On October 18, 1967 the sole 1V spacecraft to survive launch for the Soviet “V-67” mission to Venus, called Venera 4, finally reached its target. The carrier released its lander which subsequently entered the Venusian atmosphere and began to transmit data on the properties of an extraterrestrial atmosphere for the first time. After a 93-minute descent, contact with the Venera 4 lander was lost apparently just as it hit the surface where the temperature was measured as 262° C with an estimated pressure of around 18 bars (where one bar is about the Earth’s atmospheric pressure on the surface – see “Venera 4: Probing the Atmosphere of Venus”). Even as the Soviet press hailed the achievement of their first-ever planetary probe that not only returned data from its target but also landed on it, the Soviet design bureau responsible for building the 1V spacecraft, NPO Lavochkin run by Chief Designer Georgi Babakin, was already busy at work designing and building a pair of 2V spacecraft for launch in less than 15 months for the new “V-69” landing mission.

The First Venus Lander?

One of the facts that was readily apparent from the initial analysis of the findings of Venera 4 was that the atmosphere of Venus was much denser, deeper and hotter than originally expected by the staff at Lavochkin. When the 1V lander was being designed and built, the 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. The surface pressure was estimated to be somewhere between 5 to 300 bars with 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.

Still influenced by the work of Soviet scientists like Gavril Tikov who embraced the view of a more Earth-like Venus, Babakin and the engineers at NPO Lavochkin still favored the less dense and cooler estimates of the surface conditions. Under the conditions assumed with a surface pressure of several bars at temperatures in excess of 270° C, the spherical 383-kilogram 1 V lander was expected to spend about a half an hour or so descending through the atmosphere and returning data. In order to save bandwidth, the 1V lander would send a single signal when its radar altimeter sensed it was passing through an altitude of 26 kilometers. After landing, the 1V would use the remaining time of its nominal 100-minute battery life transmitting data from the surface of Venus. Even with these assumed, comparatively benign conditions, the one-meter in diameter 1V lander was designed to withstand pressures and temperatures of up to 18 bars and 400° C, respectively, while the six-meter main parachute could survive temperatures as high as 450° C.

But the conditions at Venus proved to be much more hostile than anticipated. The Venera 4 lander deployed its parachute as planned when the atmospheric pressure hit 0.6 bars and began transmitting its data at one bit per second with the first measurements indicating a pressure of 0.75 bars and a temperature of 33° C – values that continued to climb steadily as the probe descended. The radio altimeter signaled that the altitude was 26±1.3 kilometers shortly afterwards. During the descent, the gas analyzer returned its two sets of measurements – one right after the main parachute had been 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. Only traces of oxygen and water vapor were detected.

The atmospheric pressure and temperature continued to climb and 50 minutes into the descent the pressure exceeded the 7-bar limit of the probe’s barometer package. 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. Based on an extrapolation from the single radar altimeter reading, transmissions apparently stopped when the 1V lander was in the immediate vicinity surface. With the longer than expected descent and an estimated surface pressure of 18 bars, there was the possibility that the lander ran out of power or was even crushed despite the ”official” pronouncement that Venera 4 had landed. These doubts only deepened when the remote sensing results of NASA’s Mariner 5 mission which flew by Venus on October 19, 1967 suggested that the surface pressure was somewhere between 75 and 100 bars with a temperature in the 377° C to 527° C range, depending on the assumptions made about the relative amounts of carbon dioxide and nitrogen in the atmosphere (see “The Return to Venus: The Mission of Mariner 5”). Whichever interpretation was correct, the 1V lander went silent near its design limits. The next Venus lander would need to be even more robust than Venera 4.

The New Venus Lander

As Soviet and American science teams compared data and attempted to reconcile their differing results, the engineers at NPO Lavochkin had little time to incorporate changes into the 2V lander design ahead of the January 1969 launch window. First, the structure of the lander was reinforced as best as possible to handle pressures as great as 25 bars and entry loads up to 450-g. The low energy approach trajectories to Venus were less favorable in 1969 than they were in 1967 with entry speeds now increased from 10.7 to 11.2 kilometers per second resulting in a more punishing entry.

Unable to squeeze more life out of the battery, the size of the pilot and main parachutes on the 2V lander were reduced in size. With the main parachute size reduced from six to just 2.8 meters across, the descent speed was now approximately doubled. Due to the higher than expected atmospheric density, however, the 2V lander would still be travelling slow enough once it approached the surface to survive landing. Combined with delaying the deployment of the parachutes, the 2V lander would reach the surface (or the limits of its design) faster with plenty of battery life still in reserve.

Based on the experience with Venera 4, the instruments were also modified. Originally the 2V landers were to carry French-built thermometers and barmometers as part of a new cooperative effort. But nationwide strikes in France in May 1968 delayed the delivery of the French hardware until September when it was too late to integrate them into the landers. Soviet-built instruments were used instead. The 2V lander carried three improved, platinum-wire resistance thermometers to cover temperatures from 0° to 440° C, a set of barometers covering pressures from 0.13 to 39 bars and a new densitometer with an operating range of 0.5 to 40 milligrams per cubic centimeter (compared to the density of around 1.2 mg/cc for Earth’s atmosphere at the surface). More refined doppler-tracking from Earth would also allow the velocity of the descending capsule to be determined more accurately providing another means of calculating the atmospheric density as well as measuring winds and confirming touchdown. A pair of improved chemical-based gas analyzers were carried which would make two measurements during descent of the amounts of carbon dioxide, nitrogen, oxygen and water vapor present with greater precision now that the rough amounts (or upper limits) were known. Although the landers for the V-69 mission were to come down on the nightside of Venus, they carried photometers in hopes of detecting a glow in Venus’ atmosphere known as ashen light. Like earlier landers, the 2V landers would also carry medals of the Soviet coat of arms and Lenin to commemorate the event.

As American and Soviet scientists argued about the interpretation of their Venus encounter results during international conferences through 1968, their focus turned to radar data – not only the Earth-based radar data used to determine the radius of Venus but also the single radar altimeter measurement returned by Venera 4. The 1V lander’s radar was a frequency-modulated continuous-wave altimeter operating at decimeter wavelengths which was based on a design commonly used on aircraft. 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 56 or even 86 kilometers. Since the original expectation was that the Venera 4 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 and deeper than Soviet scientists and engineers had originally assumed, the sole radar altimeter reading from Venera 4 could have been misinterpreted. An improved radar altimeter was included on the 2V lander which would return multiple readings of the altitude in 10 kilometer increments starting at 45 kilometers in order to resolve the ambiguity. With all the modifications, the mass of the 2V lander increased by 22 kilograms compared to its predecessor to 405 kilograms.

The 2V carrier for the lander was little changed from the earlier 1V design save for incremental improvements in its various systems based on ground testing and flight experience. About 3.5 meters tall, including the lander, the core of the carrier consisted of a 1.1 meter in diameter cylinder that was about as tall. This pressurized compartment housed the carrier’s various systems and maintained their temperature between 15° C and 25° C using a forced gas system. The circular radiator for this system was mounted on the anti-Sun side of the spacecraft and served as the hub for an umbrella-like, deployable high gain antenna 2.3 meters in diameter used to support long-range UHF band downlink and uplink. The thermal control system was also used to pre-chill the lander to a temperature of -10° C before deployment to help maximize its life in the hot atmosphere of Venus.

Mounted on top of this compartment was the propulsion system consisting of a pressure-fed KDU-414 engine and its propellant tanks. This system would be used to perform a pair of midcourse corrections (one shortly after leaving the Earth and another prior to the Venus encounter) in order to fine tune its trajectory. On the sides of the main compartment were a pair of deployable solar panels with a span of over four meters and an area of 2.5 square meters which provided electrical power to the spacecraft’s systems. Like Venera 4, the carrier was also fitted with scientific instruments: a solar wind charged particle detector, UV photometers to measure the glow of hydrogen and oxygen in Venus’ extended corona and a set of cosmic ray detectors. The three-axis magnetometer carried by Venera 4 was deleted due to the virtually nonexistent Venusian magnetic field and to save mass. The complete 2V spacecraft had a launch mass of 1,130 kilograms – just 24 kilograms heavier than the 1V spacecraft and still within the payload capability of the 8K78M “Molniya-M” launch vehicle.

Getting Underway

As the staff of NPO Lavochkin prepared a pair of 2V spacecraft for launch, the disagreement between the Soviet and American scientists about the surface conditions on Venus reached a head in May 1968 during a COSPAR (COmmittee on SPAce Research) meeting in Tokyo. With the American science team advocating more extreme surface conditions based on an extrapolation of Mariner 5 readings and the latest radar measurements of the radius of Venus, Dr. Carl Sagan of Cornell University in New York and Prof. A.D. Kuzim of the Lebedev Physical Institute in Moscow debated the issue. With Kuzim’s own modeling of Venus’ microwave spectrum supporting the more extreme model, he proposed that Venera 4 had landed on the top of a high mountain. While possible, Sagan argued that it was improbable and that Earth-based radar studies had failed to spot terrain of such extreme altitude.

Following this, it was no longer possible to support the notion that Venera 4 had landed and instead it had fallen silent still far above the surface. Over the following months it was found that Venera’s radar altimeter data had been misinterpreted (due to the 30-kilometer ambiguity in the system) and that the 1V lander was actually about 55 kilometers above the surface when transmissions began – an offset that brought the Venera 4 and Mariner 5 findings into agreement. Combining the corrected altitude with the in situ measurements by Venera 4 of the atmosphere and its composition, 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. It was now clear that the 2V landers would not reach the surface of Venus intact. Still, their new data would help to further refine the models of the Venusian atmosphere.

With a pair of 2V spacecraft prepared for the V-69 mission and NASA’s focus now turned towards Mars for the next several years, the Soviet Union would begin to take center stage in the exploration of Venus. The first up was 2V serial number 330 which lifted off from the pad at Area 1 of the Baikonur Cosmodrome at 9:28:08 AM Moscow Time (06:28:08 GMT) on January 5, 1969. The first three stages of the 8K78M successfully placed the Blok L escape stage and its 2V payload into a temporary 203 by 218 kilometer Earth parking orbit with an inclination of 51.8°. As it flew over Africa during its first orbit, the Blok L ignited its engine at 07:47 GMT for a 228-second burn which sent what was now called Venera 5 into a 0.98 by 0.72 AU solar orbit which would reach Venus on May 16 after a transit of 131 days.

The next launch carrying 2V number 331 lifted off at 8:51:52 AM Moscow Time (05:51:52 GMT) on January 10, 1969 from Baikonur in the midst of a snowstorm clearing the pad at Area 1 for the launch of the crewed Soyuz 5 mission just five days later (see “Soyuz 4 & 5: The First Crew Exchange in Space“). After the first three stages of the Molniya had finished their job, the second 2V spacecraft and its Blok L escape stage had been placed into a 184 by 193 kilometer parking orbit. Following a short coast, the Blok L stage ignited to send Venera 6 towards Venus. Since it was following a slightly faster trajectory, Venera 6 would reach Venus the day after its sister with a faster 127-day transit.

Tracking during the first few days after launch showed the two spacecraft to be in good health but in need of minor trajectory corrections to target them properly. This was only the second time that the Soviet Union had successfully launched a pair of spacecraft towards Venus (the first being Venera 2 and 3 in 1965 – see “Venera 2 & 3: Touching the Face of Venus”). With an estimated miss distance of 25,000 kilometers, Venera 5 made a course correction with a delta-v of 9.2 meters per second on March 13 when it was 15.5 million kilometers from the Earth. Venera 6 made its course correction three days later at a distance of 15.7 million kilometers from the Earth. A larger delta-v of 37.4 meters per second was required to move the probe’s aim point by 150,000 kilometers and on target for the night side of Venus. Both Veneras were now on target and would not require a second course correction.

The Venus Encounter

During the cruise to Venus, a total of 73 and 63 communication sessions were held with Venera 5 and 6, respectively with the spacecraft responding to a total of 1,500 commands. During these communication sessions, the spacecraft would turn their high gain antennas back towards Earth to transmit recorded data. The results showed that because of the increase in solar activity during this time, the cosmic ray flux was lower than it had been two years earlier during the Venera 4 cruise. A dozen periods of increased solar proton flux were recorded with four of them lasting for a period of a week or more.

After a largely uneventful cruise to Venus, the pair of 2V spacecraft were ready to study their primary target. About two hours before its encounter on May 16, Venera 5 began its last communication session with ground controllers back on Earth. Eight minutes of precision tracking data helped to refine the mass of Venus followed by observations of how Venus interacted with the solar wind. Once again, the UV photometers detected an extended corona of hydrogen surrounding Venus with no oxygen detected confirming the earlier findings of Venera 4 and Mariner 5. Venera 5 released its lander about an hour before entry at a distance of 37,000 kilometers.

As the carrier burned up in the atmosphere, the lander began its entry at 06:01 GMT travelling at 11.19 kilometers per second at an angle of 65° to the local horizontal. The pilot parachute pulled out the main canopy once the speed had dropped to 210 meters per second to begin making measurements and transmitting them directly back to the Earth at 06:02:30 GMT. During descent, Venera 5 analyzed the composition of the atmosphere at pressure levels of 0.6 and 5 bars. The measurements confirmed that carbon dioxide dominated the atmosphere at 97±4% with <0.1% molecular oxygen and <3.5% nitrogen and noble gases. The water concentration at a pressure level of 0.6 bars was found to be 4 to 11 milligrams per liter indicating that part of the atmosphere was not saturated (and unable to form water clouds).

As Venera 5 sunk deeper into the atmosphere, it continued to return a full set of temperature, pressure and density measurements every 45 seconds. Even though it was coming down during the Venusian night (the equivalent of 4:12 AM here on the Earth), the lander’s photometer returned an isolated reading of 25 Watts per square centimeter at 06:51 GMT. It might have been a transient signal caused by an instrument issue under the high pressure, or it may have been the detection of lightning. Venera 5 went silent at 06:54:56 GMT after a descent of 53 minutes. At the time the 2V lander stopped transmitting, the altitude was about 18 kilometers with a pressure of 27 bars and a temperature of 327° C. Extrapolating to the surface of Venus suggested that the pressure there was about 140 bars with a temperature of 527° C. The interior temperature of the lander had increased from 13° to 28° C during its descent. The remains of the lander came to rest at 3° South, 18° East on what is today known as Tanatin Planitia.

The next day, the Soviet Soviet ground controllers turned their attention to Venera 6. The lander was released 25,000 kilometers from Venus and began its entry into the atmosphere at 06:05 GMT on May 17 about 300 kilometers from Venera 5. Venera 6 deployed its parachute and began transmitting data at 06:08:15 GMT. During its descent, Venera 6 analyzed the atmosphere at pressures of 2 and 10 bars to stagger its measurements with those made by Venera 5. No noticeable differences in atmospheric composition were found and the results agreed well with those of the previous day. Venera 6 continued transmitting during descent until it went silent at 06:58:04 GMT as the pressure hit 27 bars after 50 minutes of operation. The altitude at the time was estimated to be 10 to 12 kilometers and extrapolations to the surface suggested a temperature of 400° C and a pressure of 60 bars. The remains of the Venera 6 lander came down at 5° South, 23° East in the southeastern part of Tanatin Planitia. The photometer failed to return any data during descent.

The differences between the results from Venera 5 and 6 were initially explained by the latter coming down over a high altitude plateau or mountain range. Today we know that the pair of “landers” had come down over a fairly flat, low lying terrain. It seems more likely that the radar altimeter data were in error and that the Venera 6 lander went silent at an altitude of about 18 kilometers like its predecessor. The tentative model of the Venusian atmosphere created using the total of 70 temperature and 50 pressure measurements returned by Venera 5 and 6 (plus the findings from Venera 4 and Mariner 5) found a surface temperature of 497°±60° C and a pressure of 67 to 135 bars with 90 bars being the most likely value. Further analysis continued to refine the accuracy of the Venusian atmosphere model, but it was clear that Venus was not a likely abode for life and manned missions to the surface would be impractical for the foreseeable future.

But as the view that Venus was an Earth-like planet was finally dismissed thanks to the in situ measurements from three Venera descent capsules, plans were already in the works for a new “V-70” mission. This time, the engineers at NPO Lavochkin were developing a pair of much more robust “3V” landers for launch in August 1970. The 3V landers, which would be built to withstand 90 minutes at 540° C with a pressure of 180 bars, were expected to finally reach the hostile surface of Venus.

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Related Video

Here is some silent footage of the Venera 5 and 6 missions.

Related Reading

“Venera 4: Probing the Atmosphere of Venus”, Drew Ex Machina, October 21, 2017 [Post]

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

V.S. Avduevsky, M.Ya. Marov and M.K. Rozhdestvensky, “A Tentative Model of the Venus Atmosphere Based on the Measurements of Venera 5 and 6”, Journal of the Atmospheric Sciences, Vol. 27, No. 7, pp. 561-568, July 1970

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

Robert Reeves, The Superpower Space Race: An Exploration Rivalry through the Solar System, Plenum Publishing, 2003

Carl Sagan, The Cosmic Connection, Dell Publishing, 1973