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Measuring Physical Properties at the Surface of a Comet Nucleus

Ball, Andrew J. (1997) Measuring Physical Properties at the Surface of a Comet Nucleus. Doctor of Philosophy (PhD) thesis, University of Kent. (KAR id:23088)

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Abstract

The European Space Agency’s cornerstone mission Rosetta is due for launch in January 2003. It will perform a rendezvous with comet 46P/Wirtanen beyond 3 AU and, following an initial mapping phase, deploy a lander to a selected site on the nucleus surface. The Rosetta Lander will provide unprecedented access to cometary material. Some of the most uncertain characteristics of the nucleus material are physical properties such as its density, the structure of the surface layers and its mechanical strength. MUPUS (Multi-Purpose Sensors for Surface and Sub-Surface Science) is one of the experiment packages selected for the Lander payload which will address certain physical properties and their evolution with time. This thesis focuses on the in situ measurement of

the tip of the probe and semiconductor radiation detectors situated at the top of the probe will monitor the transmitted count rate during probe insertion, as the intervening material

Although the use of the gamma ray backscatter type of densitometer was eventually rejected in favour of the aforementioned attenuation technique, investigation of the backscatter technique resulted in an extension to the Single Scattering Model– an analytic approximation of its operation. This extended model adds to our understanding of these devices' response to spatial inhomogeneity. Calculations show that anchoring of the Lander is necessary to avoid possible ejection from the nucleus by gas drag in the case of a landing in an active area. The use of

the Lander’s anchoring harpoon to perform penetrometry measurements is reported, including the results of preliminary experiments and techniques for analysing the accelerometry data. It is shown that layers with distinctly different strengths may be identified, and that the mean deviatoric stress– a strength parameter– may be constrained to within a factor of about 2.2. This would be a significant improvement on current estimates, which vary by several orders of magnitude. Together with other investigations on the Rosetta mission the densitometry and penetrometry measurements will serve to constrain models of the physical state and evolution of the cometary material found at the landing site. In particular both instruments are sensitive to near-surface layering, which may be expected from theoretical models of cometary activity.

Item Type: Thesis (Doctor of Philosophy (PhD))
Subjects: Q Science > QB Astronomy
Divisions: Faculties > Sciences > School of Physical Sciences > Centre for Astrophysics and Planetary Sciences
Depositing User: James Chaffin
Date Deposited: 21 Oct 2009 14:31 UTC
Last Modified: 07 Jul 2020 13:59 UTC
Resource URI: https://kar.kent.ac.uk/id/eprint/23088 (The current URI for this page, for reference purposes)
Ball, Andrew J.: https://orcid.org/0000-0003-1593-3279
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