Vega was the last successful deep space mission of the Soviet space program. It was built on the legacy of the Venera-9 to Venera-16 series of spacecraft launched, starting in 1975, to study Venus. In its last encounter (1986), Halley’s comet’s perihelion was much closer to Venus than to Earth. That created a unique opportunity to launch Vega during the astronomical window for Venus (December 1984) and to use that planet’s gravity assist for redirecting the craft to the comet. Using such a choice of spacecraft trajectory, it was decided to combine a scenario to explore Venus with a Halley’s comet encounter by employing a two-element space vehicle: a planetary reentry module, carrying the Venus Lander and balloon, and a Halley flyby probe. The integrated mission was called Vega, a contraction of the Russian words “Venera” (Venus) and “Gallei” (Halley) and was conducted by the Soviet Union and a number of other countries within the framework of an Intercosmos program.
The Vega mission comprised two identical spacecraft, Vega 1 and Vega 2. This was a standard approach in the Soviet Union, aimed primarily at increasing the overall reliability of the mission. In addition, if both flybys were successful, there would be a significant increase in the scientific return, which was particularly valuable considering the expected variability of the comet’s activity.
The Vega project was truly international. The spacecraft production and launch operation were controlled by the Soviet aerospace agency (at that time called the Ministry of General Machine Building), but the scientific program and its payload were coordinated by the International Science and Technical Committee (ISTC), representing scientific institutions and space agencies from nine countries. The ISTC designed the Vega mission program to optimize international efforts related to the Halley’s comet campaign, which included the European Giotto and the Japanese Suisei and Sakigake space missions.
The two spacecraft were launched by Proton rockets from the Baikonur launching site on 15 and 21 December 1984, respectively. On 11 and 15 June 1985 at the Venus flybys, the planetary packages were ejected and reentered the atmosphere of the planet (the scientific results are summarized in the Venus article elsewhere in this topic). The delivery and deployment of balloons into the Venusian atmosphere was the first operation of this kind outside of the terrestrial atmosphere (and it stands alone to this day).
After Venus gravity assist maneuvers, the Vega 1 and Vega 2 trajectories were changed to encounter comet Halley on 6 and 9 March 1986, respectively.
Each Vega spacecraft, comprised of a Halley flyby probe and a Venus descent module, weighed about 4.51 at launch. The spacecraft configuration at the beginning of its interplanetary journey is shown in the center of Fig. 1. The Venus descent module, ejected at the planet’s flyby, had a package carrying the balloon, a minigondola (that had a radio transmitter, a basic atmospheric science package, and a lithium battery power source), and a pressurized helium tank which was mounted on the upper part of the descent module. The inflated balloon is presented on the right side of Fig. 1. The spacecraft was triaxially stabilized and had a span of solar panels of about 10 m. It carried 14 experiments, subdivided into three topical groups:
Vega configuration. This figure is available in full color at http://www.mrw.
Figure 1. Vega configuration.
* a space physics package for studying the solar wind versus cometary plasma interaction;
* a set of dust particle detectors for in situ measurements at the encounter with the comet;
* an imaging and spectral package of instruments, which included a TV system (TVS) for tracking and imaging the inner coma and the comet nucleus.
Taking the comet’s coma images during the fast flyby (the relative velocity was close to 80 km/s) required mounting a steerable platform on the spacecraft body. This platform, shown in the lower part of Fig. 1, was mounted on the lower left side of the spacecraft, had a mass of 82 kg, and carried a 64-kg payload.
The Vega spacecraft design used as its baseline the Venera series of spacecraft. However, a number of modifications had to be introduced to improve the survivability of the probe in the harsh environment of the comet’s coma. A special shield with a net surface area of about 5 m2 was added to protect the most essential subsystems against the bombardment of dust particles. The design of such a shield underwent a series of tests, simulating the impact of projectiles at a velocity of 80 km/s by using a high-power pulsed laser. The results have confirmed computer simulation predictions for super high velocity impacts. Finally, a double-sheet bumper was adopted, composed of a thin metallic front sheet (0.4 mm) and a thicker rear sheet, separated by several centimeters.
The solar array had a total area of 10 m2. Triaxial stabilization was achieved by a gyroscopic system in combination with a number of gas nozzles. The telemetry system consisted of a high-data-rate channel of 65,536 bps (BRL) and a low-data-rate channel of 3072 bps (BTM). The BRL channel was used only for real-time data transmission, including the data from the imaging instruments and the science payload. The housekeeping data were received via the BTM. The data could also be stored on board on a tape recorder and subsequently delivered by the BTM channel. During high-data-rate transmission periods, the spacecraft was oriented so that the high-gain antenna pointed toward Earth.

The imaging experiment on board the Vega spacecraft had the following scientific objectives:

* to detect and take images of the comet’s nucleus (this was never done before, and Vega had to become the first to do it a few days before Giotto) and to determine its shape, size, and the albedo of surface material of the nucleus;
* to assess the composition and surface morphology of the nucleus;
* to identify the spatial pattern and temporal variation of its activity; and
* to study the structure and dynamics of the near-nucleus coma, including the jets of cometary material escaping from the nucleus.
The design of the imaging camera system for the Vega mission was based on using a Ritchey-Chretien telescope that had a focal length of 1200 mm, an effective aperture of f 6.5 and a field of view of 26.4 x 39.6 minutes of arc. It was immediately obvious that the field of view of this narrow angle camera (NAC) was too small to locate the nucleus autonomously. It was also clear that ground control could not provide the pointing commands for the telescope in real time. That task was assigned to the wide angle scanning camera (WAC), that served as an addition to the imaging payload.
The type of the imaging sensors also required careful consideration. Charge-coupled devices (CCDs) were just becoming available in the late 1970s for spaceflight application and only limited information could be obtained on their reliability in space. The Vega and Giotto missions were among the first space science missions that used this new technology. We were able to use CCDs for Vega that were manufactured in the Soviet Union.
The selection of the filters was also discussed extensively. Ideally, we would have liked to use narrowband filters for spectroscopy. However, the sensitivity of the CCDs available for the project was not high enough, and we had to select wideband filters. Due to the lack of advance knowledge of brightness conditions around the nucleus, the exposure time of the system was set up to provide under-and overexposed images in sequence. That assumed the image processing procedure described subsequently.
A major concern was identification of the comet nucleus, the main target for pointing the axis of the imaging instruments. The traditional “quadrant” type sensors, widely used in pointing technology, would integrate the light over an extended area around the target to find the center of brightness. Though quite appropriate for a point object, these sensors could be misled by the jets escaping from the comet’s nucleus. Being diffuse-type objects, they are not necessarily brighter than a nucleus, but considering their large extensions, the net signal could prevail. In sum, the center of brightness does not necessarily coincide with the position of the nucleus. The pointing system might well have locked on to a bright dust jet and been steered away from the nucleus. As there was no way to define an offset in advance and with sufficient reliability, the decision was to employ a special microprocessor, capable of performing a quick cluster analysis of images taken by the WAC in real flight to identify the brightest spot, attributed to the nucleus per se. In the end, this targeting strategy worked well for Vega.
The pointing platform was based on a precise servomechanism with two degrees of freedom. Final precision of pointing, achieved with such a platform even at its maximal angular velocity, ensured that smearing of images due to the motion of the platform and residual wobbling of the spacecraft body attitude was always less than 1 pixel. The pointing platform used on the spacecraft was designed and manufactured as a contribution from Czechoslovakia. Due to the absolutely critical importance of steering the platform for the success of the encounter with the comet, a major Soviet aerospace enterprise was assigned to build a backup version. Extensive tests of the Czech designed platform were sufficiently persuasive ultimately for deciding to use it on both spacecraft.
The default navigation system for platform pointing was the imaging system (WAC) itself, but Vega had a complementary means for backup. The second choice was an eight-segmented, light-sensitive sensor mounted on the pointing platform. Its functional principle was extremely simple: if the center of brightness of the comet moved away from the center of the sensor, a change in platform orientation would be initiated to compensate for this offset. The inner envelope of four sensors had to pick up at a closer flyby to reduce the offset. Fairly sophisticated computer simulations proved that this backup was reliable. This algorithm performed well in an emergency when Vega 2 was close to the comet and the microprocessor in charge of pointing control was temporarily knocked out presumably by cosmic radiation.
Planning and manufacturing the telescope was also a major task. It was decided that this effort should be duplicated, so consequently, French and Soviet versions were prepared. Both worked satisfactorily, the French telescope was selected for Vega 1, and the Soviet telescope flew on board Vega 2.
To have critical backups was a major element in our design approach. A third navigational backup, which used analog signals from a completely independent CCD sensor, was added to the TV system. The output signal was processed similarly to the eight- segmented, light-sensitive sensor, but at much higher precision.
There was one major setback during camera development. We planned to use 8-bit analog-to-digital converters (ADCs) in the camera electronics. Suitable 8-bit ADCs were not available in Eastern Europe, and we had intended to buy them in the West. However, as we discovered only during the manufacturing phase, we were unable to buy space qualified 8-bit ADCs in the West due to technology transfer regulations. It was then decided to use the simpler Soviet design, which in the end could not provide equivalent contrast resolution. Unfortunately, this affected the scientific performance of the camera by damaging its contrast resolution.
After the encounters, both Vega spacecraft still remained largely functional despite exposure to intense bombardment by dust particles flying at a speed of 80km/s. However, this did inflict some damage on the payload. The spacecraft attitude during the encounters was determined by the requirements to point the high-gain antenna at Earth and, at the same time, to obtain maximum power from the solar panels. Consequently, the solar panels could not be aligned with the relative-velocity vector, and damage to the panels was inevitable. After the encounters, the power from the solar panels was reduced by about 50%.
The deep space communication center was located in Evpatoria (Crimea, now part of Ukraine), and most of the science investigators worked on-line from Moscow at the Space Research Institute, where they were able to obtain all ofthe data in real time. Deep-space antennas in Evpatoria (70 m) and Medvezy Ozera (64 m), near Moscow, received the telemetry data.
During the cruise phase, the pointing platform was clamped down. The clamping mechanism on Vega 1 was released on 14 February 1986 and that on Vega 2 on 18 February 1986, and the operation of the TVS and the pointing platform and imaging system were tested and calibrated by observing Jupiter and Saturn. Two days before the encounters, the cameras were oriented toward Halley’s comet and switched on for 2 hours. At last, a few minutes before 7:20 UT on 6 March 1986, the nucleus of comet Halley was unveiled to the human eye for the first time in the history of humankind.
The second encounter took place almost exactly 3 days later. Both encounters were watched live by large audiences worldwide. Such openness was unprecedented for the Soviet space program.
The Vega mission operation was discontinued a few weeks after the encounters. The general condition of the spacecraft would have allowed further operation. The solar panels were partially damaged by dust impacts, but could still have provided enough power. The camera performance was tested by observing Jupiter, and no essential degradation was registered. Even the amount of propellant on board was considerable. However, after extensive searches and debates, no interesting object was identified for a possible second encounter. Image processing commenced immediately after the flybys and was carried out in parallel by four cooperating teams located in Moscow, Budapest, Paris, and Berlin.
The Vega mission to Halley’s comet was scientifically highly successful, and it also helped to forge international cooperation in space science at an unprecedented level, both within the Vega Project and in terms of worldwide cooperation with the other major space organizations. Together with the Giotto project, it led to the formation of the Inter-Agency Consultative Group for Space Science (IACG), which coordinated all major Halley’s comet efforts by several Space Agencies: Intercosmos, NASA, ESA and ISAS (Japan) from 1981 to 1986. Following that initiative, the IACG continued international coordination for some other projects, even after 1986.
The best example of the usefulness of this cooperation was the ”Pathfinder Concept.” The idea was to target ESA’s Giotto spacecraft as precisely as possible using of the Halley nucleus observations from the two earlier arriving Vega spacecraft. Before the encounters, the Halley nucleus position was known only with an accuracy of about a few hundred kilometers. That was all that could be achieved by combining ground-based astronomical observations over a long period with a model, which included nongravitational forces on the nucleus. This was sufficient for the Vega spacecraft, flying by some 10,000 km away, but not for Giotto. For Giotto, which was supposed to pass the nucleus at a distance of only 500 km on the sunward side, significantly better targeting uncertainty was highly desirable. Fortunately, Giotto had to be the last of the spacecraft to encounter the comet, and the information obtained by the cameras on board the earlier arriving Vega 1 and Vega 2 spacecraft could be passed on from the Vega to the Giotto Project. This was to become known as the ”Pathfinder Concept.”
Based on Pathfinder data, Giotto had to make its last fine-tuned orbit correction at least 2 days before the encounter on 11 March. Vega 2 encountered Halley on 9 March, so there was very little time between the evaluation of the Vega Pathfinder data, which included positional determination of both Vega spacecraft; cross-checking by three different groups at Moscow, Pasadena, and Darmstadt; and executing Giotto’s maneuver. Both the Giotto and the Vega spacecraft uncertainties contributed to the error. Using conventional (6 GHz) ranging and Doppler techniques, the technical means available to Soviets expected a geocentric Vega positional accuracy of several hundred kilometers. Using precise L-band very long baseline interferometry (VLBI) techniques and NASA’s widely separated tracking stations of the Deep Space Network (DSN), the positional uncertainties of both spacecraft could be reduced to about 40 km. After processing all of the data, Giotto was finally aimed at 540 km on the sunward side and actually achieved a flyby distance of 596 km. As a result, the Giotto camera was able to get a truly high resolution image of the Halley’s comet nucleus.
The best of Vega images of the Halley’s comet nucleus was taken by the NAC camera on board the second craft at the closest encounter on 9 March 1986 (at a distance of 8000 km) from the nucleus (see Fig. 2). Combined results of the imaging/spectral instruments revealed the shape and size of the nucleus, its anomalously low albedo even for a ”dirty snowball” model (only 3-4%), the configuration of the jets, and the basic chemical components of escaping gas. The irregular shape of the nucleus could be best approximated by an ellipsoid with dimensions of 15 km x 8 km x 7 km.
 Halley main image.
Figure 2. Halley main image.
In situ data of the space physics package included information on the intrinsic structure of the bow shock front, the product of specific ”collisionless” interaction of the solar wind with plasma of cometary origin that generates hydromagnetic waves and accelerates the ions.
The various dust counters registered impacts of particles in the range from submicron to hundreds of microns. The ingeniuous design of the time-of-flight type mass spectrometer (shared with the Giotto spacecraft) helped to reveal several chemically different families of dust particle populations escaping from the nucleus.
The in-flight data from the various experiments on board the flyby spacecraft were complemented by a large number of remote observations both from space and from the ground; the latter were coordinated by the International Halley Watch (IHW). The IACG and its counterpart on the ground, the IHW, formed the cornerstones of a global effort to explore Halley’s comet as completely as possible during its 1985/86 encounter.

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