The Cassini Spacecraft

School bus vs. Cassini-Huygens

With a set of sophisticated instruments that rivals the equipment of small laboratories, Cassini is one of the most powerful spacecraft ever sent into space. It's one of the largest and most massive too. To connect the various high-tech instruments with the spacecraft hardware and the communication equipment, a maze of cables and connectors gives the spacecraft the complexity of a modern metropolis.

The Cassini spacecraft, which includes the Huygens probe, the launch adapter and the propellant, is roughly equal in mass to an empty 30- passenger school bus. With a total mass of about 5,650 kilograms (6 tons), Cassini is the third heaviest spacecraft ever launched into space. Only the two Phobos spacecraft sent to Mars by the former Soviet Union were heavier.

Two main factors dictated the complexity and size of the spacecraft: The mission's ambitious science goals and the distance between Saturn and Earth.

The Cassini-Huygens mission's ambitious science goals include a detailed, in-depth exploration of Saturn, its composition, its moons and its atmosphere. The distance between Saturn and Earth was an obvious challenge. To fly directly to Saturn would seem like the best way to go, but such a trajectory would have required a monumental technological endeavor to allow something as heavy as Cassini to fly directly to Saturn. To fly to Saturn using as little fuel as possible, the spacecraft used a technique called gravity assist, which consists of using the gravitational force of a planet to increase velocity. Cassini thus has had to endure a six-and-half-year trip that included four planetary gravity assists to gain the momentum needed to reach the ringed planet.

High Gain Antenna

To send and receive data, Cassini boasts a set of three antennas: a high-gain antenna with a diameter of 4 meters (13.1 feet), and two low-gain antennas. Because of the huge distance involved, it will take almost an hour and a half for the data to travel from the spacecraft to Earth or vice versa. The exact time, between 68 and 84 minutes, depends on Saturn's position with respect to Earth. In case of unexpected events or anomalies, it will take nearly three hours for ground controllers to become aware of a problem and to respond to it. To deal with this kind of event, many autonomous safety features were built into the spacecraft to protect the overall mission; another factor that contributes to its complexity.

The Orbiter Cassini

Cabling inside the orbiter

At 6.8 meters (22.3 feet) high, the orbiter Cassini boasts cutting-edge instruments capable of collecting sophisticated data sets. It was also designed to piggyback the Huygens probe all the way to Titan's orbit, and to be able to eject the probe properly. The orbiter will also act as a communication relay station, forwarding to Earth the information it receives from Huygens during the probe's twirling descent through the thick atmosphere of Titan.

There are more than 12 kilometers (almost 7.5 miles) of cabling inside the orbiter, some 20,000 wire connections and 1,630 interconnect circuits. This intricate electronic system is needed to operate the abundance of instruments on board, which includes various optical and microwave remote sensing devices.

Instruments on Cassini

More than 250 scientists from 17 countries will study the data collected by the extensive instruments onboard the Cassini orbiter and Huygens probe. The optical remote sensing instruments on the Cassini orbiter are the Composite Infrared Spectrometer, the Imaging Science Subsystem, the Ultraviolet Imaging Spectrograph and the Visual and Infrared Mapping Spectrometer.

Two instruments, the Cassini Radar and the Radio Science Subsystem, are microwave remote sensing devices.

The Cassini Plasma Spectrometer, the Cosmic Dust Analyzer, the Ion and Neutral Mass Spectrometer, the Dual Technique Magnetometer, the Magnetospheric Imaging Instrument and the Radio & Plasma Wave Science instrument will capture data about fields, particles and waves.

Such a wealth of information-gathering tools makes deciding which instrument to favor at which time and for which target a complex task.

Fortunately, there are 74 planned orbits around Saturn, enough to likely satisfy the diverse curiosities of the science community at large.

The Huygens Probe

Huygens probe heat shield

With a diameter of 2.7 meters (less than 9 feet), the 350-kilogram (772- pound) Huygens probe is considerably smaller than the Cassini orbiter. It is loaded with instruments however, and it will play the leading role in another thrilling part of the mission. Almost six months after arriving in the Saturn system, Huygens will separate from Cassini to begin a 21-day cruise to Titan, the largest of the moons orbiting Saturn.

The cone-shaped probe will enter Titan's murky atmosphere traveling at 20,000 kilometers (almost 12,450 miles) per hour. Atmospheric drag will cause friction that will serve to slow the probe down as it descends onto Titan's surface. The probe's heat shield is designed to protect the probe from temperatures of more than 12,000 Celsius (more than 21,600 Fahrenheit). Once the probe is about 160 kilometers (about 60 miles) from the surface of Titan -- and by then traveling at a speed of 1,400 kilometers (870 miles) per hour -- the first of three parachutes will deploy to provide a stable platform for scientific measurements to be taken.

During its descent the probe will be sending data and images back to the Cassini orbiter. The six instruments onboard will collect information about the atmosphere's chemical composition and the weather conditions and clouds surrounding Titan. Two hours after entering Titan's atmosphere, the probe will land south of the moon's equator. It will hit the surface at about 25 kilometers (15 miles) per hour. If the probe survives the impact, it will continue to collect information on Titan's mysterious surface for up to 30 minutes. The probe will send data back to Cassini until its batteries run out or the orbiter is out of communication range.

Instruments on the Huygens Probe

The six instruments on the Huygens probe are:

• the Aerosol Collector Pyrolyser
• the Descent Imager and Spectral Radiometer
• the Doppler Wind Experiment
• the Gas Chromatograph and Mass Spectrometer
• the Huygens Atmospheric Structure Instrument
• the Surface Science Package

Learn more about the Huygens probe instruments.