Galileo Orbiter

Description

The Galileo mission consists of two spacecraft: an orbiter and an atmospheric probe. The orbiter will be the sixth spacecraft to explore the Jovian magnetosphere, but the first to be placed into orbit around the giant planet. Scientific objectives addressed by the orbiter are to: (1) investigate the circulation and dynamics of the Jovian atmosphere; (2) investigate the upper Jovian atmosphere and ionosphere; (3) characterize the morphology, geology, and physical state of the Galilean satellites; (4) investigate the composition and distribution of surface minerals on the Galilean satellites; (5) determine the gravitational and magnetic fields and dynamic properties of the Galilean satellites; (6) study the atmospheres, ionospheres, and extended gas clouds of the Galilean satellites; (7) study the interaction of the Jovian magnetosphere with the Galilean satellites; and, (8) characterize the vector magnetic field and the energy spectra, composition, and angular distribution of energetic particles and plasma to a distance of 150 Rj.

The structure of the orbiter is divided into two sections. The main body of the spacecraft, comprised of the electronics bays, propellant system, RTG and science booms, and high-gain antenna, rotates at rates of 3.25 or 10.5 rpm. The despun section, aft of the main body, uses an electric motor to drive it counter to the rotation of the main section. This dual spin attitude control system accommodates instruments which require stable, accurate pointing (the imaging instruments) and those which benefit from repetitive, broad-angular coverage (the various particles and fields instruments). The length of the spacecraft is 9 m and, with the high-gain antenna (HGA) deployed, is 4.6 m in diameter.

Power is provided to the spacecraft through the use of two radioisotope thermal generators (RTGs), each of which is located at the end of a short boom. The magnetometer sensors and plasma wave antenna are located on yet another boom, 10.9 m in length.

Although it was intended that communications with the Deep Space Network (DSN) would be primarily through the HGA (which would remain pointing toward the Earth at all times), thermal constraints forced the use of the two low-gain antennas prior to the first Earth flyby. HGA deployment was planned thereafter, but at least three of the HGA "ribs" were unable to be moved much beyond their launch configurations, thereby jeopardizing the total science return of the mission. Several attempts have been made to deploy the antenna through a variety of techniques.