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Instrument Host Overview
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The Giotto spacecraft design was derived from the Geos concept.
In 1978, ESA was invited by NASA to plan a joint mission
consisting of a comet Halley fly-by in November 1985 and a
rendezvous with comet Tempel 2 in 1988. The mission comprised an
American main spacecraft which would carry a European probe. The
main spacecraft, with its array of sophisticated cameras and
experiments, would complete a fly-by of comet Halley at a safe
distance. Shortly before fly-by, the probe would be released
towards the nucleus to make detailed in-situ observations in the
innermost coma. In January 1980, however, it became clear that
financial support for the Halley Fly-by/Tempel 2 Rendezvous
mission could not be secured in the USA. By that time the
interest of European scientists had built up such momentum that
ESA considered the possibility of a purely European mission. The
support for a fly-by mission was strong in Europe and went far
beyond the small section of scientists specialised in cometary
research. A fly-by of comet Halley was suggested to ESA by the
scientific community in February 1980. Rather than having the
American spacecraft deliver the probe to the comet as in the
earlier concept, the Europeans proposed that the capabilities of
the small probe be increased by building an independent, self-
sufficient spacecraft to be launched using the European Ariane
rocket. The limited time available for development and the small
financial resources made it advisable to use a spin-stabilised
spacecraft derived from the European Earth orbiting spacecraft
Geos. This proposal was studied by ESA in the first half of 1980.
Details of the selected experiments are shown in the Table below.
The Giotto spacecraft carried 10 instruments to carry out the
scientific objectives. In addition, it was intended to extract
from the radio signal information on the columnar electron content
of comet Halley's ionosphere and the mass fluence of the cometary
atmosphere. The mass and power allocations to the Halley
Multicolour Camera (HMC) were similar to those of several other
experiments. Half of the total data transmission rate of 40
kbit/s was allocated to HMC.
The spin of the spacecraft had some advantages. The stability of
the orientation (attitude) of the spacecraft was based on its
angular momentum and was not so sensitive to the inevitable
impacts by dust grains. This concept is also more compact than a
three-axis stabilised spacecraft (such as the Vega and Voyager
spacecraft) and, therefore, it was easier to protect the
spacecraft. The high fly-by velocity caused by the combination of
the retrograde orbit of comet Halley and the direct orbit of the
Giotto spacecraft was almost 100 times higher than the speed of
material leaving the cometary nucleus. This implied that the
velocity of the dust and gas relative to the spacecraft would
hardly change during the fly-by. The experiments measuring
cometary species in-situ only had to look in the forward (ram)
direction along which the symmetry (spin) axis of the spacecraft
was oriented. Most of the experiments were situated on the
experiment platform near the front end of the spacecraft and
directly behind the dust shield. The communications link between
the spacecraft and Earth was supported by a despun high gain dish
antenna which was kept pointing towards Earth throughout the
mission. The angle (135.7 degrees) between the antenna beam and
the spin axis of the spacecraft was determined by the geometry of
the fly-by and was fixed during the mission design phase.
A unique feature of the Giotto spacecraft was its dust shield
which was used to protect Giotto from the high velocity dust
particle impacts. A 1mm thick plate of aluminium was mounted 23cm
in front of the main body of the spacecraft and covered the full
cross-section of the cylindrical body. Particles of mass >10**-6
g that penetrated this aluminium plate disintegrated and
vaporised. A cloud of vapour (neutral and ionised gas) then hit
the thick rear shield (made mostly of Kevlar) with a much larger
cross-section than the original particle, thereby reducing the
pressure.
Only masses >/= 1 g were expected to penetrate the Kevlar shield.
However, it had been calculated that smaller dust particles could
already perturb the spacecraft attitude so severely that the
communications link to Earth could be lost. The most realistic
dust models led to the prediction that the probability of such an
attitude perturbation was considerably higher than that of
spacecraft destruction.
Summary of Giotto experiments, as well as the Radio Science
Experiment:
HMC
---
The Halley Multicolor Camera is a CCD narrow-angle camera used
for imaging the inner coma and nucleus with high resolution
(11m at a close approach of 500 km). The camera baffle was
eroded during the encounter and the camera is no longer
operational. The HMC successfully imaged the nucleus up to 10
s before closest approach.
NMS
---
Neutral Mass Spectrometer consists of a mass analyzer and
energy analyzer that seeks to determine the neutral gas
composition. Both CCDs of the detectors ceased operation
during the dust episode that disabled the HMC.
IMS/HIS
-------
The Ion Mass Spectrometer has two sensors optimized to
different parts of the coma. For the inner coma, the HIS
(High Intensity Spectrometer) measures the relative abundance
of ions in the energy range 300 to 1400 eV/e. This instrument
remains functional.
IMS/HERS
--------
The Ion Mass Spectrometer sensor optimized for the outer coma
is called HERS (High Energy Range Spectrometer). It measures
the relative abundance of ions from 10 eV/e to 2.0 (or 4.5)
keV/e depending on the M/Q ratio. There was high voltage
damage to this isntrument and it is no longer useable.
PIA
---
The Particle Imapct Analyzer measures dust particle flux and
composition between 1-110 AMU. Although the instrument
recevied no damage in the encounter, it was turned off.
DID/MSM
-------
The Dust Impact Detector is used to determine the dust
particle flux and mass distribution. It consists of three
sensors one of which is the meteroid shield momentum sensor
(MSM) that can be used to sample a large or small shield
sector. No damage was sustained during encounter.
DID/RSM
-------
A similar type of DID measurement to determine dust particle
flux and mass distribution is possible by using the rear
momentum shield (RSM). No damage to this sensor was detected
post encounter.
DID/CIS
-------
The second type of sensor used in DID is the capacitor impact
sensor (CIS) which is sensitive to particles in the 1-5 micron
range. The sensor is still operational.
DID/IPM
-------
The final type of DID sensor is for impact plasma and momentum
(IPM) determination of the dust particle flux and mass
distribution. The sensor consists of either an impact
ionization detector (IPM-P) or a piezoelectric detector (IPM-
M). This instrument shows some anomalous behavior.
JPA/FIS
-------
The Johnstone Particle Analyzer measures the 3-dimesnional
velocity distribution of positive ions near the comet. The
Fast Ion Sensor (FIS) ristricts the energy range to 10 eV/q to
20 KeV/q for all directions, once every rotation of the
spacecraft. The high voltage to this sensor stopped working
1.5 hours from encounter.
JPA/IIS
-------
The JPA also consists of an Implanted Ion Sensor (IIS) which
measures the energy per charge distribution from 90 eV/q to 90
KeV/q with discrimination into five mass groups. No damage to
this instrument was detected post encounter.
RPA/EESA
--------
The RPA-Copernic plasma experiment for the Giotto mission
seeks to measure the 3-dimensional distributions of electrons
and thermal positive ions near the comet. The spectrometer
sensor (EESA) measures both the flux and energy spectra of
electrons from 10 eV to 30 KeV in 4 pi directions. Some
damage to the sensor was detected post encounter.
RPA/PICCA
---------
In the plasma experiment, the electrostatic mass analyzer
(PICCA) is designed to measure the thermal positive ions in
the mass range 10-203 AMU near the comet. The high voltage
damage sustained by this instrument makes it unuseable.
EPA
---
The Energetic Particle Analyzer (EPA) determines the flux of
particle with energy greater than 20 KeV, thus including both
electrons and accelerated ions. This instrument has been
operating in the cruise phase and sustained no damage in the
encounter.
MAG
---
The Magnetometer consists of a triaxial and separate biaxial
system of fluxgate sensors of the ring-core type. This
instrument was also switched-on during the cruise phase and
continues to operate flawlessly post encounter.
OPE
---
The Optical Probe Experiment measures the flux and polarized
brightness in various colors in the direction opposite to the
direction of motion of the spacecraft through the comet's
coma. This instrument sustained no damage in the encounter.
GRE
---
The Giotto Radio Experiment takes the measurements of the
phase (Doppler) shifts of the downlink carrier, e.g. X-band,
as a function of time to infer the total mass content along
the trajectory. This experiment continues to function.
Launch Date: 1985-07-02
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