Venera 8: The First Characterization of the Surface of Venus

Recent years has seen a marked increase in the planetary community’s interest in Venus after decades of near-neglect. Part of this renewed interest is to understand how Venus evolved from possibly more Earth-like conditions in the distant past into the arid hellscape it is today. Looking beyond the confines of our Solar System, there is a growing list of potentially Venus-like exoplanets being discovered so that understanding Venus better will give us fresh insights into the conditions on these remote worlds.

In response to this growing interest, three new missions to our sister planet have recently been approved for development: ESA’s EnVision as well as DAVINCI (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging) and VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) missions being developed by NASA. But this interest in Venus is hardly new. During the late-1960s to mid-1980s, the Soviet Union flew a series of increasingly capable spacecraft to study our nearest planetary neighbor. The last (and arguably, the most successful) of the Soviet’s first generation missions to Venus was the Venera 8 lander flown a half of a century ago.

Earlier Landing Missions

After a series of disappointingly unsuccessful attempts to reach Venus between 1960 and 1965, responsibility for development and construction of the Soviet Union’s automated lunar and planetary spacecraft was transferred in April 1965 from OKB-1 (Experimental Design Bureau 1) to the newly independent design bureau called NPO Lavochkin. Run by Chief Designer Georgi Babakin, NPO Lavochkin was known for its meticulous testing and the quality of its workmanship. After the official transfer, the engineers at Lavochkin set about totally redesigning the earlier 3MV-series spacecraft to create the much more capable 1V for the V-67 mission for the 1967 launch opportunity to Venus. While only Venera 4 survived launch in June 1967, the spacecraft successfully deployed its lander which then returned the first in situ measurements of the Venusian atmosphere during a 93-minute descent. Originally thought to have gone silent when it hit the surface of Venus, it was later determined that it instead suffered structural failure at an altitude of 26 kilometers when the pressure exceeded 18 bars (with one bar being approximately equal to the atmospheric pressure on Earth’s surface). The surface conditions on Venus had turned out to be much more hostile than Soviet scientists and engineers had assumed when designing their lander (see “Venera 4: Probing the Atmosphere of Venus”).

Diagram of the V-67 spacecraft, Venera 4. Click on image to enlarge.

With only enough time to make minor upgrades to the 2V landers for the next launch window in January 1969, the V-69 landers for the Venera 5 and 6 mission were able to penetrate more deeply into the Venusian atmosphere building on the findings of Venera 4 but they still failed about a dozen kilometers above the surface (see “Venera 5 & 6: Diving Towards the Surface of Venus”). With more time available to prepare for the V-70 landing mission, the engineers at Lavochkin created the even more robust 3V lander design which was capable of withstanding pressures of 180 bars and temperatures as great as 540° C for up to 90 minutes. With only Venera 7 surviving launch, the new lander finally reached the surface of Venus on December 15, 1970 to become the first spacecraft to land on the surface of another planet.

While the Venera 7 lander survived its descent and transmitted data from the Venusian surface for 23 minutes, the mission did experience a number of problems. First, the commutator which continuously cycled inputs of the transmitter between the lander’s various instruments got stuck resulting in only temperature measurements being returned following entry into the atmosphere. Worse yet, the probe’s parachute ripped during descent and failed completely at an estimated altitude of 3 kilometers. As a result, the lander was in freefall for the last three minutes of its plunge through Venusian atmosphere and impacted the surface at a speed of 16.5 meters per second (the equivalent of a fall off of a five-story building here on Earth). Between the hard landing and the terrain, Venera 7 came to rest on its side so that its antenna was not pointing towards the Earth. Initially feared to be another failure, it was only during a later detailed analysis that Venera’s weak signal was detected confirming the success of the landing. The temperature was measured to be 474°±20° C on the surface with the atmospheric pressure estimated to be 90±15 bars, based on the descent data from the earlier Venera probes (see “Venera 7: The First Landing on Another Planet”).

The New Venus Lander

With the conditions on the surface of Venus now known, the engineers and scientists at NPO Lavochkin set out to build a lander better tailored for the expected environment for the upcoming V-72 mission. Like the earlier landers, the new 3V lander was a spheroid about a meter in diameter with an offset center of gravity that would keep the blunt end pointing forward during entry without the need for an attitude control system. The spherical titanium pressure vessel at the heart of the lander, which housed most of its systems, was modified to withstand pressures of 105 bars, instead of 180 bars as before, with the weight savings used to increase the mass of instruments and other system modifications.

A cutaway diagram of the V-72 mission lander. The components include: 1) parachute housing cover, 2) drogue parachute, 3) main parachute, 4) deployable radar altimeter antenna, 5) heat exchanger, 6) thermal accumulator, 7) internal thermal insulation, 8) program timer, 9) thermal accumulator, 10) shock absorber, 11) external heat insulation, 12) transmitter, 13) pressure vessel, 14) commutation unit, 15) fan, 16) cooling conduit from carrier, 17) deployable secondary antenna, 18) parachute housing, 19) primary antenna, 20) electric umbilical, 21) antenna feed system, 22) cover explosive bolts, 23) telemetry unit, 24) stable quartz oscillator and 25) commutation unit. Click on image to enlarge.

The modified 3V lander employed a new honeycomb composite material as insulation against the heat at the surface. The lander also incorporated “thermal accumulators” to absorb heat and maintain the internal temperature. These consisted of blocks of lithium nitrate trihydrate with a melting point of 30° C that would absorb tremendous amounts of thermal energy during its phase change. The lander also carried an improved pulse modulated radar altimeter which would return a reading about every minute and a half. In order to avoid a repeat of the failures experienced by Venera 7, the 2.5-square-meter parachute was strengthened and a backup antenna for the transmitter, which would be deployed after landing, ensured contact would not be lost. The transmission pattern of the primary antenna was also altered. The previous landing attempts took place during the Venusian night with the Earth high in the sky. This mission would attempt a landings on either side of the Venus’ morning terminator where the Earth would appear lower on the horizon.

A Soviet artist depiction of the V-72 lander on the surface of Venus. Not the deployable secondary antenna in the lower left. (Lavochkin)

In addition to these modifications, the 3V lander for the V-72 mission carried an expanded suite of improved instruments to make measurements from an altitude of 55 kilometers to the surface. Carried were four resistance thermometers of various types sensitive to 47° to 587° C, 207° to 437° C, 397° to 537° C, and 17° to 607° C temperature ranges. The lander also carried three aneroid barometers (with maximum readings of 224, 153 and 102 bars) and a capacitance barometer (with an upper limit of 82 bars) to measure atmospheric pressure with an accuracy of 1.5% of the full scale. A chemical analyzer was also carried to measure the atmospheric composition including ammonia. Since one of the two probes of the V-72 missions was to land on the dayside, a pair of cadmium sulfide photometers with 60° field of view were carried to measure light levels during descent and on the surface in 520 to 720 nanometer wavelength range. Doppler tracking of the descending probe would allow line-of-sight winds to be measured to an accuracy of 0.2 meters per second. With the Earth just 30° above the horizon, these Doppler measurements would provide estimates of a horizontal component of the atmosphere’s zonal winds. Also carried was a gamma ray spectrometer sensitive to the 0.3 to 3.0 MeV range. This would measure the concentrations of radioactive uranium, thorium and potassium-40 in the surface rocks allowing them to be characterized for the first time. As a result of all the modifications and improvements, the mass of the new 3V lander increased slightly from 490 to 495 kilograms.

The 3V carrier for the V-72 lander was little changed from its immediate predecessors 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 -15° C before deployment to help maximize its life in the hot atmosphere of Venus.

Venera 8 being prepared for launch.

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 up to two 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. The carrier also sported its own suite of instruments to characterize the solar wind and cosmic rays as well as an ultraviolet spectrometer to measure Lyman-α emissions from hydrogen. Altogether, the upgraded 3V spacecraft had a launch mass of 1,184 kilograms.

The V-72 Mission

As was done for the earlier Soviet Venus missions, the Soviet V-72 mission to Venus would launch a pair of the improved 3V spacecraft towards Venus during the March 1972 launch window. The first, 3V No. 670, successfully lifted off from the pad at Area 31/6 in the Baikonur Cosmodrome at 7:15:01 AM Moscow Time (04:15:01 GMT) on March 27, 1972. The first three stages of the 8K78M Molniya launch vehicle successfully placed the Blok L escape stage and its 3V payload into a temporary 194 by 246 kilometer Earth parking orbit with an inclination of 51.7°. After coasting in orbit to reach the optimum injection point, the Blok L main engine ignited for a 243-second burn to accelerate to a speed of 11.509 kilometers meters per second to send what was now called Venera 8 into a solar orbit which would reach Venus on July 22 after a transit of 117 days.

The second V-72 launch, carrying spacecraft 3V No. 671, took place at 7:02:33 AM Moscow Time (04:02:33 GMT) on March 31, 1972. After travelling in its 194 by 215 kilometer orbit with an inclination of 51.8°, the Blok L escape stage ignited at the prescribed point to accelerate its payload to Venus. Unfortunately, a miss set timer resulted in the stage firing for only 125 seconds resulting in a velocity change of only about 1.5 kilometers per second instead of the desired value of 3.4 kilometers per second. The payload, designated Kosmos 482 by Soviet authorities, was stranded in a geocentric 205 by 9,805 kilometer orbit with an inclination of 52.2°. About three months after launch, it appears that the 3V bus, with the international designation of 1972-023A, ejected its lander into a separate orbit receiving the designation 1972-023E. The main bus is generally believed to have reentered the Earth’s atmosphere on May 5, 1981. The wayward Venus lander is still in orbit and is expected to reenter the atmosphere in 2025 or 2026.

Travelling alone to Venus just like its predecessor, Venera 8 made a single course correction on April 6, 1972 to fine tune its aim for a 500-kilometer wide target area on the morning side of Venus. A total of 86 separate communications sessions were held with the receding probe during its uneventful transit to Venus. As Venera 8 approached its target, it chilled the lander to -15° C before releasing it at 07:40 GMT on July 22, 1972. As the carrier burned up in the Venusian atmosphere, the lander hit the atmosphere at 08:37 GMT at a speed of 11.6 kilometers per second at an angle of 77° to the local horizontal. Within 18 seconds, the lander had slowed to just 250 meters per second before deploying its pilot chute. The main parachute, initially reefed by 70%, was deployed at an altitude of 60 kilometers and the spacecraft began to make its measurements and transmit them back to Earth at the rate of one bit per second. The main parachute fully opened at an altitude of 30 kilometers to further slow the descending lander. The Venera 8 lander touched down on the surface of Venus at 09:29 GMT at 10.70° S, 335.25° E in what is today known as Vasilisa Regio. The local solar time was 06:24 with the Sun just 5.5° above the local horizon.

A Magellan radar map of the region around the Venera 8 landing site. The 300-km wide circle shows the probe’s landing ellipse. Click on image to enlarge. (Abdrakhimov & Basilevsky)

Immediately after landing, Venera 8 cut its parachute loose and deployed its backup antenna. The lander transmitted data via its main antenna for the first 13.3 minutes on the surface before switching to its backup antenna for 20 minutes. Afterwards, it switched back to its main antenna for the remainder of its life on the surface. Venera 8 finally went silent 50 minutes after landing. The data on temperature and pressure during descent agreed closely with the measurements made by the previous four Venera probes with the temperature at the surface pegged at 470±8° C with an atmospheric pressure of 93±1.5 bars. These direct measurements confirmed the estimates based on earlier descent data.

Plots of the temperature and pressure readings from Venera 8 during its descent to the surface of Venus. The horizontal bars indicate the typical measurement uncertainties at various altitudes. Click on image to enlarge. (Marov et al.)

The results from the analysis of the atmospheric composition indicated it was 97% carbon dioxide, 2% nitrogen, 0.9% water vapor and 0.15% oxygen. Ammonia was shown to be present at concentrations of 0.1% and 0.01% at altitudes of 46 and 33 kilometers, respectively, where the analyses took place (although these figures are now thought to be inflated above the true values due to the presence of sulfuric acid at these altitudes). The Doppler measurements during descent showed that the horizontal winds blew at a speed of 100 meters per second above an altitude of 48 kilometers but quickly decreased from 70 to 20 meters per second by an altitude of 42 kilometers. Below 10 kilometers, the winds were a much slower meter per second. As a result of these winds, Venera 8 had drifted some 60 kilometers during its 53-minute descent.

A plot of wind speed reading by Venera 8 during its descent. The horizontal bars indicate the typical measurement uncertainties at various altitudes. Click on image to enlarge. (Marov et al.)

The 27 photometer readings made during the descent provided the first in situ measurements of the structure of the clouds shrouding Venus. They showed a steady decrease in the ambient light level to an altitude of about 32 kilometers followed by a much slower decrease down to the surface where just 1% of the Sun’s light reached during this time early in the morning. The data suggested that the base of the cloud layer was at an altitude of 35 kilometers with the atmosphere below 32 kilometers being transparent. Above 35 kilometers to an altitude of 48 kilometers was a thin, almost fog-like haze with a thick, optically dense cloud layer above that reaching up to 65 kilometers from the surface.

A plot of photometer readings returned by Venera 8 during its descent. Click on image to enlarge. (Avduevsky et al.)

Finally, Venera 8 provided first of its kind data on the surface of Venus itself. The radar altimeter, which took its last reading at an altitude of 900 meters, suggested a loosely compacted soil with a density of 1.5 g/cc. The gamma ray spectrometer’s measurements indicated that the surface rocks had 4±1.2% potassium by weight as well as uranium and thorium present at the 2.2±0.7 ppm and 6.5±0.2 ppm levels, respectively. Comparing these potassium and uranium concentrations to terrestrial rocks, the best match appears to be an alkali basalt like those found in Earth’s oceanic islands, continental rifts and volcanic fields. Alternate interpretations of the data include the possibility that the rocks could be granite instead. This is an especially intriguing possibility because geologic mapping of the landing area shows an early episode of tessera terrain formation. Some models of tessera suggest that they are regions of ancient continental crust rich in granite created when Venus was much wetter in the distant past. This hypothesis will be tested as part of NASA’s DAVINCI mission currently under development which will land in the tessera terrain of Alpha Regio.

An artist’s depiction of the DAVINCI probe during its descent to the surface of Venus. (NASA GSFC visualization by CI Labs Michael Lentz and others)

With the successful completion of the Venera 8 mission, the stage was set for missions using the much larger and more capable 4V spacecraft then under development. These five-metric ton probes would be launched starting in 1975 using the larger Proton rocket and perform much more detailed exploration of Venus including the return of the first images from the surface of another planet (see “Venera 9 and 10 to Venus”).

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General References

A. M. Abdrakhimov and A.T. Basilevsky, “Geology of the Venera and Vega Landing-Site Regions”, Solar System Research, Vol. 36, No. 2, pp 136-159, 2002

V. S. Avduevsky et al., “Venera 8: Measurements of Solar Illumination Through the Atmosphere of Venus”, Journal of Atmospheric Sciences, Vol. 30, pp 1215-1218, September 1973

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

V. Kerzhanovich and M. Ya. Marov, “The Atmospheric Dynamics of Venus According to Doppler Measurements by the Venera Entry Probes”, pp 766-778 in Venus (D.M. Huten, L. Colin, T.M. Donahue and V.I. Moroz, ed.), The University of Arizona Press, 1983

Andrew A. Lacis and James E. Hansen, “Atmosphere of Venus: Implications of Venera 8 Sunlight Measurements”, Science, Vol 184, No. 4140, pp 979-982, May 31, 1974

Marco Langbroek, “Kosmos 482: questions around a failed Venera lander from 1972 still orbiting Earth (but not for long)”, The Space Review, Article No. 4384, May 16, 2022

M. Ya. Marov et al., “Venera 8 Measurement of Temperature, Pressure and Wind Velocity on the Illuminated Sid of Venus”, Journal of Atmospheric Sciences, Vol. 30, pp 1210-1214, September 1973

Yu. A. Surkov, “Studies of Venus Rocks by Veneras 8, 9, and 10”, pp 154-158 in Venus (D.M. Huten, L. Colin, T.M. Donahue and V.I. Moroz, ed.), The University of Arizona Press, 1983