Search

When reconstructing natural satellites' ephemerides from space missions' tracking data, the dynamics of the spacecraft and natural bodies are often solved for separately, in a decoupled manner. Alternatively, the ephemeris generation and spacecraft orbit determination can be performed concurrently. This method directly maps the available data set to the estimated parameters' covariances while fully accounting for all dynamical couplings. It thus provides a statistically consistent solution to the estimation problem, whereas this is not directly ensured with the decoupled strategy. For the Galilean moons in particular, the JUICE mission provides a unique, although challenging, opportunity for ephemerides improvement. For such a dynamically coupled problem, choosing between the two state estimation strategies will be influential. This paper quantifies the Galilean moons' state uncertainties attainable when applying a coupled estimation strategy to simulated JUICE data, and discusses the challenges that remain to be addressed to achieve such a coupled solution from real observations. We first provide a detailed, explicit formulation for the coupled approach, which was still missing in the literature although already used in past studies. We then assessed the relative performances of the two ephemerides generation techniques for the JUICE test case. To this end, we used both decoupled and coupled models on simulated JUICE radiometric data. We compared the resulting covariances for the Galilean moons' states, and showed that the decoupled approach yields slightly lower formal errors for the moons' tangential positions. However, the coupled model can reduce the state uncertainties by more than one order of magnitude in the radial direction (i.e. towards the central body). It also proved more sensitive to the dynamical coupling between Io, Europa and Ganymede, allowing the state solutions for the first two moons to fully benefit from JUICE orbital phase around Ganymede., Astrodynamics & Space Missions, Space Engineering

Plumes spewing water high above Europa's surface have been inferred from several observation campaigns in the past decade. Whereas the occasional existence of plumes on Europa has thus been confirmed, the origin of the plumes remains uncertain. Most notably, it is still unclear whether the inferred plumes are of oceanic origin, possibly containing information about the ocean's habitability, or if they are of surficial nature, offering information on the highly processed surface instead. In this study, we use a Monte-Carlo model to analyze three different plume models, two of which are surficial (near-surface liquid inclusion and diapir), and one of which originates in the sub-surface ocean (oceanic plume). We analyze all relevant Lyman- (Formula presented.) and OI 1304 (Formula presented.) emission mechanisms to determine the three models' emission profiles. These profiles are then compared to the Hubble Space Telescope/Space Telescope Imaging Spectograph measurements presented by Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) with the goal of determining which ab initio model fits these measurements best. Our analyses show that all three models investigated produce similar Lyman- (Formula presented.) and OI 130.4 nm emission profiles, with differences being perceivable only on scales well below the HST resolution and sensitivity. Since none of the models contradict the Roth, Retherford, et al. (2014, https://doi.org/10.1073/pnas.1416671111) and Roth, Saur, et al. (2014, https://doi.org/10.1126/science.1247051) observations, none of them can be ruled out as being the acting force behind the observed plumes. The currently available optical measurements are thus not sufficient to determine the physical nature of Europa's plumes, identification of which can only be achieved through higher resolution images or in situ measurements.

The origin of the Galilean moons have puzzled the scientists for centuries. The most recent theories affirm that they formed within a circumplanetary disk (CPD), a disk of gas and dust very similar to the protoplanetary disk (PPD), from which the planets formed. However, there are still many uncertainties on this CPD, such as its size, mass, the contribution of the young Jupiter, the young Sun and many more. In this project, an innovative way of simulating this CPD has been undertaken. With the help of the software ProDiMo, thermo-chemical simulations of the disk have been carried out. Originally designed to simulate PPDs, for the first time it has been applied to the tinier CPD. The software has proven itself to work well even with CPDs. The results are coherent with the current theories and, with this new approach, more clarity has been added to the formation process of the Galilean moons. This helps us to understand the origin of our solar system and to refine the research methods for possible habitable exomoons outside of the solar system border., Aerospace Engineering

The Jupiter Icy moons Explorer (JUICE) will generate ephemerides of the Galilean moons to assess their ability to sustain live. However, determining the ephemerides only using radiometric tracking (acquired using PRIDE and 3GM) results in an unstable solution which is related to the absence of flybys of especially Io, such that its dynamics can only be estimated indirectly (through the Laplace resonance). This problem is partially mitigated in this thesis by optimizing the observation schedule for optical astrometry of Io (provided by JANUS) using the NSGA-II algorithm. The optimal epochs for space-based astrometry are found to be preferentially distributed around the closest approaches of JUICE with respect to Io. During these events, the observations show a high sensitivity with respect to the dynamics of Io and have a low positional uncertainty. Using the optimized astrometry, both the condition number and formal errors can be substantially reduced., Aerospace Engineering

The search for life in other celestial bodies has always been a topic of great interest within the scientific community. The curiosity to know if we are the only ones in the universe or not has driven the development of many engineering projects. This Master Thesis is meant to do its bit in this research field. Europa, a moon of Jupiter, is thought to have an ocean made of water underneath its icy surface. This ocean could be the perfect environment to find life. The hypotheses of the existence of an ocean gained more strength with the discovery of a water vapour plume in the southern hemisphere of Europa. This plume could be expelling particles from the underneath ocean. The main idea of the project is to design a mission of a pseudo-orbiter (a spacecraft orbiting Jupiter and doing flybys of Europa) which could collect particles from this plume during its flyby trajectory. The objective of this thesis is to determine the maximum number of particles that can be collected with a pseudo-orbiter strategy. To achieve this, first, a plume particle model has been simulated. The outcome of this simulation is a 3D density profile of the water vapour molecules of the plume. The results of this part are needed in order to be able to determine the number of particles that the spacecraft can collect when crossing the plume. After this, the trajectory of the pseudo-orbiter has been designed. The Graphical Method for Same-body Transfers has been used in order to select potential trajectories that could lead to a maximum number of collected particles. This method allows the engineers to identify resonant orbits such that the ground-track of the flyby crosses regions of potential interest. Finally, coupling the two parts of the project has led to the determination of the number of collected particles for the selected trajectories., Aerospace Engineering

This research aims at modeling the trajectory followed by the Galileo spacecraft during a variety of flybys about the Galilean moons. The chosen flybys have good Earth’s elevation angles and either low or high closest-approach altitudes, so that the comparison of the two can give relevant insight into the accuracy of the corresponding trajectories. By propagating the state of the spacecraft during these flybys, optimizing the nominal initial state of the spacecraft (obtained through the SPICE program) and the spherical harmonics of the moons, and estimating new harmonics, the minimum root mean square error between the resulting trajectory and the ephemerides by Jet Propulsion Laboratory (JPL) is found. The analysis of its components along the Local Orbital axes gives insight into the existing relation between them and the Earth's elevation and azimuth angles. In particular, a low-altitude flyby implies in general a higher error, but when two flybys have similar altitudes, then the Earth's elevation plays a relevant role and the flyby with the largest one is more likely to have a larger error too. The root mean square error of the fitted trajectories can vary from 15 cm to 7 m, so always less than the 9 m declared by National Aeronautics and Space Administration (NASA) as the maximum error of the moons ephemerides. Furthermore, the flybys about Ganymede and Callisto show a high error in the along-track and cross-track directions, since the radius of their sphere of influence is quite larger than that of the inner moons' ones, hence there is more time for the perturbations to influence the orbit. A by-product of this research is the estimate of the Galilean moons' gravity field, in particular the new values for their J2 and C2,2 coefficients led to the conclusion that Io is less hydrostatic, while Europa and Callisto are more hydrostatic than previously thought., Aerospace Engineering

Around 1610 Galileo Galilei made his discovery of the four large moons orbiting Jupiter which are referred to now as the Galilean moons. The moons Europa and Ganymede attract a significant amount of scientific interest due to potential present subsurface oceans. As consequence, the design of missions to go there and explore the moon system are increasing. This culminated in the sixth edition of the Global Trajectory Optimisation Competition (GTOC6) which is focussed on solving low-thrust multiple gravity-assist trajectories to map the Galilean moons. The aim of the thesis is to understand the complexity of the GTOC6 problem and to explore and evaluate the quality of various optimisation strategies to solve flyby sequences with low-thrust arcs. First, insight to the complexity of the problem was gained by analysing the best solution to GTOC6 so far by the Chinese Centre for Space Utilisation (CSU). From the results a clear picture was drawn from what the trajectory model should be capable of. The low-thrust trajectory model is based on the spherical shaping method that is part of the Tudat astrodynamics toolbox. A full analysis of the shaping method was performed to identify the capabilities and shortcomings of the algorithm. One of the main shortcomings is the limited accuracy for trajectories where the departure and arrival conditions differ with several degrees and more for the right ascension of the ascending node (RAAN). For optimisation use was made of differential evolution (DE). An extensive test was performed to determine the optimal settings. The result was that defining the control parameters randomly during the evolution was the best option with respect to quality and convergence. What followed was defining the optimisation model for a variable number of flybys. Furthermore, a framework was developed with six different optimisation strategies. A sequence of maximum five flybys was set to test the strategies. The strategies define the amount of freedom aro, Space Exploration, Astrodynamics and Space Missions, Aerospace Engineering