Vega program
The
Vega mission was a
Venus mission which also took advantage of the appearance of
Comet Halley in
1986. Vega 1 and 2 were unmanned
spacecraft launched by the
Soviet Union in December
1984. They had a two part mission to investigate
Venus and also flyby
Comet Halley.
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" (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.
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 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 five
Venera missions and were to study the atmosphere and surface, each had instruments to study temperature, 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 20 km from the surface by an especially hard wind jolt and so failed to provide results. The Vega 2 lander transmitted data from the surface for 56 minutes.
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 Venusian cloud system. The 5 kg instrument pack had sixty hour batteries 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 Vega aerobots
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 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.
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.
Vega 1 arrived first, returning images starting on
March 4,
1986, and these images were used to help pinpoint
Giotto's upcoming close flyby of the comet. The early images from Vega that showed two bright areas on the comet, which were initially interpreted as a double nucleus. The bright areas would later turn out to be two jets emitting from the comet. The images also showed the nucleus to be dark, and the infrared spectrometer readings measured a nucleus temperature of 300 K to 400 K, much warmer than expected for an ice body. The conclusion was that the comet had a thin layer on its surface covering an icy body. The Vega images also showed the nucleus to be about 14 km long with a rotation period of about 53 hours. The dust mass spectrometer detected material similar to the composition of
carbonaceous chondrites meteorites and also detected
clathrate ice. Vega 1 made its closest approach to the comet on
March 6 at a distance of 8,890 km.
Vega 2 was just three days behind its twin for its Comet Halley encounter. Vega 2 flew in closer to the comet nucleus at a distance of 8,030 km on
March 9,
1986. It returned similar data, but returned images with better clarity due to its closer approach.
In total Vega 1 and Vega 2 returned about 1500 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 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 stablized spacecraft. The spacecraft were equipped with a dual bumper shield for dust protection from the comet.
Vega 1 and 2 are currently in
heliocentric orbits.
| Vega 1 & 2 | | Spacecraft | Vega 1 | Vega 2 |
| Country | Soviet Union | Soviet Union |
| Mission | Venus Lander & Balloon,
Comet Halley Flyby | Venus Lander & Balloon,
Comet Halley Flyby |
| Launch Date | December 15, 1984 | December 21, 1984 |
| Launch Vehicle | Proton | Proton |
| Spacecraft Mass | 4920 kg | 4920 kg |
| Key Dates | June 11, 1985 - Venus Encounter
March 6, 1986 - Comet Halley Flyby | June 15, 1985 - Venus Encounter
March 9, 1986 - Comet Halley Flyby |
| End of Mission | 1986 | 1986 |
| Comments | First Venus Balloon
First Comet Halley Flyby | Second Venus Balloon
Second Comet Halley Flyby |
*
Vega mission images from the Space Research Institute (IKI)*
Raw data from Vega 1 and Vega 2 on board instruments*
Soviet Exploration of Venus
