Interplanetary missions have relied on Radio Science (RS) for recovering gravity fields via detecting their signature on the spacecraft’s trajectory. Yet the weak gravitational fields of small bodies, coupled with the prominent influence of confounding accelerations, hinder the efficacy of this method. Meanwhile, quantum sensors based on Cold Atom Interferometry (CAI) have demonstrated absolute measurements with an inherent stability and repeatability, reaching the utmost accuracy in microgravity. This work addresses the potential of CAI-based Gradiometry (CG) as a means to strengthen the RS gravity experiment for small-body missions. Phobos represents an ideal science case as astronomic observations and recent flybys have conferred enough information to define a robust orbiting strategy, whilst promoting studies linking its geodesic observables to its origin. A covariance analysis was adopted to evaluate the contribution of RS and CG in the gravity field solution, for a mission duration of one week. The favourable observational geometry and the small characteristic period of the gravity signal add to the competitiveness of Doppler observables. Provided that empirical accelerations can be modelled below the nm/s2 level, RS is able to infer the 6x6 spherical harmonic spectrum to an accuracy of 0.1-1% w.r.t. the homogeneous interior values. If this correlates to a density anomaly beneath the Stickney crater, RS would suce to constrain Phobos’ origin. Yet, in event of a rubble pile or icy moon interior (or a combination thereof) CG remains imperative, enabling an accuracy below 0.1% for most of the 10x10 spectrum. Nevertheless, technological advancements will be needed to alleviate the current logistical challenges associated with CG operation. This work also reflects on the sensitivity of the candidate orbits with regards to dynamical model uncertainties, which are common in small-body environments. This brings confidence in the applicability of the identified geodetic estimation strategy for missions targeting other moons, particularly those of the Giant planets, which are targets for robotic exploration in the coming decades.

This research focuses on GNSS users interested in a faster position fix in a situation where the receiver holds limited data. This occurs when turning on a mobile phone or sport watch to use a location-based service, driving out of a parking garage with a navigational device or activating a search & rescue beacon. In these cases, users care more about directly setting a certain action into motion, than waiting until a sub-meter level of accuracy is acquired. For this purpose, we aim to decrease the time to first fix (TTFF) of a receiver at the expense of position accuracy by focusing on its most dominant component: the time to read the first fix data, i.e. the TTFFD. In turn, the TTFFD is decreased by reducing the size of the clock correction and ephemeris data (CED) and integrating this reduced set into the existing Galileo I/NAV message while maintaining backward compatibility. Three reduction strategies are presented to reduce the size of the CED. First, the expression of the CED parameters relative to the matching almanac parameters, which requires downloading the most recent almanac. Second, the absorption of the clock and ephemeris time reference into related parameters by leveraging redundancies in the user algorithm. Third, the truncation of the CED parameters, where the optimal parameter bit allocation per CED size is determined by means of a Monte Carlo simulation. Subsequently, the optimal reduced set is integrated in the existing Galileo I/NAV message, since it is not only size but also the location and repetition rate of the reduced CED in the navigation message that determine the final decrease in TTFFD. A simulation is developed where the three reduction strategies are combined and applied to one year of Galileo navigation messages and almanacs starting from October 2017. The current I/NAV message has a size of 428 bits and an average TTFFD of 25.4 seconds. The relative expression of the CED parameters, the absorption of the clock time reference and ephemeris time reference yield a 76, 15 and 10 bit reduction respectively, without a significant decrease in position accuracy expressed as the signal-in-space ranging error (SISRE). The reduction in size that can be realized by truncation results from a trade-off with the maximum acceptable level of SISRE. When combining all three reduction strategies a 296 bit, i.e. 69%, size reduction can be realized at a SISRE level of 10.04 meters, yielding a 132 bit CED. 66% of the size reduction is in this case achieved by parameter truncation. To maintain backward compatibility, the spare and reserved bits of the I/NAV message are used to integrate the reduced set. This finally results in an average TTFFD of 6 seconds, which is a decrease of almost 20 seconds. With the reduced CED the user gains full independence of out-of-band channels for the computation of a fast position fix. In addition, it decreases the impact of bit errors on the TTFF and increases the chance of receiving an error free set of first fix data. Moreover, the reduced CED is not only of interest to fast fix applications that desire a decrease in waiting time. Positioning based on the reduced set also decreases the time the receiver has to be "on" to read the navigation data and consequently the receiver's power usage. This leads to a second type of applications that benefits from the reduced CED: low-power applications that do not require a high accuracy, such as freight management, wildlife tracking and stolen goods trackers. To conclude, the constructed reduced CED provides multiple advantages for a wide range of GNSS users and should therefore be considered as a purpose for the spare and reserved bits of the Galileo I/NAV message.

Space debris and micrometeoroids are abundant around our home planet and offer both an opportunity of research for scientists and a threat to human’s endeavours in space. Impacts with even the smallest particles can happen at such high velocities that they can damage or destroy an operational spacecraft, creating even more debris and possibly endangering its crew, if manned. Detecting and cataloguing the debris and micrometeoroid population has thus been a priority for many space agencies around the world to ensure safe space access. Unfortunately, small debris in high orbits cannot be detected by ground based observatories that can detect objects with a minimum diameter of 10 cm in LEO and 1m in GEO. This is why space based detectors are needed. Current detectors are small in size and can often detect only high velocity impacts due to relying on ionisation of particles upon impact. This MSc thesis investigated the possibility of using the whole spacecraft as a detector, by employing vibration sensors on its structure to detect impact-induced oscillations. These vibrations could contain information on the impactor’s mass, speed, material class and other features. Impacts of pieces of debris were simulated in a laboratory environment and the vibrations generated in a spacecraft-like structure were measured and processed to study the effectiveness of such a detection method. Three types of projectiles were used, beads of glass (3mm diameter, 0.04g average mass), teflon (3.2 mm average diameter and 0.033g average mass) and steel (3 mm average diameter and 0.11 g average mass). As target, to represent a spacecraft, a 3mm thick aluminium panel, a 5 mm thick aluminium- skin aluminium-honeycomb sandwich panel and a flight spare model of a solar array were used. An impact-location estimation algorithm was developed that can successfully determine the point of impact based on the signal of at least 4 sensors with centimetre accuracy. Analysis of the waveforms acquired by the sensors showed that deformation phenomena upon impact affect the frequency content of the signal, showing that to preserve as much as possible the high frequency content of the signal stiffer parts of a satellite are more indicated for this type of technology. Lastly, it was observed that the maximum voltage measured grows linearly with the momentum of the particles, making it possible to estimate the impact momentum solely based on the signal acquired. The dependence of voltage from the impact momentum together with the experimentally calculated coefficient of vibration dissipation of -0.05 cm^(−1) was used to determine the maximum distance of sensors from the point of impact as a function of particle speed and mass for successful detection of the impact. Particles as light as 0.03 g (the lightest used in the experiments) travelling at speeds as low as 15 m/s are expected to be detectable from sensors as far as 50cm, while for objects of 0.2 g travelling at speeds exceeding 200 m/s it is expected that a sensor can be as far as 1 meter away from the point of impact. The method, for its simplicity and low-cost provides an easy way to greatly increase the amount of data on small pieces of debris and micrometeoroid around our planet, enhancing space exploration safety and supporting future missions.

Aims: Analysing historical disk-integrated polarization data of Venus with the help of numerical simulations to learn about the time variability in cloud and haze properties, the position of the UV-absorber and the polar cap regions. Method: With numerical simulations, polarization curves will be calculated for specific input conditions. We compare numerically computed polarization curves to the historical polarization data to derive the microphysical properties of Venus's cloud and haze particles. Results: From our research we obtain values for the imaginary part of the refractive index of the cloud particles of the order 10^(-4). It is most likely that the absorber is mainly located at the equator or in patchy clouds covering 80% of the planet. It is found that in the presence of precise disk-integrated polarization data, it is possible to detect variations in the polar regions. For Venus we found that it is likely that there exist larger particles at higher altitudes at the Venus poles. Unfortunately, it is very hard to say something about the long term variations on the planet. For the Venus case, it can be roughly stated that: the polarization was higher in 1975/1976 than in 1965/1968; the polarization was also higher in 1968/1970 than 1965/1968 but not higher than in 1976; there was a decrease in the polarization of the ultraviolet region in the years 1964/1965. Conclusions: Future observations should be made in a broad range of wavelengths, at as many phase angles as possible and in time frames similar to the orbital period of the (exo)planet. Since Venus can be observed as if it were an exoplanet (broad range of observable phase angles and disk-integrated data), we applied this research to exoplanets as well. We concluded with the help of the Venus data and simple models that it is possible to obtain cloud properties, such as the refractive index and the optical thickness of the cloud, of exoplanets using polarization observations. It was found that those properties are harder to distinguish using flux measurements since the geometric albedo and the distance to the planet are unknown.

This thesis work has been motivated by the growing interest in small-spacecraft small-body missions and more specifically inspired by a proposed mission to the largest Martian Trojan (5261) Eureka. This binary asteroid represents a promising target for a planetary mission, as a better understanding of its formation and evolution would provide valuable insights into the history of both the Martian system and binary asteroids. This work has focused on identifying orbital designs which would optimise the geodetic parameters estimate, and thus best help characterise the asteroid's internal structure. The orbit determination solutions have been computed from simulated tracking data in various orbital configurations. To compensate the highly perturbed nature of the binary asteroid's environment, pseudo-periodic solutions identified around Eureka have been used as stable initial conditions for the spacecraft orbits. As Eureka's shape, gravity field and rotational state are not well characterised yet, large resulting uncertainties have been accounted for. They have been propagated to assess the robustness of the spacecraft orbits and subsequent orbit determination solutions to dynamical mismodelling, in order to eventually limit the science risk of the mission. For single CubeSat configurations, an optimal semi-major axis has been identified, being the one closest to the asteroid before orbital instability would start deteriorating the orbit determination solution. Non-equatorial retrograde orbits have been found to provide the lowest formal errors for geodetic parameters. Moreover, the benefit of simultaneously deploying two or even three CubeSats around the asteroid has been demonstrated. In both the two and three CubeSats configurations, promising orbital designs have been identified. They provide an estimation of the geodetic parameters which is accurate enough to bring insights into Eureka’s internal structure, and has been proven to be robust to dynamical mismodelling. The methodology developed in this work is of interest to design other small-spacecraft small-body missions, and optimise their achievable geodetic parameters estimate. Additionally, Eureka's dynamical model and the large uncertainties assigned to it have been mostly based on models available for other binary asteroids. This brings confidence in the possible applicability of the identified optimal orbital designs to CubeSats missions targeting other binary asteroids.

In the last fifty years, the spacemissions Voyager, Galileo, Cassini-Huygens and Juno explored the moons of the outer Solar System and revealed a wide spectrum of worlds. While some of these worlds are barren, others are among the most geologically active of the Solar System. The innermost Jovian moon, Io, showcases spectacular volcanic activity; its three companions, Europa, Ganymede and Callisto, are likely ocean worlds that harbour subsurface oceans beneath their icy crusts. Similarly, the biggest Saturnian moon, Titan, is believed to have a subsurface ocean concealed beneath its icy surface and dense atmosphere. Saturn’s collection of smaller icy bodies feature different levels of activity, Enceladus being the most remarkable. Above its limb, water plumes rise more than hundred kilometers spilling its internal ocean into Saturn’s E-ring. In Neptune, the captured moon Triton orbits in a peculiar retrograde orbit; its barely cratered surface is similar to that of other ocean worlds and shows signs of cryovolcanic activity. The spectrum of geological activity displayed by the moons of the outer Solar System is thought to bemainly the consequence of tides.@en

The icy satellites of the giant planets Jupiter and Saturn are among the most interesting celestial bodies in our Solar System. The interpretation of various remote sensing observations performed by the Voyager, Galileo and Cassini-Huygens missions strongly suggests that many icy satellites harbor a subsurface water ocean underneath the ice shell covering the satellites. Since the availability of liquid water is one of the prerequisites for the origin and evolution of life as we know it, the characterization of the physical properties of the putative subsurface oceans (and the overlying ice shells) has become a key research topic in planetary sciences. Due to the unavailability of direct means to explore these internal oceans, the relevant physical properties of the interior need to be derived from the combined interpretation of remote sensing measurements of observables such as the strength of the induced magnetic field, the amplitude of tidal deformations, the strength of gravity perturbations due to tides and the amplitude of forced longitudinal librations. This dissertation concentrates on the development of a unified and self-consistent physical model to determine the tidal and rotational response of a viscoelastic icy satellite with an internal water ocean. The developed tidal-rotational model is then applied to analyze the relation between the aforementioned observables (with the exception of the induced magnetic field) and the physical properties that characterize the internal structure of an icy satellite. The results in this thesis strongly suggest that the measurement of the libration amplitude of Europa’s shell with an accuracy of a few meters has the potential to provide a reasonable constraint on the rigidity of the ice-I shell in combination with measurements of the tidal response (Love numbers) at the surface. However, the modelling presented here is not extensive enough to support a strong conclusion regarding whether the ice shell thickness could be inferred from combined measurements of the libration amplitude and tidal Love numbers at the surface of Europa.@en

As a result of climate change, sea level is changing all over the world at unprecedented rates. Sea-level change can have significant impacts on coastal communities, infrastructure and global economy, as most of the major cities are located near to or at the coast. Rising sea levels can lead to, for instance, more severe and more frequent flooding, increasing coastal erosion and salt water intrusion. In addition, sea-level change can also influence coastal ecosystems, by altering the habitats of many plant and animals species. Therefore, it is crucial that we understand what is causing sea-level change and at what rate sea levels are changing. Global mean sea level has been rising at a rate of about 3.4 millimetres per year over the last 30 years. Regionally, however, sea level can be changing at a much higher or lower rate. That is because local processes, such as ocean dynamics and gravitational effects associated with continental ice mass changes, cause regional deviations from the global average. But what is causing sea level to change at a specific location? Is sea level changing because the oceans are warming, and thus expanding? Or because the ice from glaciers and ice sheets are melting? The attribution of sea-level change to these and other drivers can be done using a sea-level budget approach. Sea-level budget studies can be used to constrain missing or poorly known contributions and to validate climate models. While the global mean sea-level budget is considered closed within uncertainties, closing the budget on a regional to local scale is still challenging. In this thesis, I focused on the question: Can we close the regional sea-level budget in the satellite altimetry era on a sub-basin scale consistently for the entire world? For this, we need not only high quality observations of sea-level change and each component, but also of the uncertainties within each process. Therefore, in Chapter 2 and 3, I explored the main drivers of regional sea-level change, focusing on the uncertainty characterization of each component. I then looked at which spatial scale is optimal for analysing the regional sea-level budget, and compared the sum of the drivers with the total observed change in these regions in Chapter 4.@en

The thesis that you are about to read deals with the implementation of a technique to study atmospheres of planets or moons in the Solar System. We use radio telescopes on Earth to track spacecraft that are orbiting planets, and use the signal the spacecraft emits, as it crosses the planet’s atmosphere, to investigate its physical characteristics. Amazing, isn’t it? Often, we get lost in our daily routines and we lose sight of the overall picture. We forget how truly astounding the experiments we are able to undertake are, using the universe as our lab. I feel very privileged to have been able to do this as part of the work that led to this dissertation. @en

Europa is one of the most interesting celestial world that has been ever observed. The habitability condition, met for the liquid water layer covering the moon, yields to astonishing speculations concerning what might exist in the interior of the tiniest moon of Jupiter. The Voyager and Galileo programs detected a frozen and brittle layer, deeply battered by lineament features. The observed crevasses on the moon's icy surface can be considered as results of a strong and variating stress field applied to the brittle icy shell that eventually reaches critical deformation conditions locally, and favors the process of crevasse propagation. The superimposition of secular widening to diurnal components is the source of stress that continuously deforms the brittle surface of Europa and induces the ice to crack, similarly to the processes observed with crevasses in large terrestrial ice sheets. The research's aim is to improve the existing models of fracture propagation for the Europa ice shell, dealing with analogs observed in Earth's crevasses on large ice shelves, by the implementation and the usage of linear elastic fracture mechanics (LEFM). Two different LEFM approaches are included in the document, one dealing with the estimation of global areas on the moon that are more favorable to host propagation and one dealing with the estimation of fractures' lengths for specific observed features. Results describe the existence of critical and non-critical areas centered in the equatorial zone which are respectively prone or not to host vertical propagation. Maximum critical depths for surface crevasses reach values of 120 meters, while critical heights for bottom crevasses show values up to 1.5 kilometers. Beside the outcomes of the vertical simulation, a mathematical manipulation of the LEFM analysis allowed the determination of horizontal cracks’ growth. Knowing the aspect of an observed lineament, the current model could calculate fracturing events’ intensity. These reach propagation rates of kilometers per second, namely almost instantaneous episodes. The outcomes of the current research are particularly interesting when seen in relation with the future exploration missions to Europa: ESA's JUICE, NASA Europa Clipper and its potential lander. Specific areas that are more prone to host propagation and the determination of growth rates are helpful elements in the preliminary description of target landing areas and the fracturing events’ detection possibility. The built model yields to a further and more accurate understanding of the dynamics for the interior of one of the most promising celestial object, in term of searching for a biosphere, hence extraterrestrial life.

Since the first radio occultation measurements were performed to study planetary atmospheres, the calculations returning refractivity profiles always adopted the so-called "spherical symmetry assumption". This hypothesis imposes a null horizontal gradient of the atmospheric parameters within the region sampled during the occultation: a scenario that can be quite unrealistic for regions in proximity of the terminator line. This study suggests that another way of calculating refractivity profiles is doable, dropping the spherical symmetry assumption and assuming a straight line propagation of the radio signal through the atmospheric medium (which, according to the laws of refraction, should travel in a curved line, corresponding to the fastest trajectory according to Fermat's law). The study proved that the theoretical difference between the two methods is negligible (within 1%) and that numerical methods can be developed to account for the asymmetric regions within the atmosphere.

The practice of astronomy is in many ways an intrinsically introspective endeavour. A significant fraction of astronomical motivation is derived from the desire to understand whether a habitable planet such as the Earth is a unique object, and, by extension, whether the inhabitants of the Earth themselves collectively represent a unique phenomenon. By definition, a world is considered potentially habitable if it is theoretically capable of supporting liquid water at its surface. But by focusing solely on strictly Earth-like planets, we risk overlooking other potentially habitable options. In fact, the majority of the worlds known to host liquid water oceans in the solar system are not Earth-like at all. These other worlds do however share a singular defining characteristic: they are the icy moons that orbit the gas giant planets. Their oceans are concealed below kilometers of frozen crust. In the solar system at least three moons are known to host an ocean with a high degree of confidence (Europa, Enceladus, and Titan), and another four are suspected (Ganymede, Callisto, Mimas, and Dione). Hence, any hope of answering the question as to how unique the phenomena of life on Earth really is may hinge predominantly on answering a seemingly unrelated question: how unique are the icy moons? The formation of gas giants appears to be accompanied by the formation of moons, as, at least in the solar system, the two appear inseparable. The gaseous planets Jupiter, Saturn, and Uranus, are each attended by a retinue of moons both regular and irregular. The regular satellites tend to orbit in a single plane, in the same direction, and on nearly circular paths. These peculiar properties are also exhibited by the planets, and hence it is considered likely that some similar process has been at play to form them both. That process is the formation within a swirling disk of gas and dust. Planets form within disks that surrounded a star (a circumstellar disk) while the moons would have formed within a disk surrounding their planet (a circumplanetary disk, or CPD). The last decade of strides made in the observation of circumstellar disks has revolutionized our understanding of the planet formation process. To what extent might the moon and planet formation process be similar? To what extent might we be able to extrapolate our understanding of circumstellar disks down to the scales characteristic of circumplanetary disks? Is there a smooth continuum in physical processes, connecting the formation of large moons, with the formation of the smallest planets? In this work we have extended the theoretical tools used to explore planet formation down into this new regime. On the observational side, as the scale of the astrophysical object shrinks, the capabilities of the instrument must rise commensurately to observe it. We are now at the earliest possible stage of directly observing CPDs to gain insights beyond the theoretical into how giant planet moon formation actually proceeds… @en