Adham Alkhaja’s research while affiliated with Khalifa University and other places

All our knowledge about Saturn and its icy ring system comes from the data obtained during the flybys of Pioneer 11, Voyager 1, Voyager 2 and Cassini, as well as from the observations carried out by Hubble Space Telescope. The discovery of water vapor plumes at the poles of Enceladus and other compelling evidence of the existence of subsurface water in the major moons of Saturn has driven scientific interest and revived plans to return to Saturn. In order to gain insight into the features of this planet and its Inner Large Moons (ILMs)--Mimas, Enceladus, Tethys and Dione--an in situ mission is needed. In general, orbit insertion at a giant planet is very demanding in terms of propellant, due to the large impulse required to achieve capture. It is even more challenging to achieve orbits around moons deep inside the planetary gravitational well, like the ILMs. The majority of the proposed solutions to tour the system of icy moons is based on the patched conics technique with impulsive maneuvers (i.e., chemical propulsion). The more efficient approach presented here is the concept of a lunar tour of the ILMs based on low-thrust (LT) propulsion and low-energy transfers in the circular restricted three-body problems (CR3BP) corresponding to Saturn and each moon. The hyperbolic invariant manifolds of planar Lyapunov orbits around the equilibrium points L1 and L2 of each Saturn-moon system are used to loop around the corresponding moon and to provide initial conditions to move between neighboring moons. These moon-to-moon transfers use a LT control law designed to minimize propellant consumption. LT, combined with a gravity assist with Jupiter, is also applied to reduce the hyperbolic excess speed at Saturn. This enables unpowered capture at Saturn by means of a Titan flyby. Results show that this mission concept saves a significant amount of propellant compared to the Cassini mission. Although LT yields longer transfer times than impulsive maneuvers, the spiraling transfers between moons can be exploited to collect data of the inter-moon environment, rings and moonlets

This contribution investigates the properties of a category of orbits around Enceladus. The motivation is the interest in the in situ exploration of this moon following Cassini’s detection of plumes of water and organic compounds close to its south pole. In a previous investigation, a set of heteroclinic connections were designed between halo orbits around the equilibrium points L1 and L2 of the circular restricted three-body problem with Saturn and Enceladus as primaries. The kinematical and geometrical characteristics of those trajectories makes them good candidates as science orbits for the extended observation of the surface of Enceladus: they are highly inclined, they approach the moon and they are maneuver-free. However, the low heights above the surface and the strong perturbing effect of Saturn impose a more careful look at their dynamics, in particular regarding the influence of the polar flattening of the primaries. Therefore, those solutions are here reconsidered by employing a dynamical model that includes the effect of the oblateness of Saturn and Enceladus, individually and in combination. The dynamical equivalents of the halo orbits around the equilibrium points L1 and L2 and their stable and unstable hyperbolic invariant manifolds are obtained in the perturbed models, and maneuver-free heteroclinic connections are identified in the new framework. A systematic comparison with the corresponding solutions of the unperturbed problem shows that qualitative and quantitative features are not significantly altered when the oblateness of the primaries is taken into account. Furthermore, it is found that the J2 coefficient of Saturn plays a larger role than that of Enceladus. From a mission perspective, the results confirm the scientific value of the solutions obtained in the classical circular restricted three-body problem and suggests that this simpler model can be used in a preliminary feasibility analysis.

The discoveries made by Cassini of geyser-like jets of vapour and organic compounds at the southern polar region of Enceladus have given impulse to a detailed study of this moon. As a result, a number of mission plans for the in-situ robotic exploration of Enceladus have been proposed by scientific communities and leading space agencies. The mission objectives of those plans can only be accomplished with orbits that provide extended observations of the southern polar surface of Enceladus. In a previous contribution, heteroclinic connections between halo orbits around the collinear equilibrium points 𝐿1 and 𝐿2 of the unperturbed Saturn-Enceladus circular restricted three-body problem have been proposed for the purpose. Due to the low altitude of these orbits with respect to the surface of Enceladus and the perturbations of the gravity field of Saturn, the effect of the second zonal harmonics of the two bodies on these low-energy solutions need to be assessed. The present contribution refines the previously computed low-energy trajectories in the J2-perturbed circular restricted three-body problem in which the primaries are Saturn and Enceladus. Halo orbits and their stable and unstable hyperbolic invariant manifolds are obtained in this new framework and used to construct heteroclinic connections in the enhanced dynamical model. Maneuver-free trajectories are obtained and compared with their unperturbed counterparts. Eventually, the performance of these solutions as science orbits is assessed by evaluating their speed, lunar surface coverage, time of flight and height above the lunar surface over the transfers. The results show a good agreement with the solutions obtained with the unperturbed model, suggesting that these trajectories can serve the purpose of observing Enceladus and the unperturbed model is a valid tool for a mission preliminary analysis. Furthermore, the second zonal harmonic term of Saturn is found to have a larger effect than the oblateness of Enceladus.

This contribution investigates the properties of a category of orbits around Enceladus. The motivation is the interest in the in situ exploration of this moon following the detection on behalf of Cassini of plumes of water and organic compounds close to its south pole. In a previous investigation, a set of heteroclinic transfers were designed between Halo orbits around the equilibrium points L1 and L2 of the circular restricted three-body problem with Saturn and Enceladus as primaries. The kinematical and geometrical characteristics of those trajectories makes them good candidates as science orbits for the extended observation of the surface of Enceladus: they are highly inclined, they approach the moon and they are maneuver free. However, the low heights above the surface and the strong perturbing effect of Saturn impose a more careful look at their dynamics, in particular regarding the influence of the polar flattening of the primaries. Therefore, those solutions are here reconsidered by employing a dynamical model that includes the effect of the oblateness of Saturn and Enceladus, individually and in combination. Substitutes of the Halo orbits around the equilibrium points L1 and L2 and their stable and unstable hyperbolic invariant manifolds are obtained in the perturbed models, and maneuver-free heteroclinic transfers are identified in the new framework. A systematic comparison with the corresponding solutions of the unperturbed problem shows that qualitative and quantitative features are not significantly altered when the oblateness of the primaries is taken into account, and that J2 of Saturn plays a larger role than the oblateness of Enceladus. From a mission perspective, the results confirm the scientific value of the solutions obtained in the classical circular restricted three-body problem and suggests that this simpler model can be used in a preliminary feasibility analysis.