Huygens
Titan, the second largest moon of the Solar System (after Jupiter's Ganymede), is a fascinating world because its thick nitrogen-atmosphere is very rich in organic compounds which are constantly reacting. If they were found on a planet with Earth-like conditions, their presence would be considered as a sign of life. If water exists on Titan, it cannot be in liquid form because its surface is far too cold (at -180°C). In fact very little is yet known about the surface and scientists speculate that Huygens may find lakes or even oceans of a mixture of liquid ethane, methane and nitrogen.
Huygens is making the seven-year journey to Titan on NASA's Cassini spacecraft. It spends most of the trip in a 'dormant' state, but it will be activated to perform its mission once the pair reach Saturn. There Huygens will be released to parachute through the atmosphere of Titan, whilst Cassini will continue to explore Saturn and its rings.
ESA's Huygens mission is named after the Dutch astronomer Christiaan Huygens (1629-1695), who discovered Saturn's rings and Titan.
Objectives
Huygens will make a detailed on-the-spot study of Titan's atmosphere. It will also characterise the surface along the descent ground track and near the landing site.
Some of the questions scientists would like to answer are:
- Which chemical reactions exactly are occurring in Titan atmosphere?
- What is the source of the very abundant methane, an organic compound that on Earth is associated to biological activity?
- Are there any oceans?
- Do more complex organic compounds and pre-biotic molecules exist on Titan?
Cost:
The Huygens mission costs about 260 million Euros. This includes everything but the payload (about 100 million Euros), which was built by universities and research institutes funded by the ESA Members States and NASA.
Launch:
15 October 1997 (Titan-IVB/Centaur at Cape Canaveral, United States).
Journey
Cassini/Huygens did four gravity-assist swing-by manoeuvres: Venus (April 1998), Venus (June 1999), Earth (August 1999), and Jupiter (December 2000). In July 2004, the spacecraft will arrive at Saturn. In December 2004, Huygens will be released on its 22-day cruise to Titan. Its entry into Titan's atmosphere is planned for 14 January 2005.
Planned mission lifetime
Huygens will be fully activated for a couple of hours. The probe will be 'woken up' just before entering the outer fringe of Titan's atmosphere. Most of the data will be collected while descending through the atmosphere, which will take 2 to 3 hours. The landing will take place at the relatively low speed of about 20 kilometres per hour. Upon a successful landing Huygens will continue to send information back to Cassini up to 2 hours until its batteries run out. In any case, Cassini will listen to Huygens for 4 ½ hours, until it disappears below Titan's horizon.
Spacecraft
Design:
Huygens is built like a shellfish: a hard shell to protect a delicate interior from extreme temperatures experienced during the descent through the atmosphere. It consists of two parts: the Entry Assembly Module and the Descent Module. The Entry Assembly Module carries the equipment to control Huygens after the separation from Cassini, and has a shield that will act as a brake and as thermal protection. The Descent Module has the scientific instruments. The probe will use three different parachutes in sequence during the descent.
Mass:
Total mass is 349 kilograms, including payload (49 kilograms) and Probe Support equipment on the Orbiter (30 kg). Launch mass of Cassini and Huygens: 5.82 tonnes.
Dimensions:
Diameter: 2.7 metres
Industrial involvement:
Prime contractor is Alcatel. Companies from 14 European countries and the United States have collaborated in the construction of Huygens and its instruments.
What's on board?
Huygens Atmosphere Structure Instrument - HASI HASI will measure 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 will analyse the chemical composition of the gas.
Principal Investigator: Hasso Niemann, NASA/GSFC, United States.
Aerosol Collector and Pyrolyser - ACP ACP will collect aerosols for chemical-composition analysis.
Principal Investigator: Guy Israel, CNRS Service d'Aéronomie, Verrières-le-Buisson, France.
Descent Imager/Spectral Radiometer - DISR DISR will take images and make spectral measurements.
Principal Investigator: Marty Tomasko, University of Arizona, Tucson, USA.
Doppler Wind Experiment - DWE DWE will study the propagation of radio signals through the atmosphere to understand its properties.
Principal Investigator: Mike Bird, Universität Bonn, Germany.
Surface-Science Package - SSP SSP will determine the physical properties of the surface at the impact site and provides unique information about its composition.
Principal Investigator: John Zarnecki, Open University, United Kingdom.
A group of interdisciplinary scientists has developed models to simulate the conditions the probe will face during the descent and is helping to coordinate the planning of Titan's observations by the orbiter. They are:
- Daniel Gautier, Observatoire de Paris-Meudon, France (Titan aeronomy)
- Jonathan Lunine, University of Arizona, Tucson, United States (surface-atmosphere interactions)
- Francois Raulin, Université-Paris 12, Creteil, France (organic chemistry and exobiology)
Operations
Before arrival at Titan: Huygens is dormant during the long journey to Saturn, but it is 'woken up' every six months by ESA's flight controllers for a complete check-up.
Descent to Titan: A system of alarm clocks will wake Huygens at a pre-programmed time a few hours before it reaches the outer fringe of Titan's atmosphere. During the first three minutes inside the atmosphere, it will have to decelerate from 18 000 to 1400 kilometres per hour. The heat developed by the friction on its shield may reach temperatures up to 1800°C. The robotic controls will then fire a pilot parachute to pull out the main parachute at a speed of about 1500 kilometres per hour. Within a minute, the speed will go down to less than 300 kilometres per hour. The shell of the Entry Assembly Module will then fall away and expose the scientific instruments to Titan's atmosphere, at a height of about 160 kilometres. The atmospheric temperature may then be about -120°C. At about 120 kilometres, the main parachute will be cut away and replaced by a smaller one. At an altitude of about 45 kilometres the probe will go through the coldest layer of the atmosphere (tropopause) where the temperature will be about -200°C. Within two and a half hours, the descent will be complete.
Ground control:
Huygens flight operations are conducted from the European Space Operations Centre (ESOC) in Darmstadt, Germany. All telecommands are prepared at ESOC and sent via NASA's Deep Space Network to Cassini which stores them on board for release to Huygens at a pre-determined time. Data are received back via the reverse path. Data about the on-board systems are distributed to Alcatel for performance analysis of the probe. Experiment data are distributed by ESOC to the payload teams, which perform the analysis remotely during the in-flight checkout activities. During the mission phase, data analysis will be done at ESOC.
ESA Mission Manager and Project Scientist: Jean-Pierre Lebreton
For further information please contact:
ESA Science Programme Communication Service
tel: +31 71 565 3273
fax: +31 71 565 4101
ESA Media Relations Service
tel: +33 1 5369 7155
fax: +33 1 5369 7690