Figure: Orbit to Saturn

The Cassini-Huygens mission to Saturn is the most ambitious effort in planetary space exploration ever mounted.

A joint endeavour of ESA, NASA and the Italian space agency, Agenzia Spaziale Italiana (ASI), Cassini-Huygens is a sophisticated spacecraft being sent to the ringed planet to study the Saturnian system in detail over a four-year period. A scientific probe called Huygens was released from the main spacecraft to parachute through the atmosphere to the surface of Saturn's largest and most interesting moon, Titan.

Mission objectives

Cassini-Huygens is designed to shed light on many of the unsolved mysteries arising from previous observations, such as:

• what is the source of heat inside Saturn that produces 87 per cent more energy than the planet absorbs from sunlight?


• what is the origin of Saturn's rings?


• where do the subtle colours in the rings come from?


• are there any more moons?


• why has the moon Enceladus such an abnormally smooth surface? (Has recent melting erased craters?)


• what is the origin of the dark organic material covering one side of the moon Iapetus?


• which chemical reactions are occurring in Titan's atmosphere?


• what is the source of methane, a compound associated to biological activity on Earth, which is so abundant in Titan's atmosphere?


• are there any oceans on Titan?


• do more complex organic compounds and 'pre-biotic' molecules exist on Titan?

The journey

Cassini-Huygens was launched on a Titan IV-B/Centaur launch vehicle on 15 October 1997. It is a massive spacecraft - no existing launch vehicle could have sent the 5600 kilogram craft directly to Saturn, so a technique called 'gravity assist' (or 'fly-bys') was used (see the explanation of the Slingshot effect and also the "Double ball drop" example). Gravity assist manoeuvres work because of the mutual gravitational pull between a moving planet and a spacecraft. The planet pulls on the spacecraft as it is flying past, but the spacecraft's own mass also pulls on the planet. This permits an exchange of energy.

Cassini-Huygens looped around the Sun twice. On the first loop it flew close behind Venus in its solar orbit, where it 'stole' some of the planet's orbital momentum on 26 April 1998.

Cassini-Huygens swings by Earth and accelerates towards Saturn The next orbit provided a second fly-by of Venus on 24 June 1999, and one of Earth on 18 August 1999. Given these three gravity assist boosts, Cassini-Huygens finally had enough orbital momentum to reach the outer Solar System.

One last gravity assist manoeuvre from Jupiter on 30 December 2000 gave Cassini-Huygens the final thrust of energy it needed to project itself all the way to Saturn. The mission arrived at Saturn in July 2004.

Huygens enters Titan's atmosphere Huygens was released from Cassini on 25 December 2004. On 14 January 2005, it entered the murky atmosphere of Titan, Saturn's biggest moon, and descended via parachute onto its mysterious surface.

The Huygens probe sent its measurements and images to Cassini, which then beamed them back to Earth.

Cassini-Huygens passing through the gap in the rings The Cassini orbiter is orbiting around Saturn, and will do so for four years; it will send back valuable data to Earth that will help us understand the vast Saturnian region.

During its stay, Cassini will complete 75 orbits of the ringed planet, 44 close fly-bys of the mysterious moon Titan, and numerous fly-bys of Saturn's other icy moons.

Key dates of the Cassini-Huygens mission

11 June 2004 (19:32 UT) - Fly-by of the furthest moon orbiting Saturn, Phoebe, at an altitude of 2000 km

1 July 2004 - Crossing of Saturn's ring plane during the spacecraft's critical Saturn Orbit Insertion (SOI) sequence

25 December 2004 (02:00 UT) - Huygens probe separates from the Cassini orbiter and begins its 22-day journey to Titan

14 January 2005 (09:00 UT) - Huygens encountered the upper fringes of Titan's atmosphere, beginning its descent and landing on the surface about two and half hours later.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI).

Credit: ESA, NASA and ASI

http://www.esa.int/SPECIALS/Cassini-Huygens/SEMMD2HHZTD_0.html