The Cassini Huygens Mission
The Cassini Huygens spacecraft is expected to arrive at Saturn on July.1, 2004 and the Huygens probe is due to land on Titan on January the 14th of 2005. It is one of the most ambitious missions in the history of space exploration.The overall cost is around 3.5 billion dollars.The part of the cost relating to the probe is close to 300 million dollars.The mission is the outcome of a joint effort from NASA and ESA.The Cassini spacecraft has been conceived by NASA while the Huygens probe has been designed by the Europeans.The Huygens probe weighs some 320 kg It contains a conical heat shied 2.7 m across with a 2 cm thick layer of ceramic tiles which will be crucial in the descent phase into the atmosphere of Titan (around 1270 km high) since it will be heated up to 1400 K.The Cassini spacecraft is so massive that even NASA's largest launcher, the Titan IV could not project it directly to Saturn.It would have to use the gravitational pull of several planets to gain speed and energy.The descent of the The Huygens probe into Titan's atmosphere will last 2.5 hours.If it survives the touchdown, it should transmit data to the Cassini orbiter during thirty extra minutes until it stops operating.The Huygens probe will have been slown down by a huge parachute some 8 m across so that the landing is carried out at about 10 miles per hour (around 16 km per hour).The data will be released after a year or less for scientists.Huygens carries a DVD ROM disk on which are digitally recorded some 616 400 signatures sent in on postcards by people from 81 countries, notably signatures from Jean Dominique Cassini, Christiaan Huygens and the entire crew of the movie "Contact".The Cassini spacecraft carries a similar disk.While the life span of the Huygens probe is around 3 hours, the duration of the Cassini spacecraft should extend until 2008 and more precisely from the Saturn orbit insertion in July 2004 to June 2008. During this four year tour, it will have visited and studied Phoebe in June 2004, Titan in October and december 2004, Iapetus in August 2007 and othet bodies like Enceladus,Hyperion, Dione or Tethys.
The Titan probe was selected in November 1988 and was named Huygens but the mission would be implemented only if the US Congress approved the Titan project.That was made in 1989.Two industrial bids were submitted to ESA for the construction of Huygens, one led by the french firm Aerospatiale, the other led by British Aerospace. In the end, the Aerospatiale bid was selected.It included players from elsewhere in Europe that is to say Deutsche Aerospace, CASA of Spain, Logica of the UK and italian players.The mission plan set that Cassini would arrive at Saturn in 2002.The project could have been cancelled.Indeed,in 1992, the US Congress almost forced NASA to decide between building the International Space Station and conceiving the Cassini spacecraft. After a concerted lobbying effort, Cassini was saved.Once again, the Cassini project could have disappeared in the mid 90's but the international dimension saved it.A major element that was contested was the plutonium power supply of Cassini.On that point, There was a risk of a launch accident releasing plutonium.One have to know that all rockets have around one in thirty chance of failing.Nevertheless,this mission couldn't be envisaged with no plutonium power supply since the solar energy received at this distance from the Sun ( about 1.4 billion km ) is very weak. If the launch was beyond December 1997, it would not be able to catch a Venus flyby and would take an extra two years to reach Saturn.Finally, it was launched in October 1997.In 2000, the managers of the mission realized that the receiver on the orbiter and the transmitter on the probe were not set on the correct frequencies.The signal from the probe is Doppler shifted by the orbiter's motion towards it.More testing on the ground could have identified and fixed the Huygens radio problem.It took over six months to develop a solution.The only way to solve the problem turned out to be to modify the Cassini orbiter's trajectory by flying past Titan at a higher altitude.The Doppler shift on the signal will be reduced to a level the receiver can easily cope with.The Huygens probe is scheduled to be released on Christmas Day 2004, but it will land on the 14th of January 2005.
The Instruments onboard the Huygens Cassini Spacecraft
A dozen of instruments will be used by the spacecraft.The most important experiment comes from the Gas Chromatograph Mass Spectrometer (GCMS) that will analyse both the composition of atmospheric gases and the material delivered from the aerosol collector and pyrolyser.The second US led experiment is the descent imager spectral radiometer (DISR) that will get data about properties of the haze.We can also mention the CCD detector, the infrared spectrometer, the Doppler Wind Experiment (DWE), the aerosol collector pyrolyser (ACP) to trap the aerosol particles(which will be broken in a oven so that their composition can be analysed by the GCMS), the Huygens Atmospheric Structure Instrument (HASI) to measure the basic properties of the atmosphere during the entry and descent and the Surface Science Package (SSP), with small sensors,an accelerometer to evaluate the density profile of the atmosphere and a penetrometer to characterise the impact with the surface, an oceanographer, a refractometer that will measure the ethane / methane composition of an ocean.There are also thermal, density sensors,an acoustic sensor to measure the depth in a liquid.Certain instruments onboard the Huygens spacecraft have been conceived by europeans and some other have been conceived by americans. Several experiments on the Cassini orbiter will be used to study Titan, notably the sensitive radio antennae to search for electrical phenomena and listen out for lightning on Titan.A compass needle will detect an hypothetic ocean.The Cassini spacecraft also carries a set of sensitive magnetometers, a Visual and Infrared Mapping Spectrometer (VIMS) that will enable scientists to detect subtle changes in the brightness of surfaces in specific colours which are signatures of particular materials such as methane or water ice.Cassini has a radar to measure the radar reflectivity of broad regions of the surface, to evaluate the temperature.There is also a radar altimeter to measure the gross topography: thanks to that instrument, scientists will be able, given its rotation and the effects of Saturn's gravity,to check if Titan is a little flattened along one axis and stretched in another.The camera system or ISS (Imaging Science Subsystem) comprises two telescopes which have sensitive CCD detectors. Cassini's radar will map all of Titan crudely but will image only around 25% of Titan's surface at its highest resolution.