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.  The Planet-A
      spacecraft will be targeted to pass Halley's
      nucleus on the sunward side at a distance of 150000
      km on 8 March 1986.  Solar-wind measurements will
      be carried out continuously on Planet-A during the
      cruise, except during the periods of UVI imaging,
      and this spacecraft will be in contact with the
      ground station for 7-8 h per day.  In this period,
      during which the spacecraft will be spinning at 6.3
      rpm, its bubble memory will be read out, solar-wind
      measurements made, and commands received from the
      ground.  During the remaining 15 h per day the
      spacecraft will be spinning at 0.2 rpm and taking
      images.  It takes 5 - 10 min to spin-down and 30
      min to spin-up the spacecraft.  During the 15 h,
      six images can be taken, spaced in time, to fill
      the 1 Mbit bubble memory.  At large distances from
      the Earth, data can be transmitted from the
      spacecraft at a rate of 64 bit/s.  In this mode,
      each memory read-out will require about 4 h.
 
      Imaging is planned to start in early November 1985,
      when Halley crosses the ecliptic plane at a
      heliocentric distance of 0.85 AU.  An attempt will
      be made to find the comet with the camera as soon
      as possible, despite the large distance from the
      spacecraft.  Imaging will continue at least until
      mid-April 1986, when the hydrogen corona will no
      longer be detectable.  Halley will then be at a
      heliocentric distance of 1.39 AU and at the same
      time will be reaching its closest distance to Earth
      of 0.42 AU.  During Planet-A's journey to Halley,
      an attempt may also be made to image the corona of
      Giacobini-Zinner if this comet's hydrogen
      production rate is high enough.

Mission Objectives Summary
    ==========================
 
      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.  Planet-A has
      two major scientific objectives: to study the
      growth and the decay of the comet's hydrogen corona
      and to determine the total hydrogen production rate
      by taking UV images of the corona over an extended
      period - to study the interaction of the solar wind
      with the cometary ionosphere by measuring the
      three-dimensional distributions of ions and
      electrons (energy range 30 eV-16 keV).