Titan is a fascinating world because its thick nitrogen atmosphere is very rich in organic compounds which are constantly reacting. If found on a planet with Earth-like conditions, the presence of these compounds would be a possible sign of the existence of life. In fact, very little was known about the surface of Titan when Cassini-Huygens was launched. Scientists speculated that Huygens would find lakes or even oceans of a mixture of liquid ethane, methane and nitrogen: Huygens landed on Titanian mud wet in methane, and later radar observations by Cassini confirmed the presence of seas and lakes of liquid methane or ethane on the surface.

Cassini and Huygens have produced a string of remarkable discoveries about Saturn's magnificent rings, its amazing moons, its dynamic magnetosphere and about Titan's surface and atmosphere. Some of the mission’s highlights so far include the discovery that Titan is an Earth-like world where methane plays the role of water, and that the small moon Enceladus has hot-spots at its south pole, sources of geysers that spew out ice crystals and the evidence of liquid water beneath its surface.

The Cassini-Huygens mission is named after two 17th century European astronomers. The Dutch astronomer Christiaan Huygens (1629-1695) discovered Saturn's rings and Titan. A few years later the French-Italian Astronomer Jean-Dominique Cassini (1625-1712) discovered Saturn’s four other major moons: Iapetus, Rhea, Tethys and Dione. He also discovered that Saturn’s rings are split largely into two parts by a narrow gap, known since as the 'Cassini Division'.

Objectives

The 12 scientific instruments on board the Cassini orbiter are conducting in-depth studies of the planet, its moons, rings and magnetic environment. The six instruments on the Huygens probe, which was dispatched from Cassini during its third orbit of Saturn on 25 December 2004 and eventually landed on the surface on 14 January 2005, have provided the first direct sampling of Titan’s atmospheric chemistry and the first metre-sized resolution photographs of its hidden surface. Huygens performed a detailed in situ study of Titan's atmosphere. It also characterised the surface along the descent ground track and near the landing site. Studying the complex organic chemistry at work on Titan may provide clues as to how life began on Earth.

Some selected questions addressed by the original mission:

• Mysteriously, Saturn emits 87% more energy than it absorbs from sunlight. What is the source of heat inside Saturn that produces the excess energy?
• Where do Saturn’s rings originate from?
• Where do the subtle colours in the rings come from?
• Are there any other moons to discover?
• Why does the moon Enceladus have such an abnormally smooth surface? Has it recently melted, erasing the craters?
• What is the origin of the dark organic material covering one side of the moon Iapetus?
• Which chemical reactions occur in Titan’s atmosphere?
• What is the source of abundant methane, an organic compound associated with biological activity on Earth?
• Are there any oceans on Titan?
• Do more complex organic compounds and pre-biotic molecules exist on Titan?

The major objectives of the Cassini Equinox Mission are:

• A deeper study of the moons Titan and Enceladus
• To monitor seasonal variations on Saturn and Titan
• To explore new places within Saturn's magnetosphere
• To observe the unique ring geometry of the Saturn equinox in August 2009 when sunlight will pass directly through the plane of the rings.

The major objectives of the Cassini Solstice Mission are to study the Saturnian system until the summer solstice is passed in May 2017. By the time this new extension is completed the Cassini mission will have covered (since it arrived in the system) one half of a Saturnian year. The Solstice mission is scheduled to complete an additional 155 orbits of Saturn, 54 flybys of Titan and 11 flybys of Enceladus.
 
Cost
 
NASA’s investment in Cassini represents a total of approximately 2100 million Euros. ESA’s contribution for the Huygens probe is about 360 million Euros. An additional investment of about 100 million Euros by universities and research institutes funded the development of the instruments on board Huygens. ASI’s contribution for the high-gain antenna, portions of three science instruments on board Cassini and one full instrument on board Huygens, is about 145 million Euros. All figures are adjusted to today’s economic conditions.
 
Launch
 
15 October 1997 (Titan-IVB/Centaur from Cape Canaveral, Florida, United States).
 
Journey
 
On its seven-year journey to Saturn, Cassini-Huygens performed four gravity-assist swing-by manoeuvres: Venus (April 1998), Venus (June 1999), Earth (August 1999) and Jupiter (December 2000). The gravity assists gave the mission the boost required to reach Saturn. The spacecraft arrived at Saturn on 1 July 2004, when it entered orbit and begin its scientific observations.
 
Mission lifetime
 
Cassini-Huygens’s four-year prime mission lasted until 30 June 2008. Huygens was fully activated for only a few hours in January 2005. Its data was collected during its approximately 2.5-hour descent and from the surface after a successful landing. Cassini listened to Huygens parked on the surface for another hour and 12 minutes, before it disappeared below Titan’s horizon. Huygens’s direct signal was picked up by large radio telescopes on Earth. The lander survived on the surface for more than three hours, much more than initially expected. On 1 July 2008, Cassini started its first extended mission. A second extended mission, called the 'Cassini Solstice Mission' will continue until September 2017.
 
Major operations
 
Saturn Orbit Insertion: Upon reaching Saturn on 1 July 2004, Cassini fired its main engine for 96 minutes to slow the spacecraft down to and allow it to be captured in orbit around Saturn. Passing through the dusty, outermost ring (called the E-ring), Cassini-Huygens swung in close to the planet – to an altitude only one-sixth the diameter of Saturn itself – to begin the first of 75 orbits for the rest of its four-year mission. Huygens was dormant during the long journey to Saturn, but it was awoken every six months by ESA’s flight controllers for a complete check-up. Huygens was released on 25 December 2004, and performed a 22-day fly on a ballistic trajectory towards Titan.

Descent to Titan: A system of alarm clocks awoke Huygens at a pre-programmed time four hours before it reached the outer fringe of Titan's atmosphere. During the first three minutes inside the atmosphere, Huygens had to decelerate from 18 000 to 1400 km/h. The heat generated by the friction of its shield with the upper layer of Titan’s atmosphere may have reached temperatures up to 1800°C. The robotic controls then fired a pilot parachute to pull out the main parachute at a speed of about 1500 km/h. Within a minute, the speed reduced to less than 300 km/h.

The shell of the Entry Assembly Module then fell away and exposed the scientific instruments to Titan's atmosphere, at a height of about 160 km. The atmospheric temperature was about -120°C. At about 105 km, the main parachute was cut away and replaced by a smaller one.

After Huygens's main parachute unfurled in the upper atmosphere, the probe slowed to a little over 50 m/s, or about the speed at which you might drive on a motorway.

In the lower atmosphere, the probe decelerated to approximately 5 m/s and drifted sideways at about 1.5 m/s, a leisurely walking pace.

The probe rocked more than expected in the upper atmosphere. During its descent through high-altitude haze, it tilted perhaps by up to 10 to 20°. Below the cloud layer (at about 21 km altitude), the probe was more stable, tilting less than 3°.

Scientists had predicted that the probe would drop out of the haze at between 70 and 50 km. In fact, Huygens began to emerge from the haze only at 30 km above the surface although the haze never cleared completely. When the probe landed, it was not with a thud, or a splash, but a 'splat'. It landed in Titanian 'mud'.

When the mission was designed, the DISR camera's 20-Watt surface lamp was designed to turn on 700 m above the surface and illuminate the landing site for as long as 15 minutes after touchdown. Not only did the lamp turn on at exactly 700 m, but it was still shining more than an hour later, when Cassini lost contact with Huygens and went over Titan's horizon for its ongoing exploration of the giant moon and the Saturnian system.

Overall, Huygens’s descent towards Titan lasted 2 hours and 28 minutes. When it touched the surface on 14 January 2005, it transmitted data back to Cassini for 1 hour and 12 minutes, before the orbiter itself disappeared below Titan’s horizon. Contact with Huygens via large Earth-based radiotelescopes was established for the whole descent and for more than 3 hours on the surface.

Exploration of the Saturnian system: During its mission, Cassini executed close fly-bys of several of Saturn’s moons – including more than 44 encounters with Titan and seven selected icy moons of greatest interest. Cassini’s orbit is allowing it to study Saturn’s equatorial zone as well as its polar regions. The first extended mission called Cassini Equinox mission included 60 additional orbits of Saturn and more flybys of its exotic moons, including 26 flybys of Titan, seven of Enceladus, and one each of Dione, Rhea and Helene. The Solstice mission is scheduled to complete an additional 155 orbits of Saturn, 54 flybys of Titan and 11 flybys of Enceladus.
 
Spacecraft
 
Design

The Cassini spacecraft, including the orbiter and the Huygens probe, is one of the largest, heaviest and most complex interplanetary spacecraft ever built. Of all interplanetary spacecraft, only the two Phobos spacecraft sent to Mars by the former Soviet Union were heavier.

Cassini’s antenna subsystem consists of the high-gain antenna and two low-gain antennas. The primary function of the high-gain antenna is to support communication with Earth. It is also used for scientific experiments. To shield the harmful hot rays of the sun from the spacecraft’s instruments during most of the early portion of the long journey to Saturn, the high-gain antenna was oriented toward the Sun, functioning as an umbrella. Cassini was the first planetary spacecraft to use solid-state recorders without moving parts instead of the old-fashioned tape recorder.

Huygens was built like a shellfish: a hard shell to protect a delicate interior from extreme temperatures experienced during entry through the upper atmosphere. It consisted of two parts: the Entry Assembly Module and the Descent Module. The Entry Assembly Module carried the equipment to control Huygens after separation from Cassini, and carried a shield that acted as a brake and as thermal protection. The Descent Module carried the scientific instruments, on board computers and radio equipment. The probe used three different parachutes deployed in sequence during the descent.

Mass

The Cassini orbiter alone weighs 2125 kg. Total mass of the Huygens probe was 349 kg, including payload (49 kg) and Probe Support equipment on the orbiter (30 kg). The launch mass of Cassini-Huygens was 5.82 tonnes of which 3.1 tonnes were propellant.

Dimensions

The Cassini spacecraft stood more than 6.7 m high and is more than 4 m wide. The magnetometer is mounted on an 11-m boom that extends outward from the spacecraft. The diameter of the Huygens probe is 2.7 m.

International involvement

Hundreds of scientists and engineers from 17 European countries and the United States make up the team responsible for designing, building, flying and collecting data from the Cassini orbiter and Huygens probe.

The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington DC, USA. JPL designed and assembled the Cassini orbiter. Development of the Huygens Titan lander was managed by ESA’s European Space Technology and Research Centre (ESTEC), the Netherlands. The prime contractor for the lander was Thales Alenia Space in France. The Italian Space Agency (ASI) managed the design and construction of the high-gain antenna and the other instruments of its participation. Equipment and instruments for the mission were supplied from many European countries and the USA.

What's on board?

Cassini Orbiter

Imaging Science Subsystem - ISS
Takes pictures in visible, near-ultraviolet and near-infrared light. Team Leader: Carolyn C. Porco, Space Science Institute, Boulder, CO, USA

Cassini radar - RADAR
Maps surface of Titan using radar imager to pierce veil of haze. Also used to measure heights of surface features. Team Leader: Charles Elachi, Jet Propulsion Laboratory, Pasadena, CA, USA

Radio Science Subsystem - RSS
Searches for gravitational waves in the universe; studies the atmosphere, rings and gravity fields of Saturn and its moons by measuring telltale changes in radio waves sent from the spacecraft. Team Leader: Arvydas J. Kliore, Jet Propulsion Laboratory, Pasadena, CA, USA

Ion and Neutral Mass Spectrometer -INMS
Examines neutral and charged particles near Titan, Saturn and the icy satellites to learn more about their extended atmospheres and ionospheres. Team Leader: J. Hunter Waite, Southwest Research Institute, San Antonio, TX, USA

Visible and Infrared Mapping Spectrometer - VIMS
Identifies the chemical composition of the surfaces, atmospheres and rings of Saturn and its moons by measuring colours of visible light and infrared energy given off by them. Team Leader: Robert H. Brown, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

Composite Infrared Spectrometer - CIRS Measures infrared energy from the surfaces, atmospheres and rings of Saturn and its moons to study their temperature and composition. Principal Investigator: Michael Flasar, NASA/Goddard Space Flight Center, Greenbelt, MD, USA

Cosmic Dust Analyser - CDA
Studies ice and dust grains in and near the Saturnian system. Principal Investigator: Ralf Srama, Max-Planck-Institut für Kernphysik, Heidelberg, Germany

Radio and Plasma Wave Spectrometer - RPWS
Investigates plasma waves (generated by ionised gases flowing out from the Sun or orbiting Saturn), natural emissions of radio energy and dust. Principal Investigator: Donald A. Gurnett, University of Iowa, Iowa City, IA, USA

Cassini Plasma Spectrometer - CAPS
Explores plasma (highly ionised gas) within and near Saturn’s magnetic field. Principal Investigator: David T. Young, Southwest Research Institute, San Antonio, TX, USA

Ultraviolet Imaging Spectrograph - UVIS
Measures ultraviolet energy from atmospheres and rings to study their structure, chemistry and composition. Principal Investigator: Larry Esposito, University of Colorado, Boulder, CO, USA

Magnetospheric Imaging Instrument - MIMI
Images Saturn’s magnetosphere and measures interactions between the magnetosphere and the solar wind, a flow of ionised gases streaming out from the Sun. Principal Investigator: Stamatios M. Krimigis, Johns Hopkins University, Laurel, MD, USA

Dual-technique Magnetometer - MAG
Studies Saturn’s magnetic field and its interactions with the solar wind, the rings and the moons of Saturn. Principal Investigator: Michelle Dougherty, Imperial College, University of London, United Kingdom

Huygens probe

Huygens Atmosphere Structure Instrument - HASI
HASI measured the physical and electrical properties of the atmosphere during the entry, descent and after landing. Principal Investigator: Marcello Fulchignoni, Universite de Paris VII / Dept. de Recherche Spatiale, Observatoire de Paris-Meudon, France.

Gas Chromatograph and Mass Spectrometer - GCMS
GCMS analysed the chemical composition of the gas in the atmosphere. Principal Investigator: Hasso Niemann, NASA/GSFC, MD, USA

Aerosol Collector and Pyrolyser - ACP
ACP collected aerosols for chemical-composition analysis. Principal Investigator: Guy Israel, CNRS Service d'Aéronomie, Verrières-le-Buisson, France

Descent Imager/Spectral Radiometer - DISR
DISR took images and performed spectral measurements. Principal Investigator: Marty Tomasko, University of Arizona, Tucson, AZ, USA

Doppler Wind Experiment - DWE
DWE studied the propagation of radio signals through the atmosphere to understand its properties. Principal Investigator: Michael Bird, Universität Bonn, Germany

Surface Science Package - SSP
SSP determined the physical properties of the surface at the impact site and provided unique information about its composition. Principal Investigator: John Zarnecki, Open University, Milton Keynes, United Kingdom The interdisciplinary scientists (IDS) for Cassini-Huygens are: Michel Blanc, CESR, Toulouse, France (Plasma Circulation and Magnetosphere-Ionosphere Coupling); Tamas Gombosi, SPRL, Univ. Michigan, Ann-Arbor, MI, USA (The plasma environment in Saturn's magnetosphere); Jeffrey Cuzzi, NASA Ames, Moffet Field, CA, USA (Rings and Dust in the Saturn System); Darrel Strobel, JHU, Baltimore, MD, USA (Aeronomy / Magnetosphere & Solar Wind Interaction); Tobias Owen, Institute for Astronomy, Honolulu, Hawaii, USA (Atmospheres: Composition, Origin, and Evolution); Laurence A. Soderblom , United States Geological Survey, Flagstaff, AZ, USA (Satellites); Daniel Gautier, Observatoire de Paris-Meudon, France (Titan aeronomy) Jonathan Lunine, University of Arizona, Tucson, AZ, United States (surface-atmosphere interactions) Francois Raulin, Université-Paris 12, Creteil, France (organic chemistry and exobiology).
 
Ground control
 
Cassini flight operations are conducted from JPL using stations of NASA’s Deep Space Network in California, Spain and Australia. Huygens flight operations were conducted from the European Space Operations Centre (ESOC) in Darmstadt, Germany. All Huygens telecommand sequences were prepared at ESOC and sent via NASA's Deep Space Network to Cassini which stored them on board for release to Huygens at a pre-determined time. Data were received back via the reverse path. Experiment data were distributed to the scientific teams by JPL and ESOC.

ESA Mission Manager and Project Scientist: Jean-Pierre Lebreton
NASA Mission Managers: Robert T. Mitchell (JPL) and Mark Dahl (NASA/HQ)
NASA Project Scientists:Bob Pappalardo (JPL) and Curt Niebur (NASA/HQ)
ASI Programme Manager and Programme Scientist: Enrico Flamini

For further information, please contact:

Franco Bonacina
ESA Media Relations Division, Paris, France
Tel: +33 1 5369 7155
Fax: +33 1 5369 7690

Don Savage
NASA Headquarters, Washington DC, USA
Tel: +1 202 358 1727
Fax: +1 202 358 3093

Carolina Martinez
JPL Media Relations Office, Pasadena, USA
Tel: +1 818 354 9382
Fax: +1 818 354 4537

Fabrizio Zucchini, ASI Media Relations, Rome, Italy
Tel: +39 06 856 7235
Fax: +39 06 841 6265