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At least 1,207 records · Page 67

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NASA NTRS1988

Dynamics of coorbital satellite rings

The dynamical behavior of a coorbital satellite ring is studied for N = 2-9 satellites, in terms of a simplified dynamical description, in which the motion is reduced to the separation angles between satellites. The number and stability of different kinds of stationary configurations is explored, revealing that equally spaced rings are not stable against small perturbations for N not greater than 6, while for N = 2-8 there exists another, stable compact solution. Integrations of exact equations confirm these results. Moreover, the systems are found to display chaotic characteristics for a certain range of energy. The behavior can be interpreted in terms of maximum velocity curves, defining the allowed region of the motion in the phase space.

Salo, H.↗

NASA NTRS1988

Mars - Satellite and ring search from Viking

Viking Orbiter 1 imaging data were used in a search for possible satellites and rings of Mars in 1980, giving attention to a region within the Phobos orbit and + or - 350 km about the Martian equatorial plane. After picture processing, no images other than stars and Phobos itself were found to be a few brightnesses (or more) above their surrounding, as would be expected in the case of a point-spread, 'smeared' image of either satellites or rings. There is therefore no evidence of any such phenomenon in the 78 pictures studied.

Duxbury, Thomas C.↗

NASA NTRS1989

Eigenpolarization theory of monolithic nonplanar ring oscillators

Diode-laser-pumped monolithic nonplanar ring oscillators (NPROs) in an applied magnetic field can operate as unidirectional traveling-wave lasers. The diode laser pumping, monolithic construction, and unidirectional oscillation lead to narrow linewidth radiation. Here, a comprehensive theory of the eigenpolarizations of a monolithic NPRO is presented. It is shown how the properties of the integral optical diode that forces unidirectional operation depend on the choice of the gain medium, the applied magnetic field, the output coupler, and the geometry of the nonplanar ring light path. Using optical equivalence theorems to gain insight into the polarization characteristics of the NPRO, a strategy for designing NPROs with low thresholds and large loss nonreciprocities is given. An analysis of the eigenpolarizations for one such NPRO is presented, alternative optimization approaches are considered, and the prospects for further reducing the linewidths of these lasers are briefly discussed.

Nilsson, Alan C.↗

NASA NTRS1989

MG 1654+1346 - An Einstein Ring image of a quasar radio lobe

VLA radio images of MG 1654+1346, a candidate for an Einstein Ring gravitational lens, reveal two components which are separated by about 7 arcsec. Two optical counterparts, a 19-mag galaxy coincident with the center of the radio ring and a blue 21-mag stellar object located about 3 arcsec northeast of the galaxy, are identified. A model representing the source as a quasar with two extended radio lobes and the lens as an elliptical potential at the position of the galaxy is found to reasonably reproduce the radio observations.

Langston, G. I.↗

NASA NTRS1989

Si II mapping of the neutral gas ring in the Galactic center - Evidence for dust destruction

The distribution of Si II emission in the Galactic center has been measured and found to extend beyond 4 pc from the center. The observed forbidden Si II 34.8 micron line, which is thermally excited by H I and H2 collisions, originates from the photodissociation region formed in the neutral gas ring surrounding the ionized core. Two peaks in the Si II emission are found, one lying 20 arcsec northeast of Sgr A-asterisk along the Galactic plane and the other located about 80 arcsec north of Sgr A-asterisk. The latter peak is located beyond the northern arm seen in the ionized gas. The ratio of Si II to dust mass density varies in the ring, indicating changes in the gas-to-dust ratio. The mass density of Si II and dust are anticorrelated, indicating that Si II tracks the destruction of dust.

Herter, T.↗

NASA NTRS1989

Stability of polar rings around Neptune

An analytical and numerical study of the stability of polar rings around Neptune is presented. The stability proofs are based on various methods used to study gas disks in galaxies. It is shown that stable polar rings can exist despite energy dissipation by collisions between particles. Also, four equilibrium orientations which pass nearly over the pole of Neptune are found, two of which are stable in the presence of dissipation.

Dobrovolskis, Anthony R.↗

NASA NTRS1989

Central electrical utility power for a satellite ring city in low earth orbit space

Information is given in viewgraph form on central electrical power for a satellite ring city, defined as a group of large free flyers of 10 to 20 units with perhaps 100 people in each unit, and organized in a circle so that power can be fed from a central location. The free flyers would be located at 300 to 700 miles in altitude, and spaced about a kilometer apart. Potential activities of a ring city are listed as well as the electrical power needs. Information is given on costs and individual and centralized solar arrays and nuclear reactor systems.

Myers, Ira T.↗

NASA NTRS1990

Preliminary evaluation of space station transmission line in a ring configuration

The results of a preliminary evaluation of a space station type transmission line and commercial transmission lines in a ring configuration, are reported. In a ring configuration, each node has two paths for the return current of each wire. The additional path can create an unbalanced condition, where the magnetic fields created by the forward and return currents do not cancel. This evaluation was to quantify the effects of the unbalanced case upon the external fields. The transmission lines evaluated were standard commercial coaxial cables, RG59 and RG213, and a space station designed flat Litz transmission line. Each was evaluated in a balanced and unbalanced mode of operation. Currents and their harmonic content were recorded and compared. As expected, the harmonic content of the different current (I delta) was substantial for the unbalanced case as compared to the balanced case. For the balanced case, very little difference was noted among the various transmission lines evaluated. The evaluation is discussed, and the test circuit, the measurements, and the resulting data are described.

Roth, Mary Ellen↗

NASA NTRS1990

Performance and modeling of superconducting ring resonators at millimeter-wave frequencies

Microstrip ring resonators operating at 35 GHz were fabricated from laser ablated YBCO thin films deposited on lanthanum aluminate substrates. They were measured over a range of temperatures and their performance compared to identical resonators made of evaporated gold. Below 60 Kelvin the superconducting strip performed better than the gold, reaching an unloaded Q approximately 1.5 times that of gold at 25 K. A shift in the resonant frequency follows the form predicted by the London equations. The Phenomenological Loss Equivalence Method is applied to the ring resonator and the theoretically calculated Q values are compared to the experimental results.

Bhasin, K. B.↗

NASA NTRS1989

Mass transfer during ice particle collisions in planetary rings

Experimental results are presented from laboratory environment simulations of the ice particle collisional properties defining the structure and dynamical evolution of planetary rings. It is inferred from these data that there is a dependence of the interacting volume on the impact velocity. Although the volume fraction exchanged during a collision is small, the net amount of material transferred can be substantially smaller. Attention is given to the implications of these determinations for planetary ring structure and evolution.

Mcdonald, J. S. B.↗

NASA NTRS1989

Remote sensing of the ring current using energetic neutral atoms

Energetic neutral atom images of the storm-time ring current obtained from the ISEE-1 spacecraft provide information for a 'zero-order' global model of the energetic ion distribution. With the assumption of isotropic pressure and magnetostatic, nonconvective pressure balance, the global system of electrical currents driven by the ion pressure can be calculated using Euler potentials for the divergenceless current density. Radial pressure gradients drive azimuthal currents, and azimuthal pressure gradients drive radial currents. The radial currents cause current lines in the inner magnetosphere to close in the ionosphere, forming a partial ring current. The intensities and locations of these field-aligned currents driven into and out of the ionosphere resemble those of the observed Region 2 current system, but not all observed properties of the Region 2 system are reproduced by the zero-order model.

Roelof, Edmond C.↗

NASA NTRS1990

Models of Neptune's arc rings

Models proposed to explain the dynamical confinement of such structures as incomplete arc rings in orbit about the planet Neptune are reviewed and tested against the currently available observational data. It is shown that satellites can confine arc rings to very limited radial and azimuthal extents via a combination of corotation and Lindblad resonances. The resonances can be produced by the same moon or by different moons.

Lissauer, Jack J.↗

NASA NTRS1989

Photometry from Voyager 2 - Initial results from the Neptunian atmosphere, satellites, and rings

Voyager 2 has performed photopolarimetric measurements for the planet, several of its satellites, and its ring system. The Great Dark Spot is noted to appear as a low-albedo region, while the bright companion appear to lie at a higher altitude than the Great Dark Spot. Triton's linear phase coefficients are consistent with a solid-surface object having high reflectivity; its rotational phase curve indicates two major compositional units at its surface. The deficiency of material in Neptune's ring system by comparison with that of Uranus may indicate the absence of a recent moon-shattering event.

Lane, Arthur L.↗

NASA NTRS1990

A simplified ring-vortex downburst model

Three dimensional incompressible vortex rings are used to model downburst flow field, and modifications are made to express the induced velocities analytically. As a result, this three dimensional downburst model can be used in trajectory optimization, as well as in flight simulations. Two such vortex rings are superposed to match the JAWS AB Corridor wind profiles and the DFW Downburst wind profiles. Model parameters are determined through nonlinear least square fit.

Zhao, Yiyuan↗

NASA NTRS1990

Particle simulation of ion heating in the ring current

Heating of heavy ions has been observed in the equatorial magnetosphere in GEOS 1 and 2 and ATS 6 data due to ion cyclotron waves generated by anisotropic hot ring current ions. A one-dimensional hybrid-Darwin code has been developed to study ion heating in the ring current. Here, a strong instability and heating of thermal ions is investigated in a plasma with a los cone distribution of hot ions. The linear growth rate calculation and particle simulations are conducted for cases with different loss cones and relative ion densities. The linear instability of the waves, the quasi-linear heating of cold ions and dependence on the thermal H(+)/He(+) density ratio are analyzed, as well as nonlinear parallel heating of thermal ions. Effects of thermal oxygen and hot oxygen are also studied.

Qian, S.↗

NASA NTRS1990

A numerical model of the Uranian dust rings

The present numerical simulations of the Uranian rings discovered by Voyager 2 indicate that they may be the visible component of low optical depth moonlet belts made up of 10 to 100,000 cm objects; these objects would serve as the sources of dust, via both micrometeoroid bombardment and the collisional release of ejecta. It is proposed that the dust bands are generated by continuous ejection and reabsorption of micron-scale regolith material within the hypothesized moolet belts, and it is noted that numerical simulations of such a system in a Markov-chain formalism can reproduce Uranian dust ring characteristics.

Colwell, Joshua E.↗

NASA NTRS1990

Five stellar occultations by Neptune - Further observations of ring arcs

Diffraction model fits to the light curves obtained during April 18, 1984 and August 20, 1985 stellar occultations that revealed incomplete rings, or 'arcs' around Neptune, are found to indicate arc radial widths of 8-26 km, and normal optical depths of 0.07-0.14. On the basis of the Neptune pole direction ascertained from Voyager navigation analyses, it is confirmed that the arc observed on August 20, 1985, together with the Hubbard et al. (1986) and Covault et al. (1986) arcs, seem to match the radius of the outermost ring found in Voyager images analyzed by Smith et al. (1989).

Nicholson, Philip D.↗

NASA NTRS1989

Planetary rings - Theory

Theoretical models of planetary-ring dynamics are examined in a brief analytical review. The mathematical description of streamlines and streamline interactions is outlined; the redistribution of angular momentum due to collisions between particles is explained; and problems in the modeling of broad, narrow, and arc rings are discussed.

Borderies, Nicole↗