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.