The Venera program was undoubtedly the most successful and best known series of Soviet planetary missions. During the 1970s and early 1980s, a succession of spacecraft studied Venus from orbit as well as from its hostile surface for up to an hour at a time. The ten successful Venera landers secured the majority of the in situ scientific data we have today from the surface of Venus including direct measurements of atmospheric conditions, surface composition and four sets of unique images.

These successes, however, came after a series of frustrating failures starting with Venera 1 launched in February 1961 (see “Venera 1: The First Venus Mission Attempt”). The lessons learned from each failure were incorporated into the design and manufacture of each succeeding group of spacecraft designed to study Venus. It was not until the fifth group of Venus-bound spacecraft launched during the 1967 window that one of them, called Venera 4, finally reached its target and successfully returned data from our sister planet.

The New Venera Design

Like many of the early Soviet spacecraft, the first Soviet probes to Venus were designed and built by OKB-1 (the Russian acronym for Experimental Design Bureau-1) under the leadership of Chief Designer Sergei Korolev. The culmination of their work on planetary spacecraft was the 3MV series. Consisting of an orbital compartment, containing all of the main spacecraft systems to support spaceflight, and a planetary compartment carrying either a camera with a suite of instruments to study its target during a flyby or a lander, these metric ton spacecraft had performed poorly with only the Zond 3 engineering test flight which flew past the Moon in July 1965 being entirely successful (see “The Soviet Zond Missions of 1963-65: Planetary Probe Test Flights”). The last pair of 3MV missions, Venera 2 and 3 launched in November 1965, were meant to flyby and land on Venus, respectively, but they both failed in transit due to a problem with their thermal control systems. Although Venera 3 managed to become the first spacecraft to impact another planet on March 1, 1966, neither spacecraft were able to return any data from Venus (see “Venera 2 & 3: Touching the Face of Venus”).

Frustrated by the failures and with resources at OKB-1 stretched thin by its commitments to develop the new 7K Soyuz for crewed missions in Earth orbit and eventually to the Moon (see “The Avoidable Tragedy of Soyuz 1”), in April 1965 Korolev transferred all responsibilities for future automated lunar and planetary missions to the newly independent design bureau called NPO Lavochkin run by Chief Designer Georgi Babakin. Known for their meticulous testing and the quality of their workmanship, NPO Lavochkin scored an early success when Luna 9 landed on the Moon in January 1966 using E-6 spacecraft hardware they inherited from OKB-1 and subsequently modified (see “Luna 9: The First Lunar Landing”). After the dual failures of Venera 2 and 3, NPO Lavochkin was ready to tackle the task of improving the 3MV design to take advantage of the upcoming 1967 Venus launch window.

With the official approval of NASA’s proposed Mariner Venus 1967 flyby mission in December 1965 (see “The Return to Venus: The Mission of Mariner 5”), Babakin decided to concentrate all efforts on building a pair of landers for the upcoming Soviet V-67 mission in order to outdo his American competitors. For the new 3.5-meter tall “1V” spacecraft, Lavochkin engineers retained the basic configuration and dimensions of the 3MV spacecraft, which was designed to handle missions to Venus or Mars, but incorporated a laundry list of improvements specifically to support a Venus landing mission. The troublesome 3MV thermal control system, which used a circulating liquid to transfer heat, was replaced with a forced gas system to keep the temperature inside of the pressurized orbital compartment between 15° C and 25° C. The hemispherical radiators which were originally mounted on the ends of the solar panels on the 3MV were replaced with a single disk-shaped radiator on the 1V spacecraft’s anti-Sun side. This radiator also served as the center of an umbrella-like, deployable high gain antenna which was increased from 2 to 2.3 meters in diameter to support UHF band downlink and uplink. The configuration of the low gain antennas was also changed to help ensure contact with the 1V could be maintained.

The square solar panels of the 3MV were replaced with rectangular units with a span of over four meters and an area of 2.5 square meters. While the 1V retained the pressure-fed KDU-414 engine mounted on the top of the orbital compartment to make midcourse corrections and fine tune the spacecraft’s approach to its target, the configuration of the attitude control jets and the various sensors to lock onto the Sun, Earth and stars for attitude reference was altered for better performance. A large dark shade was also added to the sunward facing side of the orbital compartment to reduce the risk that these optical attitude sensors would be affected by stray light from the spacecraft. Many of the internal systems were also upgraded based on previous flight experience and ground testing of 3MV legacy hardware as well as extensive testing of new systems. In order to aid in the diagnosis of problems encountered during flight, a duplicate 1V spacecraft was kept in an environmental chamber back on Earth during the V-67 missions.

Like the earlier 3MV missions, the orbital compartment served as a carrier for the lander which would be released just prior to reaching Venus and would burn up during entry into the Venusian atmosphere. The release would occur upon ground command or by an on board timer or if the lock on Earth was lost as would happen during entry. In case the release mechanism failed, the straps holding the lander to the orbital compartment were designed to burn away during entry as a final fail safe to ensure deployment. Like its predecessors, the 1V orbital compartment also carried instruments to make its own measurements during the cruise to Venus. The instruments included a magnetometer, detectors to study the solar wind and energetic charged particles as well as an ultraviolet photometer to detect emissions from hydrogen and oxygen. With the improved lander, the total launch mass of the 1V was now 1,106 kilograms – 146 kilograms more than the earlier 3MV-3 lander design and much larger than its 245-kilogram American competitor, Mariner 5.

Like the other Soviet lunar and planetary missions, the launch vehicle for the 1V was the 8K78M also known as the Molniya after the series of communication satellites which regularly used this rocket. The 8K78M, introduced in 1964, was an improved version of the 8K78 which was originally developed by OKB-1 to launch the first Soviet probe to Venus and Mars starting in the fall of 1960 (see “The First Mars Mission Attempts”). The first three stages of this rocket would eventually serve as the basis of the Soyuz launch vehicle still in use today. The first two stages of the 8K78M consisted of the Blok A core surrounded by four tapered boosters designated Blok B, V, G, and D. The engines of the four boosters and core would ignite on the launch pad to generate 4,054 kilonewtons of thrust. After two minutes of flight, the four boosters would shut down after consuming their kerosene and liquid oxygen propellants and separate from the rising rocket. After another 175 seconds of flight, the Blok A core would exhaust its propellants leaving the Blok I third stage to take over. The Blok I would burn for four minutes to place the payload and its Blok L escape stage into a temporary Earth parking orbit. After a short coast in orbit to reach the optimum injection point, the Blok L escape stage would ignite its engine to send the spacecraft towards its planetary target. The 8K78 was 42.1 meters tall and had a liftoff mass of about 306 metric tons.

The New Lander

By far the greatest changes were made to the 1V lander. Influenced by the work of Soviet scientists like Gavril Tikov, the earlier 2MV and 3MV Venus landers launched in 1962, 1964 and 1966 were designed under the assumption that the surface pressure of Venus was in the 1.5 to 5 bar range (where one bar is approximately equal to the atmospheric pressure at Earth’s surface) with a temperature of up to 75° C – conditions which would allow for the existence of open bodies of water on the Venusian surface. By the mid-1960s, the general consensus in the world’s planetary science community was that Venus had an atmosphere composed mainly of nitrogen with substantial amounts of carbon dioxide present with a surface pressure of somewhere between 5 to 300 bars and temperatures in the range of 267° C to 480° C or more. Unfortunately the true values were not known with any certainty despite observations from increasingly sophisticated ground-based instruments and a distant flyby performed by the American Mariner 2 in December 1962. Whatever the surface conditions truly were, it was obvious that the 3MV lander design was likely inadequate to reach the surface. Ground-based centrifuge tests of leftover 3MV-3 lander hardware which subjected the craft to loads of up to 500 g (the upper bounds that a Venus lander would experience during a steep entry into Venus’ atmosphere) also revealed shortcomings in the original lander further underscoring the need for a more robust design.

The new lander for the 1V spacecraft was a spheroid like its predecessors with an offset center of gravity that would keep the blunt end pointing forward during entry without the need for an attitude control system. With the outside diameter increased from 0.9 to 1 meter, the 1V lander included a thicker ablative heat shield, more insulation and a more durable structure compared to its predecessors not only to withstand entry into the atmosphere but also the more hostile conditions expected on the surface. Still influenced by Soviet scientists who believed in a more benign surface environment with a surface pressure of probably no more than about ten bars, the new lander was designed to withstand pressures of at least 18 bars and temperatures up to 400° C, in light of the uncertainties in the true surface conditions. Like the earlier 3MV landers, the 1V lander was designed to float and even included a sugar lock which would dissolve upon a water landing to trigger a signal sent back to Earth.

Prior to separation from the orbital compartment, the interior of the 383-kilogram 1V lander was cooled to -10° C to help maximize its life in the hot atmosphere of Venus. After the worst of the entry into the atmosphere of Venus was over, the 1V lander would deploy 1.7-meter in diameter drogue chute while still travelling at supersonic speed. This would be followed by the deployment of a 8.4-meter main parachute which was designed to withstand temperatures of up to 450° C. At the same time, the antenna for the decimeter-band radar altimeter was also deployed at a point that was expected to be less than 30 kilometers above the Venusian surface, assuming a surface pressure of just a few bars. This frequency-modulated continuous-wave altimeter was based on a design commonly used on aircraft and would return a single reading when the altitude was determined to be about 26 kilometers above the surface in order to conserve the limited communication bandwidth.

When the ambient pressure reached about 0.6 bars during the lander’s descent, the system would automatically start transmitting data via primary and backup transmitters from each of its instruments in a continuous repeating sequence every 48 seconds in order to determine how the atmospheric properties changed with altitude. With the UHF uplink directly to Earth limited to a data rate of just one bit per second, the lander was expected to return a total of only a few kilobits of data during its descent and subsequent surface operations within the nominal 100-minute lifetime of the probe’s battery. The instruments carried by the 1V lander included a barometer with an operating range of 0.13 to 6.9 bars, a pair of thermometers covering a range of -63° C to +457° C and a densitometer to measure atmospheric density in the 0.5 to 15 milligrams per cubic centimeter range (compared to the density of around 1.2 mg/cc for Earth’s atmosphere at the surface). Also carried were a pair of chemical gas analyzers which could make two measurements during descent of the amounts of carbon dioxide, nitrogen, oxygen and water vapor present – the gases thought to be the major constituents of the Venusian atmosphere. Tracking would allow the velocity of the descending capsule to be determined providing another means of calculating the atmospheric density and confirm landing. For this first attempt by NPO Lavochkin to land on Venus, no instruments were carried specifically to study the surface itself.

The Mission

With a pair of 1V spacecraft prepared for the V-67 mission, the Soviet Union was once again ready to attempt to land on Venus. The first up was 1V serial number 310 which lifted off from Launch Complex 1 at the Baikonur Cosmodrome at 5:39:45 AM Moscow Time (02:39:45 GMT) on June 12, 1967. The first three stages of the 8K78M successfully placed the Blok L escape stage and its 1V payload into a temporary 171 by 210 kilometer Earth parking orbit with an inclination of 51.2°. After a short coast, the Blok L ignited its engine to send what was now called Venera 4 on its way to Venus with an October 18 encounter date.

As preparations for the launch of the second V-67 spacecraft were proceeding, NASA successfully launched its Mariner 5 spacecraft towards Venus on June 14, 1967. For the first time, both a Soviet and American spacecraft were simultaneously heading to Venus. Travelling along a slightly faster trajectory, Mariner 5 would flyby Venus just 37 hours after Venera 4 despite the latter’s 51½ hour head start. The second 1V, serial number 311, lifted off from the Baikonur Cosmodrome at 5:36:38 Moscow Time (02:36:38 GMT) on June 17 and was placed into a 201 by 286 kilometer parking orbit with an inclination of 51.8°. Unfortunately, the turbopump on the Blok L escape stage’s main engine failed to be cooled as required and did not ignite when commanded stranding the stage and its payload in low Earth orbit. The orbit of what was now designated Kosmos 167 decayed eight days later. Venera 4 and Mariner 5 would make the journey to Venus alone.

Venera 4 quickly settled into its cruise routine collecting data on the interplanetary environment and turning its high gain antenna towards the Earth every few days for periodic communication sessions with controllers back in the Soviet Union. On July 29, Venera 4 made a course correction using its KDU-414 engine at a distance of 12 million kilometers from the Earth. The burn successfully changed the trajectory from a miss of 60,000 kilometers to an impact near the equator on the night side of Venus without the need for a second scheduled course correction.

As Venera 4 made its approach to Venus on October 18 after travelling 338 million kilometers and conducting 115 communication sessions during its 128-day transit, instruments on board the carrier found that Venus had no magnetic field or regions of trapped radiation like those found on the Earth although there were hints of a shock wave in the flow of the solar wind past Venus. The UV photometer found a weak hydrogen corona at a distance of 10,000 kilometers but no hint of oxygen – findings largely consistent with those of Mariner 5 during its encounter the following day.

At 04:34 GMT on October 18, 1967, the Venera 4 carrier released its lander while 44,800 kilometers from Venus. The lander hit the atmosphere over the night side of Venus at a speed of 10.7 kilometers per second and an angle of 80° to the horizon. After experiencing reentry temperatures of up to 11,000° C and peak braking loads of 350 g, the lander quickly slowed to 300 meters per second when it deployed its drogue chute. As the atmospheric pressure hit 0.6 bars, the main parachute was deployed and, at 04:39 GMT, Venera 4 began transmitting. At 04:40:52 GMT, Venera 4 started returning data back to Earth as the lander was initially descending at a speed of ten meters per second. The first measurements returned from the lander indicated a pressure of 0.75 bars and a temperature of 33° C – values that continued to climb steadily as the probe approached the surface.

During the descent, the gas analyzer returned its two sets of measurements – one right after the main parachute deployed and another 347 seconds later – indicating that carbon dioxide made up about 90% of Venus’ atmosphere with less than 2.5% nitrogen and inert gases present. This was totally unexpected by the scientific community which had assumed Venus’ atmosphere would be dominated by nitrogen like the Earth with moderate amounts of carbon dioxide. Only traces of oxygen and water vapor were detected. The atmospheric pressure and temperatures continued to climb and at 04:30:31 GMT the pressure exceeded the 7 bar limit of the probe’s barometer. After another 19½ minutes, the atmospheric densitometer readings went off scale. After a descent of 93 minutes and returning 23 sets of instrument readings, transmissions from the Venera 4 lander ceased at 06:13:17 GMT as the atmospheric temperature reached 262° C with the pressure estimated to be around 18 bars.

Based on tracking data and the sole radar altimeter reading of 26±1.3 kilometers, Soviet authorities claimed that Venera 4 had stopped transmitting when it reached the surface at about 19° N, 38° E roughly between what we know today as Eistla Regio and Bell Regio. Initially, this finding was independently supported by scientists at England’s Jodrell Bank radiotelescope who were listening in on the transmissions from Venera 4 during its encounter. After seven years of frustrating failures, a Soviet spacecraft had finally succeeded in returning data from another planet – and all the way to the surface of another planet for the first time, no less.

Mystery & Disappointment

As the Soviet propaganda machine hailed the accomplishment of Venera 4, Mariner 5 made its closest approach of 4,094 kilometers above the Venusian surface at 17:34:56 GMT on October 19, 1967. The next day, Mainer 5 began transmitting its recorded data back to Earth. But as the results from NASA’s mission were being analyzed and compared to the in situ measurements made by Venera 4, a genuine mystery arose as a result of disagreement between these independent data sets.

While Mariner 5 did not come in direct contact with Venus’ atmosphere like Venera 4 did, the flyby spacecraft did use its S-band transmissions back to Earth to sound it indirectly as the spacecraft passed behind Venus as viewed from the Earth and then reemerged. Observed changes in the phase, frequency and amplitude were used to infer the electron density as well as the temperature and pressure of the atmosphere as a function of altitude. Because of the density of the atmosphere, S-band transmissions could only probe down to a point 6,085 kilometers from the planet’s center or about 32 kilometers above the radius of Venus derived from Earth-based radar observations. At lower altitudes, the atmosphere becomes super-refractive and radio signals are bent around the planet. Extrapolating the results of Mariner’s readings down to the surface, the temperature was expected to be between 377° C and 527° C with a pressure somewhere between 75 and 100 bars, depending on the assumptions made about how much carbon dioxide and nitrogen were in the atmosphere. These values were far in excess of what Venera 4 had recorded when it was apparently near the surface of Venus. So what happened?

A detailed comparison of the Venera 4 in situ and Mariner 5 remote measurements of the atmosphere showed that they roughly coincided when the altitude of the former’s measurements were offset about 26 kilometers upwards. Given the unambiguous nature of Mariner’s results and the Earth-based radius determination, there were some who initially claimed that Venera 4 had come down on top of a 26-kilometer tall mountain – not impossible but extremely unlikely. A reexamination of Venera’s measurements, however, eventually demonstrated that the radar altimeter data had been misinterpreted. Because of how the radar altimeter worked, there was a 30-kilometer ambiguity in any measurements it returned. A reading of, for example, 26 kilometers from the altimeter would be the same if the actual altitude were around 56 or even 86 kilometers. Since the original expectation was that the Venera lander’s radio altimeter would not start taking readings until it was within 30 kilometers of the surface, this was not expected to be a problem. But with the atmosphere of Venus being so much denser than Soviet scientists and engineers had originally assumed, the first measurements had actually been returned by Venera 4 at an altitude of about 55 kilometers.

So instead of transmitting data all the way to the surface, Venera 4 had actually stopped transmitting data at an altitude of 26 kilometers probably because of structural failure caused by excessive atmospheric pressure (although it is possible that the battery, which had a lifetime nominally rated at 100 minutes, was exhausted by the longer-than-expected 93-minute descent). While Venera 4 was the first Soviet spacecraft to reach another planet in operational condition and the first launched by any nation to probe the atmosphere of Venus directly, it was not the first spacecraft to transmit data from the surface of another planet. Although disappointing, the in situ measurements returned by Venera 4 were still of immense scientific value. Combining the data on the atmosphere and its composition as measured by Venera 4, by 1969 Soviet planetary scientists had extrapolated their data to show that the surface temperature as about 442° C with a pressure of 90 bars – very close to today’s accepted values and a bit closer to the mark than those of Mariner 5.

The complimentary data sets of Venera 4 and Mariner 5 had finally resolved one of the long standing mysteries of Venus. And with the realization that the surface conditions on the surface of Venus were far more hostile than had been originally believed, the engineers at NPO Lavochkin had their work cut out for them as they prepared a second pair of Venera landers for the V-69 mission set for launch in January 1969 (see “Venera 5 & 6: Diving Towards the Surface of Venus“).

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Related Video

Here is a Pathé newsreel about the Venera 4 mission.

Here is an excellent Russian-language documentary (with English subtitles) about Venera 4 with a lot of footage showing the testing and preparation made for this mission.

Related Reading

“The Return to Venus: The Mission of Mariner 5”, Drew Ex Machina, June 15, 2017 [Post]

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

General References

V. S. Avduevsky, M. Ya. Marov and M.K. Rozhdestvensky, “Model of the Atmosphere of the Planet Venus Based on Results of Measurements made by the Soviet Interplanetary Station Venera 4”, Journal of the Atmospheric Sciences, Vol. 25, No. 7, pp. 537-545, July 1968

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

Wesley T. Huntress, Jr. 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

L.R. Koenig, F.W. Murray, C.M. Michaux and H.A. Hyatt, Handbook of the Physical Properties of the Planet Venus, NASA SP-3029, 1967

Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System: Part 1 – The Golden Age 1957-1982, Springer-Praxis, 2007

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

Andrew Wilson, Solar System Log, Jane’s Publishing, 1987