PDS_VERSION_ID = PDS3
RECORD_TYPE = STREAM
LABEL_REVISION_NOTE = "NULL"
OBJECT = INSTRUMENT_HOST
INSTRUMENT_HOST_ID = "VEGA1"
OBJECT = INSTRUMENT_HOST_INFORMATION
INSTRUMENT_HOST_NAME = "VEGA 1"
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 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
---------------------------------------------------------------------
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'
---------------------------------------------------------------------"
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
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