Instrument Host Overview
    ========================
 
      The third International Sun-Earth Explorer (ISEE-3)
      was launched on 12 August 1978 as one element of a
      three-spacecraft mission that began in 1977.  The
      original purpose was to study the solar-wind
      interaction with the Earth's magnetosphere.  The
      spacecraft was maintained in a 'halo orbit' about
      the libration point, L1, where it monitored the
      solar-wind input.  It completed four years of
      uninterrupted operation at that location.  Several
      opportunities to use ISEE-3 in an extended mission
      phase were avallable.  Among the most attractive
      scientifically were exploration of the distant
      geotail and an intercept of periodic Comet
      Giacobini-Zinner.  Either or both of these options
      were available.  The comet option was constrained
      to an intercept of Giacobini-Zinner in September
      1985; specificaily, an intercept of Comet Halley
      was not possible.  The manoeuvres necessary to
      achieve the trajectory that would send the
      spacecraft into the distant geomagnetic tail and to
      an intercept of Comet Giacobini-Zinner are not
      simple.  They are the brainchild of R.  Farquhar,
      Flight Director for the Project.  Basically, five
      gravitationai encounters with the Moon were
      required to change the spacecraft's orbit.  The
      last encounter was on 22 December 1983 wh en the
      spacecraft  made a close swingbyu passing oniy 120
      km above the lunar surface.  This manoeuvre
      effectively 'launched' the spacecraft from the
      Earth-Moon system.  At the same time, the
      spacecraft was renamed the Internationai Cometary
      Explorer (ICE) to correspond to its new mission.
 
      The spacecraft is a developed version of the
      earlier Interplanetary Monitoring Platform (IMP).
      The spacecraft body is a 16-sided cylinder, 1.74 m
      in diameter and 1.6 m high.  Solar arrays mounted
      on the faces of the cylinder provide 160 W of
      primary power at 28 V.  A distinctive feature is a
      superstructure, or tower, which elevates the
      telemetry antenna above the spacecraft body and
      provides a clear field of view for several
      cosmic-ray detectors.  The spacecraft body supports
      a total of ten appendages.  Two equatorial
      experiment booms (3 m long) support the
      magnetometer and plasma-wave sensors.  Four wire
      antennas (each 49 m long) are deployed in the spin
      plane as part of the radio-wave and plasma-wave
      investigations.  Two axial antennas (7 m each)
      extend above and below the spacecraft parallel to
      the spin axis to render the radio-wave measurements
      three dimensional.  Finally, two short inertial
      booms provide stability.  The radio system consists
      of two redundant S-band transponders operating at
      2217 and 2270 MHz.  There are two low-gain antennas
      and a telemetry antenna having medium gain (7 dB)
      and a fan beam of +/-6deg.  The radiated power is
      5W.  The standard telemetry rate in halo orbit was
      2048 and 512 bit/s.  The spacecraft is
      spin-stabilised at 19.75 rpm.  Attitude information
      and control is provided by two fine Sun sensors and
      a panoramic attitude sensor.  The spin axis is
      maintained perpendicular to the ecliptic plane to
      within