Venera 5 and Venera 6

Venera 4 left a question behind. On 18 October 1967 its capsule hung under parachutes for 93 minutes sending temperature and pressure, and Soviet scientists first concluded it had measured all the way down: the radio altimeter read 28 km as the instruments came on (26 km in Mitchell's account), which matched the descent length they computed. Months of joint work with radar results and with the radio occultation data of the American Mariner 5 flyby pointed the other way. The altimeter's frequency modulation let any single reading correspond to two heights some 30 to 40 km apart, and the Soviet summary printed in Pravda on 4 June 1969 conceded that the capsule may have stopped well above the ground once the pressure pushed in the lid of its instrument container (Mitchell thinks its battery more likely ran out; Marov and Huntress call that less likely). The Great Soviet Encyclopedia's 1970 yearbook gave the revised result plainly: Venera 4 had measured pressure down to about 35 km and temperature down to about 26 km.

The next launch window came in January 1969, one Earth to Venus cycle of about 584 days later, which gave the Lavochkin design bureau under Georgy Babakin no time for a new capsule. It strengthened the one it had. According to NPO Lavochkin's own project page, the walls of the descent vehicle were thickened, its internal gas pressure was raised from 2 to 2.5 atmospheres to stiffen it, and it gained 33 kg; to keep within what the Molniya-M rocket and its Blok VL stage could send to Venus, the cruise stage shed 14 kg by giving up its magnetometer. The capsule was rated for 25 atmospheres outside. The June 1969 Pravda summary adds that it had to take decelerations of up to 450 g, one and a half times the Venera 4 figure, because the 1969 positions of Earth and Venus meant a faster arrival (Siddiqi gives the earlier figure as 300 g). Its parachutes shrank from 2.2 to 1.9 square metres for the drogue and from 55 to just 12 for the main canopy, so it would fall faster and, in Marov and Huntress's account, reach deeper layers while its battery lasted, and the canopy cloth was rated for more than 500 °C. The instruments were reworked too, according to Marov and Huntress and to Mitchell: six barometers in three overlapping ranges, three platinum thermometers, a tuning-fork densitometer, an improved altimeter and gas analysers retuned around what Venera 4 had found.

Accounts differ on what the engineers expected. Lavochkin's page says the Venera 4 data produced a revised atmosphere model and that the new capsule was designed for 25 atmospheres and a surface temperature of 290 °C, give or take 10, against 10 atmospheres and 425 °C in the earlier model, and that the smaller parachutes were meant to shorten the descent to touchdown, which reads as if the 1969 capsules might have reached the ground. Mikhail Marov, writing with William Huntress, remembers otherwise: it was already clear that the pressure far exceeded 18 bar, the bureau had no time for fundamental changes and could raise the capsule's strength only to 25 bar, and so the 1969 flights were treated as a chance to sound the atmosphere with better instruments while a new design waited for the next window. Don P. Mitchell's history of the programme is blunter still, saying no one expected these probes to reach the surface intact and that Soviet planetary scientists then put the surface pressure at 75 to 100 atmospheres. The account by Shubin that epizodsspace reproduces agrees that it was clear at launch they would not get down.

Venera 5 lifted off from Baikonur on 5 January 1969 at 06:28:08 UT and Venera 6 followed on 10 January at 05:51:52 UT. Each went first to a parking orbit; the Soviet yearbook puts Venera 5's departure for Venus at 10:47 (it names no time zone; its entry times are Moscow time), over Africa, after a 228 second burn that added about 3.6 km/s. The launch dates were picked within a window about a month long, and the resulting flight times of 131 and 127 days meant that launches five days apart would arrive one day apart. The same yearbook spells out the precision involved: an error of 1 m/s at departure, under 0.01 percent of the speed, meant missing Venus by about 70,000 km. Venera 5 was sent off with an aiming error of 25,000 km and Venera 6 with 150,000 km. Both were corrected in March, Venera 5 by 9.2 m/s on the 14th and Venera 6 by 37.4 m/s on the 16th, each some 15.5 million km from Earth in the yearbook's figures (Siddiqi and Marov with Huntress give 15.7 million km for Venera 6), and each after turning to hold the Sun and the star Sirius. Ground stations held 73 radio sessions with Venera 5 and 63 with Venera 6 during the cruise.

Along the way the cruise stages did science of their own. Their counters found galactic cosmic rays about 15 percent weaker than Venera 4 had measured in 1967 and about 40 percent weaker than in 1965, which the Soviet summary tied to the solar cycle, and over four months they logged a dozen significant bursts of solar protons. Close to the planet both stations saw the sudden change in the solar wind where it flows around Venus, at about 28,000 km from the surface in Lavochkin's account (the 1969 summary gives about 30,000 km without naming the reference point, next to a Venera 4 figure it measures from the centre), farther out than Venera 4 had met it nearer the terminator. Their ultraviolet photometers picked up hydrogen around Venus from about 25,000 km from its centre inward, reaching roughly 100 atoms per cubic centimetre at 10,000 km.

It was Venera 5 that arrived first, on 16 May. The best place to enter, the 1970 yearbook explains, lay in the middle of the planet's disc as seen from Earth, where a capsule hanging from its parachute would point its antenna home; both stations came in from the shadow side and entered on the night side, about 2,700 km from the morning terminator. At 07:08 Moscow time, 50,000 km out, the ground sent the command that opened the final session. The capsule separated 37,000 km from Venus and struck the atmosphere at 09:01 Moscow time, 06:01 UT, at about 11.2 km/s and 62 to 65 degrees to the local horizon. Braking cut the speed to 210 m/s; the parachutes opened, the transmitters switched on, the altimeter antennas unfolded, and data began to arrive from about 54 km up in NSSDCA's figure (55 km in Lavochkin's pages). For 53 minutes by the TASS count, 52.5 by Lavochkin's and Siddiqi's, readings came in. The altimeter gave three marks, at 40.4, 31.9 and 23.8 km, and the gas analysers sampled twice, near 0.6 atmospheres and 25 °C and again near 5 atmospheres and 150 °C. The densitometer returned nothing because of a fault in the instrument. Reception stopped at 27 atmospheres, beyond the 25 the capsule was built for.

Venera 6 did it again a day later. Lavochkin calls it a complete analogue of its sister in design, tasks, flight plan and instruments, and its capsule separated 25,000 km out and entered at 09:05 Moscow time, 06:05 UT, on 17 May. It transmitted for 51 minutes by the TASS count, 50 by Lavochkin's, and its altimeter gave two marks, at 32.1 and 21.2 km. Its gas analysers had been set to work lower in the atmosphere, and they sampled at about 2 atmospheres and 85 °C, then at about 10 atmospheres and 225 °C, although the June 1969 Pravda summary put the first sample at 1 atmosphere and 60 °C. It too fell silent at 27 atmospheres. Both cruise stages, left on course for the planet, kept transmitting until they reached the dense atmosphere and were destroyed there; no source read for this page gives a position for either.

Each capsule returned more than 70 pressure readings and more than 50 temperature readings, through a layer where the temperature climbed from about 25 °C to 320 °C and the pressure from about half an atmosphere to 27. The profile, the yearbook noted, stayed close to adiabatic. Mitchell describes the gas analysers as reconfigured after Venera 4 to work within narrower ranges; the yearbook's combined table put carbon dioxide at 97 percent (plus 3, minus 4), nitrogen with the inert gases at no more than 2 percent, oxygen at no more than 0.1 percent, and water at about 11 mg per litre at the 0.6 atmosphere level. The capsules also reported on themselves: the temperature inside rose only from 13 °C to 28 °C during the descent. That says the insulation worked and it was the outside pressure that broke them, which is Mitchell's reading of it.

Set beside today's values, the 1969 chemistry was close on carbon dioxide and off on the minor gases. NASA's current Venus fact sheet gives 96.5 percent carbon dioxide and 3.5 percent nitrogen, so the yearbook's ceiling of 2 percent nitrogen was too low (the wider range in the June 1969 summary, given below, brackets today's value). It lists water at 20 parts per million near the surface, while Marov and Huntress convert Venera 5's reading to about 1.1 percent, several hundred times more; the two figures refer to different heights, and none of the sources read for this page explains the gap. The preliminary Pravda summary had carried wider ranges (carbon dioxide 93 to 97 percent, nitrogen with inert gases 2 to 5 percent, oxygen up to 0.4 percent, water 4 to 11 mg per litre). NSSDCA's records (in copies archived in 2024 and 2025) say the missions confirmed a lack of water vapour, which is not what the Soviet results said: the same summary that gave 4 to 11 mg per litre concluded only that the air was not saturated with water vapour.

The height at which the signals stopped has been restated several times, and every version still circulates. The preliminary scientific summary, in Pravda on 4 June 1969, said the altimeters put the 27 atmosphere level at 24 to 26 km for Venera 5 and at 10 to 12 km for Venera 6. Because the same pressure should mark the same level in the atmosphere, the summary suggested the two capsules had come down over very uneven relief. The 1970 yearbook gave the difference as 13 km on average, called it a result needing further study, and placed the end of both descents at about 20 km above a mean surface defined as the average of the two altimeter solutions. Lavochkin's pages today say each capsule stopped with about 18 km still to fall, while keeping the 13 km difference and the relief explanation. NSSDCA held both generations at once in its most recent archived copies (2024 and 2025; the live catalogue returned an error when checked on 5 November 2026): its records for the two buses repeat the 1969 figures, and its descent craft records say Venera 5's 24 to 26 km was later determined to be closer to 18 km and that Venera 6's 10 to 12 km was revised to roughly 18 km.

Two explanations survive. Asif Siddiqi's NASA chronicle keeps the terrain reading, noting that Venera 6 reached 27 atmospheres lower than Venera 5 and so may have come down over a mountain or high plateau. Mitchell argues that the altimeters worked. They had been fitted with filters that triggered a message at set ranges, which he gives as about 35, 25 and 15 km with an ambiguity of about 30 km; Marov and Huntress give the design marks as 45, 35 and 25 km, accurate to about 1.3 km. When the temperatures and pressures reported at each mark are compared with a modern atmosphere model, Mitchell finds the heights come out normal and consistent, and he attributes the 10 or 11 km discrepancy of 1969 to the cruder atmosphere model of the day and possibly to confusion over which filter had fired. Marov and Huntress take the same side: the altimeter worked normally, and the temperatures and pressures at its marks agreed well with the atmosphere model built by then.

Magellan's radar altimetry, gathered two decades later, allows the terrain reading to be tested against the published positions. Read at 3 S, 18 E and 5 S, 23 E in the USGS release of the Magellan global topography, the two points lie about 76 m below and 196 m above the grid's 6,051 km reference sphere, some 270 m apart in height. Within 3 degrees of latitude and longitude of either point nothing in the grid rises more than about 2.4 km above that sphere, while the whole planet spans about 14.6 km from its lowest to its highest point. On these positions a 13 km step between the two descent areas finds no support, which fits the reading of Mitchell and of Marov and Huntress. The test is only as good as the coordinates, which are whole degrees of unstated origin in a frame that may differ from Magellan's; azmth ran it on 26 September 2026.

Where the wreckage lies is known only roughly. Every source read gives the same positions, 3 S, 18 E for Venera 5 and 5 S, 23 E for Venera 6, and none says how they were derived. Lavochkin, RGANTD and the 1970 yearbook say the two entry points were about 300 km apart; on a sphere of Venus's radius the published coordinates are about 570 km apart, and allowing each to be rounded by half a degree still leaves at least about 430 km, so the two statements cannot describe the same pair of points. Marov and Huntress give local solar times of 4:12 and 4:18 with the Sun 117 and 115 degrees from the zenith, before dawn, which fits the 2,700 km to the morning terminator; azmth's check with JPL Horizons at the entry times puts the published points about 330 km and 440 km from the centre of the Earth-facing disc, which the yearbook names as the aim. Doppler tracking of the capsules, analysed by V. V. Kerzhanovich, B. N. Andreyev and V. M. Gotlib, found vertical gusts within 0.3 to 0.5 m/s and bounded the horizontal wind at 3 to 25 m/s; even at the upper bound, 50 minutes of drift is about 75 km. After the signals stopped, the broken capsules fell the last 18 km or so unheard.

Below the last readings, conditions at the ground could only be extrapolated. Assuming the temperature went on rising adiabatically, the 1970 yearbook estimated 770 K, about 497 °C, and 100 atmospheres at the mean surface, with 60 K and 40 atmospheres of possible deviation for ground 7 km higher or lower. Siddiqi reports separate extrapolations giving 530 °C and 140 atmospheres from Venera 5 and about 400 °C and 60 atmospheres from Venera 6. Direct measurement at the ground came with Venera 7 in December 1970, and the values have been refined since; NASA's fact sheet now gives 92 bar and 464 °C, inside the yearbook's range.

One photometer reading remains unexplained. The light sensors were carried despite the night-side arrival because a faint glow of Venus's dark hemisphere had been reported from Earth and its source was unknown; they responded above 0.5 W per square metre, roughly twilight on Earth. In the 1969 and 1970 Soviet summaries nothing crossed that threshold except a single Venera 5 reading of about 25 W per square metre some four minutes before its signal ended. The 1969 summary left open whether it was chance or an atmospheric phenomenon, and Lavochkin's pages today mention only readings below the threshold. Marov and Huntress allow that it could have been lightning but think an electrical discharge from the capsule breaking up more likely, given the timing, and Mitchell offers the same two options. None of the sources read here settles it.

Soviet announcements left a phrase that still causes confusion. The TASS communique printed in Pravda on 17 May 1969, in the English translation NASA commissioned two days later, has Venera 5 effecting a soft landing in the atmosphere and delivering to Venus pennants bearing a bas-relief of Lenin and the Soviet coat of arms. The 1970 yearbook likewise says the pennants were delivered to the surface, and a 4 kopeck stamp of 1969 announced the two stations as on Venus. The pennants were real: the Russian State Archive of Scientific and Technical Documentation (RGANTD) holds 1968 photographs of the pentagonal emblem mounted on the Venera 5 capsule. What reached the surface, though, was whatever remained of two capsules broken open about 18 km up, and as late as 2006 the magazine Nauka i Zhizn still wrote that both had made a smooth landing on the night side.

After Venera 5 and 6, Lavochkin stopped strengthening the Venera 4 capsule and started over. Mitchell writes that Babakin and his deputy V. G. Perminov, no longer trusting the scientists' estimates of surface pressure, privately set the next capsule to survive 180 atmospheres, built a test chamber for 150 atmospheres and 540 °C, and put the instruments inside a titanium sphere. That capsule flew as Venera 7. How the Soviet engineers learned to get down alive is told on landing on Venus, and what the 1969 profiles contributed to the modern picture of the air they fell through is on the atmosphere of Venus.