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Galileo EPD Handbook

Purposes of This Document

The purpose of this document is to provide a concise collection of essential facts about the Energetic Particle Detector (EPD) instrument for the purpose of data analysis and interpretation. This handbook compiles the characteristics of the EPD instrument suitable for supporting the scientific analysis of its data. It is intended to supplement the already published literature. Key items needed to establish the physical observations are included. Geometries, passbands, thresholds, look angles, efficiencies, etc., are given for nominal instrument performance. It is intended that sufficient information be provided here to allow correct interpretation and/or recognition of both anomalous and nominal performance.

Preface

Jupiter possesses the largest planetary magnetosphere in the solar system. It is the largest in spatial dimension, has the highest trapped particle energies and intensities, has the greatest compositional variety in its major particle populations, displays the largest co-rotational effects, and has the largest number of moons within the magnetosphere which provide both strong sources for and losses of the observed particle populations. These characteristics, uncovered by the Pioneer and Voyager flybys, demand an instrument design capable of accommodating the great range in parametric values established by these extremes.

Amongst its complement of particles and fields instruments, the Galileo spacecraft carried an Energetic Particles Detector (EPD) designed to measure the characteristics of particle populations important in determining the size, shape and dynamics of the Jovian magnetosphere. To do this, the EPD provided 4p angular coverage and spectral measurements for Z >= 1 ions from 20 keV to 55 MeV, for electrons from 15 keV to >11 MeV, and for the elemental species helium through iron from approximately 10 keV/nucl to 15 MeV/Nucl. Two bi-directional telescopes, mounted on a stepping platform, employed magnetic deflection, energy loss versus energy, and time-of-flight techniques to provide 64 rate channels and pulse height analysis of priority selected events.

The Galileo EPD  provided major extensions to the Jovian energetic particle data base obtained from the Pioneer and Voyager flybys.  For example:  (1) Galileo was placed into a highly elliptical orbit around Jupiter.  The nominal two-year mission lifetime allowed both a direct measure of time variations in the Jovian magnetosphere and a significantly larger spatial sample of the system than had been possible with the previous flybys.  (2) The nominal mission included several close (<1000 km) flybys of the Galilean satellites, thereby providing the best opportunity to date to observe details of the satellite/magnetospheric interactions.  (3) The EPD provided the first 4p steradian angular coverage for Jovian energetic particles, thereby assuring that the necessary energetic particle measurements were being obtained independent of satellite orientation and magnetic field direction. (4) The low energy thresholds of the EPD effectively closed the energy gap between plasma and energetic particle measurements that had existed in previous observations and assured that processes thought to operate in that gap have been tested by direct observation.  For example, it has been suggested that the particles powering the Jovian aurora are ions of energies ≤100 keV/nucl, a composition energy range measured by Galileo instrumentation at Jupiter.

The EPD data system provided a large number of possible operational modes from which a small number were selected to optimize data collection during the many encounter and cruise phases of the mission. The EPD employed a number of "safeing" algorithms to be used in the event that its self-checking procedures indicated a problem. The EPD demonstrated its operational flexibility throughout the long evolution of the Galileo program by readily accommodating a variety of secondary mission objectives occasioned by the changing mission profile, such as the Venus flyby and the Earth 1 and 2 encounters.

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Updated 10/30/03, T. Hunt-Ward
tizby@ftecs.com