Mission Overview
    ================
 
      Planet-A and MS-T5 were launched separately by
      M-3SII launchers.  The M-3SII launcher is a new
      three-stage solid-propellant rocket with - a solid
      kick-stage motor attached.  Both spacecraft were
      injected directly into a heliocentric comet
      transfer trajectory.  Immediately after launch,
      each spacecraft was tracked for about 8 h per day.
      They were be despun from 120 rpm, to about 30 rpm,
      using the thruster subsystem.  The spacecraft's
      attitude was initially automatically adjusted such
      that its spin axis was perpendicular to the
      Sun-spacecraft line.  During the subsequent ground
      contact, the spin rate was further reduced to 6.3
      rpm and the spin axis reoriented to be
      perpendicular to the ecliptic plane.  After the
      first 4-5 d of ranging and accurate orbit
      determination, a mid-course correction manoeuvre
      was carried out.  During the interplanetary cruise,
      attitude maintenance operations will have to be
      performed at 1Od intervals.
 
      Both spacecraft will make their closest approach to
      Halley near the time of the comet's post-perihelion
      crossing of the ecliptic plane.  MS-T5 has as its
      scientific objective: - to study the solar-wind
      plasma and, if possible, determine the signatures
      of the solar-wind/comet interaction at large
      distances from the comet.

Mission Objectives Overview
    ===========================
 
      Japan's Institute of Space and Astronautical
      Science (ISAS) first decided in 1979 to send a
      spacecraft to Comet Halley.  This decision was
      supported both by Japanese scientists who were
      anxious to carry out an interplanetary mission and
      by those Institute engineers who wanted to
      demonstrate the capabilities of a new launch
      vehicle.  Comet Halley was considered a
      scientifically meaningful target because it has
      never been explored previously from close-range, it
      displays a wide range of interesting phenomena, and
      it returns only once every 76 years.
 
      The Japanese mission to Comet Halley, called the
      `Planet-A' mission, involved the launch of the
      `Suisei', or Planet-A, spacecraft on 19 August
      1985, to encounter Comet Halley in March 1986.  In
      keeping with our normal procedure of launching a
      test spacecraft to confirm the flight performance
      of a newly developed launch system and the
      necessary technology for the main mission, a test
      spacecraft called `Sakigake' (i.e.  Pioneer), or
      MS-T5, was launched on 8 January 1985.  Hence, the
      Planet-A project actually involves two spacecraft,
      Planet-A and MS-T5.
 
      Initial calculations based on launcher performance
      and mission requirements led to a mass of about 140
      kg for each spacecraft, allowing a scientific
      payload consisting of two or three experiments,
      each typically weighing 5 kg.  An EUV imaging
      experiment and a solar-wind plasma experiment were
      selected from many proposed experiments for
      Planet-A, while MS-T5 carries three experiments for
      measurements of plasma waves, solar-wind plasma and
      the interplanetary magnetic field.  The MS-T5
      spacecraft will be targeted to pass within 7
      million kilmeters of Comet Halley on 11 March 1986.
      About one month of this test spacecraft's initial
      cruise phase will be used for technical tests,
      including the operation of the new 64 m deep-space
      station at Usuda.  Later, its three scientific
      instruments will study the solar-wind plasma and,
      if possible, detect the signatures of the
      solar-wind/comet interaction.  When the other
      spacecraft make their closest approach to Halley,
      MS-T5 will be used to make complementary solar-wind
      measurements.