Modelling the behaviour of granular material on the surface of asteroids and under different gravity conditions (e.g., Mars, the Moon)

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

Masses of Small Bodies: Mass estimation of small solar system bodies using Radio Science data from close flybys

The Radio Science technique enables to estimate the mass and other gravitational parameters of a solar system body from spacecraft observations very precisely. It uses the radio link between ground station and spacecraft. The frequency shift of the radio signal is proportional to the relative velocity change between spacecraft and ground station. If a spacecraft performs a close flyby at a solar system body, the velocity of the spacecraft is changed by the gravitational attraction of the body. If all other contributions on the radio signal are known, the remaining frequency change is solely due to the gravitational attraction. A least square fit can be performed on the frequency residuals to derive from it gravitational parameters. Within this thesis models were developed and merged into a software package with which it is possible to determine the orbit of a spacecraft precisely and to predict accurately the frequency to be observed at a ground station. Models for extracting the frequency shift caused by the propagation of the radio signal through the ionosphere and troposphere of the Earth were incorporated. The accuracy of the predicted frequency, i.e. the difference between measurement and predict, is in the same order as the total Doppler velocity error in X-band from the thermal noise of the ground station and the transponder phase noise. Filtering techniques were established improving the signal to noise ratio at least by a factor of three. A numerical stable least square fitting procedure was introduced to fit the frequency change due to the gravitational attraction of a body onto the measured frequency residuals. Measurements from the close flyby of the Rosetta spacecraft at the asteroid Steins were analyzed with the developed method. Due to the large flyby distance no mass estimate was possible. A feasibility study was carried out for the upcoming flyby of Rosetta in July 2010 at the asteroid Lutetia. It is possible to estimate from this flyby the mass of Lutetia with an error of 1 %. Moreover, the developed method was applied to measurements of the Radio Science Experiment onboard Mars Express MaRS from two close flybys at the Mars moon Phobos in March 2006 and July 2008. The mass of Phobos was estimated from these flybys. The solution provides the most accurate value currently available for the mass of Phobos from close flybys. Information about the interior were derived from the precise mass estimate. Phobos has a high porosity which is discussed with respect to its origin. It seems to be unlikely that Phobos is a captured asteroid as suggested from first spectral measurements. It seems to be more likely that Phobos is the remnant of the collision between a body originating from the asteroid belt and a body remaining from the formation process of Mars. Mars Express will perform another flyby in March 2010 with a closest distance of 62 km. A feasibility study was performed from which it was derived that the C20 term of the gravity field of Phobos can be estimated with an error of 1 % with the developed method

Publications of the Jet Propulsion Laboratory, 1978

This bibliography cites 958 externally distributed technical papers released during calendar year 1978, that resulted from scientific and engineering work performed, or managed, by the Jet Propulsion Laboratory. The publications are indexed by author, subject, publication type and number. A descriptive entry appears under the name of each author of each publication; an abstract is included with the entry for the primary (first-listed) author

Reports of planetary geology and geophysics program, 1989

Abstracts of reports from Principal Investigators of NASA's Planetary Geology and Geophysics Program are compiled. The research conducted under this program during 1989 is summarized. Each report includes significant accomplishments in the area of the author's funded grant or contract

A Comprehensive Survey of Deep Learning in Remote Sensing: Theories, Tools and Challenges for the Community

In recent years, deep learning (DL), a re-branding of neural networks (NNs), has risen to the top in numerous areas, namely computer vision (CV), speech recognition, natural language processing, etc. Whereas remote sensing (RS) possesses a number of unique challenges, primarily related to sensors and applications, inevitably RS draws from many of the same theories as CV; e.g., statistics, fusion, and machine learning, to name a few. This means that the RS community should be aware of, if not at the leading edge of, of advancements like DL. Herein, we provide the most comprehensive survey of state-of-the-art RS DL research. We also review recent new developments in the DL field that can be used in DL for RS. Namely, we focus on theories, tools and challenges for the RS community. Specifically, we focus on unsolved challenges and opportunities as it relates to (i) inadequate data sets, (ii) human-understandable solutions for modelling physical phenomena, (iii) Big Data, (iv) non-traditional heterogeneous data sources, (v) DL architectures and learning algorithms for spectral, spatial and temporal data, (vi) transfer learning, (vii) an improved theoretical understanding of DL systems, (viii) high barriers to entry, and (ix) training and optimizing the DL.Comment: 64 pages, 411 references. To appear in Journal of Applied Remote Sensin