The Galileo mission and systems were designed to investigate three broad aspects of the Jovian system: the planet's atmosphere, the satellites and the magnetosphere. The spacecraft was constructed in three segments, which help focus on these areas: 1) the atmospheric probe; 2) a non-spinning section of the orbiter carrying cameras and other remote sensors; 3) the spinning main section of the orbiter spacecraft which includes the fields and particles instruments, designed to sense and measure the environment directly as the spacecraft flies through it.
This innovative "dual spin" design allows part of the orbiter to rotate constantly at three rpm, and part of the spacecraft to remain fixed. This means that the orbiter can easily accommodate magnetospheric experiments (which need to take measurements while rapidly sweeping about) while also providing stability and a fixed orientation for cameras and other sensors.
Scientific instruments to measure fields and particles, together with the main antenna, the power supply, the propulsion module, most of the computers and control electronics, are mounted on the spinning section. The instruments include magnetometer sensors, mounted on an 11-meter (36-foot) boom to minimize interference from the spacecraft; a plasma instrument detecting low-energy charged particles and a plasma-wave detector to study waves generated by the particles; a high-energy particle detector; and a detector of cosmic and Jovian dust. It also carries the Heavy Ion Counter, an engineering experiment added to assess the potentially hazardous charged-particle environments the spacecraft flies through, and an added Extreme Ultraviolet detector associated with the UV spectrometer on the scan platform.
The despun section carries instruments and other equipment whose operation depends on a steady pointing capability. The instruments include the camera system; the near-infrared mapping spectrometer to make multispectral images for atmospheric and moon surface chemical analysis; the ultraviolet spectrometer to study gases; and the photopolarimeter-radiometer to measure radiant and reflected energy. The camera system will obtain images of Jupiter's satellites at resolutions from 20 to 1,000 times better than Voyager's best, largely because it will be closer. The CCD sensor in Galileo's camera is more sensitive and has a broader color detection band than the vidicons of Voyager.
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