Mission Overview
================

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.

The European mission to comet Halley was named Giotto after the
Italian painter Giotto di Bondone who depicted comet Halley as
the `Star of Bethlehem' in one of his frescoes in the Scrovegni
chapel in Padua in 1304. The Giotto mission was finally
approved as ESA's first interplanetary mission on 7 July 1980.
An Announcement of Opportunity was issued shortly thereafter
requesting proposals for scientific payload instrumentation.
NASA was still interested at this stage but could not decide
whether to participate or not, partly because the American
scientific community did not whole-heartedly support a cometary
fly-by mission. Some scientists believed that the scientific
return would not be worth the effort. Finally, NASA declined
to participate and refused to provide direct financial support
for any American hardware involvement. By the end of January
1981, 11 European experiments were selected to perform the
diagnostic measurements during a close fly-by of comet Halley
in March 1986.

The mission was a fast flyby in March 1986 after the comet's
perihelion, when it is most active. The scientific payload
consists of 10 experiments with a total mass of about 60 KG: a
camera for imaging the comet nucleus, three mass spectrometers
for analysis of the elemental and isotopic composition of the
cometary gas and dust environment, various dust impact
detectors, a photo- polarimeter for measurements of the coma
brightness, and a set of plasma in- struments for studies of
the solar wind/comet interaction. In view of the high flyby
velocity of 68.4 km/sec, the experiment active time is only 4 h
and all data are transmitted back to Earth in real time at a
rate of 40 kbits/s. The Giotto spacecraft is spin-stabilized
with a despun, high-gain parabolic antenna inclined at 44.3
degrees to point at the Earth during the encounter. A
specially designed dual-sheet bumper shield protects the
forward end of the spacecraft from being destroyed by
hypervelocity dust impacts. The spacecraft passed the nucleus
at a distance of 596+/-2 km on the sunward side. The time of
Closest approach occurred at 00:03:01.84 UT on March 14
(spacecraft event time). However, at 7.6 s before closest
approach, Giotto was hit by a large dust particle, whose impact
caused the spacecraft angular momentum vector to shift by 1
degree. The effect of the impact was that the next 32 minutes
of scientific data were received only intermittently. It is
concluded that the spacecraft traversed a region of high dust
concentration (dust jet). A few hours after closest approach,
a number of the instruments were determined to be inoperable,
probably from the passage through the dust jet. About half of
the experiments worked flawlessly during the encounter, while
the other half suffered damage due to dust impacts. The
spacecraft also suffered some damage but it was possible to
redirect it to the Earth before it was put into hibernation.

Spacecraft ID : GIO
Target name : Halley
Spacecraft Operations Type : FLYBY


Mission Phases
==============

Launch
------
The Giotto spacecraft was launched on July 2, 1985 onboard an
Ariane-1 rocket from Kourou, French Guyana.

Mission phase start time: 1985-07-02
Mission phase stop time: 1985-07-02


Cruise
------
The Giotto spacecraft was initially injected into a
Geostationary Transfer Orbit. After three revolutions in
orbit, the onboard motor was fired near perigee to inject
Giotto into a heliocentric orbit. The high gain antenna was
despun three days later. The HMC was switched on in Format 3
on August 10, 1985 to monitog of its barrel, followed by the
Magnetometer Experimeter and Energetic Particles Experiment
switch-on on August 22, 1985. After a cruise pahse of 8
months, Giotto encountered Comet Halley on Mar 14, 1986.
Along its trajectory, the Magnetometer and Energetic Particle
experiments remained on. The other instruments followed a
on/pyro firing test sequence from Sep through Oct, 1985. The
science instruments will take data at various times starting
on March 9, but only the magnetometer and energetic particle
experiments will be able to make use of this continuous
coverage. Continuous data coverage was provided in a high-
data-rate mode about 50 hours before and 26.5 hours after
encounter, at which point the last experiment was
switched-off.

Mission phase start time: 1985-07-02
Mission phase stop time: 1986-03-12


Encounter
---------
There were specific periods of science data availability
after the last orbit correction manoeuver that occurred on
March 12 at 05:00. The time of closest approach on March 14
is 00:03:01.84 UT, given in SCET or spacecraft event time.
(This time can be related to GSRT or ground station received
time by the equation GSRT = SCET + 8 min 0.1 s.) Some
instruments, such as EPA, MAG, and GRE, ran continuously
during the encounter which lasted approximately 4 hours.
Other instruments were switched-on for some intervals between
March 12 and March 13, but by 20:18 on that day all
instruments were functioning. Unfortunately, 7.6 s before
closest approach, Giotto was hit by a large dust particle in
a dust jet. Only intermittent data was received for the next
32 minutes of the encounter and damage to a number of
instruments was substantial.

Mission phase start time: 1986-03-12
Mission phase stop time: 1986-03-15