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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Multiple ring structures and the problem of correlation between lunar basins

Comparison of multi-ringed, mare-filled basins on the nearside of the moon indicates that a simple one-to-one correspondence of all rings between lunar basins does not exist. The only rings which can be correlated reasonably well between such basins are concentric systems of mare ridges and basin rims, and the frequently cited square-root-of-2 spacing relationship does not correctly describe their spacing. It is concluded that the Cordillera is the rim of the Orientale Basin and is equivalent to the 1340 km diameter ring of the Imbrium Basin and the rims of the other mare-filled basins. The area between the Rook and Cordillera rings is believed equivalent to the marginal shelf areas of the more completely mare-filled basins. A circular arc approximately 520 km diameter located just inside the outer Rook Ring would have been the most likely site for formation of a concentric system of mare ridges, had filling of the Orientale Basin been complete.

Brennan, W. J.↗
NASA NTRS1977

Saturn's rings - A new survey

Observations of radar returns from Saturn's rings, together with radio interferometry of their absorption of radiation from the disk, combine to require an effective radius of ring particles of about 6 cm or larger. It is suggested that the ring particles may also include, in addition to the known ice constituent, a mixture of the clathrated hydrate of methane and ammonia hydrate. A two-density model for ring particles is possible in which a matrix of low density contains many nodules of higher-density ice particles; in this case, radii nearly as large as the observed ring thickness would be possible. Improved resolution in radio observations at 21 cm or, if necessary, at longer wavelengths for narrow ring openings is perhaps the most useful method for determining upper limits on the particle size.

Cook, A. F.↗
NASA NTRS1978

A measurement of the brightness temperature of Saturn's rings at 8-mm wavelength

The brightness temperature of Saturn's rings has been measured at 8-mm wavelength using a millimeter-wavelength interferometer. A ring brightness temperature of 12.7 + or -2 K is obtained with the assumption that the rings are of uniform brightness and the region of emission coincides with the visible A and B rings. This result is higher than comparable results obtained at centimeter wavelengths and may indicate a small increase in the thermal emission from the rings at 8 mm. The low brightness temperature places significant constraints on the nature of the ring particles and implies that they must be either highly metallic or of limited size and composed of a low-loss dielectric material such as water ice.

Janssen, M. A.↗
NASA NTRS1978

On the azimuthal brightness variations of Saturn's rings

A simple semiquantitative explanation is presented which accounts both for the presence of the azimuthal brightness variations in Saturn's ring A and for their absence in ring B. This explanation avoids any ad hoc reliance on albedo variations and/or synchronous rotation of ring particles. Instead, it requires only some degree of self-gravitation between nearby orbiting bodies. A bias in the particle distribution and corresponding photometric effects are thereby produced the latter corresponding very closely to the variations observed in ring A. Their absence in ring B is primarily a consequence of the higher optical thickness and decreasing importance of self-gravitation in that ring.

Franklin, F. A.↗
NASA NTRS1978

A census of Gulf Stream rings, spring 1975

During 1975 several shipboard expendable bathythermograph surveys plus satellite infrared imagery provided a nearly synoptic view of the distribution and number of Gulf Stream rings in the western North Atlantic. Twelve rings were identified; nine were cyclonic (cold core) rings and three were anticyclonic (warm core) rings. This is the largest number of rings ever observed during a short period of time (4 months). Evidence suggests that the mean movement of these rings was southwestward.

Richardson, P. L.↗
NASA NTRS1979

Interferometric observations of Saturn and its rings at a wavelength of 3.71 per cm

Interferometric observations of Saturn and its rings obtained at the Owens Valley Radio Observatory at a wavelength of 3.71 cm (8085 MHz) are presented. Models of the microwave brightness structure of the Saturn system are fit to the observations in order to estimate the brightness temperatures of the planet and its rings. The models allowed making estimates of the brightness temperatures and optical depths of the A, B, and C rings. The ring brightness temperatures and optical depths are compared with a physical ring model of isotropic scatterers in a layer which is many particles thick. The observations are consistent with particles that conservatively scatter the thermal emission from Saturn to the earth and emit no thermal radiation of their own. However, the particle single-scattering albedo that would be most consistent with the observations is slightly less than unity but probably greater than 0.95. There is evidence indicating that the ring particles must be at least a few centimeters in size.

Schloerb, F. P.↗
NASA NTRS1979

International planetary patrol observations of Saturn's rings. II - Four color phase curves and their analysis

New phase curves for Saturn's rings at an intermediate tilt angle B of about 17 deg are presented. Quantitative results for each of the A and B rings are reported in terms of the opposition effect, phase coefficient, and best logarithmic fit to the phase curve. There was no significant difference between the shape of the phase curves for the two rings in each of the four colors, and a four-parameter multiple scattering model of the rings was consistent with the observations. In this model, the difference in the phase curves for different colors can be explained by a variation in the single scattering albedo with wavelength. The observations allow the particles to have the same composition in the A and B rings, so that their different photometric behavior is explained by differences in optical depth and volume density in the two rings.

Esposito, L. W.↗
NASA NTRS1980

Interferometry of Saturn and its rings at 1.30-cm wavelength

Interferometric observations of Saturn and its rings at a wavelength of 1.30 cm are presented in an attempt to place constraints upon the amount of thermal radiation emitted by the ring particles. Model-fitting and aperture synthesis techniques were used to analyze the data obtained on nine baselines at a frequency of 23 GHz. Ring optical depth is found to be close to that observed at visible wavelengths, while ring brightness temperature is only 7 + or - 1 K, requiring the ring particles to be nearly conservative scatterers at this wavelength and implying an upper limit of 2.4 m to the radius of a typical ring particle with a lower limit of 0.95 to its single scattering albedo. An observed difference between planetary radii observed at 1.30 and 3.71 cm is interpreted in terms of limb darkening and found to be marginally different from the predictions of atmospheric models in which NH3 is the principal source of microwave opacity.

Schloerb, F. P.↗
NASA NTRS1980

Electromagnetic scattering lifetimes for dust in Jupiter's ring

The lifetime of the charged, micron-sized dust particles found to lie in a ring at 1.8 Jupiter radii from Jupiter against electromagnetic scattering is estimated. Interactions of the particles with the Jovian magnetic field dipole are found to spread particle orbits out to a much greater extent than observed for particle charges greater than -10 V, and to result in slightly eccentric, open orbits for the dust particles, with eccentricities dependent on particle size. In addition, fluctuations within the magnetic field are found to lead to significant changes in the eccentricity and inclination of the ring particle orbits. It is concluded that Jovian ring particles, if charged to -10 V as is consistent with Voyager data, will be strongly perturbed and scattered on a short time scale (100 yr), so the ring must be constantly replenished or it is a temporary phenomenon. Finally, the dust cloud observed to extend above, below, and beyond the Jupiter ring is attributed to the electromagnetic scattering of particles smaller than the ring particles.

Consolmagno, G. J.↗
NASA NTRS1980

On sound transmission into a stiffened cylindrical shell with rings and stringers treated as discrete elements

In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model is presented for the transmission of an oblique plane sound wave into a finite cylindrical shell stiffened by stringers and ring frames. The rings and stringers are modeled as discrete structural elements. The numerical case studied was typical of a narrow-bodied jet transport fuselage. The numerical results show that the ring-frequency dip in the transmission loss curve that is present for a monocoque shell is still present in the case of a stiffened shell. The ring frequency effect is a result of the cylindrical geometry of the shell. Below the ring frequency, stiffening does not appear to have any significant effect on transmission loss, but above the ring frequency, stiffeners can enhance the transmission loss of a cylindrical shell.

Koval, L. R.↗
NASA NTRS1980

Vibration characteristics of a steadily rotating slender ring

Partial differential equations are derived to describe the structural vibrations of a uniform homogeneous ring which is very flexible because the radius is very large compared with the cross sectional dimensions. Elementary beam theory is used and small deflections are assumed in the derivation. Four sets of structural modes are examined: bending and compression modes in the plane of the ring; bending modes perpendicular to the plane of the ring; and twisting modes about the centroid of the ring cross section. Spatial and temporal characteristics of these modes, presented in terms of vibration frequencies and ratios between vibration amplitudes, are demonstrated in several figures. Given a sufficiently high rotational rate, the dynamics of the ring approach those of a vibrating string. In this case, the velocity of traveling wave in the material of the ring approaches in velocity of the material relative to inertial space, resulting in structural modes which are almost stationary in space.

Lallman, F. J.↗
NASA NTRS1981

Model exospheres of the ringed planets

The theory of rotating ion exospheres in dipolar magnetic field geometries is used to predict certain effects of the rings on the inner plasmaspheres of the ringed planets Jupiter, Saturn, and Uranus. The analysis here assumes a purely ionospheric source. It is found that if rings affect only the trapped plasma, the cold plasma density at Saturn will be either reduced or increased by a factor of approximately 2 at L = 1.65, depending on whether the ring populates or empties the trapped trajectories. It is found that if all cold plasma particles on trapped orbits are absorbed and the rings are not a source of plasma, this simple exospheric theory can explain the ionospheric density profile inferred from the Pioneer 11 Saturn radio occultation experiment, in which a localized peak in the profile was observed on field lines threading the Guerin division between the C and D rings.

Luhmann, J. G.↗
NASA NTRS1980

Planetary rings

Observations of the Rings of Saturn from the Pioneer spacecraft, discovery of the Ring of Jupiter, ground based polarimetry of the Rings of Saturn and some theoretical studies may be combined to markedly advance our understanding of the Rings of Jupiter, Saturn and Uranus. In particular, narrow rings can be self-gravitatingly stable inside Roche's limit and outside another closer limit. They can be created from a satellite which evolves across its Roche limit either by inward tidal drift or by growth of the planet by accretion. These considerations suggest that Neptune may well be surrounded by one or more narrow rings like those of Uranus.

Cook, A. F.↗
NASA NTRS1980

Physical processes in Jupiter's ring - Clues to its origin by Jove

It is shown that many of the observed properties of the Jovian ring can be explained by the presence of numerious small and unseen parent bodies, or 'mooms', residing within the ring; whose radii are less than 1 km. The small visible ring grains, which are destroyed in short times by sputtering and meteoroid erosion, are derived from these parent bodies largely through meteoroid impacts, and partly from Io's dust. Substantial orbit modification results from plasma drag, and the charge carried by the grains will influence their dynamics and may modify their shapes. It is concluded that the processes discussed, though present in other planetary ring systems, may be highlighted in Jupiter's ring because of its low optical depth and the small size of some of its particles. It is suggested that hidden reservoirs similar to the Jovian 'mooms' proposed may be present in the rings of Saturn and Uranus.

Burns, J. A.↗
NASA NTRS1981

Pioneer fly-by of Saturn and its rings

Results acquired by the imaging photopolarimeter on board Pioneer 11 during the spacecraft fly-by of Saturn and its rings on September 1, 1979 are reviewed. Analysis of the broadband photometry and polarimetry obtained of the Saturn atmosphere has been used to determine a cloud top height of 300 mb and a scale height of the aerosol distribution about 1/4 that of the ambient gas, and to point out differences between the forward scattering and belt and zone characteristics of the Saturn and Jupiter atmospheres. Images of Saturn's rings have been used to derive a profile of ring optical depth between 1.22 and 2.35 Saturn radii, and reveal new divisions and thin rings and azimuthal variations in the brightness of the A ring not observable from earth. Linear polarization observations of Titan in red and blue light reveal that the aerosols near the top of the atmosphere have radii less than about 0.09 micron and that the optical thickness of the small aerosol layer is about 0.6 above an effectively depolarizing surface, and indicate radii of 2845 + or - 25 km and 2880 + or - 22 km in red and blue light, respectively. Earth-based and spacecraft data are consistent with the formation of rings structures as a result of Poynting-Robertson drag and gravitational satellite resonances with the original ice and rock particles.

Gehrels, T.↗
NASA NTRS1981

The F-ring of Saturn

The existence of a split and twisted F-ring, located outside of Saturn's A-ring, as observed during the Voyager Saturn fly-by, is reported. The peculiarities in the appearance of the F-ring are described, and include: (1) the presence of one or more loose strands of matter reaching up to 100 km away from it; (2) a fairly sharp kink or step in the main ring; and (3) the presence of local bright clumps and streaks in the otherwise rather dark ring. It is proposed that, the loose strands are probably the result of the action of the local magnetic field on sufficiently small charged particles of the ring; and that the sharp kinks and steps are a result of sudden changes in the solar magnetic field, carried by the solar wind, which compress temporarily the planetary magnetosphere.

Smoluchowski, R.↗
NASA NTRS1981

Saturn's E ring

Observations of the tenuous E ring of Saturn made by an earth-based CCD system at the time of the ring-plane crossing of March 1980 are presented. The observations were made with the CCD system attached to the 1.8-m Perkins reflector at Lowell Observatory using a pupil mask behind a focal plane mask to suppress telescopic diffraction. Photometric analysis of the CCD images reveal the edge-on brightness profile of the ring, beginning at a distance of 3 Saturn radii, to peak sharply in the vicinity of the orbit of Enceladus at about 4 Saturn radii, then decrease to a distance of over 8 Saturn radii. In addition, beyond Enceladus, the edge-on width of the ring is observed to increase with radial distance, reaching nearly 5 arcsec at 7 Saturn radii. Observations suggest, on the one hand, that the E ring is associated with Enceladus and possibly represents material ejected from the satellite, and on the other, that the ring is at an early stage in its evolution.

Baum, W. A.↗
NASA NTRS1981

No evidence of rings around Neptune

The results of two observations of stellar occultations of Neptune to determine if the planet has a ring system are reported. The sightings were made from Mt. Stromlo, Mauna Kea, and Cerro Tololo, noting that an equatorial ring would subtend only two arcsec of view. An upper accretion limit was defined to set the region around Neptune where rings, rather than satellites, could form. The intensities of the starlight from the two selected stars were recorded by photometers on magnetic tape during the occultation period. One of the stars did not occult, but passed through the entire region where a ring system might be present. No definitive evidence for rings was found, although an optical depth for a Neptunian ring was calculated at 0.07, with a width of more than 5 km and a radius of 31,400 km.

Elliot, J. L.↗