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