This thesis, at the interface between the scientific disciplines of planetary science and granular physics, has two key components, both of which intend to increase our understanding of granular dynamics in varying gravitational conditions. The dynamics of granular materials are involved in the evolution of solid planets and small bodies in our Solar System, whose surfaces are generally covered with regolith. Understanding granular dynamics is also critical for the design and/or operations of landers, sampling devices and rovers to be included in space missions. The first component of this thesis is the validation of the hard-sphere discrete element method implementation in the N-body code pkdgrav to model the dynamics of granular material. By direct comparison with results from laboratory experiments, it is demonstrated that the hard-sphere discrete element method implementation in pkdgrav is valid for modelling granular material in dilute regimes and is capable of reproducing the complex dynamical behaviour of a specific dense system as well. The second component is focussed on the AstEx parabolic flight experiment. This experiment, with the aim of characterising the response of granular material to rotational shear forces in a microgravity environment, was designed, constructed, flown and the data were analysed as part of this thesis. It was found that the effect of constant shearing on a granular material in a direction perpendicular to the gravity field is not strongly influenced by gravity. The AstEx experiment has demonstrated, for the first time, that the efficiency of granular convection may decrease in the presence of a weak gravitational field, similar to that on the surface of small bodies. The first measurements of transient weakening of granular material after shear reversal in microgravity are also reported. Results suggest that the force contact network may be weaker in microgravity, although the influence of any change in the contact network is felt by the granular material over much larger distances. This may have important implications for our interpretation of asteroid surfaces. Continued advancement of our understanding of granular materials in varying gravitational conditions requires futher experiments and the development of the soft-sphere discrete element method implementation in pkdgrav in order to model the granular regimes that are inaccessbile to the hard-sphere implementation.

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The smallest bodies of our Solar System, such as asteroids and comets, are characterised by very irregular sizes and shapes and therefore, very irregular gravitational fields. Moreover, asteroids are also found in binary or multiple systems, which allow for complicated dynamics with coupling between translational and rotational motion. Classical problems used to study astrodynamics such as Kepler's problem, Hill's equations or the Restricted Three Body Problem cannot describe the dynamics near asteroids and comets as they do not take into account the non-spherical shape of the bodies. In this thesis, the non-linear dynamical environment around rotating non-spherical bodies or around binary systems when at least one of the bodies is not spherical has been studied. The study consists of the analysis of different mathematical models that can be used to describe the movement of massless particles, such as dust or a spacecraft, orbiting an elongated body, the dynamics between the components of a binary system, or a spacecraft orbiting the vicinity of a binary asteroid. In order to do this an analysis and development of gravitational potentials has been performed. A gravitational potential that takes the shape of the non-spherical bodies into account has allowed us to describe the movement of the dusty environment of an asteroid, to design trajectories to approach and observe an asteroid, and even land on it. Furthermore, the effect of the shape and rotation period of asteroids and binaries on the dynamics has been studied. The fact that asteroids and comets are not point masses but elongated irregular bodies leads to rich dynamics around them. Equilibrium points, periodic orbits and invariant manifolds exist in their vicinity. These are used in this thesis to design low cost landing missions to asteroids, understand the dynamics of binary systems or to explain a possible mechanism for the accretion of mass and formation of the Solar System.

Asteroid impacts are a hazard to human populations. A method to assess the impact risk of hazardous asteroids was developed in this work, making use of the universal concept of risk culminating in the Asteroid Risk Mitigation Optimization and Research (ARMOR) tool. Using this tool, the global spatial risk distribution of a threatening asteroid can be calculated and expressed in the units of expected casualties (= fatalities). Risk distribution knowledge enables disaster managers to plan for a potential asteroid impact through identification of high risk regions and estimation of total risk as a scalar value. Expressing the risk in terms of expected casualties would allow the placement of the asteroid threat on the same scale as other human hazards. Thus, this unit provides an accessible way of defining thresholds for asteroid threat response protocols, of communicating the threat utilizing a new hazard scale, and of allocating adequate resources to address the hazard by comparison with other natural disasters. To accomplish risk estimation, vulnerability models were needed that relate the severity of impact effects (wind blast, overpressure shock, thermal radiation, cratering, seismic shaking, ejecta out-throw, and tsunami) on the human population and a novel comprehensive suite of such models were derived and presented. The need for high fidelity impact effect and vulnerability modelling, as opposed to a simplified, impact location based approach, for risk estimation of a specific asteroid threat was analysed and confirmed. Subsequently, the method of ARMOR was applied to asteroid 2015 RN35 to produce an example risk distribution output. Additional analysis shows that the general impact location distribution of asteroids is approximately uniform, confirming, for the first time, a common assumption made in planetary defense. Extensive global simulations were performed utilizing an artificial sample of 50,000 impactors with sizes up to 400m to identify which impact effects are most hazardous to the human population. Aerothermal effects are most hazardous while tsunamis only contribute moderately to the overall hazard. The average land impactor is an order of magnitude more dangerous than a similar water impactor and asteroids smaller than 50-60m (density ≈ 3100 kg/m3) are expected to airburst rather than reach the surface. Furthermore, the average loss estimate for asteroid impactors enables fast threat analysis of newly discovered asteroids and helps determine the asteroid size that contributes most to the residual asteroid impact risk. These results provide new insights to inform efficient preparation for a future asteroid threat. In the future, ARMOR can be used to perform on-ground risk driven asteroid detection mission design which would reduce risk of an incoming asteroid progressively and this is not accomplished with current methods.

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In this thesis, I present a study of the rotation and shape properties of small asteroids using both sparse survey observations from PanSTARRS 1 and dense light curve observations. The rotation al state of asteroids is controlled by various physical mechanisms including collisions, internal damping and the Yarkovsky O'Keefe Radzievskii Paddack (YORP) effect. I have analysed the changes in magnitude between consecutive detections of over 60,000 asteroids measured by the PanSTARRS 1 survey during its first 18 months of operations. Using this sparse photometric data I have attempted to explain the derived brightness changes physically and through the application of a Monte Carlo simulation. I have found a tendency toward smaller magnitude variations with decreasing diameter for objects of 1<D<8 km. I applied simple shape models to this sparse data to produce a best fit shape distribution for small main belt asteroids. Assuming the shape distribution of objects in this size range to be independent of size and composition our model suggests a population with average axial ratios 1:0.85 ± 0.13:0.71 ± 0.13. Using the first 1.5 years of the PanSTARRS 1 survey I have identified a list of potential 'extreme asteroids', with either rotation period P<2.2 h or light curve amplitude A > 1.0 mag. I carried out follow up photometric observations telescopes on La Palma and in La Silla, Chile. In the course of this work I have studied the photometry of 21 objects. 9 asteroids produced light curves with amplitude A > 1 mag with no objects exhibiting a spin period P<2.2 h. I discuss the different possible shape configurations for each of the 9 high amplitude asteroids and constrain their shape and density information. Of the observed asteroids, 2 had sufficient observations at different orbital geometries to allow shape models and spin pole axes to be determined through light curve inversion.

Collisions is one of the most important processes in the Solar System that have played a significant role in its evolution for 4.5 Gy. They are responsible for the formation of asteroid families, craters and regolith production on bodies surfaces. Moreover they pose a hazard for our planet's environment, human civilisation and space assets. Impacts have shaped the asteroids and their surfaces and recently there are indications that they are also responsible for the creation of multi-lithology asteroids. The effectiveness of this process lies, apart from the collisional speed and angle, on the physical parameters of both the target and the impactor. A plethora of laboratory experiments are devoted to study the outcome of impacts, from low speeds of a few m/s to greater speeds of several km/s. In addition space missions; such as Deep Impact (NASA) in the past and AIDA (ESA/NASA) hopefully in the near future, are aiming to perform hyper-velocity impact experiments at large scales. Although there is advance in our understanding of crater formation, target fragmentation and ejecta speeds, however the fate of the impactor is still very poorly constrained. Experiments so far were focused using materials not directly relevant to the composition of asteroids. We start an investigation for the impactors' fate, by using lithological projectiles that impacted three different types of targets with different material and bulk porosities. For this experimental campaign was used the Light Gas Gun (LGG) of the Impact Group at the University of Kent. The study was focused on three main topics: i) the fragmentation of the impactor, ii) the implantation of exogenous material onto the target and iii) the inspection of the final state of the projectile.\\ This Thesis is divided in six Chapters. The first two, Chapters 1 and 2, are giving a review of recent advances of small bodies studies, the importance of collisions in the Solar System, and a brief description of the laboratory impact experiments, providing the current state of research on the fate of projectiles. Some open questions lead to the explanation of the aim of this study. In Chapter 3 are described the series of experiments performed, explaining the analysis methods were developed and the way that the main topics of fragmentation, implantation and characterisation of the impactor were studied. All the results for each one of these topics, along with the difficulties during the experimental procedure are provided in Chapter 4. In Chapter 5 we discuss the results giving the implications, attempting to place the outcome in the big picture of the small bodies collisions. In the last Chapter 6 there is a summary of this work, providing also possible future ideas for the continuation of this study.

Laser ablation has been investigated as a possible technique for the contactless deflection of Near Earth Asteroids. It is achieved by irradiating the surface of an asteroid with a laser light source. The absorbed heat from the laser beam sublimates the surface, transforming the illuminated material directly from a solid to a gas. The ablated material then forms into a plume of ejecta. This acts against the asteroid, providing a controllable low thrust, which pushes the asteroid away from an Earth-threatening trajectory. The potential of laser ablation is dependent on understanding the physical and chemical properties of the ablation process. The ablation model is based on the energy balance of sublimation and was developed from three fundamental assumptions. Experimental verification was used to assess the viability of the ablation model and its performance in inducing a deflection action. It was achieved by ablating a magnesium-iron silicate rock, under vacuum, with a 90 W continuous wave laser. The laser operated at a wavelength of 808 nm and provided intensities that were below the threshold of plasma formation. The experiment measured the average mass flow rate, divergence geometry and temperature of the ejecta plume and the contaminating effects - absorptivity, height and density - of the deposited ejecta. Results were used to improve the ablation model. A critical discrepancy was in the variation between the previously predicted and experimentally measured mass flow rate of the ablated ejecta. Other improvements have also included the energy absorption within the Knudsen layer, the variation of sublimation temperature with local pressure, the temperature of the target material and the partial re-condensation of the ablated material. These improvements have enabled the performance of the ablation process and the specifications of the laser to be revised. Performance exceeded other forms of electric propulsion that provided an alternative contactless, low thrust deflection method. The experimental results also demonstrated the opportunistic potential of laser ablation. Using existing technologies, with a high technology readiness level, a small and low-cost mission design could demonstrate the technologies, approaches and synergies of a laser ablation mission. The performance of the spacecraft was evaluated by its ability to deflect a small and irregular 4 m diameter asteroid by at least 1 m/s. It was found to be an achievable and measurable objective. The laser ablation system could be successfully sized and integrated into a conventional solar-power spacecraft. Mission mass and complexity is saved by the direct ablation of the asteroid's surface. It also avoids any complex landing and surface operations. Analysis therefore supports the general diversity and durability of using space-based lasers and the applicability of the model's experimental verification.

Of all the natural hazards that could befall on Earth, only an Earth impact of a large comet or asteroid has the potential to wipe out the entire civilisation in a single event while that of a small object could be mitigated by a space program. Over the last decade, Near Earth Objects (NEOs) have rapidly become of interest to scientists and engineers as these small celestial bodies offer tantalising clues to the origins of the solar system and one day they could be used as stepping-stones for further space exploration. The purpose of this dissertation is to present a comprehensive study of such asteroid impact hazards and their mitigation. During the early stages of hazardous Near Earth Asteroid (NEA) mitigation campaign planning, the fundamental asteroid characteristics (e.g., mass, size, albedo, etc.) should be accurately determined to increase the chance of successful mitigation. However, given a limited warning time, an asteroid impact mitigation campaign would hinge upon uncertainty-based information consisting of remote observational data of the identified Earth-threatening object, general knowledge of NEAs, and engineering judgment. Due to this ambiguity, the campaign credibility could be profoundly compromised. It is therefore imperative to comprehensively evaluate the inherent uncertainty in deflection and properly plan the campaign in order to ensure successful mitigation. In the thesis, three different (ground-based, space-based, and proximity) preliminary characterisation approaches to the identified threatening object are defined. Their corresponding uncertain information about the fundamental asteroid characteristics is quantified through Evidence Theory based on the existing literature about the NEO population as well as the capability of the three different characterisation approaches. The outcomes of four active hazard mitigation/asteroid deflection techniques (kinetic impactor, nuclear interceptor, gravity tractor, and solar collector) are then evaluated under the uncertainty-based information. In addition, the thesis investigates the influence of internal density inhomogeneity of the target asteroid on the outcome of the instantaneous deflection approach: kinetic impactor whose deflection efficiency is subject to the actual position of the asteroid’s centre of mass with respect to the actual kinetic impact site. Following this, perturbations of a gravity tractor spacecraft orbiting an irregularly-shaped asteroid due to the inhomogeneous asteroid gravitational field are analysed. The effects of asteroid shape and rotational state on a solar collector mission are also briefly evaluated, assuming a rotating ellipsoidal asteroid. Finally, the thesis extends to the study of a multi-deflection mitigation approach that aims for a high confidence level on successful mitigation and, more specifically, explores a dual-deflection campaign consisting of an instantaneous/quasi-instantaneous deflection technique (kinetic impactor, nuclear interceptor, or solar collector) as a primary mission and a slow-push deflection technique (gravity tractor) as a secondary mission. Here, both deflection efficiency and campaign credibility are taken into consideration. The results of dual-deflection campaign planning show that there are trade-offs between the competing aspects: the total mitigation system mass, mission duration, deflection distance, and the confidence in successful mitigation. The design approach is found to be useful for multi-deflection campaign planning under the uncertainty-based information, allowing to select the best possible combination of deflection missions from a catalogue of various mitigation campaign options, without compromising the campaign credibility.