the 3MV-2 Venus lander, it would still allow any problems with the basic 3MV design to be uncovered early enough to make changes a few months before the Venus launches. The 3MV-4A Zond mission was scheduled for launch in March 1964 with sufficient time to make any needed changes to the Mars-bound 3MV design to be launched eight months later.

The Zond Missions

Continued delays in the preparation of the first 3MV-1A ended up pushing its launch date out by several weeks, but by early November 1963 it was finally ready. Less time would be available to correct any problems uncovered by this flight, but it was felt to be vitally important. On November 4, the Central Committee officially approved two TASS press releases for the upcoming mission. If the test craft was successfully placed on its interplanetary trajectory, it would be called “Zond 1”. But if it got stranded in its short-lived parking orbit by another launch vehicle failure, it would receive a generic “Kosmos” designation to avoid the international treaty and public relations issues that had arisen as a result of the unannounced 2MV launch failures towards Venus and Mars a year earlier.

At 06:23:35 GMT on November 11, 1963, 8K78 serial number G103-18 lifted off from Site 1/5 at the Baikonur Cosmodrome in Soviet Kazakhstan carrying the 800-kilogram 3MV-1A No. 2 on an Earth-return mission. The first three stages of the launch vehicle successfully placed the Blok L escape stage and its payload into a 195 by 229 kilometer parking orbit with an inclination of 64.8°. But as had happened eight times before, the Blok L stage failed to function as intended. During its coast in parking orbit, all telemetry stopped at about 06:45:45 GMT as attitude control was lost. When the time came to make its burn to escape its parking orbit, the Blok L ended up firing its engine in the wrong direction stranding the rocket and its payload in Earth orbit. The orbit of what was now called Kosmos 21 decayed three days later. This would be the first of many failed lunar and planetary missions to be hidden (rather poorly, I might add) by the Kosmos designation.

With the launch failure of the first Zond engineering test mission and the continued unsatisfactory condition of the 3MV spacecraft under final assembly and test, by the end of December 1963 a second 3MV-1A mission was officially approved for launch in January 1964 on the first improved 8K78M, with the 3MV-4A test flight to follow in the April-May time frame after the launch of the Venus missions. While a longer test flight would have been preferred, at least the new 8K78M could be tested and a few weeks of flight experience could be gained before the launch of the Venus missions in two months. Since the problems with Venera 1 (see “Venera 1: The First Venus Mission Attempt”) and Mars 1 occurred during the first few days of their missions, it was hoped that any hidden problems with the 3MV design would become apparent early enough – and be minor enough – so that the operational 3MV craft could be modified before launch. Eventually it was decided that no 3MV-2 flyby probes would be launched towards Venus during the 1964 window and that all resources would be instead concentrated on preparing just a pair of 3MV-1 landers.

Still more delays pushed the launch of the 3MV-1A test flight out further, towards the very beginning of the 1964 Venus window. Additional issues with the scheduling of E-6 lunar landing missions, also being developed at OKB-1 at the same time, complicated matters further. Officials finally decided that the 3MV-1A test flight would now take place in late February 1964 – at the very beginning of the Venus launch window. While the target of this flight has yet to be revealed, an analysis of the trajectory options at this time suggests that the 8K78M had sufficient payload capability to launch the lightweight 3MV-1A on a longer Type II trajectory towards Venus that could reach the planet sometime around the end of August (see “Trajectory Analysis of the 1964 Soviet Venus Missions”). Although it is likely that there was little expectation that this test mission would actually survive the six-plus months needed to reach Venus, it would still provide a vital test of the new 8K78M launch vehicle and a month’s worth of in-flight engineering data that could help improve the chances of success for the pair of operational 3MV-1 landers. After the next E-6 lunar lander was launched during its narrow window around March 21, launch opportunities to Venus on the faster Type I trajectories that would get the landers to Venus in the last half of July became available over the course of the following couple of weeks.

The Venus Missions

At 05:45:40 GMT on February 19, 1964, the first of the improved 8K78M rockets, serial number T15000-19, lifted off carrying 3MV-1A No. 4A on the last chance at a test flight of the Venus probe design. But just as the Blok I third stage was supposed to ignite, failure occurred once again. Liquid oxygen leaking from a bad valve seal froze a kerosene fuel line, causing it to break. This resulted in an explosion and the loss of the launch vehicle with the debris falling to Earth 85 kilometers north of Barabinsk in Siberia.

With the loss of the last available 3MV-1A test craft, it appears that a rather drastic decision was made. Apparently desperate to get some test flight data to improve the chances of reaching Venus, it seems that officials decided to launch one of the pair of operational 3MV-1 spacecraft early at the beginning of March on a slower Type II trajectory towards Venus that would provide three weeks of flight data before the opening of the faster Type I launch window later in the month (see “Trajectory Analysis of the 1964 Soviet Venus Missions”). While the longer, six-month trip time increased the chances that the 3MV-1 would fail before reaching Venus in early September, it would seem that it was felt to be less risky than launching a pair of untried spacecraft with absolutely no flight testing. But the launch of 8K78M serial number T15000-22 for March 1, 1964 was ultimately scrubbed because of problems encountered during prelaunch integration of the 3MV-1 with the 8K78M. In order to resolve the problem, the launch was postponed to the opening of the Type I launch window at the end of the month. The 3MV-1 would now have to fly untested.

As the final preparations for the pair of 3MV-1 landers were being made, the focus shifted to the launch of the unmanned lunar lander, E-6 No. 6. The E-6 Luna program was coming out of a self-imposed year-long stand down as major problems with the E-6 uncovered during the first three attempts in early 1963 were fixed. It was hoped that the E-6 problems were resolved and that this flight would beat America’s delayed Surveyor lunar lander by a year or more (see “Surveyor 1: America’s First Lunar Landing”). At 08:15:35 GMT on March 21, 1963, 8K78M serial number T15000-20 lifted off carrying the new E-6. But a series of problems with the 8D715P engine on the Blok I third stage culminated in its premature shutdown 487 seconds into the flight. The rocket and its payload were destroyed during reentry (for more details on this and other early E-6 failures, see “The Mission of Luna 5”).

With two successive failures of the new 8K78M, it was now the turn of the 950-kilogram 3MV-1 Venus landers. With its earlier problems now resolved, 3MV-1 No. 5 was launched at 03:24:42 GMT on March 27, 1964, on 8K78M serial number T15000-22. This time the first three stages of the new 8K78M worked as intended to place the Blok L escape stage and its Venus-bound payload into a 191 by 237 kilometer parking orbit with an inclination of 64.8°. But during the unpowered coast, an electrical fault caused attitude control to be lost and the Blok L never ignited its 11D33 engine. Stranded in Earth orbit, the rocket and its payload were designated Kosmos 27. Despite the failure, one of the many improvements made to the new Blok L paid off this time. Data from the various systems gathered by the new telemetry system were recorded and radioed back to ground controllers on the next orbit allowing engineers to diagnose the failure like never before. This failure (and probably a couple of earlier failures) were found to be the result of a fault in the design of the wiring of a key control system in the Blok L. Fortunately, the fix involved only 20 minutes of a technician’s time with a soldering iron to resolve for the next launch attempt.

With its wiring fault fixed just in time for the end of the Venus launch window, 8K78M serial number T15000-23 lifted off the morning of April 2, 1964, at 02:42:40 GMT carrying 3MV-1 No. 4 and its escape stage into a 187 by 213 kilometer parking orbit with an inclination of 64.8°. This time all four stages of the 8K78M worked sending the spacecraft out of Earth orbit and into a 0.65 by 1.06 AU solar orbit with an inclination of 3.92° to the ecliptic and a period of 290 days. What was to be called “Venera 2” was now on its way to Venus and the first attempted landing on another planet. However, before the triumphant launch announcement could be made, ground controllers discovered a major problem during the first communication session with the receding probe. The pressurized orbital compartment was leaking and all its gas would be lost within a week severely compromising the ability of its equipment to operate. With bleak prospects for success, the Soviets announced the probe simply as “Zond 1” making no mention of its mission to Venus (see “Zond 1: First Lander Sent to Venus“).

Diagnosing the Failure of Zond 1

Because of the torque from the leak in the orbital compartment, Soviet engineers were able to pin down the problem quickly as a bad weld near the quartz window for the probe’s star and Sun sensors. Although it would not help Zond 1, future 3MV craft would have their welds X-rayed as a new quality control check. While the probe was still fully functional and engineers formulated a contingency plan to keep the spacecraft operating as long as possible, Zond 1 made a course correction maneuver the day after launch at a distance of 563,780 kilometers to refine its trajectory. This was the first time the KDU-414-based propulsion system had been used on a Soviet planetary mission.

By April 9 the pressure in the orbital compartment had dropped to the point where it became unreadable by the onboard sensors. Since the Zond’s main transmitter required a pressurized compartment to maintain thermal control and suppress arcing in its high voltage circuitry, ground controllers routed communications through one of the redundant pair of transmitters in its 290-kilogram lander. Obviously Soviet engineers had learned the same hard lessons of redundancy that their American counterparts had during the failures in NASA’s early Ranger missions (see “NASA’s First Lunar Lander”). With some measure of command, control, and communications, Zond 1 proceeded to gather limited data from its instruments about the interplanetary environment. Its new ion thrusters were also tested but were found to operate erratically, possibly due to the loss of pressure in the orbital compartment where the thrusters’ control systems were located.

Continued tracking of Zond 1 showed that it would still miss Venus by a large margin, so on May 14, a second course correction was attempted. At a range of 14 million kilometers from Earth, the KDU-414 engine ignited for a second time changing the velocity of Zond 1 by 50 meters per second before the engine apparently cut off early. Still 20 meters per second shy of the required velocity change, Zond 1 would still miss Venus by about 100,000 kilometers.

As the cruise to Venus continued, more problems cropped up including the apparent loss of one of its star sensors required for attitude control. Zond 1 was placed into a flat spin to stabilize its orientation and keep the solar panels pointing towards the Sun. Unfortunately, the high gain antenna could no longer be used so contact via the lander’s communication systems could only be maintained until about the middle of June before it was too far away – a full month before reaching Venus. As luck would have it, though, Zond 1 would not last that long. The last public announcement about Zond 1 came on May 19 and all communications were lost on May 24. The now silent Zond 1 flew by Venus on July 19.

A Change of Plans for Mars

With this latest failure, Korolev and his team at OKB-1 were left scrambling to diagnose and correct the problems uncovered with the 3MV and the 8K78M. Based on revised plans for the upcoming Mars launch opportunity, Korolev and his team intended to launch a pair of 3MV-3 landers and another pair of 3MV-4 flyby spacecraft towards Mars in November 1964. But in addition to the ongoing hardware issues, Soviet ambitions for this Mars launch window were now being threatened by the latest revelations about Mars gleaned from ground-based observations.

When design work on the first generation Soviet Mars landers started in 1960, the general consensus of the astronomical community was that Mars had an atmosphere dominated by nitrogen, much like the Earth’s, with traces of carbon dioxide. Based on decades of photometric and polarimetric observations of how the Martian atmosphere scattered sunlight, it was estimated that the atmospheric surface pressure on Mars was about 85 millibars compared to Earth’s 1,013 millibars. Of course, today we now know that the Martian atmosphere is composed primarily of carbon dioxide with a mean surface pressure of only 6 millibars – less than a tenth as dense as had been generally assumed at the beginning of the Space Age. As a result, the original Soviet Mars lander design was simply inadequate for such a thin atmosphere and would crash during any landing attempt.

The low pressure of the Martian atmosphere started to become apparent at about the same time as the 3MV-3 landers were in the process of being developed and manufactured. The first indications of trouble came from Soviet astronomer Vassili I. Moroz at the Sternberg State Astronomical Institute in Moscow. A pioneer in infrared spectral studies of bodies in the solar system, his analysis of the infrared spectra he had obtained of Mars during its opposition in early 1963 showed that the surface pressure of Mars was likely only about 24 millibars and possibly much less. While Korolev and his engineers must have been aware of this work, which had been submitted for publication in September of 1963 and might have dismissed it as an isolated anomalous result, similar results were being published in peer-reviewed scientific journals in the West starting on New Year’s Day 1964. By the summer of 1964, NASA-sponsored studies for Mars landing missions were now reflecting this new reality of a much thinner Martian atmosphere with a surface pressure possibly as low as 10 millibars. Only later did scientists determine that the large amounts of fine dust in the Martian atmosphere had biased earlier measurements making the atmosphere scatter more light and appear denser than it actually is. The original Soviet Mars lander design was simply inadequate for such a thin atmosphere and would crash during any landing attempt (for a full discussion, see “Zond 2: Old Mysteries Solved & New Questions Raised”).

With the continuing delays in the development of the Mars-bound 3MV spacecraft and the realization that the 3MV-3 landers were doomed to fail, Soviet officials eventually scrapped the original plans to launch four spacecraft to Mars in November 1964. Since it was already in an advanced state of preparation, officials decided instead that only the single 950-kilogram 3MV-4A test craft would be launched towards Mars. But given the poor track record of the design during previous planetary missions, they recognized that the chances of this spacecraft actually surviving all the way to Mars were slim even if it followed a faster 195-day trajectory that would allow it to reach Mars a full month before NASA’s Mariner spacecraft. As a result, Mars would be only a secondary objective for this mission with the primary objective being the original engineering test flight of the Zond series. The 3MV-4A, which would receive a “Zond” designation, would be launched into a slow trajectory with a 249-day transit time. This would simulate a future Mars lander mission profile with the spacecraft essentially performing a very long-duration engineering test flight.

Unlike NASA’s Mariner-Mars 1964 spacecraft, this Zond mission would attempt no Mars photography despite the fact that its more advanced camera system, equipped with 35 and 750 mm focal length lenses to expose up to 40 images on 25 mm format film, could return more than an order of magnitude more imaging data. Instead, the Zond would presumably photograph the Earth shortly after its departure as planned for the original test flight plan. Aside from engineering data and exercising the new imaging system, the spacecraft would acquire scientific data on fields and particles during its interplanetary cruise. If the test craft managed to survive its long flight to the Red Planet in good condition, it would be redirected to impact Mars and deliver a set of commemorative pennants it was carrying. While Mariner 4 would be the first spacecraft to flyby Mars and return images of its surface, the Soviet probe could be the first to impact its surface to at least satisfy propaganda purposes.

Zond 2 and Its Aftermath

The unsuccessful launch of NASA’s first Mars-bound spacecraft, Mariner 3 on November 5, 1964 (see “The Launch of Mariner 3”), was followed by a crash program to correct the fault with the rocket’s new launch shroud that caused the failure. With its new shroud barely ready in time, Mariner 4 successfully lifted off on November 28, 1964 with a scheduled encounter date of July 14, 1965 (see “Mariner 4 to Mars”). The Soviet’s Mars-bound engineering test craft, 3MV-4A No. 2, lifted off two days later on November 30 from the Baikonur Cosmodrome at 13:12 GMT with its 8K78M launch vehicle placing it into a 153 by 219 kilometer parking orbit. After a short coast, the Blok L escape stage came to life and successfully injected Zond 2 out of its initial low Earth orbit and into a 0.98 by 1.52 AU solar orbit with an inclination of 6.4° to the ecliptic and a period of 508 days. This much slower trajectory would not reach Mars until three weeks after Mariner 4.

As had happened during earlier Soviet planetary missions, problems were already apparent during the first communication session with Zond 2 when the spacecraft reported that only half of the expected power was being generated by its solar panels. As the problem was being diagnosed, controllers took measures to conserve power including the cancellation of the Earth imaging session as well as postponing the first scheduled course correction maneuver. In the end, they determined that one of the two solar panels on Zond 2 failed to deploy as intended. When the Blok L finished its burn, protective shrouds that were connected to the solar panels with lines were supposed to jettison pulling the solar panels out of their stowed position. It seems that one of the two pull-cords had broken resulting in the failure of one of the solar panels to deploy. After several engine firings to shake the spacecraft, the stuck panel finally deployed on December 15. But by this point it was already too late to perform the first planned midcourse correction.

This would prove to be only the beginning of Zond’s problems. A failed timer resulted in the thermal control system not functioning properly hampering operation of onboard equipment. While the new plasma engines were test-fired successfully, communications with the probe apparently became increasingly erratic after its scheduled December 18 communication session. Some accounts of the mission suggest that a course correction was finally made around February 17, 1965 that further refined the path of Zond 2 towards Mars. At some point in time after this maneuver, and possibly as late as May 2, controllers finally lost contact with Zond 2 with an official public announcement being made by Soviet authorities on May 5, 1965. Three months later on August 6, Zond 2 flew silently past Mars at a reported distance of 1,500 kilometers (although some later sources claim the flyby was at a much greater distance of 650,000 kilometers suggesting that no course correction was ever made).

New Missions

In addition to the Zond 2 engineering test flight towards Mars, the Soviet government had also approved more missions of the 3MV design. The latest plans called for a pair of landers and a pair of flyby spacecraft to be launched towards Venus during the November 1965 launch opportunity. Eventually these spacecraft were assigned the defunct 3MV-3 and 3MV-4 designations of the Mars spacecraft that were never completed and launched. In addition, the launch of one or two more 3MV-4 engineering flights was authorized in the first half of 1965 that would follow the earlier 3MV-4A mission profile to test the improvements made to the 3MV design in the wake of the Zond 2 failure.

As before, delays in the preparation of the improved 3MV spacecraft pushed the launch of the Zond test flight into the summer of 1965. But the delay also opened up a genuine opportunity for discovery for this mission as a result. As luck would have it, the Moon could be easily reached during this time of year via Zond’s departure trajectory towards the orbit of Mars and the lighting conditions would be ideal to observe the Moon’s western hemisphere. It was proposed that instead of photographing the Earth on its way into solar orbit, the new Zond mission could be directed to photograph the Moon including most of the lunar far side that had not been photographed earlier by the Soviet Luna 3 mission in 1959 (see “Luna 3: Shedding Light on the ‘Dark Side’ of the Moon“). This would provide a perfect opportunity for engineers to perform a complete end-to-end test the 3MV systems during an actual planetary encounter and provide new data on the Moon in the process.

The instrument complement of this 3MV-4 flight included the advanced photo-television system capable of taking either photographs or ultraviolet spectra in the range of 250 to 350 nanometers. This system was housed inside of the planetary compartment behind portholes at the base that allowed the instruments to view the target. Mounted on the exterior of the compartment were ultraviolet and infrared spectrophotometers sensitive to bands of 190 to 270 nanometers and 3 to 4 microns, respectively. The spectrophotometers and ultraviolet spectrometer were originally designed to study planetary atmospheres and thus were of little use in a lunar mission, but the photo-television system was going to be the star of this mission.

The 6.5-kilogram photo-television system was basically a much-improved version of that employed six years earlier by Luna 3 and a later model flown on the unsuccessful Mars 1 mission. Images from a single 106.4-millimeter focal length, f/8 lens were focused onto 25 mm photographic film. A total of 25 exposures of one-thirtieth or one one-hundredth of a second were made. Using the same film, the ultraviolet spectrometer would expose the eighth, ninth, and tenth frames bringing the total number of exposures up to 28. After the film was exposed, it was automatically developed on board.

The dried negatives were then scanned and transmitted back to Earth in one of two formats. A quick-look format broke the photograph into 67 lines that could be transmitted back to Earth via a high-power C-band transmitter using the spacecraft’s two-meter high gain antenna in 135 seconds. A more detailed scanning of the photographs was also possible. In this mode, each photograph was broken into 1,100 lines of 860 points each that were comparable in quality to Ranger’s full-scan television images of the Moon (see “The Mission of Ranger 7”) but far superior to the digital imaging system used by NASA Mariner 4 mission to Mars. In this high-resolution mode, a single photograph could be transmitted over interplanetary distances in 34 minutes. Each photograph could be scanned multiple times to help increase the signal-to-noise ratio of the images reconstructed back on Earth.

The Flight of Zond 3

For this test flight, the 960-kilogram 3MV-4 number 3 was prepared for launch (some sources claim this was a 3MV-4A spacecraft but the difference was probably irrelevant at this stage.) At 14:38:00 GMT on July 18, 1965, what became Zond 3 was launched on an 8K78 Molniya rocket into a temporary 164 by 210 kilometer parking orbit with an inclination of 64.8°. After a short coast, the Blok L escape stage ignited sending Zond 3 to the Moon and beyond.

Once on its way, engineers discovered that Zond 3 was experiencing no major system malfunctions unlike all of its predecessors where problems were immediately apparent during the first communication session. Since the 3MV-4 spacecraft was so much lighter than the 1,500-kilogram E-6 lunar landers the Soviets had been launching towards the Moon at that time using the same rocket (unfortunately, with little success up to this point in history), Zond 3 reached the vicinity of the Moon in just half the time: a relatively short 33 hours.

At 01:24 GMT on July 20, at a distance of 11,570 kilometers, Zond 3 pointed its camera and other instruments towards the Moon and started taking one photographic exposure every 134 seconds. The initial images included not only the unmapped far side but also the near side so that newly discovered features could be tied into the already existing lunar mapping control net. This continued as the fast-moving probe reached its closest point to the Moon of 9,219 kilometers and then receded to a distance of 9,960 kilometers at the end of its photography session at 02:32 GMT with the Moon having been viewed through an angle of about 60 degrees. After this 68-minute photography session, Zond 3 immediately developed its film as it headed into a 0.9 by 1.5 AU solar orbit which simulated trajectory to Mars – simulated since Mars was not in position for a low-energy encounter and would not be for another year and a half.

On July 29, at a distance of 2.25 million kilometers, Zond 3 was out far enough for the sensors on its high gain antenna to lock onto the Earth and start transmitting the recorded images to waiting scientists. The images were spectacular and far superior to the ones returned six years earlier by Luna 3. Details as small as five kilometers across could be seen in the photographs which showed little more than a cratered wasteland. These photographs confirmed that there was a lack of maria on the Moon’s far side compared to the more familiar near side, which was dominated by these dark and relatively flat basaltic flows.

The photographs also showed no signs of the purported Mare Parvum which some ground-based observers claimed to have seen near Mare Orientale during especially extreme librations of the Moon. Zond 3 did discover a new type of lunar feature that Soviet scientists proposed be called thalassoids. These were the battered concave-shaped remnants of basins over 500 kilometers across and were thought to be the precursors of maria. For some reason these far side structures were never flooded with basaltic lava to form true maria (the term “thalassoids” fell out of official use in 1967 since these features were found to be indistinguishable from other large, unflooded impact basins). The spectral instruments onboard Zond 3 showed that the Moon reflected just 1% of the ultraviolet radiation hitting its barren surface. In contrast, the lunar surface reflected 80% to 90% in the incident infrared light, with a broad peak around 3.6 microns.

With these photographs in hand, the Soviets had mapped all but 5% of the Moon’s surface. While NASA’s Mariner 4 mission had just sent back the first images of Mars, Zond 3 had managed to send back the best images of the lunar far side with far superior quality. Even at this early stage of the mission, Zond 3 was a propaganda success after a long string of failed planetary probes and Luna missions.

Zond 3 continued to operate as it travelled farther from Earth and towards the general direction of the orbit of Mars. In addition to engineering information, Zond 3 sent back data from a complement of instruments designed to study the interplanetary environment during the long cruise. On September 16, at a distance of 12.5 million kilometers, Zond 3 used its propulsion system to change the probe’s velocity by 50 meters per second to simulate a mid-course correction. The spacecraft successfully retransmitted its lunar images in mid-August, mid-September, and for the last time on October 23, when it was 31.5 million kilometers away from the Earth.

Afterwards

By the time of the launch of Venera 2 on November 12, 1965, the first of four spacecraft planned to be launched during this Venus window, Zond 3 had been operating for 117 days in solar orbit confirming that the 3MV design could survive the 107-day transit to Venus. Regular communication sessions with Zond 3 were maintained until March 3, 1966, at a range of 153.5 million kilometers. Although the spacecraft was not heard from afterwards, Zond 3 had managed to operate for 228 days – barely long enough to survive the typical flight time to Mars but vindicating the soundness of the improving 3MV design. Unfortunately, contact with Venera 2 and 3 (the only two of the four spacecraft to be successfully launched towards Venus the previous November) had been lost days before the spacecraft were to begin their encounter activities at Venus due to thermal control issues (see “Venera 2 & 3: Touching the Face of Venus”).

Even before the final 3MV launches (and their eventual failures), responsibility for future Soviet probes to Venus and Mars had been transferred in April 1965 to NPO Lavochkin under Chief Designer Georgi Babakin – an organization known for its intensive testing and quality control of the hardware it built. Because of the limitations of the 3MV design and the new realization of how thin the atmosphere of Mars actually is, it was decided to scrap any further plans to send these probes to the Red Planet and instead design a new spacecraft that would use the much more capable Proton launch vehicle (see “The Largest Launch Vehicles Through History”). For the Venus landing missions, Babakin and his team would significantly upgrade the 3MV spacecraft to create the new 1V design to be launched in June 1967 (see “Venera 4: Probing the Atmosphere of Venus”). The hope was that these new spacecraft would fare better than their unsuccessful predecessors.

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Related Reading

“Luna 3: Shedding Light on the ‘Dark Side’ of the Moon”, Drew Ex Machina, October 4, 2019 [Post]

“You Can’t Fail Unless You Try: The Soviet Venus & Mars Missions of 1962”, Drew Ex Machina, November 1, 2017 [Post]

“Trajectory Analysis of Soviet 1964 Venus Missions”, Drew Ex Machina, April 2, 2014 [Post]

“Zond 1: The First Lander Sent to Venus”, Drew Ex Machina, April 2, 2020 [Post]

“Zond 2: Old Mysteries Solved & New Questions Raised”, Drew Ex Machina, July 17, 2014 [Post]

“Venera 2 & 3: Touching the Face of Venus”, Drew Ex Machina, March 1, 2016 [Post]

General References

Boris Chertok, Rockets and People Volume III: Hot Days of the Cold War (ed. Asif Siddiqi), SP-2009-4110, NASA History Division, 2009

Brian Harvey, Race Into Space: The Soviet Space Programme, Halsted Press, 1988

Brian Harvey, Russian Planetary Exploration: History, Development, Legacy and Prospects, Springer-Praxis, 2007

Bart Hendrickx, “Managing the News: Analyzing TASS Announcements on the Soviet Space Program (1957-1964)”, Quest, Vol. 19, No. 3. pp. 44–58, 2012

Wesley T. Huntress and Mikhail Ya. Marov, Soviet Robots in the Solar System: Mission Technologies and Discoveries, Springer-Praxis, 2011

Nicholas L. Johnson, Handbook of Soviet Lunar and Planetary Exploration, Univelt, 1979

Yuri N. Lipsky, “Zond 3 Photographs of the Moon’s Far Side”, Sky & Telescope, Vol. 30, No. 6, pp. 338-341, December 1965

Timothy Varfolomeyev, “The Soviet Venus Programme”, Spaceflight, Vol. 35, No. 2, pp. 42–43, February 1993

Timothy Varfolomeyev, “Soviet Rocketry that Conquered Space Part 5: The First Planetary Probe Attempts, 1960–1964”, Spaceflight, Vol. 40, No. 3, pp. 85–88, March 1998

Timothy Varfolomeyev, “Soviet Rocketry that Conquered Space Part 6: The Improved Four-Stage Launch Vehicle, 1964–1972”, Spaceflight, Vol. 40, No. 5, pp. 181–184, May 1998