Venera 11 and Venera 12

The 1978 launch window posed a harder problem than the one solved in 1975. According to NPO Lavochkin, changed ballistic conditions that year led the bureau to choose a flyby and landing scheme for the pair it calls series 4V1: each station would release its lander two days out and swing past the planet relaying the lander's signals, where Venera 9 and 10 had braked into orbit. NSSDCA explains the choice in terms of fuel: the craft would reach Venus much faster than in 1975, and the propellant needed to enter orbit was too great. Don P. Mitchell, whose history of the Soviet landers draws on Russian books and interviews, puts the arrival energy at three times that of the 1975 craft. Siddiqi's chronicle for NASA adds a second motive, that a passing carrier gave a lander more radio time.

The carriers changed little. NSSDCA describes the same 4V platform used for the 1975 orbiters: a 110 cm cylinder of propellant tanks, a pressurised ring of electronics and instruments, two solar wings spanning 6.7 m and a 1.6 m dish, with the entry sphere mounted on top. It gives Venera 12's bus a dry mass of 2,127 kg. At launch Venera 11 weighed 4,447.3 kg and Venera 12 4,457.9 kg, with 687.4 kg and 685.3 kg of propellant, and each carried a 1,645 kg descent craft whose lander would stand on the surface at 750 kg.

Below the clouds the new landers were meant to fall almost free. In NPO Lavochkin's description a 1.4 m² pilot chute and a 6 m² chute pulled away the top half of the heat shield, and a 24 m² braking chute lowered the lander through the clouds for nine or ten minutes before being cut loose at 46 to 48 km. The 180 m² three-canopy main parachute of Venera 9 and 10 was left off, which the bureau says shortened the descent by 15 minutes, and the lander finished the fall on its disc-shaped brake, designed to meet the ground at 7.3 to 8.5 m/s. The plan on the ground was for the soil sampler and both cameras to start 32 seconds after touchdown and keep going until radio contact was lost.

Siddiqi lists thirteen lander instruments: an imaging system, the Sigma gas chromatograph, a mass spectrometer, a gamma-ray spectrometer, the Groza lightning detector, temperature and pressure sensors, a nephelometer, an anemometer, an optical spectrophotometer, a remote soil collector, an X-ray fluorescence analyser for cloud particles, the Arakhis X-ray fluorescence spectrometer with its drill, and the PrOP-V penetrometer. Mitchell adds a scanning photometer fed by a rotating prism in the aerobrake and a microphone, and describes the mass spectrometer's metre-long intake, meant to avoid the contamination by cloud material that had affected Venera 9's. The two cameras were new colour telephotometers. In Mitchell's figures one would have sent 252 by 1,000 pixels of 10 bits in 14 minutes, over a lander link twelve times faster than in 1975, with the relays received by a new 70 m dish at Yevpatoria in Crimea.

The buses carried their own payload for the cruise and beyond. Siddiqi lists a plasma spectrometer, the Konus gamma-ray detector, the Signe-2MZ1 gamma and X-ray burst detector, the DUMS-1 ultraviolet spectrometer, a magnetometer, solar wind detectors and cosmic ray detectors. Konus was the experiment of E. P. Mazets and his colleagues at the Ioffe Institute; SIGNE 2 came out of the programme that CESR, the space radiation research centre in Toulouse, ran with the Soviet side under the French space agency CNES, according to the historian Philippe Varnoteaux. Mitchell describes an extreme-ultraviolet spectrometer on the bus as an extension of V. G. Kurt's work on atomic hydrogen, built with French colleagues to record the Lyman-alpha lines of hydrogen, helium, oxygen and other elements.

Venera 11 left Baikonur on 9 September 1978 and was in trouble almost at once. NPO Lavochkin records that its Sun-pointing attitude dropped out in the first communication session, as the antennas unfolded, and was quickly restored. The worse fault was that both the main and the reserve stage of the fan in the descent craft's cooling loop had failed, so the lander could not be chilled in the usual way. Controllers closed the shutter of the lander's radiative heater on 17 October, almost a month early, and tilted the station 40 degrees away from the Sun. Even so the lander was at 12 °C when it separated, where the norm was minus 10 to minus 15 °C.

Venera 12, launched on 14 September, had a smoother flight on a path of 98 days against Venera 11's 107, which let it arrive first. One of its telemetry tape recorders failed on the way, and a precision Sun sensor had to be swapped for its backup, but its cooling worked: the chill-down before release brought the lander to minus 12.3 °C, and it had warmed only to 5 °C by the time it entered the atmosphere.

On 19 December Venera 12 released its lander and fired its engine for 212.83 m/s to swing the bus onto a flyby path. Two days later the entry sphere hit the atmosphere at 11.2 km/s at an angle of minus 19 degrees, braked for 35 seconds, rode its parachute down to 46 km and then fell for 52 minutes 11 seconds on the disc brake alone. It landed on 21 December 1978 at 6 h 30 min Moscow time, which NSSDCA and Siddiqi give as 03:30 UT, after 61 minutes 53 seconds in the atmosphere, and it transmitted for 110 minutes through its bus, then passing about 34,000 km above the planet. Siddiqi calls it a record. NPO Lavochkin gives 470 °C and 90 atm at the site. Mitchell prints 468 °C and 93.6 atm, and writes that the landing raised a cloud of dust that darkened the sky for 20 to 30 seconds until winds of about 1 m/s cleared it.

Four days later it was Venera 11's turn. Its lander, released on 23 December, entered on 25 December at 11.2 km/s and minus 18 degrees, spent 52 minutes 58 seconds on the brake after the parachute let go at 46 km, and landed after 62 minutes 41 seconds, at 6 h 24 min Moscow time (03:24 UT). The bus passed 34,370 km from the surface and relayed for 95 minutes. The failed cooling showed: by the end the instrument container was above 70 °C, though NPO Lavochkin notes that the equipment was still working. Surface conditions were 462 °C and 92 atm according to the bureau, or 452 °C and 92.6 atm in Mitchell's figures. The Soviet yearbook for 1979 puts the site about 800 km from Venera 12's and Mitchell about 850 km. The whole-degree positions, 14° S, 299° E and 7° S, 294° E, are about 900 km apart on a sphere of Venus's size, a gap that rounding can account for.

NSSDCA's pages for both landers say they entered the nightside atmosphere, which is hard to square with the Soviet record. NPO Lavochkin lists a landing on the day side among the expedition's aims, the 1979 yearbook describes the spectrometer recording the daytime sky during descent, and V. I. Moroz and colleagues at IKI published those spectra in Nature in March 1980 as spectra of the Venus day sky. A JPL Horizons calculation for the published whole-degree sites, using the current IAU rotation model, puts the Sun about 73 degrees above the horizon at Venera 11's site at touchdown, a little after 11 in the morning local solar time, and about 69 degrees at Venera 12's. A 2018 JPL conference abstract gives Venera 11's entry at 11:10 local solar time, which fits that geometry, but also a solar zenith angle of 37 degrees, which does not. Every reading except NSSDCA's puts both landings in daylight.

Mitchell describes the moment in the control centre when the pictures should have arrived: there was a strong signal, but no modulation. The main scientific tasks were not accomplished, NPO Lavochkin says: no photo panoramas and nothing on the composition of the soil, because on each lander both telephotometer covers, redesigned after Venera 9 and 10, failed to jettison. On both landers the outside atmosphere also broke into the vacuum cavity of the VB01 soil sampler, so the sample never reached the detector, and the PrOP-V penetrometer returned nothing because its readings were to be judged from the pictures. Mitchell writes that a later study found a seal had formed that stopped the pressure equalising, so tons of atmospheric pressure held the covers down against their pyrotechnic charges. The bureau also lists smaller losses, including no data from the Sigma chromatograph and the IOAV optical instrument after Venera 11 touched down. The Soviet yearbook for 1979 told a different story: it names only the atmospheric instruments, says all of them worked to plan, and never mentions the cameras or the soil analyser.

Most of the descent science worked. According to the Soviet yearbook, the mass spectrometers switched on at about 24 km and ran to the ground, taking 22 gas samples and sending 220 spectra, and the two landers agreed completely; the gas chromatograph made nine analyses. NPO Lavochkin summarises the finding as a significant amount of nitrogen, about 5%, beside the main carbon dioxide, with traces of water vapour, chlorine, sulfur and their compounds, carbon isotopes roughly like Earth's, and isotopes of argon, neon and krypton. The yearbook adds that the isotopic make-up of Venus's argon shows a strong anomaly against Earth's, which NSSDCA records as a high ratio of argon-36 to argon-40. Mitchell's table shows how far the two methods could differ: nitrogen at 4.0 ± 0.3% by mass spectrometer and 2.5 ± 0.3% by chromatograph, argon at 110 against 40 on a row with no unit, where the yearbook gives about 0.01% (some 100 parts per million) from both instruments. In the clouds, Mitchell writes, the X-ray analyser found ten times more chlorine than sulfur, and according to NPO Lavochkin the nephelometer confirmed the cloud base at 48 to 49 km found by Venera 9 and 10.

Every 10 seconds, the yearbook says, the optical spectrometer recorded the sky spectrum from 450 to 1,200 nm and the angular spread of scattered light through four filters between 490 and 1,330 nm. Siddiqi reports that the Venera 11 spectrophotometer found only 3 to 6% of the sunlight reaching the surface. Mitchell draws the picture that emerged: the sky grows steadily more orange with depth, the deep atmosphere scatters so strongly that the surface would not be clearly visible from more than a few kilometres up, and an observer on the ground would see the rocks fade into a featureless orange haze.

The Groza receiver, designed by Leonid Ksanfomaliti of the Space Research Institute (IKI) around a 25 cm loop antenna, listened at kilohertz frequencies for electrical discharges. On 21 December 1978 it recorded many electromagnetic pulses on the Venera 12 lander, which Ksanfomaliti interpreted as electrical discharges, according to a 2025 Literaturnaya Gazeta piece on a manuscript biography of him; he reported them in Nature in March 1980 as frequent lightning discharges in the clouds. NPO Lavochkin places the discharges in the cloud layer, with energy roughly equal to terrestrial lightning and a rate of up to 30 a second. Mitchell differs twice over: he calls the bursts significantly more intense than terrestrial storms, with some evidence of a source far below the clouds, and he says Venera 11 heard intense activity while Venera 12 heard much less. The question never closed. A 2023 study of whistler waves seen by Parker Solar Probe calls Venus lightning debated for decades and shows that some whistlers need no lightning at all; the atmosphere page follows the argument.

The twins did not have Venus to themselves. NASA's Pioneer Venus orbiter had entered orbit on 4 December 1978, and on 9 December the four probes of the Pioneer Venus multiprobe had entered the atmosphere. Mitchell notes that the American probes returned only about 1% as much data as Venera 11 and 12, that their results largely agreed, and that all four suffered damage and odd readings at about 12.5 km, the height at which Venera 12 saw anomalous Groza readings and unusual phase shifts in its Doppler signal.

After the flybys both buses were reassigned. For the Soviet-French experiment on locating gamma-ray bursts, NPO Lavochkin writes, the two stations were used to give the largest possible measuring base: Venera 12 fired about 350 m/s on 5 February 1979 and Venera 11 did the same on 7 February, burning nearly all their remaining propellant. A burst reaches widely separated detectors at slightly different times, so a longer baseline gives a tighter position. HEASARC, NASA's high-energy archive, describes SIGNE 2 as two sodium iodide detectors on each craft covering the whole sky, which with a third set on Prognoz 7 in Earth orbit formed a three-point interplanetary triangulation network, and it credits the arrangement as the first time identical detectors had been used simultaneously for burst studies on spacecraft separated by interplanetary distances. Varnoteaux writes that the detectors on Prognoz 7, Venera 11 and Venera 12 together located several dozen gamma-ray sources and yielded a first catalogue of the phenomenon.

One burst stood apart from the rest. The Konus catalogue of soft gamma repeater activity, compiled at the Ioffe Institute from the Konus detectors on Venera 11 to 14 and later craft, lists an event on 5 March 1979 at 15:51:39 UT from the source SGR 0526-66: an initial pulse 0.25 seconds long followed by a pulsating tail lasting more than 64 seconds. Mazets and his colleagues reported in Nature in December 1979 that Konus on both Veneras had caught a very intense burst on 5 March and a much weaker one on 6 March from the same source, that several other spacecraft saw the first too, and that they could place the source on the sky. The catalogue lists sixteen more bursts from that source up to 1983. The Ioffe laboratory's memorial to its later head Rafail Aptekar says that soft gamma repeaters, objects it describes as having extremely strong magnetic fields, were discovered in the Konus experiments on the Venera stations at the turn of the 1980s.

Venera 12 had one more target. About a year and a half after launch, NPO Lavochkin writes, it could study the newly found comet Bradfield, catalogued by Siddiqi as C/1979 Y1. On 13 February 1980 the bus, then crossing Earth's orbit about 150 million km from the Sun, turned on its gyro platform toward the comet, which lay inside the orbit of Venus, and its DUMS-1 ultraviolet spectrometer, never meant for comets, recorded a spectrum; the session was repeated on 17 March. Venera 11 was already gone. As it moved away from the Sun its Sun sensors became unreliable and it fell back on gyroscopic spin, and after a last session on 1 February 1980 it did not answer on 8 February. One possible cause, the bureau says, was a poor spin of a craft unbalanced by the loss of its lander and nearly all its propellant, which left the solar array's current below the load until the battery ran down. Venera 12 met the same end in mid-April 1980, 174 million km from the Sun, and was last heard on 18 April.

SIGNE 2 belongs to a longer French thread. France and the Soviet Union signed a space cooperation agreement on 30 June 1966, Varnoteaux writes, and CESR, under CNES, proposed a first high-energy astronomy experiment to the Soviet side. SIGNE 1 flew on Prognoz 2 from 29 June 1972, and its more capable successor, SIGNE 2, went on Prognoz 6, 7 and 9 and on four Venus probes: Venera 11, 12, 13 and 14. HEASARC spells the version on Venera 13 and 14 SNEG-2MG and says those buses returned gamma-ray data until March 1983.

Other European instruments also reached Venus on other nations' spacecraft before ESA sent its own Venus Express. NSSDCA's page for the Venera 9 orbiter of 1975 (see Venera 9 and 10) lists a French ultraviolet photometer at 350 nm, and Jean-Loup Bertaux, Jacques Blamont, Vladimir Kurt and colleagues published Lyman-alpha observations of hydrogen around Venus from the same orbiters in 1978; the two descriptions may refer to one French instrument or to two. The Space Research Institute (IWF) in Graz says the first magnetometer built there flew on Venera 13 at the end of 1981, and that its instruments on Venera 13 and 14 gave new insight into the solar wind. The East German Academy of Sciences built an infrared Fourier spectrometer for Venera 15 and 16 as its first deep-space experiment, to map temperature and pressure above the clouds, according to a 2009 German history. NSSDCA states that the ultraviolet spectrometer, the mass spectrograph and the pressure and temperature instruments on the Vega landers of 1985 were developed jointly by French and Soviet investigators. On Pioneer Venus, Ulf von Zahn of the University of Bonn led the multiprobe bus's neutral mass spectrometer, and Jacques Blamont of the CNRS aeronomy laboratory shared the lead of the large probe's nephelometer with Boris Ragent of NASA Ames.

Both landers remain on the surface at positions known only to the nearest degree, and the halves of their entry spheres and their parachutes also came down on Venus, at unrecorded positions; the ledger of what reached Venus tracks them. The buses circle the Sun, silent since 1980. Some of their remote science was later revised: in 1996 S. A. Stern and colleagues reported that far-ultraviolet emissions the Venera 11 and 12 spectrophotometers had attributed to argon were not argon, removing a conflict with the argon measured in place. Mitchell writes that Venera 13 and 14, launched in 1981, would complete the missions attempted by Venera 11 and 12, and in March 1982 they did, with colour pictures and soil analyses from two new sites.