The Attitude and Orbit Control System for GIOTTO, ESA's Halley Encounter Mission

Abstract

GIOTTO is ESA’s spacecraft which will encounter the comet Halley in March 1986 shortly after the comet will have passed its perihelion. GIOTTO will transport a payload containing some ten experiments of about 50 kg total, within a range of a few hundred kilometers from Halley. The high encounter velocity of about 69 km/s is due to the combination of the spacecraft prograde orbit and the comets retrograde orbit.The GIOTTO spacecraft is a dual spin spacecraft consisting of a cylindrical rotor of about two meter diameter, spinning at 15 rpm, and a sizeable high gain antenna which points continuously at the earth to provide S and X band communication. The dual spin design has been selected to provide gyroscopic stability in the unknown comet environment during encounter and to offer at the same time sufficient pointing accuracy for the high gain antenna during the complete mission.The GIOTTO attitude and orbit control and measurement system has a blowdown hydrazine reaction control system with eight thrusters, a star mapper, X beam sun sensors, pencil beam infrared earth sensors operating with a microprocessor based attitude and orbit control electronics. The GIOTTO AOCMS is designed to perform most of the required spacecraft manoeuvres either by direct ground command or time tagged command. An attitude recovery mode is also built in to autonomously recover the nominal attitude in case of emergency.