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 of a 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
 
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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 degrees
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'
 
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