CASSINI SPACECRAFT

Cassini, a joint NASA and European Space Agency mission, comprises two parts. The main part is an orbiter named Cassini, after the Italian astronomer Giovanni Cassini who observed Saturn and its rings, discovering the main gap in them (the Cassini Division) and four of the planet's moons. The second is a probe, named Huygens after Christiaan Huygens, Dutch astronomer and discoverer of Titan, Saturn's largest moon.

The Cassini spacecraft was launched in 1997, and arrived at Saturn on 1 July 2004. The orbiter will spend four years surveying the planet and its moons, during which time it will make 70 orbits of Saturn and its satellites. Cassini will collect data on Saturn's magnetosphere, examine the ring system and study the gas giant's composition and atmosphere. Instruments on board Cassini will measure atomic particle density, mass, electrical charge in the environment of the spacecraft, as well as dust particles, plasma and radio waves. The flybys of the moons will provide detailed new pictures of their surfaces, including Titan at wavelengths which will 'see-through' the murky atmosphere.

Mysterious Titan was imaged by Voyager, but no details of its surface were visible. Ever since there has been much speculation as to what lies below the thick orange-coloured atmosphere. In perhaps the most eagerly waited part of the mission, Huygens, released by Cassini 22 days earlier, will descend to Titan's surface, its fall braked by a three large parachutes. As it travels through the atmosphere Huygens will make scientific measurements, collecting data on the nature and composition of the atmosphere. The journey to the surface is expected to take about two hours, if the probe survives the landing it is hoped it will be still operable and return pictures of the icy surface.

Signals from Huygens will be directed toward the Cassini Orbiter which will relay them to Earth. Each time the Cassini's own data recorder reaches capacity, the spacecraft is programmed to orientate its antenna to Earth, and transmit the information for reception through the antennas of the Deep Space Network (DSN). Throughout the mission Cassini is expected to return daily several gigabytes of fresh science data.