Kent Academic Repository Full text document (pdf) Citation for published version Ball, Andrew J. (1997) Measuring Physical Properties at the Surface of a Comet Nucleus. Doctor of Philosophy (PhD) thesis, University of Kent. DOI Link to record in KAR https://kar.kent.ac.uk/23088/ Document Version UNSPECIFIED Copyright & reuse Content in the Kent Academic Repository is made available for research purposes. Unless otherwise stated all content is protected by copyright and in the absence of an open licence (eg Creative Commons), permissions for further reuse of content should be sought from the publisher, author or other copyright holder. Versions of research The version in the Kent Academic Repository may differ from the final published version. Users are advised to check http://kar.kent.ac.uk for the status of the paper. Users should always cite the published version of record. Enquiries For any further enquiries regarding the licence status of this document, please contact: [email protected] If you believe this document infringes copyright then please contact the KAR admin team with the take-down information provided at http://kar.kent.ac.uk/contact.html Measuring Physical Properties at the Surface of a Comet Nucleus by Andrew Jonathan Ball December 1997 A thesis submitted for the degree of Doctor of Philosophy Unit for Space Sciences and Astrophysics University of Kent Canterbury, UK Acknowledgements First of all I would like to thank my principal co-workers, Dr. Norbert Kömle of the Institut für Weltraumforschung, Graz, and Matt Whyndham of University College London’s Mullard Space Science Laboratory, Holmbury St. Mary. I enjoyed working in Graz– a total of eight weeks– and appreciate the welcome I received from Norbert, his family and colleagues. Thanks are of course due to the Austrian Academy of Sciences for funding my stays there, and to Masara Dziruni and Günter Kargl with whom I also worked. At MSSL Matt Whyndham (backed by Dr. Alan Smith) has provided much-needed assistance on the experimental front over the past three years, working with me on the densitometer as it evolved as well as all aspects of the experiments presented here. Dr. Ann Chadwick at UKC guided us safely though the radiation protection procedures, and led us to obtain the 137Cs source from Bristol at no cost. The CdTe detector used in the attenuation experiments was kindly lent to us for evaluation by Eurorad, Strasbourg. I would also like to thank Prof. Tilman Spohn and Dr. Karsten Seiferlin at the Institut für Planetologie, Münster for their support and skilful management of MUPUS along the way. It has been a pleasure to work with the international MUPUS team. I would like to thank those at UKC who have contributed to our MUPUS activities, in particular Mark Leese, Harjinder Jolly and Malcolm Wright, as well as the University’s Research Fund which helped us through the early stages of preparing and submitting the proposal. More recently, James Garry and Dr. Martin Towner have been very helpful in developing our penetrometry research activities. Dr. Chris Solomon of the Applied Optics group introduced me to both Compton scattering and Matlab, as well as being there to discuss the details of my work on backscatter densitometry and Monte Carlo simulation. Andy Salmon of the Midlands Spaceflight Society provided me with useful information on some of the more obscure aspects of the Soviet / Russian space programme. Maxim Jacobson at the Moscow Aviation Institute has also been helpful in this respect. The Space Education Trust enabled me to attend the 1996 Summer Session of the International Space University in Vienna– thanks are due to my many ISU friends around the world and to Mrs. Sue Bayford, Executive Secretary of the SET. I would also like to thank the ERASMUS scheme for funding my travel to Austria that year. The Particle Physics and Astronomy Research Council gave me their support in the form of a three-year quota award, as well as financial assistance for attendance at the ii Alpbach Summer School in 1995 and the European Geophysical Society conference in Vienna, 1997. My supervisor, Dr. John Zarnecki, has provided friendly encouragement and advice throughout. Without John I would not have had the opportunity to work with the MUPUS team or test my map-reading skills on the motorways of France, Belgium, the Netherlands and Germany. Thanks are due to my fellow students in and around Room 164 for their comradeship during my time here– in particular Emma Taylor, Jon Marchant and Nick Shrine for sharing the ups and downs of the writing-up process, whether that be in the project room, Possums Bistro, Woody’s or simply slumped happily on the sofa after a large meal. I would also like to thank Nick Shrine for the sterling help he has always provided with our computer systems– I’m not sure how we’d have coped otherwise. Sarah Dunkin and the restaurants around UCL have also been very supportive. My family have of course supported and encouraged me all the way. iii 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 density of the surface layers by a radiation densitometer incorporated into the MUPUS thermal probe, and on the penetrometry measurements to be performed by an accelerometer mounted in the Lander’s anchoring harpoon. A concept for incorporation of a gamma ray attenuation densitometer into the thermal probe is presented and explored. A 137Cs radioisotope source will be mounted near 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 attenuates the radiation. Preliminary experiments to evaluate cadmium telluride (CdTe) detectors for this purpose are presented, as well as results from a specially-developed Monte Carlo computer code designed to model the absorption and scattering of photons in bulk material. Also presented is a control algorithm to dynamically re-budget the integration time and depth resolution of the instrument as it is inserted by the hammering mechanism. This is required due to: a) the wide range of possible densities the instrument may encounter, b) the variation vs. depth of required integration time, and c) the limited time in which the measurement must be performed. For lower than nominal densities, integration time may be wasted when it could be used to improve the accuracy and depth resolution. For higher densities the integration time at particular depths may not be sufficient to obtain acceptable accuracy; in this case some depth resolution could be sacrificed to improve the accuracy. The proposed algorithm uses the density measured at each point to update the time budget and depth resolution for the remaining stages of penetration. 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. iv Table of Contents ACKNOWLEDGEMENTS II ABSTRACT IV TABLE OF CONTENTS V LIST OF ACRONYMS VIII 1 INTRODUCTION 1 1.1 COMET NUCLEUS MATERIAL 1 1.1.1 Past, present and future investigations of cometary material 3 1.2 HISTORY OF THE ROSETTA MISSION 4 1.3 HISTORY OF THE ROSETTA LANDER 10 1.4 HISTORY OF MUPUS 20 1.5 OVERVIEW OF THIS THESIS IN THE CONTEXT OF COMETARY SCIENCE 25 2 BACKGROUND 28 2.1 SCIENTIFIC BACKGROUND 28 2.1.1 The physical nature of cometary material 30 2.1.2 Rationale for density measurement 33 2.1.3 Cometary nucleus density measurement 39 2.1.4 Questions to be answered 41 2.2 TECHNOLOGICAL BACKGROUND OF BACKSCATTER DENSITY MEASUREMENT 42 2.2.1