21,196 research outputs found
Asymptotic analysis of displaced lunar orbits
The design of spacecraft trajectories is a crucial task in space mission design. Solar sail technology appears as a promising form of advanced spacecraft propulsion which can enable exciting new space science mission concepts such as solar system exploration and deep space observation. Although solar sailing has been considered as a practical means of spacecraft propulsion only relatively recently, the fundamental ideas are by no means new (see McInnes1 for a detailed description). A solar sail is propelled by reflecting solar photons and therefore can transform the momentum of the photons into a propulsive force. Solar sails can also be utilised for highly non-Keplerian orbits, such as orbits displaced high above the ecliptic plane (see Waters and McInnes2). Solar sails are especially suited for such non-Keplerian orbits, since they can apply a propulsive force continuously. In such trajectories, a sail can be used as a communication satellite for high latitudes. For example, the orbital plane of the sail can be displaced above the orbital plane of the Earth, so that the sail can stay fixed above the Earth at some distance, if the orbital periods are equal (see Forward3). Orbits around the collinear points of the Earth-Moon system are also of great interest because their unique positions are advantageous for several important applications in space mission design (see e.g. Szebehely4, Roy,5 Vonbun,6 Thurman et al.,7 Gomez et al.8, 9). Several authors have tried to determine more accurate approximations (quasi-Halo orbits) of such equilibrium orbits10. These orbits were first studied by Farquhar11, Farquhar and Kamel10, Breakwell and Brown12, Richardson13, Howell14, 15.If an orbit maintains visibility from Earth, a spacecraft on it (near the L2 point) can be used to provide communications between the equatorial regions of the Earth and the lunar poles. The establishment of a bridge for radio communications is crucial for forthcoming space missions, which plan to use the lunar poles.McInnes16 investigated a new family of displaced solar sail orbits near the Earth-Moon libration points.Displaced orbits have more recently been developed by Ozimek et al.17 using collocation methods. In Baoyin and McInnes18, 19, 20 and McInnes16, 21, the authors describe new orbits which are associated with artificial Lagrange points in the Earth-Sun system. These artificial equilibria have potential applications for future space physics and Earth observation missions. In McInnes and Simmons22, the authors investigate large new families of solar sail orbits, such as Sun-centered halo-type trajectories, with the sail executing a circular orbit of a chosen period above the ecliptic plane. We have recently investigated displaced periodic orbits at linear order in the Earth-Moon restricted three-body system, where the third massless body is a solar sail (see Simo and McInnes23). These highly non-Keplerian orbits are achieved using an extremely small sail acceleration. It was found that for a given displacement distance above/below the Earth-Moon plane it is easier by a factor of order 3.19 to do so at L4=L5 compared to L1=L2 - ie. for a fixed sail acceleration the displacement distance at L4=L5 is greater than that at L1=L2. In addition, displaced L4=L5 orbits are passively stable, making them more forgiving to sail pointing errors than highly unstable orbits at L1=L2.The drawback of the new family of orbits is the increased telecommunications path-length, particularly the Moon-L4 distance compared to the Moon-L2 distance
Thermal van der Waals Interaction between Graphene Layers
The van de Waals interaction between two graphene sheets is studied at finite
temperatures. Graphene's thermal length controls
the force versus distance as a crossover from the zero temperature
results for , to a linear-in-temperature, universal regime for
. The large separation regime is shown to be a consequence of the
classical behavior of graphene's plasmons at finite temperature. Retardation
effects are largely irrelevant, both in the zero and finite temperature
regimes. Thermal effects should be noticeable in the van de Waals interaction
already for distances of tens of nanometers at room temperature.Comment: enlarged version, 9 pages, 4 figures, updated reference
CFD and aeroelastic analysis of the MEXICO wind turbine
This paper presents an aerodynamic and aeroelastic analysis of the MEXICO wind turbine, using the compressible HMB solver of Liverpool. The aeroelasticity of the blade, as well as the effect of a low-Mach scheme were studied for the zero-yaw 15m/s wind case and steady- state computations. The wake developed behind the rotor was also extracted and compared with the experimental data, using the compressible solver and a low-Mach scheme.
It was found that the loads were not sensitive to the Mach number effects, although the low-Mach scheme improved the wake predictions. The sensitivity of the results to the blade structural properties was also highlighted
Electron concentration effects on the Shastry-Sutherland phase stability in Ce_{2-x}Pd_{2+y}In_{1-z} solid solutions
The stability of a Shastry-Sutherland ShSu phase as a function of electron
concentration is investigated through the field dependence of thermal and
magnetic properties of the solid solution Ce_{2-x}Pd_{2+y}In_{1-z} in the
antiferromagnetic branch. In these alloys the electronic (holes) variation is
realized by increasing concentration. The AF transition T_M decreases from
3.5K to 2.8K as concentration increases from y=0.2 to y=0.4. By applying
magnetic field, the ShSu phase is suppressed once the field induced
ferromagnetic polarization takes over at a critical field B_{cr} which
increases with content. A detailed analysis around the critical point
reveals a structure in the maximum of the dM/dB derivative, which is related
with incipient steps in the magnetization M(B) as predicted by the theory for
the ShSu lattice. The crossing of M(B) isotherms, observed in ShSu prototype
compounds, is also analyzed. The effect of substitution by is
interpreted as an increase of the number of 'holes' in the conduction band and
results in a unique parameter able to describe the variation of the magnetic
properties along the studied range of concentration.Comment: 8 pages, 11 figure
Contrastive grammar : a theory and practice handbook
En consonancia con los lineamientos del programa vigente de Gramática Contrastiva, materia incluida en el programa de estudios del Traductorado de Inglés de la Facultad de Lenguas, Universidad Nacional de Córdoba, el objetivo principal de Contrastive Grammar: A Theory and Practice Handbook es brindar a los estudiantes un manual que combine las gramáticas descriptivas del inglés y del español. No pretende ser una revisión completa de todas las diferencias lingüísticas existentes entre ambas lenguas: por el contrario, el objetivo del presente manual es combinar información teórica clave con prácticas variadas respecto de estructuras dispares que representan la fuente más frecuente de interferencia entre los dos sistemas.Fil: Gómez Calvillo, M. Natalia. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Fil: Meehan, Patricia. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Fil: Díaz, M. Josefina. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina.Fil: Rolfi, Laura. Universidad Nacional de Córdoba. Facultad de Lenguas; Argentina
Designing displaced lunar orbits using low-thrust propulsion
The design of spacecraft trajectories is a crucial task in space mission design. Solar sail technology appears as a promising form of advanced spacecraft propulsion which can enable exciting new space science mission concepts such as solar system exploration and deep space observation. Although solar sailing has been considered as a practical means of spacecraft propulsion only relatively recently, the fundamental ideas are by no means new (see McInnes1 for a detailed description). A solar sail is propelled by re ecting solar photons and therefore can transform the momentum of the photons into a propulsive force. This article focuses on designing displaced lunar orbits using low-thrust propulsion
First clear evidence of quantum chaos in the bound states of an atomic nucleus
We study the spectral fluctuations of the Pb nucleus using the
complete experimental spectrum of 151 states up to excitation energies of
MeV recently identified at the Maier-Leibnitz-Laboratorium at Garching,
Germany. For natural parity states the results are very close to the
predictions of Random Matrix Theory (RMT) for the nearest-neighbor spacing
distribution. A quantitative estimate of the agreement is given by the Brody
parameter , which takes the value for regular systems and
for chaotic systems. We obtain which
is, to our knowledge, the closest value to chaos ever observed in experimental
bound states of nuclei. By contrast, the results for unnatural parity states
are far from RMT behavior. We interpret these results as a consequence of the
strength of the residual interaction in Pb, which, according to
experimental data, is much stronger for natural than for unnatural parity
states. In addition our results show that chaotic and non-chaotic nuclear
states coexist in the same energy region of the spectrum.Comment: 9 pages, 1 figur
Mission design for LISA Pathfinder
Here we describe the mission design for SMART-2/LISA Pathfinder. The best
trade-off between the requirements of a low-disturbance environment and
communications distance is found to be a free-insertion Lissajous orbit around
the first co-linear Lagrange point of the Sun-Earth system L1, 1.5x 10^6 km
from Earth. In order to transfer SMART-2/LISA Pathfinder from a low Earth
orbit, where it will be placed by a small launcher, the spacecraft carries out
a number of apogee-raise manoeuvres, which ultimatively place it to a parabolic
escape trajectory towards L1. The challenges of the design of a small mission
are met, fulfilling the very demanding technology demonstration requirements
without creating excessive requirements on the launch system or the ground
segment.Comment: 7 pages, 6 figures, 5th International LISA Symposium, see
http://www.landisoft.de/Markus-Landgra
Large Scale Morphological Segregation in Optically Selected Galaxy Redshift Catalogs
We present the results of an exhaustive analysis of the morphological
segregation of galaxies in the CfA and SSRS catalogs through the scaling
formalism. Morphological segregation between ellipticals and spirals has been
detected at scales up to 15-20 h Mpc in the CfA catalog, and up to 20-30
h Mpc in the SSRS catalog. Moreover, it is present not only in the
densest areas of the galaxy distribution, but also in zones of moderate
density.Comment: 9 pages, (1 figure included), uuencode compressed Postscript,
(accepted for publication in ApJ Letters), FTUAM-93-2
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