Swarm Keeping Strategies for Spacecraft under J_2 and Atmospheric Drag Perturbations

This paper presents several new open-loop guidance methods for spacecraft swarms composed of hundreds to thousands of agents with each spacecraft having modest capabilities. These methods have three main goals: preventing relative drift of the swarm, preventing collisions within the swarm, and minimizing the propellant used throughout the mission. The development of these methods progresses by eliminating drift using the Hill-Clohessy-Wiltshire equations, removing drift due to nonlinearity, and minimizing the J_2 drift. In order to verify these guidance methods, a new dynamic model for the relative motion of spacecraft is developed. These dynamics include the two main disturbances for spacecraft in Low Earth Orbit (LEO), J_2 and atmospheric drag. Using this dynamic model, numerical simulations are provided at each step to show the effectiveness of each method and to see where improvements can be made. The main result is a set of initial conditions for each spacecraft in the swarm which provides the trajectories for hundreds of collision-free orbits in the presence of J_2. Finally, a multi-burn strategy is developed in order to provide hundreds of collision-free orbits under the influence of atmospheric drag. This last method works by enforcing the initial conditions multiple times throughout the mission thereby providing collision-free trajectories for the duration of the mission

Ice-VII inclusions in diamonds: Evidence for aqueous fluid in Earth’s deep mantle

Water-rich regions in Earth’s deeper mantle are suspected to play a key role in the global water budget and the mobility of heat-generating elements. We show that ice-VII occurs as inclusions in natural diamond and serves as an indicator for such water-rich regions. Ice-VII, the residue of aqueous fluid present during growth of diamond, crystallizes upon ascent of the host diamonds but remains at pressures as high as 24 gigapascals; it is now recognized as a mineral by the International Mineralogical Association. In particular, ice-VII in diamonds points toward fluid-rich locations in the upper transition zone and around the 660-kilometer boundary

High-pressure structural study of the scheelite tungstates CaWO4 and SrWO4

Angle-dispersive x-ray diffraction (ADXRD) and x-ray absorption near edge structure (XANES) measurements have been performed in the AWO4 tungstates CaWO4 and SrWO4 under high pressure up to approximately 20 GPa. Similar phase transitions and phase transition pressures have been observed for both tungstates using the two techniques in the studied pressure range. Both materials are found to undergo a pressure-induced scheelite-to-fergusonite phase transition under sufficiently hydrostatic conditions. Our results are compared to those found previously in the literature and supported by ab initio total energy calculations. From the total energy calculations we have also predicted a second phase transition from the fergusonite structure to a new structure identified as Cmca. Finally, a linear relationship between the charge density in the AO8 polyhedra of ABO4 scheelite-related structures and the bulk modulus is discussed and used to predict the bulk modulus of other materials, like zircon.Comment: 52 pages, 9 figure, 4 table

Satellite Formation Control Using Continuous Adaptive Sliding Mode Controller

peer reviewedThe focus of this study is on the development of a robust controller with a simple gain adaptation for satellite formation control. The complete nonlinear dynamics of the motion of the follower satellite relative to the leader satellite is considered and the rigorous proof for the stability of the controlled formation system is given in the presence of unknown external disturbances and unknown mass of the follower satellite. Although the controller design is based on the concept of sliding mode control, the proposed control strategy is free from chattering and guarantees a finite-time convergence of the controlled system to the target area. Furthermore, a simple adaptive law to automatically update the control gain is suggested that does not require a priori knowledge of the uncertainties of the system. In addition, to guarantee the robustness from the beginning and to improve the transient performance, a new sliding surface is also constructed. Numerical simulations are carried out to demonstrate the effectiveness of the proposed adaptive controller to maintain a desired formation configuration by compensating for the initial offset errors and external disturbance effects including gravitational perturbations and atmospheric drag