5,157 research outputs found
Design of Phasing Trajectories From a Low Lunar Orbit To a Near Rectilinear Halo Orbit
The paper presents three different approaches to the design of a phasing trajectory in a cislunar
environment, where the third body perturbation is considered non-negligible. The working
framework is the one proposed by the ESA’s Heracles mission in witch the passive target
spacecraft -LOP-G- is orbiting on a Near Rectilinear Halo Orbit and the Lunar Ascent Element
must reach that orbit from a Low Lunar Parking Orbit to start the rendezvous procedure.
In this scenario the authors propose three different ways to design such phasing manoeuvre
under the Circular Restricted Three Body Problem hypotheses: Lambert/Differential correction,
Hohmann/Differential correction and Optimization. The three approaches are compared
in therms ofV consumption, accuracy and time of flight. Finally, a selected solution is also
validated under the Restricted Elliptic Three Body Problem Hypotheses
Thrust Expenditure Feasibility Analysis for Rendezvous Operations in Cis-Lunar Space
In recent years, Moon exploration has become a primary objective within most space agencies worldwide. The Lunar Space Gateway program ARTEMIS (or LOP-G) is an example of mission proposal for technology feasibility in terms of autonomous (and later manned) operations of a space station orbiting the L2 Earth – Moon Lagrangian point. Rendezvous and docking (berthing) are tasks that are envisioned to be performed fully autonomously. The focus of the paper falls in this category, whereby an active module called Lunar Ascender Element (LAE), returning from the lunar surface, shall be able to operate an automatic rendezvous mission with the LOP-G station. The paper concentrates, in particular, with the feasibility analysis needed to assess the engines’ thrust capabilities to provide appropriate propulsion for open loop and closed loop control during rendezvous. The capability of providing the desired amount of thrust is not only linked to the actual guidance commands, but also to the nature of the motors. The rendezvous maneuver sequence, dynamics and hold points are first defined, and the thrust distribution and configuration detailed for the specific mission. The guidance logics are described, and the implementation of a passively safe trajectory outlined. Based on the dynamic model of the system, and the assumed actuator model, the main causes of unfeasibility are listed. The paper continues by analyzing the sensitivity of the thrust profile at each motor with respect to the control allocation algorithm, the duration of the maneuver, the duration of each impulse (assuming a two-impulse maneuver), and the location of the berthing port within a selected near rectilinear halo orbit around the Moon. The tests take into account how the parameters influence the Delta V required to perform the mission. The authors wish to remark that this analysis is critical to the design of rendezvous and berthing (docking) operations, since feasibility is necessary for the success of the mission, and it provides a structured computation of a realistic parameter space in the relative motion in the presence of a third body perturbation
A model for liquid-striped liquid phase separation in liquids of anisotropic polarons
The phase separation between a striped polaron liquid at the particular
density and a high density polaron liquid is described by a modified Van der
Waals scheme. The striped polaron liquid represents the pseudo gap matter or
Wigner-like polaron phase at 1/8 doping in cuprate superconductors. The model
includes the tendency of pseudo- Jahn-Teller polarons to form anisotropic
directional bonds at a preferential volume with the formation of different
liquid phases. The model gives the coexistence of a first low density polaron
striped liquid and a second high density liquid that appears in cuprate
superconductors for doping larger than 1/8. We discuss how the strength of
anisotropic bonds controls the variation the phase separation scenarios for
complex systems in the presence of a quantum critical point where the phase
separation vanishes.Comment: 10 pages, 3 figure
Analysis of an Experimental Cortical Network: i) Architectonics of Visual Areas 17 and 18 After Neonatal Injections of Ibotenic Acid; Similarities with Human Microgyria
Lesions of cortical areas 17 and 18 have been produced in newborn kittens by local injections of the excitotoxin ibotenic acid (ibo). Twenty-four hours after an injection on postnatal days 2 or 3, the gray matter of areas 17 and 18 near the center of the injection appears completely destroyed, with the exception of a one-to-two cell-thick layer at the bottom of layer I. Intact migrating neurons and radial glia can be found light- and electron-microscopically in the region affected. During the following weeks a several hundred micron thick cortex reforms. In the adult, this cortex consists of superficial layers I, II and III as proven by cytoarchitectonics, continuity with the corresponding layers of the normal cortex and cellular composition. We believe that the recovery is due to completion of migration by neurons spared by the ibo injection. More severe destruction of cerebral cortex, i.e. complete loss of the neuronal layers or their reduction to a few cell-thick mantles can be obtained with ibo injections at the end of the second or, respectively, first postnatal week. Severity of lesion also depends on the dose of ibo injected. There are interesting similarities between the ibo-injured cortex and two human neocortical displasias: microgyria and ulegyria
Analysis of an Experimental Cortical Network: ii) Connections of Visual Areas 17 and 18 After Neonatal Injections of Ibotenic Acid
Lesions of cortical areas 17 and 18 were produced
in newborn kittens by local injections of
the excitotoxin ibotenic acid. In the adult this
results in a microcortex which consists of superficial
layers I, II and III, in the absence of granular
and infragranular layers. Horseradish peroxidase,
alone or wheat germ agglutinin conjugated,
was injected in the microcortex or in the
contralateral, intact areas 17 and 18. The microcortex
maintains several connections characteristic
of normal areas 17 and 18 of the cat. It receives
afferents from the dLGN, and several visual
areas of the ipsilateral and contralateral
hemisphere. However, it has lost its projections
to dLGN, superior colliculus, and, at least in
part, those to contralateral visual areas. Thus some parts of the microcortex receive from, but
do not project into, the corpus callosum. In addition,
the microcortex maintains afferents from
ipsilateral and contralateral auditory areas AI
and AII which are normally eliminated in development
Bearings-Only Guidance in Cis-Lunar Rendezvous
The current plans of returning to the Moon involve rendezvous and docking/berthing operations
with a passive permanent station, whose targeted orbit is a Near Rectilinear Halo
Orbit. In this work a preliminary guidance algorithm for the close-range rendezvous phase
is proposed, based on a sample mission application, assuming the active chaser can measure
relative angles only to the target. The navigation performance is affected by the maneuvers
performed because their execution is necessary to make the problem observable. The relative
motion equations with third body perturbation are considered directly in the guidance
algorithm, with the additional aim of improving observability during the approach trajectory.
Numerical simulations are presented to validate the proposed methodology
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