Huygens Payload

The Huygens payload consists of six complex instruments provided by the Principal Investigators. A payload description is available in: The Huygens Probe: Science, Payload and Mission Overview (Lebreton and Matson), 1997. A brief description of each of the instrumentis given below:

Huygens Atmospheric Structure Instrument (HASI):

HASI is a multi-sensor instrument that will measure the physical and electrical properties of Titan's atmosphere. Its set of sensors consists of a 3-axis accelerometer, a temperature sensor, a multi-range pressure sensor, a microphone, and a electric field sensor array. The set of accelerometers is specifically optimised to measure entry deceleration for the purpose of inferring the atmosphere structure during the entry. The electric field sensor consists of a relaxation probe to measure the atmosphere's ionic conductivity and a quadripole array of electrodes fpr measuring the permittivity of both the atmosphere and of the surface. The sensor will also be used to detect atmosheric electromagnetic waves. In order to infer information on the surface, HASI will also process the reflected signal of the radar altimeter.

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The Doppler Wind Experiment (DWE):

The primary scientific objective of this experiment is to determine the direction and strengh of Titan's zonal winds. A height profile of wind velocity will be derived from the residual Doppler shift of Huygens radio relay signal as received by CASSINI. This will be corrected for all known Probe and Orbiter motion and signal propagation effects. Wind-induced motion of the Probe will be measured to a precision better than 1 m/s starting from parachute deployment at an altitude of about 165 km down to the surface. A secondary objective is to investigate the probe dynamics (spin rate, spin phase) during the descent, and the probe's location and orientation up to and after impact.

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The Descent Imager/Spectral Radiometer (DISR):

DISR is the optical remote sensing instrument aboard Huygens. It includes a set of upward and downward looking photometers, visible and IR spectrometers, a solar aureole sensor, a side-looking imager, and two down-looking imagers: a medium-resolution and a high-resolution imager. There is also a sun sensor that will measure the spin rate. DISR will make measurements in the 0.3 to 1.7 µm range.

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The Aerosol Collector and Pyrolyser (ACP):

This instrument will collect aerosol that will be analysed by the Gas Chromatograph and Mass Spectrometer experiment. It is equipped with a deployable sampling device that will be operated twice during the descent: the first sample from the top of the atmosphere down to about 40 km; the second sample from about 23 km down to 17 km. After extension of the sampling device, a pump draws the atmosphere and its aerosols through a filter in order to capture the aerosols. At the end of each collection period, the filter is retracted into a pyrolysis furnace where the effluent from the captured aerosols is analysed, first at ambiant temperature (about 0 °C), subsequently heated to 250 and then to 600 °C in order to conduct a step-wise pyrolysis. The pyrolysed products are flushed into GCMS for analysis.

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The Gas Chromatograph and Mass Spectrometer (GCMS):

GCMS is designed to measure the chemical composition of Titan's atmosphere from 170 km to the surface abd determine the isotope ratios of the major gaseous constituents. It will also analyse gas sample from the ACP experiment, and will investigate the composition of several candidate surface materials.

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The Surface Science Package (SSP):

SSP is a suite of laboratory-type sensors for determining the physical properties of the surface at the impact site, and for providing information on the composition of the surface material. The instrument includes a force transducer for measuring the impact deceleration, and sensors to measure the refraction index, temperature, thermal conductivity, heat capacity, speed of sound and dielectric constant of the surface material. It includes an acoustic sounder for sounding the atmosphere's bottom layer and the surface's physical properties before impact. If the Probe lands in a liquid, the sounder will be used to probe the liquid depth. A tilt sensor is included to indicate the Probe's attitude after impact.

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