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Cassini-Huygens overview 
Objective Heading for Titan on board the Cassini spacecraft, Huygens will help solve the great mystery of how life began on Earth by probing Saturn’s mysterious moon.
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Huygens will be the first probe to land on a world in the outer Solar System - on the surface of Titan, Saturn’s largest moon. The Huygens data may offer clues about how life began on Earth. Huygens is currently in space, hitching a ride on NASA’s Cassini spacecraft. While the Cassini orbiter continues to explore Saturn and its rings, the Huygens probe will be released to parachute through the atmosphere of Titan. Shrouded in an orange haze that hides its surface, Titan is one of the most mysterious objects in our solar system. It is the second largest moon (only Jupiter's Ganymede is bigger), and the only one with a thick atmosphere. It is this atmosphere that excites scientific interest, since it is thought to resemble that of a very young Earth.
Huygens's six instruments will take measurements throughout its descent, providing details on the chemical composition of Titan's atmosphere, its weather and clouds, and then the surface itself. Spectacular data and images are already expected from the descent and, if the Huygens probe survives the impact with the mysterious surface, it will continue to send unique information back to the Cassini orbiter until its batteries expire or it is out of range.
What's special? Huygens's goals are to study the atmosphere and the surface of Titan along the descent ground track and near the landing site. The Huygens instruments will make detailed on-the-spot measurements of Titan's atmospheric, looking at its structure, composition and dynamics. Images and other remote-sensing measurements of the surface of Titan will also be made during the descent. After a descent of about 137 minutes, the probe will impact the surface at about 5-6 metres per second. It is hoped that the probe will survive this impact for at least a few minutes and that the instruments are also able to make direct measurements of the state and composition of the landing site surface.
Preserved in the deep freeze of Titan's atmosphere are chemical, carbon-rich compounds thought to be similar to those of Earth's primeval soup. The in-situ results from Huygens, combined with Cassini's global observations from repeated flybys of Titan, will provide vital information towards the great mystery of how life began on Earth.
Spacecraft The first element of the Huygens system consists of the 318-kilogram Huygens probe itself, which enters Titan's atmosphere after separating from the Cassini orbiter. It consists of two parts: the Entry Assembly (ENA) which cocoons the Descent Module (DM). The ENA connects the Huygens probe to the Cassini orbiter until it initiates and performs the ejection of the probe. It then controls the cruise to Titan, provides thermal protection during entry and the parachutes decelerate the probe in time for the landing on Titan. It is then jettisoned, releasing the DM. The DM comprises an aluminium shell and inner structure containing all the experiments and probe support subsystems, including the parachute descent and spin control devices. The second element of the Huygens system is the 30-kilogram Probe Support Equipment (PSE), which remains attached to the orbiter after probe separation.
The PSE consists of four electronic boxes aboard the orbiter providing control and communications. It provides power and data links between the probe and orbiter before the probe is launched.
Journey The large Cassini-Huygens spacecraft used four gravity-assist swing-by manoeuvres: Venus (April 1998), Venus (June 1999), Earth (August 1999) and Jupiter (December 2000) on its journey towards Saturn and Titan. In early 2005, towards the end of Cassini-Huygens' third orbit around Saturn, the probe is ejected on a 22-day cruise to Titan. History The development of the Cassini-Huygens mission, a complex and ambitious venture between NASA and ESA, required substantial scientific, technical and programme planning efforts over several years. In the late 1970s, NASA studied several scenarios for a mission to Saturn as the next natural step after the Galileo orbiter/probe mission to Jupiter in the detailed exploration of the giant planets. The Cassini mission was originally proposed in November 1982 by a team of European and American scientists as a collaborative initiative with NASA in response to a regular call for mission ideas by ESA. Very early in the study phase, the Titan probe was identified as ESA’s potential contribution to the international Cassini mission. It was within the technical capabilities of the European space industry, which had limited experience in planetary missions, mainly acquired with the Giotto mission. During the evaluation studies, the need for using planetary gravity-assist manoeuvres was identified in order to send the spacecraft towards Saturn, as no launcher existed that was powerful enough to send it directly to Saturn. Three launch opportunities were identified; each included a Jupiter fly-by in addition to Venus and Earth fly-bys. A fly-by of Jupiter is required to reach Saturn in a reasonable time: 6.7 years, instead of 9–10 years.
The Titan probe was named Huygens in honour of the Dutch scientist who discovered Titan in 1655.
Partnerships There are six instruments on board the Huygens probe, each developed by different teams. The Huygens Atmosphere Structure Instrument comprises sensors for measuring the physical and electrical properties of the atmosphere and an on-board microphone that will send back sounds from Titan. This is led by the Université de Paris VII, Observatoire de Paris-Meudon, France, with contributions from Italy, Austria, Germany, Spain, France, the United Kingdom, Norway, Finland, and the United States. The Gas Chromatograph and Mass Spectrometer is a versatile gas chemical analyser designed to identify and quantify various atmospheric constituents. It is also equipped with gas samplers which will be filled at high altitude for analysis later in the descent when more time is available. This is led by NASA and supported by contributions from the Austria and France. The Aerosol Collector and Pyrolyser will collect aerosols for chemical-composition analysis. After extension of the sampling device, a pump will draw the atmosphere through filters which capture aerosols. Each sampling device can collect about 30 micrograms of material. This is managed by the French CNRS Service d'Aéronomie, with support from Austria and the United States. The Descent Imager/Spectral Radiometer can take images and make spectral measurements using sensors covering a wide spectral range. A few hundred metres before impact, the instrument will switch on its lamp in order to acquire spectra of the surface material. This is provided by the University of Arizona, United States, with support from Germany and France. The Doppler Wind Experiment uses radio signals to deduce atmospheric properties. The probe drift caused by winds in Titan's atmosphere will induce a measurable Doppler shift in the carrier signal. The swinging motion of the probe beneath its parachute and other radio-signal-perturbing effects, such as atmospheric attenuation, may also be detectable from the signal. This is managed by the Universität Bonn, Germany, with collaboration from Italy and the United States.
The Surface-Science Package is a suite of sensors to determine the physical properties of the surface at the impact site and to provide unique information about its composition. The package includes an accelerometer to measure the impact deceleration, and other sensors to measure the index of refraction, temperature, thermal conductivity, heat capacity, speed of sound, and dielectric constant of the (liquid) material at the impact site. The Open University (UK), manages this with support from ESA, Italy and the United States.
Last update: 3 June 2004
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