The Cassini Orbiter mission consists of delivering the Huygens probe, which was provided by the European Space Agency, ESA, to Titan, and then remaining in orbit around Saturn for detailed studies of the planet and its rings and satellites. The principal objectives are to:

• 1. Determine the three-dimensional structure and dynamical behavior of the rings
• 2. Determine the composition of the satellite surfaces and the geological history of each object
• 3. Determine the nature and origin of the dark material on the Iapetus leading hemisphere
• 4. Measure the three-dimensional structure and dynamical behavior of the magnetosphere
• 5. Study the dynamical behavior of Cronian atmosphere at cloud level
• 6. Study the time variability of Titan's clouds and hazes
• 7. Characterize Titan's surface on a regional scale

• Spacecraft and Subsystems

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The spacecraft was originally planned to be the second three-axis stabilized, radioisotope thermoelectric generator, RTG, powered Mariner Mark II, a class of spacecraft developed for missions beyond the orbit of Mars. However, various budget cuts and rescopings of the project forced a more specialized design, postponing indefinitely any implementation of the Mariner Mark II series.

Cassini is the largest interplanetary spacecraft ever constructed by NASA. It measured 6.8 m in length with a 4 m high gain antenna. At launch, the spacecraft had a mass of 5,655 kg of which 3,132 kg were propellant.

The orientation of Cassini was maintained through the use of either three reaction wheel assemblies mounted along orthogonal axes on the spacecraft, for fine pointing control or via 16 0.5 N thrusters for coarse pointing control. The thrusters were arranged in four groups of four and used hydrazine. The orientation was determined through the use of either three inertial reference units that used solid-state gyroscopes or a star tracker, which detects stars in its field of view and compares them with an on-board catalog of 5,000 stars. The thrusters were also used in the alteration of the spacecraft trajectory of less than 5 m/s. For trajectory corrections of greater than 5 m/s, one of two identical main engines, of which one served as a backup, was used. The engines were gimbaled so that thrust could can be maintained through the spacecraft center of mass and burn the bipropellants nitrogen tetroxide and monomethyl hydrazine.

Power was provided to the spacecraft through the use of three RTGs. Each RTG used the heat generated by the decay of 10.9 kg of plutonium dioxide, PuO2, to generate electrical power to be used throughout the spacecraft. At the beginning of the mission, each RTG was capable of producing 300 W of electrical power. By the end of the nominal 11 yr mission, the output was expected to degrade to around 210 W per RTG. The resultant electricity, a regulated 30 V DC source, was used not only to provide power to the various science instruments and spacecraft subsystems, but also to one-time pyrotechnic devices that were used in the course of the mission, such as to separate the the spacecraft from the Centaur launch vehicle or to separate the Huygens probe from Cassini.

Although some data will be transmitted in real time, much science data and spacecraft health and status information were recorded on a solid-state data recorder. Although such systems had been used previously on other missions, Cassini was the first to use one in deep space. The spacecraft was equipped with two recorders, each of which had a capacity of 2 Gb in the form of dynamic random access memory, DRAM. Because such memory is vulnerable to radiation effects, the recorders were encased in half-inch thick aluminum. Nonetheless, degradation of the recorders due to solar and cosmic ray activity was expected to reduce their capacity by about 10% by the end of the mission. Recorded data were then periodically transmitted to Earth via the HGA and erased.