The origin of the Martian dichotomy is subject to question and no substantial evidence exists. Some surface and interior features that are not visible in, e.g., topography data, can show up in gravity data. Therefore, this research inverts gravity data to find a crustal and mantle global density model. Previous research performed a one-layer inversion, assuming equal mass in all columns. Also, missions like InSight do not provide global interior information, but only at the landing site. The aim of this research is to provide a global density model of both the Martian crust and upper mantle. The inversion is performed using a weighted, regularized least-squares algorithm. The gravity input consists of the residual between the MRO120F data set and the state-of-the-art gravity field model of the TU Delft. The design matrix is built using Green’s functions, which define the influence of a mass element in all different directions on a measurement point. Using this least-squares algorithm, three different methods for inversion are used. The separate two-layer inversion, the combined independent two-layer inversion and the combined dependent two-layer inversion. All three inversion methods are performed on synthetic planets as well, for verification purposes. By performing all inversions on the synthetic planets, it was found that the combined independent two-layer inversion results in a strong decoupling of short and long wavelength signals, but is not able to attribute gravity signals to different features in the crust and mantle. The combined dependent two-layer inversion does lead to a result that shows decoupling of crust and mantle features. The hypothesis is that adding different gravity components to the combined dependent two-layer inversion will further increase its accuracy. The results of the inversion methods applied to Mars are in agreement with existing research in terms of standard deviations of the crust and mantle density anomalies. The maps were also analysed geologically, where the most important conclusion is the evidence of potential impact basins in the north polar region. These can be evidence to accept the several impact theory for the origination of the Martian dichotomy. Increasing the resolution and refining the third inversion method with multiple gravity components will increase the potential of gravity inversion to define geological features of Mars.

Ceres is a small dwarf planet in the asteroid belt between the orbits of Mars and Jupiter. In this work, its interior density, interior temperature, crust thickness and crust density are modelled with the aim of determining possible formation mechanisms for the cryovolcanic bright spots on its surface. The results imply that Ceres has a muddy subsurface ocean from which the cryovolcanic material originates.

Surface heat flux, which can be defined as the heat flowing out of the interior of a planetary body, provides important constraints on the present-day thermal state of the lunar interior. Measurements, performed in situ during the Apollo program, and from orbit by Chang’E 1, Chang’E 2, and by the Diviner radiometer instrument, indicate important lateral variations in surface heat flux on the Moon (~5-180 mW/m²). The differences between Apollo 15 and Apollo 17 measurements have been explained by the presence of an anomalous region, enriched in uranium (U), thorium (Th), and KREEP (Potassium, Rare Earth Elements, and Phosphorus) elements, located on the lunar nearside. Previous modeling efforts also identified crustal thickness and thermal conductivity variations as secondary causes for surface heat flux differences. However, detailed explanations for the remaining two estimates and their implications for the evolution of the Moon remain highly debated. Additionally, the structure and properties of this putative KREEP-rich layer have remained uncertain.

Therefore, this study proposes a new global geodynamic model of lunar thermal evolution that includes lateral variability in the distribution of radiogenics, crustal thickness, and thermal conductivity. The present setup is capable of simultaneously explaining the variability between the Apollo 15, Apollo 17, and Region 5 heat flux values, while also providing further constraints on the KREEP layer structure and lunar crustal properties. The research question addressed in this work is: What is the effect of crustal structure (radiogenics, thickness, and thermal conductivity distribution) on 3D thermal evolution models of the Moon?

Here, we model the interior dynamics and thermal evolution of the Moon after magma ocean solidification using the fluid solver GAIA. We investigate the abundance and distribution of radiogenics on the Moon, and how they shape the interior temperature distribution through time. Additionally, we account for a spatially variable crustal thickness, derived from gravity and topography data. We also include a laterally variable thermal conductivity model, derived from porosity data, which we constrain using the nearside-farside differential thermal state of lunar basins. We model and vary the extent of a putative KREEP layer underlying the PKT (Procellarum KREEP Terrane) region. We enrich the KREEP layer and crust in heat-producing elements compared to the mantle, simulating an asymmetrical distribution of radiogenics as an initial condition.

We find that measurably lower heat flux values at the lunar south pole compared to Apollo 15 and 17 require KREEP material to extend at least partly beneath Mare Serenitatis. In this case, the Apollo 15 measurement would be representative of the KREEP region average heat flux, while Apollo 17 would lie on its edge. On the other hand, a smaller KREEP region (<1200 km in diameter) would make the Apollo 17 location representative of non-KREEP terrane, and show heat flux comparable to south pole values. This is incompatible with estimates based on the Diviner Lunar Radiometer Experiment onboard Lunar Reconnaissance Orbiter, although uncertainties associated with these estimates are unclear. Heat flux measurements that will be performed by the NASA CLPS-CP12 mission at Schrödinger crater will provide key information to exclude one of these two scenarios, and thus potentially constrain the extent of the KREEP layer underneath the PKT region.

Our results also show that a laterally variable thermal conductivity helps reduce the interior temperatures, while maintaining surface heat flux distribution unchanged. We find an effective farside crustal conductivity of ~2 W/(mK) (comparable to that of compact anorthosite) to best match the differential basin relaxation constraints, implying negligible effect of a porous megaregolith layer on the thermal conductivity. In the KREEP region, we favour models with effective crustal conductivity below 2 W/(mK), suggesting that lunar volcanic basalts may have an even lower bulk conductivity than the 2.6-2.7 W/(mK) values considered here. This could be due to the porosity of lunar volcanic material, a more complex layering of basaltic eruptions, or the effect of the temperature and pressure dependence of thermal conductivity. Although our model setup is simplified, it provides novel insights on the distribution of radiogenics and crustal properties on the Moon, and shows the potential to further constrain the asymmetrical character of lunar evolution.

This thesis analyses the possibility of creating a portable Doppler tracking ground station using commercial of the shelf components. This solution is called DopTrackBox and is based on TU Delft’s DopTrack. This system uses the Doppler shift of radio signals received from satellites to analyse their location and velocity. Different experiments were conducted to analyse the effects of various hardware components on the system. From the experiments can be concluded that the most impactful hardware change is switching from an omnidirectional antenna to a manually pointed directional one, as this introduces more variance to the system. When using the same omnidirectional antennas as DopTrack, DopTrackBox performs on par with the bigger system; achieving a higher SNR and lower range rate differences. More research is needed to look into the specific effects of the GPS clock and different SDR.

Spacecraft navigation and control is difficult in deep space operations. Especially around asteroids, the irregular gravity field increases the difficulty of estimating the spacecraft trajectory. Autonomous navigation can increase the safety and accuracy for orbit proximity operations. Furthermore, it eliminates the need for continuous communication with the spacecraft. For the implementation of autonomous navigation in deep space, the onboard guidance navigation & control (GNC) should be able to accurately estimate the attitude and relative position of the spacecraft. By using sensor fusion, information from individual sensors can be combined to increase certainty and accuracy of the state estimation. A sensor fusion model is proposed, comprising an inertial measurement unit (IMU), star tracker and light detection and ranging (LiDAR) as navigation sensors. The aim of this research is to investigate the feasibility and performance applying sensor fusion for the spacecraft state estimation.

A navigation filter is applied to a benchmark scenario that orbits an asteroid at 50 km. In this scenario, asteroid 433 Eros has been selected for its unique shape, which has been mapped during the Near Earth Asteroid Rendezvous (NEAR) mission. A simulation is performed to approximate the dynamics and kinematics of the mission environment. The simulation takes a polyhedron model of the asteroid, a third-body disturbance by the sun, and an additional acceleration due to solar radiation pressure into account. The simulation forms a base for the sensor measurement simulation. As the IMU consists of an accelerometer and a gyroscope, the measurements total to two sets of available data for position as well as attitude estimation.
The navigation filter estimates the position, velocity, attitude and gravitational constant of the asteroid, by use of an extended Kalman filter (EKF). The EKF is augmented for the quaternion states to a multiplicative extended Kalman filter. The navigation filter is simulated for a benchmark scenario, as well as for different orbital heights and temporary loss of the star tracker as well as the LiDAR sensor.

As a result, it is concluded that it is feasible with the given sensor set to approximate the position and attitude of a spacecraft in proximity of 433 Eros. For the position, a root-meansquare error (RMSE) of 0.5 m is found at an orbital height of 50 km. Using a time step of 0.1 s for the EKF is recommended after a trade-off between accuracy and computational time. It is concluded that the proposed model for state estimation is sufficiently accurate for position and attitude estimation for the given benchmark scenario. With this navigation filter, we come one step closer to the development of autonomous navigation for asteroid observing spacecraft.

In 2018 the InSight mission was launched with the main objective of providing accurate 3D models of Mars’s interior. This was done by placing a seismometer on the surface of Mars to measure seismic activity. In 2021 three studies were published, one dedicated to the Martian core, one to the Martian mantle, and the last one focused on the Martian crust. Based on this seismic data, Khan et al. [2021] obtained a lithospheric thickness between the 400 and 600 km. The average Martian crust was found to lie between 24 to 72 kilometers. A restrictive range of crustal densities between 2700 to 3100 kg/m3 was used. The mantle extended up to 1560 km below the surface of Mars. The core is molten and has a radius of 1830 km. Before seismic data was available, the method to learn more about the interior of a planet was to study the topography and gravity data. This is still, on Mars, the only way to get a global picture of the subsurface. Only one seismometer is currently available on Mars, making it difficult to currently obtain full planet 3D maps from seismology alone.

The objective of this research is to study the lateral density variations within the Martian lithosphere. The lithosphere is the planet’s outermost shell, defined by its rigid mechanical properties. It is thought that the lithosphere of Mars consists of the crust and the outer part of the mantle. In previous research, the density of the Martian crust is most often taken as uniform. However, density variations exist at a small scale and potentially even at the largest Martian scale [Beuthe et al., 2012]. The thesis project aims to study these density variations within the Martian crust and upper mantle.

To study the lateral density variations in the crust and mantle, the MRO120D gravity data and MOLA topography data are used as input. The crust-mantle boundary is created based on thin shell isostasy [Qin, 2021]. A lower boundary of the lithosphere is chosen to be equal to 500 km. A mantle plume underneath Tharsis is added, based on the findings from van der Tang [2021]. This plume reached from a depth of 800 km up to a depth of 900 km and was centered around [110◦W 3◦N] and had a density variation of 400 kg/m3 with respect to the surrounding mantle. An inversion was performed from which the density of both the crust and the mantle was obtained. In this calculation, two assumptions are made. First of all, it is assumed that the mass of each column is equal. The second assumption is that the obtained gravity for the column can be split into 2 point mass sources, one corresponding to the mass of the crust, located in the center of mass of the crust, and one corresponding to the mass of the mantle, located in the center of mass of the mantle.

A reference planet with Te = 400 km, C = 105 km, ρc,ref = 2700 kg/m3, and ρm,ref = 3800 kg/m3, resulted in the smallest density differences within the crust, as well as the smallest gravitational tensor residual. When these input parameters were used for the inversion, a final density difference of approximately 1000 kg/m3 between the crust and mantle was obtained after the inversion. A mean crustal density of approximately 2750 kg/m3 and a mean mantle density of approximately 3750 kg/m3 were found. The density variations in the crust varied from -424 up to 618 kg/m3 around the mean crustal density and -228 and 133 kg/m3 around the mean mantle density. The optimal elastic thickness obtained through the inversion lies between the 450±50 km and the uniform crustal thickness between 100 ± 10 km.

For the large impact craters, high crustal densities were found compared to the mean crustal density of Mars. Densities for Hellas and Utopia lay between the 3200 and 3345 kg/m3, and for Isidis Basin, a slightly smaller impact crater, a crustal density of around 2870 kg/m3 was found. These large crustal densities, together with the large elastic thickness and density difference between the crust and mantle, resulted in the conclusion that the large impact craters may be compensated using a different isostasy method. For this study, the crust-mantle boundary was obtained by applying the flexural response function to the crust-mantle boundary obtained using Airy isostasy. A boundary based on Pratt isostasy, instead of the thin shell isostasy boundary used currently, might be a better fit for the larger impact craters. The volcanoes outside the Tharsis region were found to be well compensated using the thin shell method, and small densities were found compared to the volcanoes in the Tharsis region, as well as what has been found in previous literature. The smaller densities for these stand-alone volcanoes might have to do with the compensation of these volcanoes due to the curvature of the planet, which is taken into account with the thin shell isostasy model. The literature analyzed in this study used different isostasy methods. By studying the possibility of different isostasy methods for different regions on Mars, the current model could be improved further.

This study showed that it is possible to use gravity inversion to gain insights into the lithosphere of a planet. With these density differences, the Martian power spectrum could be fitted up to spherical harmonic degree 40. But, due to the resolution of the current gravity model, it was only possible to calculate the Martian density difference at a larger scale. To also be able to study the density differences at a smaller scale, higher resolution gravity data needs to become available.

The history of space exploration shows a widespread interest in asteroids. These small bodies were visited by multiple spacecrafts made by different space organizations through time. The asteroids give us information about the history of the solar system, due to their unchanged state since the early beginning of the Solar System. In the future, these rocky bodies could provide resources for use on Earth or during human spaceflight. Besides these benefits, so-called near-Earth asteroids can be of danger by the possibility of collision. In the history of Earth, asteroid impacts had large consequences for life on Earth.

During missions towards asteroids, navigation is challenged by the gravity field of the asteroid. Asteroids are relatively small and often have a very irregular shape. The relatively small and irregular gravity field of the asteroid on the spacecraft, makes the acceleration difficult to predict. Therefore, gaining knowledge about this gravity field during the mission improves the safety of the spacecraft.

Gravity fields of celestial bodies often modeled and estimated using the spherical harmonics model. This model has the disadvantaged that its convergence is limited to the body's circumscribing sphere, called the Brillouin sphere. Because of their often irregular shape, this model is not suitable for orbits close to the asteroid, reaching inside this reference sphere.
To eliminate this problem, the alternative mascon (mass concentration) gravity model is implemented in an extended Kalman filter (EKF). This model distributes point-masses along the asteroid's body. The gravitational parameters of each defined mascon are estimated by the EKF using noisy position measurements.

To test the performance of the EKF, a spacecraft is simulated in an asteroid environment. For this simulator, the polyhedron gravity model using the 433 Eros shape model is defined as the real-world gravity field. This high-precision and non-diverging gravity field model assumes a constant density in the asteroid. The model is adjusted such that it also can include density differences inside the asteroid shape by dividing the surface shape model into numerous volume elements. Each element is assigned a density according to the smoothing Mátern Covariance function, which depends on the Euclidean distance between the elements.

The EKF is capable of handling asteroids with a heterogeneous density distribution with the same accuracy as for homogeneous asteroids. Its performance depended highly on the chosen mascon positions, as point masses can be highly correlated. The EKF can estimate the gravity field as accurate as the spherical harmonics degree 8 using noisy position measurements with 10 meter standard deviation.

The Moon is covered by a blanket of rock fragments and loosely bound dust particles called regolith. This layer is key in deciphering the evolution of the Moon and the terrestrial planets, including Earth. It is also a protagonist in the return of humanity to the Moon after its last visit over half a century ago. The main concern for lunar regolith studies is the determination of compactness. Compactness may be expressed as porosity and can be determined by the discrepancy between bulk density estimates from the gravity field and grain density estimates from material composition. New state-of-the-art gravity models of the Moon allow for small-scale gravity studies of regolith porosity, which have resulted in better understanding of the Moon's thermal evolution. However, regional variations in compactness are currently poorly known due to the ambiguous nature of gravity data. Ideally, an additional information source is desired to provide constraints on regolith compaction. It might be that the amount of reflected sunlight from the lunar surface provides additional constraints on regolith compactness. For example, incident light can enter a more porous material deeper, increasing the probability of absorption and decreasing the amount of reflected light. Therefore, this thesis will explore the scattering behaviour of reflected sunlight from the lunar surface as a potential additional information source.

Mars has been a target for space exploration for decades. Exploring the interior of the red planet could reveal information about its formation and evolution. In this thesis, the crustal structure of Mars is investigated by a power spectra analysis of both topographic and gravitational data. With models of flexural isostasy, the best-fitting lithospheric (crust + uppermost mantle) thickness is found to be between 136 km and 158 km globally. Possible values for the thickness of the lithosphere range from 120 km to 580 km. In addition, a 3D flat Finite Element Method (FEM) model is created for Mars. The FEM Mars model incorporates the above-mentioned crustal profiles and calculates the surface stresses in the regions of interest. The calculated stresses are compared to observed faults in Tharsis, Hellas, and Utopia to reveal information about the evolution of these regions.

The gravity potential field ofMars is characterized by a long-wavelength feature located at the Tharsis region. In addition, this region houses large volcanoes that have been active until geologically recently (200 to 100 Ma ago). Research has not succeeded in pinpointing the cause of this long-wavelength areoid high, while some researchers claim it is due to a mantle plume, others suggest that it is caused by volcanic construction and the associated flexure in the lithosphere. This work aims to contribute to resolving this question with the development of a new mantle convection model for Mars. It is renewing in four aspects; (1) A Mars model is developed in SFEC (Tosi, 2007), which has been primarily used for Earth studies, (2) mantle anomalies are analysed in order to determinewhether they could be involved in the creation of the Tharsis volcanoes, (3) the solutions challenge the current views upon the (pre-dominantly isostatic) support of Tharsis and (4) a new method is developed to assess the similarity of models and observations for low degree spherical harmonics (n Ç 5) fields. The research contributes to a better understanding of the Martian interior and the volcanic activity in the Tharsis region. Seismic studies of the Earth’s interior have correlated large provinces on the core-mantle-boundary with plume generation and hotspots that arise on Earth’s surface. This research investigates whether similar structures in the Martian mantle could be connected to, or even be the cause of, the volcanism in the Tharsis region. Firstly, a crustal reduction is performed to obtain the gravitational field of the mantle and core. An Airy model is used for this as research found this to be the dominant form of compensation for long-wavelength features on Mars (Mussini, 2020). Subsequently, density anomalies are positioned in the mantle and their gravity potential is computed and compared with the observations.

The subsurface of Mars is impossible to measure directly, yet it has been the subject of many studies. An understanding of the subsurface of Mars would yield large amounts of information on the history of the planet. Two of the tools available to indirectly interact with the Martian subsurface, in particular the lithosphere, are the gravity and topography signals of the planet. These two datasets can be combined using a variety of geological theories in order to investigate the subsurface. In this study, an isostatic model (Airy-type) and two flexural isostatic models (an infinite plate model and a thin shell model) will be the methods of choice. A distinction is made between isotropic or global models, which use one set of physical parameters for the entire planet, and anisotropic or multi-region models which allow for regional variation in physical parameters. The goal of this study is to investigate the performance of the novel thin shell model as compared to the older infinite plate model.

To investigate this, the theory behind each model is explained, after which the models are validated using results from literature. Several regions of interest are defined, mostly among large geological formations or gravity anomalies. Two parameters are chiefly investigated: the average thickness of the lithosphere and the lithospheric elastic thickness, which is a measure of the strength of the lithosphere. Each model is run globally for a variety of these two parameters, and the best fitting parameters are identified. After this, the planet is split into different regions with their own physical parameters. The first study is a dichotomy study which splits the planet into a northern and southern hemisphere, aimed at characterizing the disparity between the Martian north and south. After this, each region is assigned its best fitting physical parameters and the regions are combined into a 'global' regional model. A best fitting multi-region model is obtained via manual observation of the results and adjustment of the inputs until a visual best fit is achieved.

The results are then discussed. A key takeaway is that better methods of judging the performance of models without human visual inspection of their results is necessary in order to realize the full potential of the flexural isostasy models presented in this study. The lack of suitable methods leads to a manifold of best fitting solutions for many of the problems modelled in this study, hindering firm conclusions about the subsurface of Mars. Having said this, global average lithospheric values of about 200 km combined with very low effective lithospheric elastic thickness values of 0 to 40 km are the best fits found in this study. Literature values are typically lower, but this can partially be explained by differences between the flexural isostasy models in this study and the models from literature. Regionally there are large variations, with some features (Hellas basin, Alba mons) being isostatically compensated, others being supported by locally strong lithospheres (much of the Tharsis region), and others resting on buried mass anomalies that cannot be explained with the models in this study (Isidis planitia, Argyre basin). In a dichotomy study, the best fitting values were found for a northern lithosphere zero to ten kilometers thinner than the southern lithosphere. In general, the thin shell model is more sensitive to nonzero lithospheric elastic thickness values, providing very strong lithospheres at low elastic thicknesses. This is due to its aggressive flexural response function's filtering of higher spherical harmonic degree signals. The thin shell models yields higher residuals in the global analyses, but lower residuals in the multi-region studies. At the same time, 80% of the error in all models can be attributed to spherical harmonic degrees between 1 and 10. These signals are likely not caused by flexural isostasy, and require models incorporating more physics (mantle plumes, mass anomalies, etc) to be explained.

The GRACE mission has provided unprecedented insights into mass redistribution processes in the Earth system. Following a strong call for continuation of the mass observations, the GRACE-Follow On (GRACE-FO) mission was launched in May 2018, leaving a coverage gap of ca. 1 year between GRACE and GRACE-FO. Geopotential solutions derived from data of the Swarm mission (2013-present) are candidate data to potentially bridge this gap, as well as bridge the minor discontinuities in the current GRACE time series. This study aims to validate the sensitivity of the Swarm measurement system to mass rates, by comparing GRACE and Swarm observations of the gravity trend induced by glacial isostatic adjustment (GIA). Studies inverting GIA observations (e.g., relative sea level change, surface deformation, [time variable] gravity) into mantle viscosity estimates suggest relatively high viscosity in the Hudson Bay area. This suggests that the North American GIA-induced gravity trend is linear on a multi-decadal time scale, which means we can extrapolate the GRACE-derived GIA observations into the Swarm time period for this area. We find that the Swarm-derived GIA observations correspond well in amplitude and spatial distribution to GRACE observations and conclude that both systems have a similar sensitivity to gravity trends at spatial scales to which Swarm is sensitive to (ca. 1500 km). We validate our findings via geopotential solutions of GRACE-FO and provide a brief analysis of the benefits of adding Swarm-derived information to the combined GRACE / GRACE-FO time series.

Meteors have been observed by accident from geostationary orbit (GEO). It is of interest for surveillance, meteor science and the updating of empirical models to know what instruments are needed on a GEO satellite to observe meteors and re-entering capsules in Earth ‘s atmosphere. A re-entry simulation code is written to fill the gap of missing observation data of meteors and re-entry capsules. The simulated re-entry data is used to determine what optical instrument is needed to track or detect these objects. The calculations take into account the reflected sunlight, Earth’s thermal irradiance and atmospheric transparency, and the requirement of a sufficient signal-to-noise ratio and signal-to-background ratio. The investigated meteors, with a minimal absolute magnitude of -2.9, and the two investigated Hayabusa and Stardust re-entry capsules can be tracked from GEO for at least 3 consecutive observations with a sub 1 meter aperture diameter optical instrument. A 2 to 5 second tracking time of the investigated meteors requires aperture diameters between 0.1 m and 0.87 m. For 20 second tracking of the re-entry capsules a 0.1 m aperture diameter is needed, which increases to 0.27 m when tracking for 50 seconds. To observe the whole Earth multiple optical instruments would be needed on one GEO satellite. This requires in most cases less than 1 cubic meter of space on the satellite.

In 2022, ESA plans to launch the JUICE (JUpiter ICy moons Explorer) mission which will spend at least three years making detailed observations of Jupiter and three of its largest moons, Ganymede, Callisto and Europa. These moons are currently a hot topic within the science community as their interiors might include oceans consisting of liquid water. These oceans could provide life, but at the moment little is known about the exact composition and structure of these interiors. Only Earth based observations and a few fly-by’s have been performed to measure the characteristics of these moons. The JUICE mission will provide more detailed information on the moons through fly-by’s. This thesis research will focus on Ganymede as JUICE will be the first human-developed satellite to orbit thismoon. Ganymede stands out as a potential scientific target due to several specific reasons; the most remarkable being it’s intrinsic magnetic field. Only two other solid bodies within the Solar System generate such a magnetic dipole field (Earth andMercury). The complex interactions of this magnetic fieldwith Jupiter’smagnetic field are unique and could provide a lot of new knowledge when studied. Measurements from Galileo and the Hubble space telescope suggest that a subsurface layer of (saline) water is present within the moons interior. Saline water could be a good conductor of electricity, generating the magnetic field. The magnetic field of Ganymede could also point towards a complex core, which is another possibility for the generation of this field. It could be that the core of Ganymede consists of liquid, iron rich elements which generate and maintain this magnetic field. Unfortunately, current models of the gravitational potential field and the interior of Ganymede are still uncertain. A precise gravitationalmodel of Ganymede could provide a lot of information about this interior. An orbiter or in-situ probes are required to achieve high precision gravitational potential field models. JUICE is expected to obtain a model of Ganymede’s gravitational potential field of at least degree and order 15. This thesis will provide insight in how different possible internal density distributions of Ganymede influence the gravitational potential field of the moon. Thisway,when JUICE obtains more information on the gravitational potential field of Ganymede, variations within this field can directly be utilized to determine what interior aspects could cause these variations. From 44 billion 1D homogeneous models considered during this research, only 260 adhered to current known characteristics of Ganymede. Certain elements and water phases are present in all models: a pure iron or iron-sulfide core, a silicon mantle, an ice VI layer together with an liquid ocean and a outer crust consisting of Ice Ih. Dependent on the exact layer thicknesses within a model, intermediate ice phases, ice III and V, can also be present. Layer correlations between the 260 models were analyzed and fourteen models where selected for further research. These models were combined with different boundary and density variations to obtain different 3D heterogeneous models. Gravitational potential simulations for spherical harmonics coefficients up to order/degree 48 were performed. It was found that several relations exist between gravitational potential field data and internal density distributions within Ganymede. If one can effectively correct gravitational potential field signals for measurable components within Ganymede’s interior, several sets of internal structures emerge. Furthermore, taking into account the established limitations and correlations between layers, the gradient of the gravitational signal power over spherical harmonics degree can be directly related to the thickness of an interiors ocean. Several distinguishable models show that the presence of ice III, and to a lesser extent ice V, increase the gravitational signal power of a model. When combined with the correlations found between internal layers during this research, one could even establish an accurate first order approximate of Ganymede’s internal composition. These results, together with measurements performed by JUICE, will provide numerous new insights on Ganymede’s frozen enigma.

Large Low Shear-Velocity Provinces (LLSVPs) in Earth have been recognized in seismic data for several decades. The anomalies are however still poorly understood for reasons including their inconvenient location in the deep mantle, inhibiting progress in our understanding. Comprehension of these structures is important, because of their speculated major role in the formation and evolution of the Earth, and her residents. The gravity anomaly associated with the LLSVPs, the C22-lobs, are roughly observed in the gravity field of Mercury too. Therefore, the gravity and topography data about Mercury is assessed for indications of similar deep mantle features as observed in Earth. The advantage of Mercury is its thin mantle, that allows for a relatively shallow location of the core-mantle boundary (CMB), hence shallow deep mantle anomalies. The cause of the thin mantle is yet unknown, currently suggested explanations include vaporization of the mantle due to the immense heat of the solar nebula or a hit-and-run collision scenario that ripped off the mantle of Mercury. In this research it is investigated how deep mantle structures like LLSVPs and CMB topography could account for the gravity field of Mercury. It is concluded that deep mantle structures with comparable size and density characteristics as the LLSVPs in Earth could exist. Compensation by CMB topography would require a rough CMB. The results are highly affected by the limited knowledge about crustal compensation in Mercury and the truncation of the gravity field. Better crustal models of Mercury could be obtained by investigating local gravity anomalies in relation to topographic features, considering flexural isostasy and using normal modes. In addition to that, BepiColombo is currently en route to Mercury to gather more global data of Mercury, that would highly aid in our understanding of the crust and mantle of Mercury. A relation comparable to the hypothesised relation between hotspot volcanism and LLSVPs on Earth is also investigated for Mercury. A relation was not necessarily observed, however it was also not ruled out and requires more attention as soon as the crustal models have improved and more global coverage of volcanic features on Mercury is available.

Satellite-derived gravity models have been used in modelling of several subduction zones, but not yet for the Sumatra subduction zone. There, gravity models can shed light on the geometry and density of the plate, and whether a slab tear is present under northern Sumatra, as seismic and tomographic studies present contradicting conclusions.

Ten different gravity sensitivity tests were carried out. Those tests investigated the sensitivities of the radial gravity (g_R) and the second gravity gradient tensor invariant (I₁) to different slab parameters (depth, thickness, density), three isostatic scenarios, and the inclusion of subduction-specific characteristics, e.g. a slab tear, and subducting crust. An innovative approach was implemented to determine slab thickness estimates based on oceanic lithospheric age, subduction direction, and a weighted half-space cooling model. Thereafter, a numerical model for the Sumatra subduction zone was developed which approximates the XGM2016 satellite-terrestrial gravity observations and supports a slab tear.

The gravity sensitivity test results together with the numerical model for the Sumatra subduction zone provide a valuable and universal foundation for future gravity-based subduction zone research.

Measurements in the frequency range below 30 MHz, key to a better understanding of the origins of our universe, are one of the last unexplored areas of radio astronomy. Low frequency measurements suffer greatly from disturbance from man-made signals. Therefore, satellite missions are proposed which use the Moon as a shield from these disturbances, by making use of the Earth-Moon L2 Lagrange point. Such missions require large numbers of small satellites. One of the challenges for these missions is getting the satellites into their destination orbit with a very small Delta V budget. This research looks into the use of low energy trajectories for such a mission transfer trajectory.
Low energy trajectories are found on the invariant manifolds of the CR3BP and may be classified into several categories. Different combinations of these categories lead to different types of transfer trajectories. Four possible combinations were considered in this research, and compared to see which one best suits a small satellite mission to the Earth-Moon L2 Lagrange point. One combination has been previously researched in literature, the other three are new options for a transfer to the Earth-Moon L2.
The approach known from literature resulted in transfers with a required Delta V of down to 28 m/s. Using one of the other approaches, this could be reduced to 9 m/s. Additionally, one of the approaches allows for a station-keeping orbit around the Sun-Earth L2 point, which does not only make scheduling of the transfer easier, but also allows for additional mission objectives. These are encouraging results, showing that possibilities exist for the use of low energy trajectories for a lunar L2 mission.

The 26th of November 2018, NASA's InSight mission successfully landed on Mars. InSight has a seismometer on board with as main science goal to determine the current level of seismic activity and the impact rate of meteorites on Mars. To process the incoming data of InSight as fast and efficient as possible, a-priori models of seismicity on Mars are needed. This thesis work consists out of making such models by using gravitational potential theory. Multiple interior density models of mars (crust and upper mantle) are created by use of isostatic assumptions, the Bouguer gravity or derived from literature. For each model, gravitational potential stresses are calculated which can be compared to the yield stress of the Martian lithosphere. Conclusions about the most likely sources of seismicity of Mars for varying interior density models are made.

Electric Propulsion (EP) has become one of the most efficient technologies for spacecraft propulsion. In contrast to conventional chemical propulsion, the low thrust force generated by EP thrusters can deliver a momentum transfer to the spacecraft that is up to twenty times greater, for an equivalent propellant expenditure. Despite decades of heritage, the topic of low-thrust spacecraft trajectory optimization remains an active field of research, with many approaches available yet much room for improvement. This MSc thesis presents the development of a novel methodology for low-thrust many-revolution trajectory optimization. It employs the state-of-the-art hybrid technique to harness the strengths of both indirect and direct optimization methods. Its indirect nature efficiently reduces the number of optimization variables and its direct counterpart provides an unmatchable flexibility in terms of a configurable force and perturbation model as well as operational constraints handling. This methodology was already shown in literature to be highly reliable for minimum-time trajectories. The research hereby presented maintains these results while enabling it to optimize minimum-propellant trajectories through a mechanism that allows for coasting (non-thrusting) arcs. This approach is additionally combined with an orbital averaging scheme to reduce the propagation load at the expense of accuracy for the rapidly changing variables. Nonetheless, it retains the continuous integration scheme to enable final geodetic longitude targeting in combination with propellant-minimization, which the former hybrid methodologies were incapable of solving for. The trajectory simulator is coupled with an enhanced objective function that reduces the user fine-tuning effort, and with a differential evolution algorithm that leads to a flexible global optimization process with a practical computational effort.

This research is the outcome of the cooperation between the author, Delft University of Technology, and GMV Innovating Solutions, a technology business group with a strong leadership in space engineering. The specific interest of application of this research lies in many-revolution planetocentric trajectories, such as an orbital transfer to Geostationary Earth Orbit, where there are growing market opportunities for all-electric satellite platforms. The resulting software, integrated as part of GMV's Flight Dynamics solution, allows the user to include orbital perturbations and perform a multi-objective optimization with respect to time-of-flight, propellant expenditure, and final geodetic longitude. This research constitutes a quantum leap for the hybrid optimization method because it shows that its accuracy in propellant-minimization is comparable to the analytical global optima despite the simplified co-state dynamics. Furthermore, it is a significant advancement for space mission design and satellite operations because it demonstrates the superior convergence performance of the hybrid methodology relative to GMV's implementation of the indirect approach, particularly in complex problems that combine multiple optimization objectives with varied orbital perturbations and operational constraints fulfillment.