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thumb|Vega mission description

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 German Democratic Republic, Poland, Czechoslovakia, and the Federal Republic of Germany in December 1984. They had a two-part mission to investigate Venus, where each would release a lander and a balloon probe, and later conduct a flyby of 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 balloon probes that both missions released into the Venusian atmosphere were also the first man-made aircraft to fly on a planet other than Earth, albeit unpowered, and until the operation of NASA's Ingenuity helicopter probe in 2021 the only examples of man-made extraterrestrial aircraft in history.

The Vega spacecraft

thumb|A Vega probe bus on display at the [[National Air and Space Museum]]

Vega 1 and Vega 2 were identical sister ships developed from the earlier Venera craft. They were of the 5VK type, designed by Babakin Space Center and constructed by Lavochkin at Khimki. The craft were powered by twin large solar panels and instruments included an antenna dish, cameras, spectrometer, infrared sounder, magnetometers (MISCHA; Austrian and Soviet), and plasma probes. The craft were launched by a Proton 8K82K rocket from Baikonur Cosmodrome, Tyuratam, Kazakh SSR (current Kazakhstan). Both Vega 1 and Vega 2 were three-axis stabilized spacecraft. The spacecraft were equipped with a dual bumper shield for dust protection from Halley's comet.

Bus Instruments

  • imaging system
  • infrared spectrometer
  • ultraviolet, visible, infrared imaging spectrometer
  • shield penetration detector
  • dust detectors
  • dust mass spectrometer
  • neutral gas mass spectrometer
  • APV-V plasma energy analyzer
  • energetic-particle analyzer
  • magnetometer (Austrian-Soviet)
  • wave and plasma analyzers

The Venus mission

thumb|300px|Position of Vega landing sites. Red points denote sites returning images from the surface, black central dots sites of surface sample analysis. Map based on mapping from [[Pioneer Venus Orbiter, Magellan, and Venera 15/16.]]

Vega 1 arrived at Venus on 11 June 1985 and Vega 2 on 15 June 1985, and each delivered a , 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.

Descent craft

thumb|Artist's impression of the Vega probe on Venus

The landers were identical to that of the previous five Venera missions and were to study the atmosphere and surface. Each had instruments to study temperature and pressure, a UV spectrometer, a water concentration meter, a gas-phase chromatograph, an X-ray spectrometer, a mass spectrometer, and a surface sampling device.

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.

Payload

  • Meteocomplex T, P sensors
  • Sigma-3 gas chromatograph
  • LSA particle size spectrometer
  • IFP aerosol analyzer
  • VM-4 hygrometer
  • ISAV-A nephelometer/scatterometer
  • Malakhit-V mass spectrometer
  • ISAV-S UV spectrometer
  • GZU VB-02 drill + BDRP-AM25 soil X-ray fluorescence spectrometer
  • GS-15-STsV gamma ray spectrometer
  • PrOP-V penetrometer
  • MSB small solar batteries

Balloon

thumb|Vega balloon probe on display at the [[Steven F. Udvar-Hazy Center|Udvar-Hazy Center of the Smithsonian Institution. Photo by Geoffrey A. Landis.]]

The two balloon aerobots were designed to float at from the surface, in the most active layer of the Venusian cloud system. The instrument pack had enough battery power for 60&nbsp;hours of operation and measured temperature, pressure, wind speed, and aerosol density. The balloon envelopes were surfaced with polytetrafluoroethylene to resist attack by the corrosive atmosphere. Both Vega-1 and Vega-2 balloons operated for more than 46&nbsp;hours from injection to the final transmission.

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:

  • An arm carrying thin-film resistance thermometers and a velocity anemometer. The anemometer consisted of a free-spinning plastic propeller, whose spin was measured by LED-photodetector optointerrupters.
  • A module containing a PIN diode photodetector to measure light levels and a vibrating-quartz-beam pressure sensor.
  • A package at the bottom carrying the batteries and a nephelometer to measure cloud density through light reflection.

The small low-power (5 watt) At this altitude, pressure and temperature conditions of Venus are similar to those of Earth at 18,000 feet MSL (5,5&nbsp;km),