The Cassini Spacecraft

Size: The spacecraft stands 6.8 meters (22.3 feet) high and is 4 meters (13 feet) wide. A magnetometer is mounted on an 11 meter-long (36-foot) boom that extends outward from the spacecraft; three other rod-like antenna booms, each measuring about 10 meters (32 feet), extend outward from the spacecraft in a Y-shape.

Weight: The Cassini orbiter weighs 2,125 kilograms (4,685 pounds). After addition of the 320-kilogram (705-pound) Huygens probe and a launch vehicle adapter, as well as 3,132 kilograms (6,905 pounds) of propellants, the spacecraft weight at launch was 5,712 kilograms (12,593 pounds, or more than six tons). More than half of the spacecraft's total mass at launch was propellant -- somewhat less than half of which was needed for Cassini's 96-minute main engine firing that braked it into orbit around Saturn. Cassini's propellant mass alone is more than the mass of the Galileo and Voyager spacecraft combined.

Appearance: Most of the spacecraft and its instrument housings are covered with multiple-layered, shiny amber-colored or matte-black blanketing material. The blankets protect Cassini against the extreme heat and cold of space, and maintain the room temperature operating environment needed for computers and other electronic systems onboard. The blanketing includes layers of mylar to afford protection against dust-size particles called micrometeoroids that zip through interplanetary space. Traveling at speeds of 5 to 40 kilometers per second (roughly 10,000 to 90,000 miles per hour), these particles could potentially penetrate portions of the spacecraft. (Basic physics equation: Force = mass times velocity. Therefore, even though the small mass of such a particle would make it seem harmless, when multiplied by such high velocities, they can cause catastrophic damage!)

The main body of the orbiter is a nearly cylindrical stack consisting of a lower equipment module, a propulsion module and an upper equipment module, and is topped by the fixed 4-meter-diameter (13-foot) high-gain antenna. Attached about halfway up the stack are a remote sensing pallet, which contains cameras and other remote sensing instruments, and a fields and particles pallet, which contains instruments that study magnetic fields and charged particles. The two pallets carry most of the Cassini orbiter's science instruments. In general the whole spacecraft must be turned to point the instruments in the proper direction, though three of the instruments provide their own articulation about one axis.

Complexity: The spacecraft's complexity is necessitated both by its flight path to Saturn and by the ambitious program of scientific observations to be accomplished at Saturn. Cassini has some 22,000 wire connections and more than 12 kilometers (7.5 miles) of cabling linking its instruments, computers and mechanical devices. Sophisticated fault protection software resides in the spacecraft's computers to continuously sample and sense the health of the onboard systems. The fault protection system automatically takes corrective action when it determines the spacecraft is at risk due to any onboard failure.

Power: The orbiter receives electrical power from three radioisotope thermoelectric generators. These generators produce power by converting heat into electrical energy. Heat is provided by the natural radioactive decay of plutonium dioxide. Devices called thermocouples convert the heat into electricity to run the spacecraft. Upon arrival at Saturn, the three generators will provide about 750 watts of power. Plutonium dioxide is also used as the heat source in 82 small radioisotope heater units on the Cassini orbiter and 35 on the Huygens probe; each produces about 1 watt of heat to keep nearby electronics at their operating temperatures. (Similar heater units were used on the Mars Exploration Rovers to keep their electronics warm during Martian nights and on the Voyager and Galileo missions.) At Saturn's distance, the intensity of sunlight is approximately one percent of that at Earth, so extreme cold was a major concern.

Propulsion: After launch on a Titan IVB booster, propulsion for major changes to Cassini's trajectory was provided by one of two main engines. These powerful engines use monomethylhydrazine as the fuel and nitrogen tetroxide as the oxidizer. Sixteen smaller thrusters use hydrazine to control Cassini's orientation and to make small adjustments to the spacecraft's flight path.

Guidance and control: The spacecraft's position is governed by sensors that recognize reference stars and the Sun, and by onboard computers that determine the spacecraft's orientation. Using a new type of gyroscope called an inertial reference unit, the spacecraft can perform turns, twists and propulsion firings while retaining continuous knowledge of its own position.

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