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The flyby of Comet Halley had been a late mission change in the Venera program following on from the cancellation of the US Halley mission in 1981. A later Venera mission was cancelled and the Venus part of the Vega 1 mission was reduced. Because of this, the craft was designated Vega, a contraction of Venera and Gallei (from Halley, as the Russian language does not have the letter "H"). The spacecraft design was based on the previous Venera 9/10 missions. The two spacecraft were launched on December 15 and December 21, 1984, respectively.
Vega 1 arrived at Venus on June 11 and Vega 2 on June 15, 1985, and each delivered a 1500 kg, 240 cm diameter spherical descent unit. The units were relased some days before each arrived at Venus and entered the atmosphere without active inclination changes. Each contained a lander and a balloon explorer.
The landers were identical to that of the previous five Venera missions and was to study the atmosphere and surface, it had instruments to study temperature, pressure, a UV spectrometer, a water concentration meter, a gas-phase chromatograph, a X-ray spectrometer, a mass spectrometer and a surface sampling device. The Vega 1 lander's surface experiments were inadvertently activated at 20 km from the surface by an especially hard wind jolt and so failed to provide results.
The balloon aerobot was a constant-pressure 3.4 metre diameter balloon with instruments, weighing 25 kg in total. It was deployed at 54 km from the surface in the most active layer of the Venuian cloud system. The 5 kg instrument pack had sixty hour batteries and measured temperature, pressure, wind speed and aerosol density. The Vega 1 balloon managed to transmit data for only 56 minutes, but the Vega 2 balloon was much more successful, transmitting data for 46.5 hours.
The balloons were spherical superpressure types with a diameter of 3.54 meters (11.6 feet) and filled with helium. A gondola assembly weighing 6.9 kilograms (15.2 pounds) and 1.3 meters (4.26 feet) long was connected to the balloon envelope by a tether 13 meters (42.6 feet) long. Total mass of the entire assembly was 21 kilograms (46.31 pounds).
The top section of the gondola assembly was capped by a conical antenna 37 centimeters (14.6 inches) tall and 13 centimeters (5.12 inches) wide at the base. Beneath the antenna was a module containing the radio transmitter and system control electronics. The lower section of the gondola assembly carried the instrument payload and batteries.
The instruments consisted of:
The small low-power transmitter only allowed a data transmission rate of 2,048 bits per second, though the system performed data compression to squeeze more information through the narrow bandwidth. Nonetheless, the sampling rate for most of the instruments was only once every 75 seconds. The balloons were tracked by an international network of 20 radio telescopes back on Earth.
The balloons were dropped onto the planet's darkside and deployed at an altitude of about 50 kilometers (31 miles). They then floated upward a few kilometers to their equilibrium altitude. At this altitude, pressure and temperature conditions of Venus are similar to those of Earth, though the planet's winds moved at hurricane velocity and the carbon-dioxide atmosphere is laced with sulfuric acid, along with smaller concentrations of hydrochloric and hydrofluoric acid.
The balloons moved swiftly across the night side of the planet into the light side, where their batteries finally ran down and contact was lost. Tracking indicated that the motion of the balloons included a surprising vertical component, revealing vertical motions of air masses that had not been detected by earlier probe missions.