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| Titel |
Numerical simulations of PP-SESAME/Philae/ROSETTA operations during the Descent Phase and at the surface of the Churyumov-Gerasimenko nucleus |
| VerfasserIn |
Anthony Lethuillier, Michel Hamelin, Alice Le Gall, Sylvain Caujolle-Bert, Walter Schmidt  , Réjean Grard |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250096777
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| Publikation (Nr.) |
 EGU/EGU2014-12296.pdf |
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| Zusammenfassung |
| The ROSETTA probe has never been so close to its target; the comet Churyumov-Gerasimenko
that it will reach later this year. Among the instruments on board the lander, Philae, the
Permittivity Probe (PP) experiment, which is part of the Surface Electric Sounding and
Acoustic Monitoring Experiment (SESAME) package, will measure the low frequency
complex permittivity (i.e. dielectric constant and electrical conductivity) of the first 2 meters
of the subsurface of the cometary nucleus. At frequencies below 10 kHz, the electrical
signature of the matter is especially sensitive to the presence of water ice and its temperature
behavior. PP will thus allow to determine the water ice content in the near-surface and to
monitor its diurnal and orbital variations thus providing essential insight on the activity and
evolution of the cometary nucleus.
The PP instrument is based on the quadrupole array technique, which employs a set of
transmitter and receiver electrodes for emitting alternating currents into a medium of interest.
The complex permittivity of the cometary surface material is determined by measuring the
magnitude and phase shift of both the emitted currents and the resulting potential difference
at a pair of receiver electrodes. This technique has been used for many decades on
Earth and recently helped to determine the electrical properties of the Huygens
landing site on Titan (PWA/HASI experiment on Cassini-Huygens). In the case of
PP, 5 electrodes can be used: 2 receiver electrodes are integrated into the lander
feet while the transmitter electrodes are mounted on the third foot and on 2 other
instruments.
In this paper we will present results from numerical simulations performed in order to
model PP operations and prepare the scientific return of this experiment.
Though simple in theory, the inference of the complex permittivity from PP
measurements is not straightforward in practice. In particular, the actual environment of the
electrodes (lander body, feet, harpoons-¦) must be accounted for since the presence of nearby
conducting objects will affect the data. We have thus developed a numerical model of the
electrodes in their environment using COMSOL Multiphysics®. A simple version of this
model was validated by comparison to laboratory measurements and analytical
calculations.
This model was then used to simulate PP operations during the Descent Phase of the
lander (i.e. in the void and as the ground gets closer) and once at the surface of the nucleus
considering different types of surfaces. The first set of simulations will be very useful to
better understand the calibration data that will be acquired after separation from the
ROSETTA Orbiter while the second will illustrate the idealistic sensitivity of PP to the
ground electrical properties. |
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