Mission Overview
    ================
      The effective launch of ICE took place on 22
      December 1983.  As the spacecraft distance from
      Earth increased, tracking and data acquisition
      involved the NASA Deep Space Network (DSN).  The
      necessity for this involvement is apparent if one
      considers that the ISEE-3 radio system was designed
      to transmit from the halo orbit at a geocentric
      distance of 0.01 AU, whereas the distance to the
      spacecraft at cometary encounter will be 0.47 AU.
      A major effort required for the ICE mission was the
      outfitting of antennas in the DSN to operate at the
      ICE frequencies.  Current plans are to utilize the
      64 m DSN (Goldstone, Madrid, Canberra) and the 300
      m dish at Arecibo as the prime station.  The
      anticipated data rate at encounter will be 1024
      bit/s, although 512 bit/s may be used at other
      times.  The acceptable performance is based on a
      bit error rate of 10**-4.  There will be additional
      coverage by the 64 m station at Usuda, Japan.
      Operations outside the month centered on the
      encounter date of 11 September 1985 are basically
      cruise-science measurements, which will be
      discussed in the next section.  The ICE spacecraft
      will be approaching the aim point on the main
      plasma tail axis 10000 km from the nucleus.  Some
      idea of the spatial scales associated with key
      instruments and their sampling times is given below
      for the expected data rate of 1024 bit/s.
 
  Instrument	Sampling period(s)	Spatial resolution (km)
  _______________________________________________________________
  Magnetometer		1/3			 7
 
  Plasma Waves
    16 channel E	   1                      21
    8 channel E,B         16                     330
 
  Plasma Electrons
    2-d distribution      24                     500
 
  Energetic Protons       32                     660
 
  Radio Waves             56                    1200
 
  Plasma Ions           1200                   25000
 
 
      For the sampling period indicated, the 'spatial
      resolution' is the distance travelled at the
      relative encounter speed of 20.7 km/s.  These
      dimensions should be compared with the estimated
      distance between bow-shock crossings of about 175
      000 km and a measured main tail diameter of 5000
      km.  Expressed in terms of time, we expect the
      spacecraft to be inside the bow shock for about 2 h
      20 min and in the main plasma tail for about 4 min.
      The magnetometer will produce many measurements
      during the encounter period and we use it to
      illustrate possible scientific product.  Current
      models of comets do not indicate a major
      amplification of the cometary magnetic field over
      the solar-wind value.  However, major changes in
      field-line direction are expected.  Well away from
      the comet the magnetic field should, on average,
      show the Archimedian spiral angle of 135deg or
      315deg to the radial appropriate for normal
      solar-wind flow.  Interior to the bow shock we
      expect a different, possibly somewhat chaotic,
      orientation tending to the ordered two-lobed
      configuration along the axis of the plasma tail.
      If the neutral sheet is encountered, a magnetic
      reversal should be recorded.  The model-dependent
      nature of this description must be stressed.  For
      example, the bow shock may or may not exist.  We
      should know after 11 September 1985, and the model
      will be tested in this and in other respects.
      Obviously, we need data from as many different
      experiments as possible to complete our model
      testing.  To enhance the science return from the
      encounter, a Guest Investigator Program has been
      established by NASA.