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At least 775 records · Page 43

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NASA NTRS1983

Polarization catastrophe model of static electrification and spokes in the B-ring of Saturn

A Clausius-Mossotti polarization catastrophe is presented as the operative charging mechanism in the Saturn B-ring. A review is undertaken of the polarization effect appearing in ice at the crystallization temperature. The ice crystals in the Saturn rings are in the 68-85 K temperature range, close to the ferroelectric transition temperature. An analytical treatment of the interaction of the ice crystals with the Saturn electric discharge field indicates that once polarization is initiated, it propagates toward the direction of the initial polarization. The spokes form in the radial direction, following the radial direction of the Saturn electric field. The polarization is cancelled by free plasma in the planet shadow. The charges are freed and are observed as the Saturn electric discharge phenomena, which has a periodicity which corresponds with the superrotation of the Saturn ionosphere.

Handel, P. H.↗

NASA NTRS1983

Local circumstances in Java during the total solar eclipse of 1983

A total solar eclipse of unusually long duration will occur on 11 June 1983. The path of totality begins in the Indian Ocean, passes over many of the islands of Indonesia and ends in the Coral Sea. The duration of totality is maximum near Java where weather prospects are also quite favourable. Local circumstances are examined in detail, presenting predictions for 34 cities on the island. A consideration of lunar-shadow geometry is given to clarify some of the confusion concerning the advantages of observing from the centre line. A comparison of the present predictions with those of the U.S. Naval Observatory reveals some differences which could have important implications for observers.

Espenak, F.↗

NASA NTRS1983

Moire deflectometry with deferred analysis

A simplified version of moire deflectometry for flow visualization is presented. The modification comprises replacement of the second grating, the ground glass screen, and the camera with a plate film holder to obtain a shadow picture. Postanalysis proceeds by placing a grid transparency of the same periodicity of the remaining glass grating on the negative for projection viewing. The rotational angle between the grating and the plate alters the moire fringe periodicity. Use of the method in studying flows is predicted to reveal weak density gradient areas and shocks.

Rhodes, D. B.↗

NASA NTRS1982

The Pinhole/Occulter Facility

The Pinhole/Occulter Facility concept uses a remote occulting mask to provide high resolution observations of the solar corona and of astronomical X-ray sources. With coded-aperture and Fourier-transform techniques, the Pinhole/Occulter makes images at a resolution of 0.2 arc sec for 2-120 keV X-rays, using a 50-m boom erected from the payload bay of the Space Shuttle or mounted on a free-flying platform. The remote occulter also creates a large shadow area for solar coronal observations; the Pinhole/Occulter concept includes separate optical and ultraviolet telescopes with 50-cm apertures. These large telescopes will provide a new order of resolution and sensitivity for diagnostic observations of faint structures in the solar corona. The Pinhole/Occulter is a powerful and versatile tool for general-purpose X-ray astronomy, with excellent performance in a broad spectral band complementary to that accessible with AXAF. The large collecting area of 1.5 sq m results in a 5 sigma detection threshold of about 0.02 microJy for the 2-10 keV band.

Hudson, H. S.↗

NASA NTRS1982

Examples of sequential radar images from Washington, Arizona and Alaska

Radar image time sequences of three areas were obtained to illustrate the capability for change detection over time intervals that range from two to fourteen years. The image sequences show geomorphic changes caused by volcanic eruption at Mount St. Helens, Washington, changes that have resulted from mining activities in the Sierrita Mining District, Arizona, and differences in the Ikpikpuk River drainage area, Alaska, that are mostly dependent on imaging system characteristics. Enhancement, suppression or distortion of features on the images are related in each case to interaction between the illumination geometry of the imaging system and the characteristics of the terrane surface. The radar images are contrasted with Landsat multispectral scanner images to illustrate the importance of resolution and shadowing for feature perception.

Ford, J. P.↗

NASA NTRS1983

Thermally induced spin rate ripple on spacecraft with long radial appendages

A thermally induced spin rate ripple hypothesis is proposed to explain the spin rate anomaly observed on ISEE-B. It involves the two radial 14.5 meter beryllium copper tape ribbons going in and out of the spacecraft hub shadow. A thermal lag time constant is applied to the thermally induced ribbon displacements which perturb the spin rate. It is inferred that the averaged thermally induced ribbon displacements are coupled to the ribbon angular motion. A possible exponential build up of the inplane motion of the ribbon which in turn causes the spin rate ripple, ultimately limited by damping in the ribbon and spacecraft is shown. It is indicated that qualitative increase in the oscillation period and the thermal lag is fundamental for the period increase. found that numerical parameter values required to agree with in orbit initial exponential build up are reasonable; those required for the ripple period are somewhat extreme.

Fedor, J. V.↗

NASA NTRS1984

Interpreting Michigan forest cover types from color infrared aerial photographs

The characteristics of 17 cover types (13 forest types or tree species and 4 nonforest cover types) in Michigan are discussed as well as their interpretation from medium scale color infrared photography. The occurrence of each type is described by region and site requirements. Those attributes of a tree or stand which are helpful when attempting to interpret the type from a vertical perspective are discussed as well as common crown types. The identification of the forest type or tree species by using image characteristics (size, shape, shadow, color, texture, pattern, or association) is discussed. Ground photographs and sketches of individual trees are included. Stereograms of typical stands are available.

Hudson, W. D.↗

NASA NTRS1984

Atmospheric model development

Spectral albedo measurements from the LANDSAT-4/5 thematic mappers require that spacecraft upwelling radiances be corrected for atmospheric absorption and scattering and for local surface illumination. A two-stream model is developed, with a lower boundary condition that varies with incidence angle. The TM data must be registered to digital terrain data. Reflectance from points in shadows can be used to estimate optical depth. The primary application is determination of the spectral albedo of snow. The TM is better-suited for this purpose than the MSS because of its larger dynamic range.

Source record↗

NASA NTRS1984

Theoretical aspects of photoclinometric terrain profiling on the Galilean satellites

For large parts of the surfaces of the Galilean satellites there is no stereoscopic high resolution imagery, and so for shadow-free regions far from the terminator, the only method of obtaining topograpic information is photoclinometry, the technique of converting brightness variations into local surface tilts and hence into topographic profiles and contour maps. The conversion from brightness changes to slopes require knowledge of the angular photometric function for the type of planetary surface terrain being analyzed, i.e., the relationship describing the brightness of the surface as a function of the angles of incidence (I) and emergence (E) of light at a surface element. In order to investigate the effects of these changes on the shapes of topographic profiles deduced from the functions, an area of grooved terrain (lines 500-505, samples 550-650 on frame 20640.27 0060J2) using wide ranges of values of all the adjustable parameters in Hapke's new photometric function were analyzed.

Wilson, L.↗

NASA NTRS1984

Morphology of buried craters: Lava thickness estimates by simulated flooding

Partially and completely buried craters on planetary surfaces are used as probes to determine the thickness and extent of the overlying material as well as the composition of underlying terrain units. Rims of craters protruding through mare basalts on the Moon have been used extensively by DeHon (1977, 1979) to develop isopach maps of both the circular and irregular maria. Such estimates are possible because of Apollo topographic data and the extensive coverage by Lunar Orbiter photographs at various sun angles. On planetary surfaces without such data, however, shadow measurements can only be made by pixel listings with appropriate assumptions concerning support data. In addition, high resolution geochemical data is not available to determine whether the exposed crater rim is composed of material dissimilar from the lavas. Because of the need for lava thickness data, flooding of lunar mare and highland craters was simulated in order to determine empirical relationships between dimensions observed in orbital images and the thickness of the lava.

Maxwell, T. A.↗

NASA NTRS1984

Wind streaks on Mars: Comparisons of production models with observations of bright streaks

To investigate the influence of crater size on martian wind streaks, the characteristics of bright streaks associated with craters ranging from approximately 100 m to 80 km in diameter were analyzed in several regions. The ratio of streak length to obstacle height (L/h) could be diagnostic of the relative effectiveness of different sized obstacles in disturbing near surface atmospheric flow; since the crater rim height to diameter relationship has not previously been determined for Mars, the rim heights of numerous craters were estimated from shadow measurements. In all areas studied, bright streak L/h values decrease with increasing h, indicating that smaller obstacles produce longer streaks relative to their height than do larger obstacles. Several models of bright streak production are used to explain this behavior are discussed.

Lee, S. W.↗

NASA NTRS1984

Interaction of atomic oxygen with solid surfaces at orbital altitudes (A0114)

The basic approach to this experiment is to expose a wide variety of material surfaces to the atomic flux in orbit. The experiment is passive and depends on preflight and postflight measurements of the test surfaces in the laboratory. The experiment will also include a reflectometer device to measure atomic beam reflection angles and thus momentum accommodations, and a unique passive spacecraft attitude sensor. Samples consisting of solid disks or thin film coatings on substrate disks will be mounted in a panel. The face of this panel will be flown on Long Duration Exposure Facility normal to the incident stream of oxygen atoms. Each disk will have part of its front surface masked so exposure to the atomic-oxygen reaction will be limited to selected areas, the shadowed areas being used as control surfaces in the measurements. A typical sample is an optically flat quartz disk overcoated with a film of the material of interest. These include Ag, Au, Pt, Nb, Ni, Al, C, Si, Ge, LiF, and a few engineering materials. Some materials for which the expected removal rate is high, such as carbon, will be solid disks rather than thin films.

Gregory, J. C.↗

NASA NTRS1984

Multiseasonal variables in digital image enhancements for geological applications

Examples of enhanced multiseasonal orbital imagery illustrate the influence of multiseasonal changes in their spatial and spectral attributes, and consequently in their application to structural geology and lithological discrimination. Shadow effects associated with appropriate solar elevation and azimuth effects enhance the spatial attributes but not the spectral. In this case, variations in illumination conditions should be minimized by selecting images with high solar elevation and by the use of techniques that minimize illumination conditions. Multiseasonal imagery should be used in the identification of spectral contrast changes of rock-soil-vegetation associations which can provide evidences of related lithological units and structural features. The extraction of maximum geological information requires, at least, a fall/winter and a spring/summer scene from which spatial, spectral and multiseasonal attributes can be adequately explored.

Parada, N. D. J.↗

NASA NTRS1984

Photometric Studies of Heat Flow at the Photosphere

Continuum photometry is carried out and the results of comparing these observations with models of photospheric heat flow are described. The main results are: (1) a possible detection of weak bright rings around some spot penumbrae; (2) no evidence is found for large scale photospheric brightness inhomogeneities exceeding 2-3 K which places tighter constraints on models of global scale convection; (3) supergranular scale continuum structures observed across the photosphere appear mainly due to random clumping of granules; (4) the one case observed of a sunspot emergence shows no thermal shadow exceeding 1.5 K rms one day prior to umbra appearance; (5) network and faculae are found to show a small excess brightness even at mu = 1, so detection of faculae at mu = 1 by differential photometry indicates a gentler temperature gradient near tau = 1 in the facular (relative to cell) atmosphere; (6) the limb darkening study shows no significant global variations to within 0.1% rms.

Foukal, P.↗

NASA NTRS1984

On-orbit performance of LIPS gallium arsenide solar cell experiment

Telemetry from the Living Plume Shield's gallium arsenide solar panel experiment was evaluated to determine degradation. The data were culled to preclude spurious results from possible shadowing or inaccurate measurements on a cold array. Two independent methods were then used to obtain the maximum power points and the various characteristics of the solar array. Fill factor, open circuit voltage, short circuit current, and series and shunt resistances were examined. The data analysis concluded that, to date, nearly all of the solar array degradation is due to the reduction in the short circuit current.

Bavaro, T.↗

NASA NTRS1984

Lageos orbit and solar eclipses

The objective was to assess the importance of solar eclipses on Lageos' orbit. Solar radiation pressure perturbs the orbit of the Lageos satellite. The GEODYN orbit determination computer program includes solar radiation pressure as one of the forces operating on the satellite as it integrates the orbit. GEODYN also takes into account the extinction of sunlight when Lageos moves into the Earth's shadow. The effect of solar eclipses on the semimajor axis of Lageos' orbit was computed analytically by assuming Lageos to be in a circular orbit, the Sun and the Moon to be in the plane of the orbit, and the Moon to be stationary in the sky in front of the Sun. Also, the magnitude of the radiation pressure is assumed to be linearly related to the angular separation of the Sun and Moon, and that Lageos is a perfect absorber of radiation. The computation indicates that an eclipse of the Sun by the Moon as seen by Lageos can affect the semimajor axis at the 1 centimeter (1 cm) level. Such a change is significant enough to include in GEODYN, in order to get an accurate orbit for Lageos.

Rubincam, D. P.↗

NASA NTRS1983

Plasma electron analysis: Voyager plasma science experiment

The Plasma Science Experiment (PLS) on the Voyager spacecraft provide data on the plasma ions and electrons in the interplanetary medium and the magnetospheres of the giant planets Jupiter and Saturn. A description of the analysis used to obtain electron parameters (density, temperature, etc.) from the plasma science experiment PLS electron measurements which cover the energy range from 10 eV to 5950 eV is presented. The electron sensor (D cup) and its transmission characteristics are described. A derivation of the fundamental analytical expression of the reduced distribution function F(e) is given. The electron distribution function F(e), used in the moment integrations, can be derived from F(e). Positive ions produce a correction current (ion feedthrough) to the measured electron current, which can be important to the measurements of the suprathermal electron component. In the case of Saturn, this correction current, which can either add to or subtract from the measured electron current, is less than 20% of the measured signal at all times. Comments about the corrections introduced by spacecraft charging to the Saturn encounter data, which can be important in regions of high density and shadow when the spacecraft can become negatively charged are introduced.

Sittler, E. C., Jr.↗

NASA NTRS1983

Measurement of vehicle glow on the Space Shuttle

From the combined data set of glow observations on STS-3, STS-4 and STS-5 some of the properties of the Shuttle glow were observed. Comparison of the STS-3 (240 km) and STS-5 (305 km) photographs show that the intensity of the glow is about a factor of 3.5 brighter on the low altitude (STS-3) flight. The orbiter was purposely rotated about the x axis in an experiment on STS-5 to observe the dependence of the intensity on the angle of incidence between the spacecraft surface normal and the velocity vector. For a relatively large angle between the velocity vector and the surface normal there is an appreciable glow, provided the surface is not shadowed by some other spacecraft structure. As the angle becomes less the glow intensifies. The grating experiments (STS-4 photography only, STS-5 image intensifier photography) provided a preliminary low resolution spectra of the spacecraft glow. Accurate wavelength calibrations of the STS-5 instrument permitted measuring of the spectrum and intensity of the earth's airglow. Previously cited in STAR as N83-30509

Mende, S. B.↗