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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Picture of Ganymede

Spin-scan images of Ganymede obtained by the imaging photopolarimeter of Pioneer 10 in December 1973 are described and presented. Intensity measurements in the blue and red scans are provided. The width of the scan elements is 0.5 mrad which is the size of the aperture in the focal plane of the telescope; the height is also 0.5 mrad (1 millisec integration - or dwell-time at 5 rpm spin rate). The distance of Pioneer 10 from Ganymede was 7.8 x 10 to the fifth km, and the resolution is therefore 390 km. The spin-scan image is in general agreement with Lyot's visual observations (Dollfus, 1961). Preliminary results on polarimetry of Ganymede obtained during the flyby of Pioneer 10 are discussed with attention to the question of whether water frost, NaCl powdered crystals, or other substances is responsible for the observed pattern.

Gehrels, T.↗
NASA NTRS1977

An explication of the radiometric method for size and albedo determination

A radiometric model for determination of diameters, albedos, and temperatures of asteroids and other airless planets is presented. Integrated photometry of planets is reviewed, and attention is given to problems of stellar calibration, monochromatic corrections, and corrections for the temperature variations across the planetary sphere. A previous model, which assumes that an asteroid emits infrared radiation as a smooth sphere, is modified to account for observed sunward beaming of infrared emissions, apparently due to craters. In contrast to some studies of the lunar emission, the observable flux ratio between a cratered sphere and a smooth sphere is calculated for large ranges of wavelength, temperature, and phase angle. Revised diameters based on the crater model are given for 84 asteroids. The systematic discrepancy between radiometrically- and polarimetrically-obtained albedos is accounted for; further areas of disagreement between radiometry and polarimetry are discussed, and research to resolve them is suggested.

Hansen, O. L.↗
NASA NTRS1977

The E asteroids and the origin of the enstatite achondrites

Color, polarization, and albedo data are summarized for the three known minor planets of optical type E: 44 Nysa, 64 Angelina, and 434 Hungaria. The inventory of E asteroids with dimensions in excess of 50 km is shown to be essentially complete. The surfaces of the E objects evidently consist of colorless translucent iron-free silicates such as plagioclase, forsterite, or enstatite. Their possible identification as the source of enstatite achondrites is consistent with laboratory polarimetry of the Norton County aubrite. Both Nysa and Hungaria seem to be rather favorably situated for the production of meteorites. Nysa has a highly aspherical shape and is dynamically associated with the metal-rich asteroid 135 Hertha and several small objects also apparently of the metal-depleted E type. The configuration is suggestive of the fragments of a differentiated parent body, in which Hertha originated as the iron core and Nysa as the largest surviving mantle fragment. The relative volumes, however, are not consistent with simple igneous differentiation from a chondritic composition.

Zellner, B.↗
NASA NTRS1978

Definition and detailed characterization of lunar surface units using remote observations

Remote sensing techniques are reviewed in terms of their application to the study of the lunar surface. These techniques include: reflectance spectroscopy (UV and IR), X-ray and gamma-ray analysis, alpha-particle spectroscopy, thermal emission, polarimetry, photography, and visual observation. Each technique is indexed with respect to its usefulness in either describing the appearance and location of a feature, classifying the feature in terms of a broader category, or supplying additional quantitative data. Special attention is given to the mare deposits of the Imbrium basin.

Head, J. W.↗
NASA NTRS1978

Io-phase motion and jovian decametre source locations

A second-order effect in the relationship between jovian decameter storms and the departure of Io from superior geocentric conjunction is explained on the basis of latitudinal variations in the earth-Jupiter viewing geometry. These variations are defined by the 12-year cycle in the jovicentric declination of the earth. In addition, it is found that the emission of the jovian decameter storm source Io-B (Io-C) is beamed from the northern (southern) magnetic latitudes. These conclusions are compatible with source positions derived from polarimetry and from considerations of planetary-limb shadowing.

Desch, M. D.↗
NASA NTRS1978

Correlated photometric and polarimetric phenomena in AM Herculis

Via simultaneous multicolor photometry and polarimetry of AM Herculis, we find correlations among polarization, flux, and color in the V and I bands in periodic and nonperiodic (i.e., flickering) activity. The primary minimum observed in both bands is accompanied by a blueward shift of the (B - R) color and a decrease in the absolute value of the percent circular polarization in the V band. Outside primary minimum, peaks of flickering activity tend to be associated with an increase in the absolute value of the circular polarization and the concurrent reddening of the (B - R) and (R - I) colors. The visual light curve of AM Her can be explained by a flickering, circularly polarized light source which is obscured at primary minimum, and is redder than the total system color in the URVRI bands.

Priedhorsky, W. C.↗
NASA NTRS1978

Taxonomy of asteroids

A taxonomic system for asteroids is discussed which is based on seven directly observable parameters from polarimetry, spectrophotometry, radiometry, and UBV photometry. The classification scheme is entirely empirical and independent of specific mineralogical interpretations. Five broad classes (designated C, S, M, E, and R), as well as an 'unclassifiable' designation, are defined on the basis of observational data for 523 asteroids. Computer-generated type classifications and derived diameters are given for the 523 asteroids, and the application of the classification procedure is illustrated. Of the 523 asteroids classified, 190 are identified as C objects, 141 as S type, 13 as type M, three as type E, three as type R, 55 as unclassifiable, and 118 as ambiguous. The present taxonomic system is compared with several other asteroid classification systems.

Bowell, E.↗
NASA NTRS1978

1580 Betulia - An unusual asteroid with an extraordinary lightcurve

Results of UBV photometry and polarimetry of 1580 Betulia during its 1976 apparition are presented. The synodic period of rotation is found to be 6.130 hr. The linear phase coefficient and absolute magnitude of the primary maximum in V are 0.032 mag/deg and 14.88, respectively. No color variations with rotation or solar phase angle were detected, the mean colors being B-V = 0.66 and U-B = 0.24. Betulia's light curve is unique among asteroids studied to date in that it displays three maxima and three minima within one rotational cycle, indicative of a region of greater roughness and/or a dark spot on one of its broad faces. Polarization results indicate a low albedo and a mean diameter of about 7 km, establishing Betulia as the first C-type asteroid to be found among the Mars crossers. A model accounting for most features of Betulia's light curve is given by a prolate spheroid rotating about one of its shorter axes having an axis ratio of 1:1.21 with a major topographic feature on one of its broad faces.

Tedesco, E.↗
NASA NTRS1977

High-resolution spectropolarimetry - A new atmospheric remote sensor

Polarimetry within a spectral line or band is proposed as a new atmospheric remote sensor, particularly for clouds and hazes. The limitations of conventional absorption/emission spectroscopy and photopolarimetry are discussed. The selective formation and behavior of intensity and polarization lines is analyzed under a variety of atmospheric and scattering conditions. The technique, limitations and advantages of Fourier Transform Spectroscopy are reviewed as a prelude to the proposed approach. Experimental results obtained at the telescope for the planet Venus are presented as evidence for the existence, behavior and effects of line polarization. Fourier Transform Spectropolarimetry provides means for separating the effects of scattering from those of absorption, probing the internal properties of clouds and hazes, and studying the detailed wavelength dependence of polarization with variable spectral resolutions from low to high.

Fymat, A. L.↗
NASA NTRS1979

Layered synthetic microstructures - Properties and applications in X-ray astronomy

Refinements in vacuum deposition technology have made it possible to produce structures in which two materials are arranged in alternating layers of uniform thickness and as thin as 5 A. Such structures act as Bragg diffractors or 'artificial crystals' for X-rays, as well as they may be regarded as multilayer interference coatings. A dynamical theory is used to show how the properties of layered synthetic microstructures are dependent on the layer materials and thicknesses and how these properties can be tailored to specific applications. Laboratory results at various X-ray wavelengths are presented. In particular, specific X-ray astronomy applications in spectroscopy, imaging, polarimetry and laboratory calibration are discussed.

Underwood, J. H.↗
NASA NTRS1979

Solar maximum ultraviolet spectrometer and polarimeter

The objectives of the UVSP experiment are to study solar ultraviolet radiations, particularly from flares and active regions, and to measure constituents in the terrestrial atmosphere by the extinction of sunlight at satellite dawn and dusk. The instrument is designed to observe the Sun at a variety of spectral and spatial resolutions in the range from 1150 to 3600 A. A Gregorian telescope with effective focal length of 1.8 m is used to feed a 1 m Ebert-Fastie spectrometer. A polarimeter containing rotatable magnesium fluoride waveplates is included behind the spectrometer entrance slit and will allow all four Stokes parameters to be determined. Velocities on the Sun can also be measured. The instrument is controlled by a computer which can interact with the data stream to modify the observing program. The observing modes, including rasters, spectral scans, velocity measurements, and polarimetry, are also described along with plans for mission operations, data handling, and analysis of the observations.

Tandberg-Hanssen, E.↗
NASA NTRS1980

Spectral evidence for a carbonaceous chondrite surface composition on Deimos

The surface compositions of Phobos and Deimos as determined by their UV-visible reflectance are compared in order to evaluate the hypothesis that the different surface morphologies of the two satellites are due to different mechanical properties. The UV-visible reflectance spectrum of Deimos is compiled from Mariner 9 UV spectrometry and Canopus star tracker photometry and ground-based colorimetry and polarimetry; the geometric albedo of Deimos is determined from Mariner 9 Canopus star tracker data. The reflectance spectra of Deimos and Phobos are found to be similar in a first approximation, exhibiting low, flat reflectivities in the visible and dropping off sharply in the UV, compatible with a probable carbonaceous chondrite nature for Deimos as well as Phobos and suggesting that their different surface morphologies are most likely due to different orbital histories.

Pang, K. D.↗
NASA NTRS1979

Radiometry of asteroids

Measurements of the thermal emission of asteroids can be used to derive albedos and diameters and to infer information on thermophysical properties of the upper layer of asteroid surfaces. Radiometric diameters and albedos, derived on the basis of standard thermal models for low-conductivity regoliths, have been determined for approximately 200 asteroids, with results in agreement with determinations by polarimetry and stellar occultations. For objects to which these standard models apply, diameters can be determined with an accuracy of plus or minus 10%. In several cases of small asteroids, however, there is evidence that the standard models do not apply, presumably due to the absence of an insulating regolith. Thus the interpretation of radiometric diameters is in question and the utility for objects with diameters less than about 30 km is diminished. In these cases the thermal observations potentially can provide information on the ability of low-gravity objects to retain regoliths.

Morrison, D.↗
NASA NTRS1980

AM Herculis - Simultaneous X-ray, optical, and near-IR coverage

A 6 hour X-ray pointing at AM Her using the HEAO 1 satellite is correlated with simultaneous broadband V and I photometry and visual circular polarimetry. The absence of correlations on either a flickering or an orbital time scale implies distinct regions for the visual and X-ray emission. Significant changes in the light curves are observed from one binary cycle to the next.

Szkody, P.↗
NASA NTRS1980

The remote-controlled spectrograph, area scanner, and spectropolarimeter for the Lick 3-m telescope

A Cassegrain spectrograph used in conjunction with an image-tube, image-dissector scanner (ITS) is discussed with reference to its design features and operation. The spectrograph has a remote control feature, and its parameters, such as slit width, decker position, filter selection, focus, and grating selection, can be easily changed by computer command or a hard-wired control box. The discussion also includes the area scanner modification which allows raster scanning and limited picture taking. Finally, the design of a two-channel polarization analyzer is presented which allows simultaneous sky and sky-plus-star observations and faint-object polarimetry.

Miller, J. S.↗
NASA NTRS1980

Spectroscopic and polarimetric observations of NGC 1333 and the surrounding dark cloud complex

Spectroscopy and polarimetry of several very red stars, noted on Near Infrared Photographic Sky Survey photographs in the region of NGC 1333, have yielded a 'map' suggesting the extent of polarization arising from a two-cloud structure in the region. Spectral types and color indices of the stars make it possible to infer that grain radii in the clouds exceed typical interstellar medium values and, further, that they increase with optical depth into the cloud. The present observations indicate that the cloud structure is far more extensive than previously realized.

Turnshek, D. A.↗
NASA NTRS1981

Algol - A further study of the optical polarization

Results of a study of the phase-locked optical polarization of the Algol triple star system are presented. Polarimetry was obtained in the B filter during 75 nights from 1979 to 1980, and results were folded on the AB orbital period for the equatorial Stokes parameters Q and U. As expected for a nearly edge-on system, the polarization variations are predominantly in one direction of the Stokes-vector plane, and are mainly second-harmonic in character. The variable polarization indicates an eigendirection for the AB system at an angle of 137 + or - 4 deg, and a projected major axis of the third star at 133 + or - 3 deg with respect to the AB system. Considerations of the lowest-order theories of polarizing mechanisms, including limb polarization and reflection, indicate that the AB and AB-C orbital planes are almost normal to each other, rather than coplanar. An angular splitting in the Q-U curve between the two quadrature phases 0.25 and 0.75 is also found which can be explained by centrifugal flattening of the secondary's Roche lobe, or reflection from an optically thick, flattened gas cloud near the L1 point.

Kemp, J. C.↗
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.↗