To Open The Sky
The Front Pages of Christopher P. Winter
The Cassini Mission to Saturn
A personal view
Like Galileo, the Cassini spacecraft consists of an orbiter (Cassini proper) built by JPL (the Jet Propulsion Laboratory) and the Huygens probe built by ESA. There are many differences in instrumentation and other spacecraft systems, however.
Cassini stands 6.8 meters (22.3 feet) tall, and its widest part is the high-gain antenna (HGA) with a diameter of 4 meters (13.1 feet.) Fully fueled, it weighs in at 5,634 kg (12,421 pounds.) Here's a breakdown of component weights:
Twelve science packages are on board. In addition to these instruments, Cassini has various support systems to permit getting the data back to Earth (via the HGA mentioned above, and two low-gain antennas), to receive commands from mission control at JPL, and to give it a degree of "smarts". About 750 Watts of power is needed for all these functions.
That power comes from three Radioisotope Thermoelectric Generators (RTGs) of the same design used on Galileo. Protected by multiple layers of cladding, the heart of an RTG is plutonium dioxide, a ceramic material. The natural radioactive decay of the plutonium atoms produces heat, raising the internal temperature to about 400 Fahrenheit. The difference between this and the outside (ambient) temperature drives the thermoelectric elements. This type of generator is compact, sturdy and long-lasting. At the end of Cassini's 11-year mission, its RTGs will still be producing 620 Watts of electric power.
The Huygens probe is designed to safely enter Titan's atmosphere and soft-land on its surface. Except for twice-yearly health checks, it will be dormant until released from the orbiter. Then it will coast for 22 days until reaching its destination. Its active life is expected to be no more than 153 minutes, and it carries a set of lithium-sulfur dioxide batteries designed to last just this long at full discharge. (However, the batteries are sized so that Huygens can still complete its mission with one failed battery.)
A Centaur upper stage will thrust Cassini out of Earth orbit and head it for Venus, after the whole assembly is lifted into orbit by the Titan IV-B booster.