When the Cassini-Huygens mission was launched on 15 October 1997, the entry probe Huygens was mounted on the side of the Cassini orbiter spacecraft, connected by a rotary plate. Huygens spent the seven-year journey to Saturn in 'sleep mode,' only waking every six months for three hours at a time to allow functional checks to be performed.
On 25 December 2004, the small, wok-shaped entry probe separated from Cassini and began its journey to Saturn's moon Titan. After 20 days, Huygens reached Titan's outer atmosphere; the moon's atmosphere stretches 600 kilometres out into space ten times further than Earth's atmosphere does.
Huygens lands on Titan
The successful landing of Huygens on Titan on 14 January 2005 was the first time in the history of space exploration that a scientific mission has reached a body in the outer Solar System.
The probe began its descent through Titan's atmosphere at 11:13 CET, at a height of about 1270 kilometres. During the next three minutes, the probe decelerated from 18 000 to 1400 kph. Next, the first small parachute opened, pulling away the upper heat shield. The larger main parachute then opened, slowing Huygens to less than 300 kph. At a height of about 160 km, the probe's scientific instruments went into action. At around 120 km, the main parachute was discarded and a smaller parachute opened to complete the landing. Huygens touched down on Titan at 13:34 CET.
Scientists received the first reassurance that Huygens was alive and well on at 11:25 CET, when the Green Bank radiotelescope in West Virginia, USA, received the first weak signal from the probe. The first scientific data from Huygens reached ESA's Space Operations Centre (ESOC), in Darmstadt, Germany, on 14 January 2005 at 17:19 CET.
During the two-hour, twenty-eight-minute descent through Titan's atmosphere, Huygens gathered information about the atmosphere and, after landing, Huygens transmitted data for another 90 minutes.
Data from Huygens, which were first transmitted to the Cassini probe to be stored in quadruplicate, were received by NASA's Deep Space Network and sent directly to ESOC in Darmstadt.
Cassini's long journey
The Cassini orbiter's long research expedition through the Saturnian system began upon arrival at Saturn on 1 July 2004. Originally, the mission was scheduled to end on July 2008, but due to its success, NASA extended the mission until 2017.
Cassini will circle Saturn a total of 76 times; these orbits will include 52 close fly-bys of seven of Saturn's 47 moons. To accomplish its orbits around Saturn, the orbiter must perform 45 'swing-by' manoeuvres near Titan to give it extra momentum. These manoeuvres will bring Cassini as close as 950 km to the surface of Titan, enabling the Cassini radar system (RADAR), which can penetrate the moon's atmosphere, to map the surface with high resolution.
Cassini will perform at least six close fly-bys of four other Saturnian moons: Iapetus, Enceladus, Dione and Rhea. These particular moons were chosen because researchers believe, on the basis of earlier data, that they could provide us with clues about the formation of the Saturn system and perhaps even the formation of our Solar System.
Cassini's mission is expected to end on 30 June 2008, four years after arriving at Saturn and 33 days after the last fly-by of Titan, scheduled for 28 May 2008.