Jupiter, as one of the brightest objects of the night sky, has always held a fascination for mankind. That fascination intensified with the invention of the telescope. The discovery by Galileo of a small `planetary' system in orbit about Jupiter revolutionized the world's view of its place in the Universe. No longer did the Universe revolve around the Earth. The Earth became but one of many worlds. Telescopic observations of the Jovian system also made a major contribution to modern physics. The timing of eclipses of the Galilean moons led to the first measurement of the velocity of light. The telescope also gives atmospheric scientists an opportunity to study a 4 century old enigma, the Great Red Spot. Is it a giant storm that has been raging for over 4 centuries or is it related to some structure in the interior of Jupiter?

          The fascination deepened once again with the invention of another type of telescope, the radio telescope. In 1955 B. F. Burke and K. L. Franklin discovered that Jupiter emitted intense radio waves. This led almost immediately to the conclusion that Jupiter had a strong magnetic field and an intense radiation belt. Jupiter soon became the target of our fledgling planetary program, with visits by Pioneer 10 in 1973, Pioneer 11 in 1974, and Voyager 1 and 2 in 1979. These four space Probes provided brief glimpses into the magnetosphere and the atmosphere of the planet and returned pictures of the Galilean satellites, worlds quite alien to our own.

          The momentum had been building through these years for the next step in the exploration of the Jovian system, an orbiter. When this Jovian orbiter was approved by NASA, it was only fitting that it be named for the first explorer of the Jovian system, Galileo. The 1980's was a decade of planning, designing, building, and rebuilding as the Galileo mission got off to a slow start because of the ever changing availability of launchers in NASA's fleet during these years. Eventually in October 1989, Galileo was launched and sent on its way via Venus and the Earth (twice) on its voyage to Jupiter with arrival in December 1995.

          The articles that follow in this volume represent our attempt to document the Galileo mission and make it accessible to the broader scientific community. The volume begins with a mission overview by the project scientist, Torrence Johnson, and his colleagues which details the science objectives of the mission and describes how these objectives will be addressed. This article is followed by one by L. A. D'Amario and colleagues on the design of the spacecraft trajectory, a key element of the mission planning process that allows the often conflicting objectives of the mission to be met. Following these articles are descriptions of investigations by the various investigator groups associated with the mission. Most of these involve dedicated hardware but some, such as the article by Interdisciplinary Scientist J. Pollack and colleagues and those by the Radio Science and the Gravitation and Celestial Mechanics teams, discuss the use of spacecraft telemetry signals to further the scientific objectives of the missions.

          We have divided the mission into three sections. The Probe articles are presented first, as appropriate for these investigations that will provide much of the initial science return in the mission as the Probe descends through the atmosphere on the day of orbit insertion. Then we present the `magnetospheric' instruments which provide local measurements of the Jovian system. Finally we present the remote sensing instruments and the radio science investigations.

          It is hoped that these articles will provide interested planetary and space scientists some insight into what Galileo is expected to achieve and how the requisite measurements will be made. The compilation of this volume is due to the efforts of many individuals, especially the referees and the authors, who worked together to develop readable and complete descriptions of the investigations. We also wish to remember Clayne Yeates of the Galileo Science Office who assisted in the very early phases of this project but whose untimely death did not allow him to see its completion.


C. T. Russell

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