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TheVega program (Russian:Вега) was a series ofVenus missions that also took advantage of the appearance of comet1P/Halley in 1986.Vega 1 andVega 2 were uncrewedspacecraft launched in a cooperative effort among theSoviet Union (who also provided the spacecraft and launch vehicle) andAustria,Bulgaria,France,Hungary, theGerman Democratic Republic,Poland,Czechoslovakia, and theFederal Republic of Germany in December 1984. They had a two-part mission to investigateVenus and also flybyHalley's Comet.
The flyby of Halley's Comet had been a late mission change in theVenera program, following on from the cancellation of the American Halley mission in 1981. A later Venera mission was canceled and the Venus part of theVega 1 mission was reduced. Because of this, the craft was designated VeGa, a contraction ofVenera andGallei (Венера and Галлей respectively, the Russian words for "Venus" and "Halley"). The spacecraft design was based on the previousVenera 9 andVenera 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 theHalley Armada, a group of space probes that studied Halley's Comet during its 1985/1986perihelion.
Vega 1 andVega 2 were identical sister ships developed from the earlierVenera craft. They were of the5VK type, designed byBabakin Space Center and constructed byLavochkin atKhimki. The craft were powered by twin large solar panels and instruments included an antenna dish, cameras,spectrometer, infrared sounder,magnetometers (MISCHA; Austrian and Soviet),[1] and plasma probes. The 4,920 kg (10,850 lb) craft were launched by aProton 8K82K rocket fromBaikonur Cosmodrome,Tyuratam,Kazakh SSR (currentKazakhstan). BothVega 1 andVega 2 were three-axis stabilized spacecraft. The spacecraft were equipped with a dual bumper shield for dust protection from Halley's comet.
Vega 1 arrived at Venus on 11 June 1985 andVega 2 on 15 June 1985, and each delivered a 1,500 kg (3,300 lb), 240 cm (94 in) diameter spherical descent unit. The units were released 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 fiveVenera missions and were to study the atmosphere and surface. Each had instruments to study temperature and pressure, aUV spectrometer, a water concentration meter, agas-phase chromatograph, anX-rayspectrometer, amass spectrometer, and a surface sampling device.
TheVega 1 lander's surface experiments were inadvertently activated at 20 km (12 mi) from the surface by an especially hard wind jolt, and so failed to provide results. It landed at 7.5°N, 177.7°E.
TheVega 2 lander touched down at 03:00:50 UT on 15 June 1985 at 8.5° S, 164.5° E, in easternAphrodite Terra. The altitude of the touchdown site was 0.1 km (0.062 mi) above the planetary mean radius. The measured pressure at the landing site was 91 atm and the temperature was 736 K (463 °C; 865 °F). The surface sample was found to be ananorthosite-troctolite. The lander transmitted data from the surface for 56 minutes.[citation needed]
The two balloonaerobots were designed to float at 54 km (34 mi) from the surface, in the most active layer of the Venusian cloud system. The instrument pack had enough battery power for 60 hours of operation and measured temperature, pressure, wind speed, and aerosol density. The balloon envelopes were surfaced withpolytetrafluoroethylene to resist attack by the corrosive atmosphere. Both Vega-1 and Vega-2 balloons operated for more than 46 hours from injection to the final transmission.[2]
The balloons were of spherical superpressure types of 3.54 m (11.6 ft) diameter, filled withhelium. A gondola assembly weighing 6.9 kg (15 lb) and 1.3 m (4 ft 3 in) long was connected to the balloon envelope by a tether 13 m (43 ft) long. Total mass of the entire assembly was 21 kg (46 lb).
The top section of the gondola assembly was capped by a conical antenna 37 cm (15 in) tall and 13 cm (5.1 in) 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)[3] 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 theSoviet Academy of Sciences, and the international network, coordinated byCentre national d'études spatiales of France (CNES).
The balloons were pulled out of the lander at 180,000 feet (55 km) above the planet's darkside surface, and fell to 164,000 feet (50 km) ASL while they were being inflated. After that, they rose to 177,000 feet (54 km) and stabilized.[3] At this altitude, pressure and temperature conditions of Venus are similar to those ofEarth at 18,000 feet MSL (5,5 km),[3] though the planet's winds move at hurricane velocity, and thecarbon dioxide atmosphere is laced withsulfuric acid, along with smaller concentrations ofhydrochloric andhydrofluoric 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.
After their encounters, the Vega motherships were redirected to interceptComet Halley, utilizing the gravity of Venus to alter their trajectory.
Vega 1 made its closest approach on 6 March, around 8,890 km (5,520 mi) from the nucleus, andVega 2 made its closest approach on 9 March at 8,030 km (4,990 mi). 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 thecoma, 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 totalVega 1 andVega 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 theUSSR,Hungary, France, andCzechoslovakia. The TV data was processed by an international team, including scientists from the Soviet Union, Hungary, France,East Germany, and theUnited States. The basic steps of data acquisition and preprocessing were performed inIKI using an image processing computer system based on aPDP-11/40 compatible host.
Vega 1 andVega 2 are currently inheliocentric orbits.
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