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Impact Hazard Associated with Large Meteoroids from Disrupted Asteroids and Comets

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Impact Hazard Associated with Large Meteoroids from Disrupted Asteroids and Comets ADVERTIMENT. Lʼaccés als continguts dʼaquesta tesi queda condicionat a lʼacceptació de les condicions dʼús establertes per la següent llicència Creative Commons: http://cat.creativecommons.org/?page_id=184 ADVERTENCIA. El acceso a los contenidos de esta tesis queda condicionado a la aceptación de las condiciones de uso establecidas por la siguiente licencia Creative Commons: http://es.creativecommons.org/blog/licencias/ WARNING. The access to the contents of this doctoral thesis it is limited to the acceptance of the use conditions set by the following Creative Commons license: https://creativecommons.org/licenses/?lang=en Impact hazard associated with large meteoroids from disrupted asteroids and comets Manuel Moreno Ib´a˜nez Institut de Ci`enciesde l’Espai (ICE, CSIC/IEEC) Advisors: Dr. Josep Ma. Trigo Rodr´ıguez & Dr. Maria Gritsevich Tutor: Prof. Dr. Jordi Mompart Penina Universitat Aut`onomade Barcelona Departament de F´ısica A thesis submitted for the degree of Doctor of Philosophy in Physics Bellaterra, September 2018 © September 2018 Science is simply common sense at its best, that is, rigidly accurate in observation, and merciless to fallacy in logic. — Thomas Huxley ABSTRACT Large meteoroid fragments disrupted from asteroids and comets may encounter the Earth along their orbits, posing extremely hazardous scenarios. Contemporary events like Chelyabinsk (2013), Carancas (2007) or Tunguska (1908) demonstrated that met- eoroids in the diameter range of 1 to 100 m can devastate large areas and injure local population through the associated energetic blast, or even produce casualties due to localized crater excavation. Despite the relatively low frequency of these events, they have become a major concern within space agencies and other planetary defense initi- atives which are currently developing impact mitigation tactics. This is in line with the growing popularity of this subject in the scientific community. The number of yet unresolved questions underlying the pre- and impact physics motivates the work carried out in this thesis. The study of meteoroids that encounter the terrestrial atmosphere provides valuable clues about their progenitors, their delivery mechanisms to Earth, and their ability to threaten our planet. This thesis starts by exploring the complexity of meteoroid dynamics through two exceptional phenomena. On the one hand, a limited number of meteoroid dust trails detached from the comet 8P/Tuttle 620 years ago impact the Earth when the parent comet is in its aphelion, thus increasing the activity of the annual Ursid meteor shower. Aphelion-related increases in a meteor shower activity are uncommon and hence the Ursids offer a new perspective of orbital mechanics. On the other hand, meteorite falls, like Annama, can be orbitally linked to celestial bodies if their atmospheric trajectories are accurately recorded. Exploring these parental relationships offer the opportunity to overcome the uncertainties emerged from the short-term orbital evolution of near-Earth objects and ultimately predict future impacts. The second part of this thesis focuses on the characterization of the atmospheric flight of a meteoroid. While up to-date re-entry models that account for the meteoroid ablation and fragmentation are common, no observational or modelling studies have re- solved the intricacies associated with the mesosphere and lower thermosphere region for meteoroids travelling at hypersonic velocities and in rarefied gas flow conditions. This thesis presents the first observational validation of the flight flow regimes of centimeter- sized meteoroids and provides a new insight into the consequences for the meteoroid flight physics due to the generation of a shock wave. Meteoroid shock waves are also intimately related to the meteoroid energy deposition at different heights, which can alternatively be stated from the analysis of the terminal height of the meteoroid’s tra- jectory. A new approach capable to predict the terminal heights is outlined in this thesis. The results show that, besides the great accuracy achieved, the calculated ter- minal heights are a valuable input to the derivation of atmospheric flight parameters. Furthermore, the approach taken provides a new way of classifying impacting meteor- v oids that improve previous classification scales. Finally, a discussion of the implications of the previous analysis to impacting bodies of different sizes is carried out. Since Earth impacts by meter-sized or smaller bodies are more frequent, the study of sub-metric meteoroids provides a wide catalogue of events that can be crucial to understand the meteor physics. Being able to extrapolate the behaviour of these bodies to asteroid sizes can provide new clues on the underlying physics and make predictions concerning the degree of hazard associated with energetic events. The results of this work also provide feedback and an alternative approach to current and foreseen numerical simulations that were seen in the past essential to deal with these challenging encounters. vi RESUMEN El impacto de grandes fragmentos desprendidos de cometas y asteroides contra la Tie- rra puede llegar a ser muy peligroso. Recientemente, sucesos como Chelyabinsk (2013), Carancas (2007) o Tunguska (1908) han demostrado la capacidad que tienen los meteo- roides con di´ametrosentre 1 y 100 m para devastar grandes extensiones y amenazar a la poblaci´onlocal, ya sea mediante la liberaci´onde energ´ıaen la atm´osferao el impacto en la superficie. A pesar de la baja frecuencia de este tipo de sucesos, la preocupaci´on entre agencias espaciales y otras iniciativas de defensa planetaria es creciente, y ya est´an elaborando t´acticasde mitigaci´onante posibles impactos. Este inter´esse extiende tam- bi´enen la comunidad cient´ıfica.Adem´as,el estudio de los meteoroides que impactan la atm´osferaterrestre revela valiosa informaci´onsobre sus progenitores, los mecanismos que siguen para llegar a la Tierra, y su habilidad para amenazar nuestro planeta. As´ı, esta tesis tiene por objeto dar respuesta a una serie de interrogantes sobre la f´ısicaaso- ciada al impacto y a las condiciones pre-impacto. Este trabajo comienza explorando aspectos complejos de la din´amicaorbital de me- teoroides a trav´esde dos fen´omenosexcepcionales. Por un lado, la actividad anual de las Ursidas´ aumenta cuando ciertos enjambres de meteoroides desprendidos del cometa 8P/Tuttle hace 620 a˜nosimpactan la Tierra encontr´andoseel cometa en su afelio orbital. Los incrementos de actividad cuando el cometa est´aen su afelio no son comunes y por tanto las Ursidas´ ofrecen nuevas claves sobre la mec´anicaorbital. Por otro lado, si la trayectoria atmosf´ericade un meteorito, como es Annama, se obtiene de manera preci- sa, ´estese puede vincular orbitalmente con un cuerpo celeste. Dada la r´apidaevoluci´on orbital de los NEA (considerados) posibles progenitores, este estudio nos permite acotar mejor sus ´orbitasy predecir futuros impactos. La segunda parte de la tesis versa sobre la caracterizaci´ondel vuelo atmosf´ericode un meteoroide. Los modelos actuales son capaces de considerar la ablaci´ony fragmentaci´on de un meteoroide en la atm´osfera,pero a´unno es posible comprender completamente, ni de manera observacional ni num´erica, el vuelo hipers´onicode un meteoroide en la regi´ondefinida por la mesosfera y baja termosfera donde el gas se considera rarificado. Esta tesis presenta la primera comprobaci´onobservacional de los reg´ımenesde vuelo para meteoroides centim´etricosa estas alturas, y discute las consecuencias que origina la formaci´onde una onda de choque en la f´ısicadel vuelo del meteoroide. Adem´as,las ondas de choque est´an´ıntimamente relacionadas con la energ´ıadepositada por el meteo- roide a distintas alturas; magnitud que tambi´ense puede acotar conociendo su altura terminal. As´ı,esta tesis se ofrece tambi´enuna nueva metodolog´ıapara calcular estas alturas. Los resultados obtenidos son muy precisos y muestran que el c´alculode las alturas terminales es muy ´utilpara derivar otros par´ametros del vuelo del meteoroide. Es m´as,el planteamiento usado ofrece una nueva manera de clasificar los impactos de vii meteoroides y mejorar escalas anteriores. Por ´ultimo,se discute la extrapolaci´onde los estudios anteriores a cuerpos de dife- rentes tama˜nos.Aunque menos peligrosos, los impactos de cuerpos inferiores a 1 metro son los mas frecuentes y, en consecuencia, nutren las bases de datos y resultan funda- mentales para abordar el estudio de meteoroides. La extrapolaci´onde las conclusiones obtenidas previamente a objetos mas grandes puede revelar claves sobre la f´ısicasubya- cente y aportar nuevas predicciones sobre el riesgo asociado a impactos energ´eticos.Los resultados de esta investigaci´onproveen tambi´enun enfoque alternativo al desarrollo de modelos num´ericos,tanto actuales como futuros, que hasta hoy han sido fundamentales para afrontar el estudio de meteoroides. viii ACKNOWLEDGMENTS The research presented in this thesis was a result of team work. From the guidance of my directors to the daily support of my family, friends and girlfriend. Every tiny bit of positive encouragement, scientific lesson or understanding of my thesis obligations (that hindered weekend plans or special appointments) has contributed to the final sub- mission of this work. First, I am very grateful to my two directors, Dr. Josep Mar´ıaTrigo Rodr´ıguezand Dr. Maria Gritsevich. I understand the difficulties of co-directing a thesis and so I appre- ciate your efforts to promote my learning on this fascinating field and to guide my work in a collaborative manner. I do also thank the support you gave me to temporally host my research in your institutes. These temporal stays were funded by the AYA 2011- 26522 Spanish national plan research project, the Catalan Institute of Space Studies (IEEC), and by the project no. 260027 of the Academy of Finland, to which I gratefully acknowledge the support. Having such a wonderful couple (something like a mother and father) with such different profiles supporting my work has offered me the opportunity to learn a diversity of skills and to gain an insight into two alternative ways of seeing science and life.
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