The Galileo Spacecraft Architecture

Abstract

This paper provides an overview of the architecture of the Galileo Spacecraft. The mission overview is provided in O’Neil (1997), and early results of this truly historic mission are given in papers throughout this volume. The Galileo Spacecraft was comprised of a Jupiter Atmospheric Entry Probe and a Jupiter Orbiter. The mission was created to make the first ever in-situ measurements of the atmosphere of Jupiter below the veiling cloud layers that prevent measuring the constituents remotely, and to make long-term observations in the Jupiter system, including particularly close encounters with the Galilean satellites. The Probe would be delivered to Jupiter by the first Jupiter Orbiter. The Orbiter would release the Probe on a ballistic trajectory to its entry corridor about five months before arrival. The five-month solo flight time was dictated by Probe battery capacity and entry control accuracy vs. propellant required for the Orbiter deflection. The Orbiter was to target the Probe with an accuracy of +1.6° and ±4.5° in entry flight path angle and angle of attack, respectively. (Ultimately, these specifications became 1.4° and 7.0°.) The Orbiter would precisely adjust the Spacecraft trajectory and then reorient the spin axis parallel to what would be the atmosphere-relative entry velocity vector at Jupiter and spinup to 10 rpm prior to release. Thus, the Probe would be spin-stabilized for its flight to Jupiter to control the entry angle of attack. The Probe itself would require no attitude or flight path control system. There would be no communication link with the Probe after release until the atmospheric descent. The Orbiter would deflect itself several days after release to target to the required Probe overflight and initial conditions for its Jupiter Orbit Insertion (JOI) maneuver. Following entry and on the main parachute, the Probe would transmit its data at L-band over a 56° circular beamwidth for nominally 60 minutes. The Orbiter would receive the Probe signal through a dedicated 1.1 m antenna and dedicated receivers. The Probe “relay” data would be interleaved with Orbiter data and transmitted to Earth in real-time over the Orbiter 4.8-m High-Gain Antenna (HGA) at X-band. The Orbiter would be the first dual-spin planetary spacecraft. A spin function was required for Probe delivery and for the Orbiter fields and particles observations. A three-axis, stable platform was required for Orbiter remote sensing. The dual-spin Galileo design provided superb and elegant support for all these functions.