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 (from Halley, as the Russian language does not have the letter "H"). The spacecraft design was based on the previous Venera 9/10 missions. The two spacecraft were launched on December 15 and December 21, 1984, respectively.

Table of contents 1 The Venus mission 1.1 The Vega aerobots 2 The Halley mission 3 The Vega spacecraft

The Venus mission

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 relased 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 was to study the atmosphere and surface, it had instruments to study temperature, pressure, a UV spectrometer, a water concentration meter, a gas-phase chromatograph, a 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 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 Venuian cloud system. The 5 kg instrument pack had sixty hour batteries and measured temperature, pressure, wind speed and aerosol density. The Vega 1 balloon managed to transmit data for only 56 minutes, but the Vega 2 balloon was much more successful, transmitting data for 46.5 hours.

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 (46.31 pounds).

The top section of the gondola assembly was capped by a conical antenna 37 centimeters (14.6 inches) tall and 13 centimeters (5.12 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 an "anemometer", or wind speed indicator. 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 an international network of 20 radio telescopes back on Earth.

The balloons were dropped onto the planet's darkside and deployed at an altitude of about 50 kilometers (31 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.

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 300K to 400K, 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.

Spacecraft operations were discontinued a few weeks after the Halley encounters.

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 4920 kg craft was launched by a Proton 8K82K rocket from Baikonur Cosmodrome, Tyuratam, Kazakhstan. 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	Jun 11, 1985 - Venus Encounter Mar 6, 1986 - Comet Halley Flyby	Jun 15, 1985 - Venus Encounter Mar 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