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The Huygens probe's descent profile through Titan's atmosphere.

Cover by fiberglass struts with pyrotechnic mechanisms. The parachutes are carried on the Top Platform. The upper and lower surfaces of the Equipment Platform accommodate the five lithium sulphide batteries of the Electrical Power Subsystem and the six science experiments.

As Titan's atmosphere is so deep, the first contact will be at an altitude of about 1,250 kilometres, travelling at 6,150 metres per second. The timer will awaken the probe's sequencer 15 minutes beforehand. Atmospheric penetration will be over the day hemisphere in order to facilitate imaging during the descent. Although the probe will be subjected to a peak deceleration of 16 times that of the Earth's gravity, this is an order of magnitude less than that endured by the Galileo probe in penetrating the Jovian atmosphere. Simulations indicate that the temperature of the shockwave that will form in front of the probe will peak at about 12,000°C as it passes from 300 to 200 kilometres altitude, and this heat may stimulate reactions in the detached layers of hydrocarbons in this altitude range.31 The deceleration phase will last about three minutes, during which the probe's velocity will fall to about 1,400 kilometres per hour. At an altitude of about 170 kilometres, towards the end of the deceleration phase, as the onboard accelerometers sense that the probe has slowed to Mach 1.5, a mortar will deploy a 2.6-metre-diameter drogue parachute, which will pull off the Back Cover in order to permit the deployment of the 8.3-metre-diameter braking parachute. About 30 seconds later, having slowed to Mach 0.6, the probe will jettison its Front Shield and expose the inlets for the sampling instruments, which will start to report data at this point. Fifteen minutes later, at an altitude of 125 kilometres, and now falling at 360 kilometres per hour, the main parachute will be released, in the process drawing out the 3-metre-diameter parachute that will be used for the rest of the descent. If the probe were to remain on its large braking parachute, it would not be able to reach the surface before its batteries expired. This smaller parachute has been scaled so that the probe will rapidly descend through the chilly tropopause without freezing, and yet reach the surface sufficiently slowly to survive the impact. On Titan's chilly surface, water ice will be as hard as steel, so the probe will hopefully strike the surface at no more than 6 metres per second. The first part of the probe to make contact with the surface should be the spear-like penetrometer on the base, which was built by Ralph Lorenz while working for his doctorate at the University of Kent. On the surface, the probe's accelerometers will indicate whether it is stationary on dry land, or has splashed down and is riding the waves.

Overall, the parachute descent is expected to take just over two hours (even with the benefit of the Voyager data it is difficult to predict the timing accurately), so the probe will have at most half an hour on the surface before its batteries expire. They are rated for a total of 350 watts, which ought to be more than enough for the descent. The probe will report the data from its instruments over an 8-kilobit S-Band radio link using a pair of redundant radio transmitters. On board Cassini, systems in the PSE will recover the data and feed this to Cassini for storage in solid-state memory for subsequent replay to Earth, and monitor the Doppler shift on the probe's relay link in order to track its motions within the atmosphere. The Deep Space Network will forward the probe's data to ESA's operations facility in Darmstadt, Germany, for analysis. The Data Relay Subsystem was built by Alenia Spazio in Italy.

A cutaway of the Descent Module of the Huygens probe snuggled within the shield that will protect it during the deceleration phase of entering Titan's atmosphere.

DASA undertook the assembly, integration and testing of the Huygens probe for shipment to NASA. Once the 2,150-kilogram 'dry' mass of the main spacecraft was augmented with 3,132 kilograms of propellant, the 373-kilogram Huygens probe and the 165-kilogram launch vehicle adapter, the total payload was 5,820 kilograms. Like all spacecraft designed to enter orbit around a giant planet in the outer Solar System, Cassini's mass is dominated by its propulsion system and propellants. The finished vehicle stood 6.8 metres tall in the Kennedy Space Center's preparation facility.

Instrumentation

Huygens carries 48 kilograms of science instruments to perform the first-ever in situ study of Titan's atmosphere and surface. In the parachute descent, the atmospheric instruments will profile the temperature, pressure, density, composition and wind as functions of altitude. As sampling will start at the 170-kilometre level, this 'sample column' will be entirely within the 200-kilometre-deep optically thick orangey haze. After landing, or perhaps splashing down - the Descent Module is designed to float in a hydrocarbon fluid - the physical properties of the surface will be measured and pictures taken of what is sure to be an intriguing out-of-this-world land(sea)scape.32,33

The Huygens Atmospheric Structure Instrument (HASI) is the counterpart of an instrument carried by the atmospheric probe dropped by the Galileo spacecraft into Jupiter's atmosphere. As the only one of the Huygens suite to be activated during the deceleration phase, it will profile the temperature and pressure. Its measurements of the state of the atmosphere at altitudes at which the detached layers of thin haze form should help to identify their composition.34 While Voyager 1 found no evidence of lightning in the atmosphere, HASI will assess this possibility. A thermodynamic study suggested that the formation and evolution of clouds in Titan's troposphere are governed by convection, in which case it will resemble the Earth's in being driven by latent heat released from the primary condensable species.35 Electrical breakdown may occur in regions of strong vertical motion, resulting in high-current discharges. Even if HASI fails to detect anything, this will not rule out the possibility. Lightning on Titan ought to be concentrated near the subsolar point where the energy input is greatest. The probe's brief in situ study will be supplemented during a succession of fly-bys by Cassini's RPWS.36 HASI's principal investigator is Marcello Fulchignoni, initially of the University of Rome but now at the Meudon Observatory in Paris.

The Gas Chromatograph Neutral Mass Spectrometer (GCMS) will draw in gas, mix it with the hydrogen 'carrier', and feed this into the mass spectrometer which will first ionise the sample then separate the ions using a magnetic field to enable them to be counted. It will be able to identify elements up to 146 atomic mass units with a sensitivity of one part in 1012. The results will profile atmospheric composition. The principal investigator is Hasso Niemann of the Goddard Space Flight Center. The Aerosol Collector and Pyrolyser (ACP) will sample the stratosphere (above 45 kilometres) and the 'middle' troposphere (in the 15- to 30-kilometre altitude range), evaporate each sample, thermally dissociate its molecular species, and feed its output into the GCMS in order to measure the aerosol composition as a function of altitude. Specifically, it will determine the proportions (a) of carbon, hydrogen, nitrogen and oxygen in aerosols, (b) of condensed hydrocarbons and nitriles in the stratosphere, and (c) of condensed methane in the troposphere. Guy Israel of Service d'Astronomie du Centre National de la Recherche Scientifique (CNES) in France is the principal investigator. About 20 per cent of the GCMS's operating time will be devoted to the analysis of the ACP's samples.

The Descent Imager and Spectral Radiometer (DISR) is a composite instrument. A set of fibre optics viewing upwards, to the side, and downwards will illuminate a silicon photodiode, a CCD array, and a pair of near-infrared linear arrays in order to

HASI deptoymenl booms (2)

HASI slud /

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