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|10 May 2012|
The Cassini-Huygens mission to Saturn is the most ambitious effort in planetary space exploration ever undertaken. A joint endeavour of the European Space Agency (ESA), NASA and the Italian Space Agency (ASI), a sophisticated robotic spacecraft was sent to orbit the ringed planet and study the Saturnian system in detail, initially over a four-year period (mid-2004 to mid-2008). On board Cassini, was a lander called Huygens that was released from the spacecraft to parachute through the atmosphere to the surface of Saturn’s largest and most interesting moon, Titan, in January 2005. In April 2008, the mission was extended for another two years, until September 2010, and was named the Cassini Equinox Mission. A second extended mission, called the 'Cassini Solstice Mission' will continue until September 2017.
Saturn is the second largest planet in the Solar System. Like the other gaseous outer planets – Jupiter, Uranus and Neptune – it has an atmosphere made up mostly of hydrogen and helium. Saturn’s distinctive, bright rings are made up of ice and rock particles ranging in size from grains of sand to a freight container. More moons of greater variety orbit Saturn than any other planet. The saturnian satellites range in size from small asteroid-size bodies to the aptly named Titan, which is the second largest moon of the Solar System (after Jupiter's Ganymede) and is larger than the planet Mercury.
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'.
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:
The major objectives of the Cassini Equinox Mission are:
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.
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.
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.
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.
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.
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?
Imaging Science Subsystem - ISS
Cassini radar - RADAR
Radio Science Subsystem - RSS
Ion and Neutral Mass Spectrometer -INMS
Visible and Infrared Mapping Spectrometer - VIMS
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
Radio and Plasma Wave Spectrometer - RPWS
Cassini Plasma Spectrometer - CAPS
Ultraviolet Imaging Spectrograph - UVIS
Magnetospheric Imaging Instrument - MIMI
Dual-technique Magnetometer - MAG
Huygens Atmosphere Structure Instrument - HASI
Gas Chromatograph and Mass Spectrometer - GCMS
Aerosol Collector and Pyrolyser - ACP
Descent Imager/Spectral Radiometer - DISR
Doppler Wind Experiment - DWE
Surface Science Package - SSP
ESA Mission Manager and Project Scientist: Jean-Pierre Lebreton
For further information, please contact:
Fabrizio Zucchini, ASI Media Relations, Rome, Italy
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