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The Vega program () was a series of Venus missions that also took advantage of the appearance of comet 1P/Halley in 1986. Vega 1 and Vega 2 were uncrewed spacecraft launched in a cooperative effort among the Soviet Union (who also provided the spacecraft and launch vehicle) and Austria, Bulgaria, France, Hungary, the East Germany, Poland, Czechoslovakia, and the West Germany in December 1984. They had a two-part mission to investigate Venus and also flyby Halley's Comet.
The flyby of Halley's Comet had been a late mission change in the Venera program, following on from the cancellation of the American Halley mission in 1981. A later Venera mission was canceled 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 (Венера and Галлей respectively, the Russian words for "Venus" and "Halley"). The spacecraft design was based on the previous Venera 9 and Venera 10 missions.
The two spacecraft were launched on 15 and 21 December 1984, respectively. With their redesignated dual missions, the Vega probes became part of the Halley Armada, a group of space probes that studied Halley's Comet during its 1985/1986 perihelion.
The Vega 1 lander's surface experiments were inadvertently activated at from the surface by an especially hard wind jolt, and so failed to provide results. It landed at 7.5°N, 177.7°E.
The Vega 2 lander touched down at 03:00:50 UT on 15 June 1985 at 8.5° S, 164.5° E, in eastern Aphrodite Terra. The altitude of the touchdown site was above the planetary mean radius. The measured pressure at the landing site was 91 atm and the temperature was . The surface sample was found to be an anorthosite-troctolite. The lander transmitted data from the surface for 56 minutes.
The balloons were of spherical superpressure types of diameter, filled with helium. A gondola assembly weighing and long was connected to the balloon envelope by a tether long. Total mass of the entire assembly was .
The top section of the gondola assembly was capped by a conical antenna tall and 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 (5 watt) transmitter only allowed a data transmission rate of 2,048 bit/s, 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 two networks (20 radio telescopes total) back on Earth: the Soviet network, coordinated by the Soviet Academy of Sciences, and the international network, coordinated by Centre national d'études spatiales of France (CNES).
The balloons were pulled out of the lander at above the planet's darkside surface, and fell to ASL while they were being inflated. After that, they rose to and stabilized. At this altitude, pressure and temperature conditions of Venus are similar to those of Earth at 18,000 feet MSL (5,5 km), though the planet's winds move at hurricane velocity, and the carbon dioxide atmosphere is laced with sulphuric 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 died, 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.
Vega 1 made its closest approach on 6 March, around from the nucleus, and Vega 2 made its closest approach on 9 March at . The data intensive examination of the comet covered only the three hours around closest approach. They were intended to measure the physical parameters of the nucleus, such as dimensions, shape, temperature, and surface properties, as well as to study the structure and dynamics of the coma, the gas composition close to the nucleus, the dust particles' composition and mass distribution as functions of distance to the nucleus, and the cometary-solar wind interaction.
In total Vega 1 and Vega 2 returned about 1,500 images of Comet Halley. Spacecraft operations were discontinued a few weeks after the Halley encounters.
The on-board TV system was created in international cooperation of the scientific and industrial facilities from the USSR, Hungary, France, and Czechoslovakia. The TV data was processed by an international team, including scientists from the Soviet Union, Hungary, France, East Germany, and the United States. The basic steps of data acquisition and preprocessing were performed in IKI using an image processing computer system based on a PDP-11/40 compatible host.
Vega 1 and Vega 2 are currently in heliocentric orbits.
The Halley mission
After their encounters, the Vega motherships were redirected to intercept Comet Halley, utilizing the gravity of Venus to alter their trajectory.
See also
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