105,177 research outputs found
Radar imaging from a spinning spacecraft
The feasibility of imaging the surface of Venus using a synthetic aperture radar in a spin stabilized Pioneer class spacecraft operating in an eccentric orbit is studied. Imaging radar fundamentals, constraints, power requirements and data processing considerations are reviewed. Additional effects due to operation from an elliptical orbit with a spinning spacecraft are covered. Recommended spin parameters are determined by simulations. Extensions to include noncoherent integration and stereo coverage are briefly reviewed. The results indicate that resolution on the order of 100 meters can be obtained from a 0.2 eccentricity orbit using a 2 meter antenna and reasonable transmitter power levels
Extension of highly elliptical Earth orbits using continuous low-thrust propulsion
The extension of highly elliptical orbits, with free selection of orbit period, using low thrust propulsion is investigated. These newly proposed orbits, termed Taranis orbits, are enabled by existing low-thrust propulsion technology, offering a radically new set of tools for mission design and facilitating new, novel Earth Observation science. One particular example considered herein, using general and special perturbation techniques, is the application of continuous low-thrust to alter the âcritical inclinationâ of an orbit from the natural values of 63.4deg or 116.6deg, to any inclination required to optimally fulfill the mission goals. This continuous acceleration is used to compensate for the drift in argument of perigee caused by Earthâs gravitational field. Pseudo-spectral optimization techniques are applied to the 90deg inclination Taranis orbit, generating fuel optimal low-thrust control profiles, with a fuel saving of ~ 4% from general perturbation results. This orbit provides an alternative solution for high latitude imaging from distances equivalent to geostationary orbits. Analysis shows that the orbit enables continuous, high elevation visibility of frigid and neighboring temperate regions using only three spacecraft, whereas a Molniya orbit would require in excess of fifteen spacecraft, thus enabling high quality imaging which would otherwise be prohibited using conventional orbits. Order of magnitude mission lifetimes for a range of mass fractions and specific impulses are also determined. Finally, a Strawman mass budget is developed, where the mission lifetimes for spacecraft with initial mass of 1000kg, 1500kg, and 2500kg, are found to be limited to 4.3 years, 6 years and 7.4 years respectively
Multiple Scattering Theory for Two-dimensional Electron Gases in the Presence of Spin-Orbit Coupling
In order to model the phase-coherent scattering of electrons in
two-dimensional electron gases in the presence of Rashba spin-orbit coupling, a
general partial-wave expansion is developed for scattering from a cylindrically
symmetric potential. The theory is applied to possible electron flow imaging
experiments using a moveable scanning probe microscope tip. In such
experiments, it is demonstrated theoretically that the Rashba spin-orbit
coupling can give rise to spin interference effects, even for unpolarized
electrons at nonzero temperature and no magnetic field.Comment: 34 pages, 7 figure
Utilizing Astrometric Orbits to Obtain Coronagraphic Images
We present an approach for utilizing astrometric orbit information to improve
the yield of planetary images and spectra from a follow-on direct detection
mission. This approach is based on the notion-strictly hypothetical-that if a
particular star could be observed continuously, the instrument would in time
observe all portions of the habitable zone so that no planet residing therein
could be missed. This strategy could not be implemented in any realistic
mission scenario. But if an exoplanet's orbit is known from astrometric
observation, then it may be possible to plan and schedule a sequence of imaging
observations that is the equivalent of continuous observation. A series of
images-optimally spaced in time-could be recorded to examine contiguous
segments of the orbit. In time, all segments would be examined, leading to the
inevitable detection of the planet. In this paper, we show how astrometric
orbit information can be used to construct such a sequence. Using stars from
astrometric and imaging target lists, we find that the number of observations
in this sequence typically ranges from 2 to 7, representing the maximum number
of observations required to find the planet. The probable number of
observations ranges from 1.5 to 3.1. This is a dramatic improvement in
efficiency over previous methods proposed for utilizing astrometric orbits. We
examine how the implementation of this approach is complicated and limited by
operational constraints. We find that it can be fully implemented for internal
coronagraph and visual nuller missions, with a success rate approaching 100%.
External occulter missions will also benefit, but to a lesser degree.Comment: 28 pages, 14 figures, submitted to PAS
Falseâpositive technetiumâ99m methylene diphosphonate bone scan activity in the orbit in a patient with a history of breast carcinoma
Metastasis of breast carcinoma to the orbit is an uncommon entity and carries a poor prognosis. This case report presents false-positive technetium-99m methylene diphosphonate activity in the right orbit of a patient with a history of a primary breast neoplasm. Orbital computed tomography imaging was obtained to further characterize the radiotracer uptake identified on the bone scan and demonstrated diffuse right globe intraocular calcifications secondary to degenerative intraocular changes. A brief literature review of orbital metastasis from breast carcinoma and causes of intraocular calcification in the context of chronic vision loss are provided
Radio Astrometry Of The Triple Systems Algol And UX Arietis
We have used multi-epoch long-baseline radio interferometry to determine the
proper motion and orbital elements of Algol and UX Arietis, two radio-bright,
close binary stellar systems with distant tertiary components. For Algol, we
refine the proper motion and outer orbit solutions, confirming the recent
result of Zavala et al. (2010) that the inner orbit is retrograde. The radio
centroid closely tracks the motion of the KIV secondary. In addition, the radio
morphology varies from double-lobed at low flux level to crescent-shaped during
active periods. These results are most easily interpreted as synchrotron
emission from a large, co-rotating meridional loop centered on the K-star. If
this is correct, it provides a radio-optical frame tie candidate with an
uncertainty {\pm}0.5 mas. For UX Arietis, we find a outer orbit solution that
accounts for previous VLBI observations of an acceleration term in the proper
motion fit. The outer orbit solution is also consistent with previously
published radial velocity curves and speckle observations of a third body. The
derived tertiary mass, 0.75 solar masses, is consistent with the K1
main-sequence star detected spectroscopically. The inner orbit solution favors
radio emission from the active K0IV primary only. The radio morphology,
consisting of a single, partially resolved emission region, may be associated
with the persistent polar spot observed using Doppler imaging
Stabilizing periodic orbits above the elliptic plane in the solar sail 3-body problem
We consider periodic orbits high above the ecliptic plane in the Elliptic Restricted Three-Body Problem where the third massless body is a solar sail. Periodic orbits above the ecliptic are of practical interest as they are ideally positioned for the year-round constant imaging of, and communication with, the poles. Initially we identify an unstable periodic orbit by using a numerical continuation from a known periodic orbit above the ecliptic in the circular case with the eccentricity as the varying parameter. This orbit is then used to construct a reference trajectory for the sail to track. In addition we illustrate an alternative method for constructing a periodic reference trajectory based on a time-delayed feedback mechanism. The reference trajectories are then tracked using a linear feedback regulator (LQR) where the control actuation is delivered by varying the solar sails orientation. Using this method it is shown that a 'near term' solar sail is capable of performing stable periodic motions high above the ecliptic
Direct Observation of the Fourth Star in the Zeta Cancri System
Direct imaging of the zeta Cnc system has resolved the fourth star in the
system, which is in orbit around zeta Cnc C. The presence of the fourth star
has been inferred for many years from irregularities in the motion of star C,
and recently from C's spectroscopic orbit. However, its mass is close to that
of C, making its non-detection puzzling. Observing at wavelengths of 1.2, 1.7,
and 2.2 microns with the adaptive-optics system of the CFHT, we have obtained
images which very clearly reveal star D and show it to have the color of an M2
star. Its brightness is consonant with its being two M stars, which are not
resolved in our observations but are likely to be in a short-period orbit,
thereby accounting for the large mass and the difficulty of detection at
optical wavelengths, where the magnitude difference is much larger. The
positions and colors of all four stars in the system are reported and are
consistent with the most recent astrometric observations.Comment: 7 pages including 3 tables, 1 figure; To appear in PAS
Orbital dynamics of high area-to-mass ratio spacecraft under the influence of J2 and solar radiation pressure
This paper investigates the effect of planetary oblateness and solar radiation pressure on the orbit of high area-to-mass spacecraft. A planar Hamiltonian model shows the existence of equilibrium orbits with the orbit apogee pointing towards or away from the Sun. These solutions are numerically continued to non-zero inclinations and considering the obliquity of the ecliptic plane relative to the equator. Quasi-frozen orbits are identified in eccentricity, inclination and angle between the Sun-line and the orbit perigee. The long-term evolution of these orbits is then verified through numerical integration. A set of âheliotropicâ orbits with apogee pointing in direction of the Sun is proposed for enhancing imaging and telecommunication on the day side of the Earth. The effects of J2 and solar radiation pressure are exploited to obtain a passive rotation of the apsides line following the Sun; moreover the effect of solar radiation pressure enables such orbits at higher eccentricities with respect to the J2 only case
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