Vega program

.

The flyby of Halley's Comet 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" (Russian words for "Venus" and "Halley", respectively). The spacecraft design was based on the previous Venera 9 and Venera 10 missions.

The two spacecraft were launched on December 15 and December 21, 1984, respectively. With their redesignated dual missions, the Vega probes became part of the Halley Armada, a group of Earth probes that studied Halley's Comet during its 1985/86 perihelion.

The Vega spacecraft

Vega 1 and 2 were identical sister ships. The spacecraft was a development of the earlier "Venera" craft. They were designed by Babakin Space Center and constructed as 5VK by Lavochkin at Khimki. The craft was powered by twin large solar panels and instruments included an antenna dish, cameras, spectrometer, infrared sounder, magnetometers (MISCHA), and plasma probes. The 4,920 kg craft was launched by a Proton 8K82K rocket from Baikonur Cosmodrome, Tyuratam, Kazakh SSR. Both Vega 1 and 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
# wave and plasma analyzers

The Venus mission

Vega 1 arrived at Venus on June 11, 1985 and Vega 2 on June 15, 1985, and each delivered a 1,500 kg, 240 cm 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

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. 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 0.1 km above the planetary mean radius. The measured pressure at the landing site was 91 atm and the temperature was 736 K. The surface sample was found to be an anorthosite-troctolite. It transmitted data from the surface for 56 minutes.

Payload

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

Balloon

The balloon aerobot was a constant-pressure balloon 3.4 m in diameter with instruments, weighing 25 kg in total. It was deployed at 54 km from the surface in the most active layer of the Venusian cloud system. The 5 kg instrument pack had enough battery power for sixty hours of operation and measured temperature, pressure, wind speed and aerosol density. Both Vega-1 and Vega-2 balloons operated for more than 46 hrs from injection to the final transmission [Science, 1986, v.231, p.1416] .

The balloons were spherical superpressure types with a diameter of 3.54 meters (11.6 ft) and filled with helium. A gondola assembly weighing 6.9 kilograms (15.2 pounds) and 1.3 meters (4.26 ft) long was connected to the balloon envelope by a tether 13 meters (42.6 ft) long. Total mass of the entire assembly was 21 kilograms (46 pounds).

The top section of the gondola assembly was capped by a conical antenna 37 centimeters (14.6 inches) tall and 13 centimeters (5 ¹⁄₈ 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:

* 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 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 two networks of 20 radio telescopes in total back on Earth: the Soviet network, coordinated by the USSR Academy of Sciences and the international network, coordinated by CNES.

The balloons were dropped onto the planet's darkside and deployed at an altitude of about 50 kilometers (30 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.

The Halley mission

After their encounters, the Vegas' motherships were redirected by Venus' gravity to intercept Comet Halley.

Vega 1 made its closest approach on March 6, around 8,890 km from the nucleus, and Vega 2 made its closest approach on March 9 at 8,030 km. 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. TV data were processed by international team, including the USSR, Hungary, France, GDR and USA scientists. The basic steps of data acquisition and preprocessing were performed in IKI using the image processing computer system, based on PDP11/40 compatible host.

Vega 1 and 2 are currently in heliocentric orbits.

ee also

*Venera program
*Pioneer Venus
*Vega 1
*Vega 2

External links

* [http://arc.iki.rssi.ru/IPL/vega.html Vega mission images from the Space Research Institute (IKI)]
* [http://starbase.jpl.nasa.gov/archive/vega2-c-tvs-5-rdr-halley-transform-v1.0/hal_0026/ Raw data from Vega 1 and Vega 2 on board instruments]
* [http://www.mentallandscape.com/V_Vega.htm Soviet Exploration of Venus]
* [http://articles.adsabs.harvard.edu/full/1986SvAL...12....7K The Vega balloons - A tool for studying atmosphere dynamics on Venus]

References