June 1983:the Japanese Halley probes

Halley's Comet will make its next flyby of the Sun
in early 1986. Several spacecraft, including the Japanese Planet-A, will approach the streaking comet on
the far side of the Sun to provide the first close up
views of the celestial visitor. An experimental version
of Planet-A, the pathfinding "MS-T5", will be Japan's
first deep space mission, launched eight months before
the main probe. Japan's Institute of Space and Astronautical Science (ISAS) has scheduled the launch of
Planet-A for 14 August 1985 from the ISAS Kagoshima
Space Center in Uchinoura, Kagoshima Prefecture.

Planet-A will be a spin-stabilised probe featuring
Charge Coupled Device (CCD) scanning instrumentation. On 8 March 1986 it will make its closest approach
to the comet, approximately one month following Halley's 9 February 1986 perihelion passage.

MS-T5 will be launched from the Kagoshima centre
on 31 December 1984 (Universal Time). Both spacecraft are being built by Nippon Electric Co. (NEC) in
Yokohama, under the direction of ISAS.
The main scientific objective of Planet-A is to make
ultraviolet observations of the comet during the immediate post-perihelion phase, when activity is expected
to be at a peak. Closest approach will be 150,000 km.
Over the course of about six weeks, mainly in the
post-perihelion phase, the probe will take a series of
pictures of the growth and decay of the hydrogen cloud
around the comet's nucleus (formed by the photodissociation of hydrogen within the nucleus).
Scientists at the Komaba-Tokyo ISAS Laboratories
will also study the influence of the solar wind on the
comet. Investigation of the interaction between the
on-coming solar wind and the comet's plasma tail is planned as one of the main objectives of the two
Japanese probes. MS-T5 will observe the magnetic
field of the solar wind, plasma waves, and solar wind
particles in the area "downstream'' of the comet
relative to the Sun. Planet-A will concentrate on actual
comet observations.

The two spacecraft will be launched by a four-stage
solid propellant rocket: the deep space version of the
"Mu-3SII" launcher. Each of the four stages uses
polybutadiene propellant, and two strap-on boosters
will be carried. The standard three-stage version is capable of injecting a 700 kg satellite into a low Earth orbit. The launch weight of Planet-A's vehicle is
62 tonnes. The manufacturer is the aerospace division
of Nissan Motors (Nissan-Datsun motorcars).

A 64 m deep space dish antenna is now under
construction at Usuda, Nagano Prefecture, for the
Tracking, Telemetry and Command (TT&C) of these
and subsequent ISAS planetary probes. This TT&C
antenna is being built by Mitsubishi Electric in the
"radio-quiet" Nagano valley approximately 170 km
northwest of Tokyo.

The Spacecraft

The spin-stabilised Planet-A is cylindrical, with a
1.4 m diameter and a height of 0.7 m. MS-T5 has the
same measurements and will weigh 141 kg; one or
two kg heavier than its successor. ISAS decided on a
spin-stabilised design in order to avoid the problems
of thermal control inherent in three-axis control spacecraft. Planet-A will feature an ultraviolet telescopic
mirror lens and image-detecting CCD scanners. During
its cruise, the spin-rate will be set at 5 r.p.m.; at the
time of imaging the rate will be decreased to 0.5 r.p.m.
The image data will be stored in magnetic bubble
memory recorders and transmitted to the ISAS deep
space centre after completion of the imaging cycle.
While imaging of the comet is underway, Planet-A will
be stabilised by a bias momentum wheel.

Both probes will use offset parabolic reflector high
gain S-band antennae with medium-gain backup antennae. The 80 cm diameter high gain antennae are
mounted on despin motor assemblies.

The two spacecraft will be injected directly into
heliocentric orbits, without the use of Earth-parking
orbits, in order to reduce the complexity and weignt
of the attitude control systems. Planet-A's orbit will have a perigee of 0.68 AU and an apogee of 1.0 AU.
MS-T5’a orbit has yet to be finalised; it may be directed
to take the probe into the comet's ion tail. Fortunately,
the launch windows conveniently fit the ISAS launch
schedule which is determined by the powerful fishermen's union in the area (launches mean that the sea
area has to be cleared for safety). Both ISAS and the
National Space Development Agency (NASDA) have to
undergo negotiations with the union for any transgressions of these windows. Planet-A is scheduled for
launch near the beginning of its launch window (lasting
20 days and extending into early September) because
typhoons occasionally disrupt the Kagoshima and
Tanegashima schedules in the August-September "fisherman launch window.”

Before Planet-A's imaging cycle of the comet's
hydrogen cloud can begin, an intricate series of manoeuvres must be made. For five hours after fourth stage
firing the trajectory will not be accurately known. After
re-tracking is completed, the spin-rate is reduced from
120 to 5 r.p.m. Next, the probe's attitude is stabilised
to be perpendicular to the ecliptic plane. An initial
course correction will then be made on the basis of the
first three days of ranging and orbital analysis. This
correction will be made by spin-synchronised pulse
jets, ensuring that the attitude remains normal to the
ecliptic.

Planet-A will carry charged particle detectors for
investigating the three dimensional distribution of the
solar wind near the ecliptic plane. MS-T5 will carry a
different package: ion temperature, ion velocity, plasma
density, and electron temperature will be monitored.