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The Cassini orbiter, with its complement of scientific instruments, is an amazing "tool" for making remote observations. However, much work has to be done to get the spacecraft to its destination, and that is the story of mission design and execution, which has many facets. For example, the spacecraft and its instrumentation must be designed within allowable weight or "mass" constraints. A trajectory must be determined, a launch system must be chosen, and the time of launch must be established based on the alignment of the planets. There needs to be some means of communication with the spacecraft in flight, which involves both on board and ground-based transmission and reception. There must be a navigation system for keeping the spacecraft on course and placing it in planetary orbit, and this system requires an on board fuel supply. There must be a power system to run the spacecraft hardware and the scientific instruments. Finally, there must be an "operations concept" to best utilize the spacecraft resources, and there must be contingency plans to cover hardware failures or other unexpected emergencies in space.
The functions of the orbiter are to carry the Huygens probe and the on-board science instruments to the Saturnian system, to serve as the platform from which the probe is launched and scientific observations are made, and to store information and relay it back to Earth. The design of the orbiter is being "driven" by a number of requirements and challenges that distinguish Cassini from most other interplanetary missions. Among these are the distance of Saturn from the Sun and from Earth, the length of the mission, the complexity and volume of the science observations, and the spacecraft trajectory to Saturn, which, as mentioned, involves four planetary gravity assists along the way.
Because of Saturn's distance from the Sun, solar arrays, as stated above, are not feasible power sources for the spacecraft. To generate enough power, such arrays would simply have to be too large and heavy. Thus, the Cassini orbiter will get its power from three radioisotope thermoelectric generators or RTGs, which use heat from the natural decay of plutonium to generate direct current electricity. These RTGs will be of the same design as those already flying on the Galileo and Ulysses spacecraft and will have a long operational lifetime. At the end of the 11-year Cassini mission, they will still be capable of producing at least 628 watts of power.
The distance of Saturn from Earth is especially important, because it affects communication with the spacecraft. Specifically, when Cassini is at Saturn it will be between 8.2 and 10.2 astronomical units or AUs from Earth (1 AU is the distance from Earth to the Sun, or 93 million miles). Because of this, it will take 68 to 84 minutes for signals to travel from Earth to the spacecraft, or vice versa. In practical terms this means that ground controllers will not be able to give "real-time" instructions to the spacecraft either for day-to-day operations or in cases of unexpected in-flight events. By the time the controllers become aware of a problem and respond, nearly three hours will have passed.
The orbiter stands 6.8 meters (22.3 feet) high, and its maximum diameter of the high-gain antenna (HGA) primary reflector, is 4 meters (13.1 feet). The orbiter carries 687 kg (1515 lbs) of science instruments, including the Huygens probe system. After separation from its Titan IV/Centaur launch vehicle and adapter, the spacecraft has a baseline total mass of 5,634 kg (12,421 lbs). The orbiter dry mass is 2150 kg (4740 lbs), including 365 kg (805 lbs) for the science payload. | |