The spacecraft was three-axis stabilized. Its main features were large solar panels, a high-gain antenna dish, and an automatic pointing platform carrying those experiments that required pointing at the comet nucleus. The automatic platform could rotate through + or -110 deg and + or -40 deg in two perpendicular directions with a pointing accuracy of 5 arc-min and a stability of 1 arc-min/s. It carried the narrow- and the wide-angle camera, the three-channel spectrometer, and the infrared sounder. All other experiments were body-mounted, with the exception of two magnetometer sensors and various plasma probes and plasma wave analyzers which were mounted on a 5-m boom. The total scientific payload weighed 125 kg and had a data rate of 65 kbs in fast telemetry mode for encounter. There was also a slow telemetry mode for the cruise mode. The comet-encounter science data-take was from 2.5 h before until 0.5 h after the closest approach, with several periods of data-take before and after, each lasting about 2 h. Continuous coverage for plasma and dust instruments was provided by an onboard memory (5-megabit tape recorder). The spacecraft was shielded from hypervelocity dust impacts by a shield consisting of a 100-micrometer multilayer sheet 20 to 30 cm from the spacecraft, and a 1-mm Al sheet 5 to 10 cm from the spacecraft. Approximately half of the VEGA spacecraft was devoted to the Halley module, and half to the Venus lander package. The total scientific payload weight was 144.3 kg.
The Venus package consisted of a sphere 240 cm in diameter, which separated two days before arrival at Venus and entered the planet's atmosphere on an inclined path, without active maneuvers, as was done on previous Venera missions. The lander probe was identical to those of Venera 9 through 14 and similarly had two objectives, the study of the atmosphere and the study of the superficial crust. In addition to temperature and pressure measuring instruments, the descent probe carried a UV spectrometer for measurement of minor atmospheric constituents, an instrument dedicated to measurement of the concentration of H2O, and other instruments for determination of the chemical composition of the condensed phase: a gas-phase chromatograph; an X-ray spectrometer observing the fluorescence of grains or drops; and a mass spectrograph measuring the chemical composition of the grains or drops. The X-ray spectrometer separated the grains according to their sizes using a laser imaging device, while the mass spectrograph separated them according to their sizes using an aerodynamical inertial separator. After landing, a small surface sample near the probe was to be analyzed by gamma spectroscopy and X-ray fluorescence. The UV spectrometer, the mass spectrograph, and the pressure- and temperature-measuring instruments were developed in cooperation between French and Soviet investigators.
In addition to the lander probe, a constant-pressure instrumented balloon aerostat was deployed immediately after entry into the atmosphere at an altitude of 54 km. The 3.4 meter diameter balloon supported a total mass of 25-kg. A 5-kg payload hung suspended 12 meters below the balloon. It floated atapproximately 50 km altitude in the middle, most active layer of the Venus three-tiered cloud system. Data from the balloon instruments were transmitted directly to Earth for the 47-hr lifetime of the mission. (The batteries had a lifetime of 60 hrs.) Onboard instruments were to measure temperature, pressure, vertical wind velocity, and visibility (density of local aerosols). Very long baseline interferometry was used to track the motion of the balloon to provide the wind velocity in the clouds. The tracking was to be done by a 6-station network on Soviet territory and by a network of 12 stations distributed world-wide (organized by France and the NASA Deep Space Network). After two days and 9000 km, the balloon entered the dayside of Venus and expanded and burst due to solar heating.
