The Vega balloons

Two spheres of Teflon fabric, each about 3.4 m across, rode the winds of Venus for two Earth days in June 1985. The Soviet stations Vega 1 and Vega 2 released them on 11 and 15 June as their landers fell toward the surface, and each towed a small instrument gondola through the middle cloud deck, 53 to 54 km up. NPO Lavochkin, which built the stations, calls the balloon probe the first ever used as a research tool in spaceflight. Each balloon transmitted for about 46 hours, until its battery ran down, and drifted west some 11,000 km. Nothing relayed their faint signals: twenty radio telescopes on Earth listened directly and fixed their positions to about 10 km. This page also covers that network, the tracking papers and Jacques Blamont. The landers and the Halley flyby are on the Vega 1 and Vega 2 page.

The idea is older than the hardware by almost two decades. According to the balloon team's 1986 account in the Soviet journal Pis'ma v Astronomicheskii Zhurnal, translated by NASA as TM-88516, Blamont first proposed balloons for studying the dynamics of the Venus atmosphere in 1967. In April 1974, at the Soviet-French EOS-Venera symposium in Moscow, L. R. Kogan, V. M. Linkin, L. I. Matveenko and colleagues proposed following a transmitting balloon with ground interferometers. From 1974 to 1980 the Interkosmos Council and CNES, the French space agency, developed a much larger balloon whose data would be relayed through an orbiter; it is covered with the Soviet unflown Venus projects. Its ending depends on who tells it. The Soviet paper describes a next step proposed by R. Z. Sagdeev and Linkin: send the data straight to Earth, measure position by very long baseline interferometry (VLBI), and cut the mass. The Société astronomique de France, in its 2020 tribute to Blamont, says the Soviets preferred to replace the French EOS balloon, 9 m across with a 220 kg gondola, by two smaller ones. The mission note in NASA's Planetary Data System (PDS) attributes the change to the decision to send the carriers on to Comet Halley. The Russian State Archive of Scientific and Technical Documentation (RGANTD) holds the technical specification for the Vega complex, index 5VK, drawn up in May to September 1981; the minister of general machine building, S. A. Afanasyev, gave the project final approval on 24 February 1982.

The flown balloon was a sealed, pressurised sphere of Teflon fabric coated with Teflon film. Filled with helium to 30 mbar above the outside pressure, it measured 3.4 m across, according to R. S. Kremnev and the design team in 1986. Their mass budget is 21 kg: 12.5 kg of envelope and fittings, 2.0 kg of helium and a 6.5 kg gondola. NSSDCA gives 21.5 kg, with 2.1 kg of helium and a 6.9 kg gondola. The PDS mission note gives the diameter as 3.54 m, and Kremnev's own volume figures (19.6 cubic metres unpressurised, about 1 more per 10 mbar) put a sphere at 30 mbar near 3.5 m, a mismatch no source explains. The gondola, about 1.2 m long, hung 13 m below the envelope in Kremnev's paper and 12 m in the team's 1992 paper. A 37 cm conical spiral antenna sat on top. The middle box held the transmitter, timer, oscillator and the Meteo assembly, with a quartz-resonator pressure sensor good to about 0.5 mbar and a light sensor looking down through a cone; a folding arm carried a film thermometer good to 0.5 K and a two-fan anemometer that counted air moving up or down past the gondola. The bottom box held about 1 kg of lithium cells, rated at 250 watt-hours for 46 to 52 hours, and a nephelometer pulsing at 930 nm to gauge the density of the cloud.

Everything the gondola learned had to cross about 100 million km on 4.5 W. The transmitter worked at 1667.9 MHz, the 18 cm band where radio astronomers already observed hydroxyl masers. In telemetry mode it sent 30 s of pure carrier for tracking, then the data gathered over the previous half hour at 4 bits per second, in a session Kremnev gives as 332 s long and JPL rounds to 330 s. In coordinate mode it sent two tones 6.5 MHz apart, whose phase difference let the interferometers fix its position. Sessions came in bursts during the first 10 hours and for about 12 hours on the second day, and less often at other times, to save the battery. To fit 12-bit readings into so few bits, the gondola usually sent only the six least significant bits of each value. The Venus air proved livelier than expected, so the missing top bits sometimes had to be estimated on the ground, and the PDS archive credits Lee Elson and David Crisp with untangling the record.

Putting a balloon into the air took 15 to 25 minutes and a chain of parachutes. The folded balloon and gondola sat in a ring-shaped container around the lander's antenna, inside the upper half of the entry sphere, with helium tanks, parachute and timer on top; the kit weighed about 120 kg. NPO Lavochkin's project page gives the sequence. At about 64 km a parachute pulled off the upper hemisphere with the balloon package inside. The package dropped free 32 s later, rode a small stabilising parachute, then a 35 square metre main parachute. The container was cut apart and its lower half, serving as ballast, drew out the gondola, tether and envelope as it fell. A pyrotechnic valve let the helium in, the filling line was cut and sealed, and parachute and filling system dropped away. At 0.9 atmospheres or 60 °C the ballast fell, near 50 km, and the balloon climbed back to float height. Lavochkin also explains the choice of entry points: the balloon had to enter on the half of Venus visible from Earth and stay there through its drift, as far as possible from the subsolar meridian, where it might not survive.

The Vega 1 balloon went in first, at 02:06:10 UT on 11 June 1985 by Earth-received time, at 8.1° N, 176.9° E and 84.6° of longitude short of the morning terminator (Sagdeev and colleagues, 1992). It was just after local midnight, and the early hours were rough: the team's first report puts its biggest vertical swings in the first few hours, when a downdraft reached 3 m/s. Across both flights, downward winds as strong as 3.5 m/s occasionally pushed the balloons as much as 2.5 km below float level, the overview in Science adds. Along its whole drift Vega 1 found air about 6.5 K warmer, at the same pressure, than its twin found south of the equator. Its light sensor picked up sunlight about three Earth hours before the balloon crossed the terminator, and the signal climbed quickly on the day side. The Doppler reconstruction puts the terminator crossing at 12:20 UT on 12 June, after 8,500 km, and the last communication session at 00:38 UT on 13 June at 68.8° E, 11,600 km from the start, at about eight in the morning local solar time. Its nephelometer returned data from every session, and the only cloud-density record from the pair comes from it.

Vega 2's balloon followed four days later, at 02:06:04 UT on 15 June, placed by the same reconstruction at 7.5° S, 179.8° E, 75.4° of longitude short of the terminator, around 1 a.m. local time. For its first 20 hours it rode in calm air, rising and falling by less than 100 m. Then its motion turned as restless as its twin's. It crossed the terminator at 09:10 UT on 16 June, after 7,400 km, and between the 35th and 40th hours counted from 0 h UT, roughly 11:00 to 16:00 UT that day, it went through what the team called the great event. The Doppler signal strayed from the constant-wind model by up to 15 m/s, and in Andreev's early estimate the balloon sank about 2 km while the air around it warmed by 20 °C; the 1986 Soviet encyclopedia yearbook gives the drop as about 1.5 km. In the PDS archive the highest pressure after the balloon first reached float height is 884 hPa, at 12:56 UT on 16 June, a reading the archive flags as good but uncertain in decommutation, with its model altitude at 51.4 km four minutes later. Early in the flight it floated near 530 hPa. By the team's calculation the envelope lost its overpressure on the way down, making the balloon more sensitive to gusts, yet the last session, at 00:38:01 UT on 17 June, showed it back at float height; the Doppler model put it at 7.5° S, 76.3° E after 11,100 km.

The team's explanations for the plunge stayed tentative. The first analysis, led by Blamont, noted that it came as the balloon crossed the mountainous region of Aphrodite Terra, over terrain far rougher in Pioneer Venus altimetry than any under Vega 1, and that Vega 1 felt nothing comparable at its own terminator crossing; the Doppler team placed Vega 2 near a peak about 5 km high. The authors judged an erupting volcano the least likely cause and favoured waves in the atmosphere raised by air flowing over the mountains, while warning that their trajectory was preliminary and the altimetry coarse. The final VLBI paper of 1992 adds that Vega 2 turned north over the northern flank of the Aphrodite mountains before running parallel to the equator, and that the gondola recorded pronounced disturbances during that sideways drift. Even the balloon model behind the archived tracks is uncertain: the PDS extras note says its fill and leakage values may or may not match what happened, and points to an alternative, cloud droplets collecting on the envelope, set out by G. E. Dorrington in a 2013 paper on possible drizzle in the middle cloud layer.

Most of the science came out in seven papers in Science on 21 March 1986 and in the Soviet journal issue that NASA translated. Doppler tracking alone gave mean westward winds of 69 ± 1 m/s for Vega 1 and 66 ± 1 m/s for Vega 2, about 250 km/h, 5 to 10 m/s faster than earlier probes had found at that height (6 to 10 m/s in Blamont's analysis and the 1992 paper), a sign that the circulation varies over time. The departures from a steady wind rose and fell in step on both balloons, peaking near 3:30 a.m. Venus local time, which the authors tentatively tied to a feature fixed relative to the Sun, perhaps a thermal tide. Vertical winds were typically 1 to 2 m/s, in line with convection. Blamont's group put the persistent 6.5 K gap between the balloons down, probably, to large-scale disturbances travelling west around the planet. Lightning was the thinnest result. In about seven hours of combined observation spread along both tracks, the flash detector registered one signal, on Vega 2, which the team said could be lightning or a brief change in the light level. The PDS tables carry no light-sensor channel at all.

Two networks worked as one to track the balloons. The Soviet network, coordinated by IKI, used 70 m dishes at Yevpatoriya in Crimea and Ussuriysk in the Far East, finished just before the experiment according to the 1992 paper, a 64 m dish at Medvezhi Ozera near Moscow, and smaller antennas at Simeiz, Pushchino and Ulan-Ude. CNES organised the international network, with Blamont and R. A. Preston of JPL as its scientific leaders. Its backbone was NASA's three 64 m Deep Space Network dishes at Goldstone, Madrid and Canberra, joined by observatories including Arecibo, Effelsberg, Jodrell Bank, Onsala, Hartebeesthoek in South Africa, Atibaia in Brazil, Penticton in Canada and Haystack in Massachusetts. The two large Soviet dishes and the three DSN dishes took the telemetry, and CNES staff in Toulouse coordinated real-time work between JPL and IKI. Over the four days of the two flights the network made about 1,200 recordings. JPL's report says about 95 percent of the telemetry was decoded and differential VLBI was obtained for 120 of the 138 balloon transmissions.

Accounts of who ran which antenna do not agree. The team's 1986 paper and NSSDCA count 6 Soviet stations and 14 abroad coordinated by CNES. JPL's report says CNES was responsible for 15 of the 20, including the DSN, with five Soviet antennas taking part, although its own map shows six on Soviet soil. The station lists differ too. The Soviet encyclopedia yearbook for 1985 listed Green Bank and Parkes, which appear in neither JPL's 1986 map nor the team's 1992 table; both name North Liberty in Iowa and Owens Valley in California instead. The post-flight 1986 yearbook still names Green Bank among those used. JPL's January 1985 release spoke of at least ten antennas plus the Soviet network, and of 3 m balloons with 60-hour batteries.

Turning the recordings into tracks took years, and two versions still circulate. The older one, published in 1986 and repeated by NSSDCA, used Doppler shifts alone. The team fitted a model of purely east-west motion at constant speed along a parallel, which fixes each balloon's latitude by construction; that is why both catalogue end points sit exactly at the start latitudes, 8.1° N and 7.5° S. The newer one came from the phase of the signals recorded across the network, processed at IKI and JPL, and was published by Sagdeev and 34 co-authors in Astronomy and Astrophysics in 1992. Using each Vega flyby spacecraft as a reference transmitter, it reached an accuracy of about 10 km. It confirmed Vega 1's track as parallel to the equator, at 68.7 m/s westward with no measurable north-south wind, and showed Vega 2 moving 65.3 m/s westward and 3.4 m/s northward (3.5 m/s in its discussion), drifting toward the equator on its first day and holding latitude on its second; the text puts the total shift at 250 km. It also found a 7.4 s wobble, explained as the gondola swinging like a pendulum, and contradicts itself on where it was commoner: ten times more often in the southern hemisphere, says the abstract, and ten times more often for Vega 1, which flew in the north, says the discussion.

Those tracks became public in February 2020, when the Planetary Data System released a restored Vega archive compiled by Ralph Lorenz, David Crisp and Lyle Huber. For each balloon it holds a telemetry table and a second table that adds position from differential VLBI, altitude from a balloon model and the derived winds. Few of its 2,040 rows carry a position. Vega 1 has 61 fixes, from 8.32° N, 176.16° E at 02:18:27 UT on 11 June to 9.08° N, 77.41° E at 20:03:12 UT on 12 June. Vega 2 has 57, from 6.18° S, 177.95° E at 02:18:22 UT on 15 June to 3.22° S, 83.99° E at 20:03:07 UT on 16 June. All fixes carry the archive's top quality flag, 4; the other rows are zero-filled. Fixes arrive every half hour to hour, with gaps of up to 2.5 hours for Vega 1 and 5 hours for Vega 2. Both tables end a little after 21:00 UT on the second day, about three and a half hours before the last session. JPL's 1986 report fits that ending: it judged two Vega 1 transmissions at the end of the balloon's life probably too weak to use, and the last two Vega 2 recordings too weak to decode.

For Vega 1 the two products agree closely. The VLBI fixes keep it between 7.96° and 9.09° N, although over its last half-day of fixes it edged about 0.9° north. For Vega 2 they part company. The VLBI track begins at 6.18° S, 1.3° north of the model's 7.5° S, climbs to 3.22° S by the last fix and spans about 3.2° of latitude, roughly 340 km at float height, more than the 250 km given in the 1992 text. The widely quoted end point of 7.5° S, 76.3° E is therefore a model position at a latitude the measured track never touches. Anyone plotting the flights must choose, because the measured fixes stop about four and a half hours, roughly 1,100 km of drift, before the last contact, whose position comes only from the model. Where the balloons went after their batteries died is unknown. JPL's January 1985 release expected daytime heat to burst them; the 1986 team concluded instead that sunlight warming the envelope would offset the slow helium loss, so they probably flew on.

The Académie des sciences lists Jacques Blamont (1926 to 2020) as scientific and technical director of CNES from 1962 to 1972 and director of the CNRS Service d'aéronomie from 1962 to 1985, and names him promoter of the Eole programme of sealed balloons in Earth's lower atmosphere (1971). The Société astronomique de France adds that he brought instrumented ballooning back from the United States and that the balloon centre at Aire-sur-l'Adour began in his laboratory in 1963. The same tribute calls him principal experimenter for the nephelometer on the four Pioneer Venus atmospheric probes, whose results he published with Boris Ragent of NASA Ames. On Vega he co-led the balloon tracking, meteorology and nephelometer experiments, and his Vénus dévoilée (Odile Jacob, 1987) tells the story of the mission. He died on 13 April 2020, ten weeks after the restored balloon archive was released.

Engineers were still developing the idea in 2024. That year Jacob Izraelevitz and colleagues described two flights over Nevada's Black Rock desert of a subscale, altitude-controlled aerobot made of materials meant to survive Venus, flown at the air density of the 54 to 55 km cloud layer, near the Vega float level. JPL and Aerostar were developing it for a mission that would circle the planet several times. Other balloon plans are under Venus concepts.