PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM LABEL_REVISION_NOTE = "NULL" OBJECT = INSTRUMENT_HOST INSTRUMENT_HOST_ID = "VEGA2" OBJECT = INSTRUMENT_HOST_INFORMATION INSTRUMENT_HOST_NAME = "VEGA 2" INSTRUMENT_HOST_TYPE = "SPACECRAFT" INSTRUMENT_HOST_DESC = " Instrument Host Overview ======================== Two spacecraft, Vega-1 and Vega-2, have been launched aboard Proton rockets from Baykonour (Kazharstan), on 15 and 21 December 1984, respectively. The two spacecraft are identical and the `redundancy' is aimed at increasing the overall reliability of the scientific mission. In June 1985, the landers were separated from the Halley probes in the vicinity of Venus. They were injected into the planet's atmosphere to perform measurements until after surface impact. During their descent, the landers each released a balloon, to drift in the planet's atmosphere. These balloons were tracked with the help of an international Very-Long-Baseline Interferometry (VLBI) network. The Halley probes are three-axis-stabilised and their orientation will be defined with an accuracy of 1 deg during Halley flyby. The optical instruments are mounted on a pointing platform which can track the nucleus with an angular accuracy of the order of 5 arcmin. The other instruments are mounted on the main structure of the probes. The data are transmitted via two independent telemetry links, with capacities of 3072 bit/s and 65 536 bit/s, respectively. Each Halley probe carries a complement of 14 experiments. The Vega spacecraft was composed of a Halley flyby probe and a Venus descent module; the whole system weighed about 4.5 t. The Halley probe is shown in Figure 15 in its nominal flyby configuration: the orientation of the vehicle velocity relative to the comet is also indicated. The spacecraft has a wingspan of the order of 10 m, and it carries 120 kg of scientific instrumentation. On its trajectory to Venus, the probe was still surmounted by the descent module, which was a spherical object with a diameter of 2.5 m and a mass of approximately 2 t. The Vega vehicle is derived from the Venera series of spacecraft. A number of modifications improve the reliability of the probe; for example, 5 m**2 of shield have been added in order to protect the most essential subsystems against the bombardment of dust particles with masses of less than 0.1 g. A dual-sheet bumper shield has been adopted; it is composed ofa thin metallic front sheet (0.4 mm) and a thicker rear sheet, separated by several centimetres. The spacecraft structure resembles a cylindrical body connected to two conical skirts. The lower skirt houses a motor for orbital manoeuvres and a toroidal pressurised utility instrument bay; the cylindrical compartment contains the fuel tanks and the upper skirt is the interface that held the Venus lander. Two pairs of deployable solar panels are mounted on each side of the cylindrical section: the solar array has a total area of nearly 10 m**2. The spacecraft is three-axis-stabilised during the cometary flyby by a gyroscopic system and a number of gas nozzles, most of which are mounted on the solar panels. The telemetry system consists of a high-data-rate channel (BRL) and a low-data-rate channel (BTM). The BRL channel is used for real-time transmission only. Its capacity of 65536 bit/s can be reduced by half if required by propagation conditions; that of the BTM channel is 3072 bit/s. The scientific data can also be stored by onboard magnetic tape recorders (capacity 5 Mbit) and subsequently telemetered through the BTM channel, once every 20 days during the interplanetary transit and once every 20 min around the time of cometary flyby. The high-gain antenna must be directed towards the Earth whenever data are transmitted via the BRL channel. The scientific instruments can be classified into three categories, characterised by common objectives: (i) The electromagnetic field sensors (MISCHA, APV-N and APV-V) are mounted on booms, as far as possible from the spacecraft to achieve the best degree of electromagnetic cleanliness. (ii) The dust, gas and plasma detectors have pointing directions generally related to the spacecraft velocity relative to the comet. (iii) The optical systems that observe the nucleus (TVS, TKS, TKS) are located on the automatic pointing platform. The pointing platform was a precise servomechanism with two degrees of freedom. Its main performance characteristics are summarised in the Table below. As the maximum angular velocities of the platform were always less than 1 degree/s the smearing due to the motion of the platform and the instability of the spacecraft attitude was always less than 1 pixel. The pointing platform used was a Czechoslovakian design. As a backup, a similar platform was also designed by a Soviet company. In principle, we could have used the Czechoslovakian platform on one spacecraft and the Soviet platform on the other, but it turned out that it would have been extremely complicated to use different platforms on Vega 1 and Vega 2 and, therefore, it was decided to use the Czechoslovakian platform on both spacecraft. Main pointing platform performance characteristics ------------------------------------------------------------------ mass of platform 82 kg mass of platform payload 80 kg power consumption 40 W turn angle in the ecliptic plane (beta) -147 degrees to +126 deg turn angle in the plane perpendicular to -60 degrees to +20 degrees the ecliptic (alpha) maximal angular velocities over alpha +/- 1 degree/s and beta accuracy of tracking the comet in terms +/- 3'/s of angular velocity tracking accuracy +/- 8' ------------------------------------------------------------------" END_OBJECT = INSTRUMENT_HOST_INFORMATION OBJECT = INSTRUMENT_HOST_REFERENCE_INFO REFERENCE_KEY_ID = "ESA-SP1066" END_OBJECT = INSTRUMENT_HOST_REFERENCE_INFO END_OBJECT = INSTRUMENT_HOST END