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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Potential for observing and discriminating impact craters and comparable volcanic landforms on Magellan radar images

Observations of small terrestrial craters by Seasat synthetic aperture radar (SAR) at high resolution (approx. 25 m) and of comparatively large Venusian craters by Venera 15/16 images at low resolution (1000 to 2000 m) and shorter wavelength show similarities in the radar responses to crater morphology. At low incidence angles, the responses are dominated by large scale slope effects on the order of meters; consequently it is difficult to locate the precise position of crater rims on the images. Abrupt contrasts in radar response to changing slope (hence incidence angle) across a crater produce sharp tonal boundaries normal to the illumination. Crater morphology that is radially symmetrical appears on images to have bilateral symmetry parallel to the illumination vector. Craters are compressed in the distal sector and drawn out in the proximal sector. At higher incidence angles obtained with the viewing geometry of SIR-A, crater morphology appears less compressed on the images. At any radar incidence angle, the distortion of a crater outline is minimal across the medial sector, in a direction normal to the illumination. Radar bright halos surround some craters imaged by SIR-A and Venera 15 and 16. The brightness probably denotes the radar response to small scale surface roughness of the surrounding ejecta blankets. Similarities in the radar responses of small terrestrial impact craters and volcanic craters of comparable dimensions emphasize the difficulties in discriminating an impact origin from a volcanic origin in the images. Similar difficulties will probably apply in discriminating the origin of small Venusian craters, if they exist. Because of orbital considerations, the nominal incidence angel of Magellan radar at the center of the imaging swath will vary from about 45 deg at 10 deg N latitude to about 16 deg at the north pole and at 70 deg S latitude. Impact craters and comparable volcanic landforms will show bilateral symmetry parallel to the illumination vector and will appear increasingly compressed toward higher latitudes.

Ford, J. P.↗
NASA NTRS1990

Mark 4-A DSCC (Magellan-era) telemetry system description

An update to the description of the Deep Space Communications Complex portion of the Mark 4-A Telemetry system is given. This system is currently implemented at all signal processing centers. The upgrade of the telemetry system was undertaken primarily in support of the Voyager-Neptune Encounter and the Magellan mission. The Mark 3 Telemetry System is the predecessor of the Mark 4-A system.

Ross, D. L.↗
NASA NTRS1990

A higher density VLBI catalog for navigating Magellan and Galileo

The density of radio sources near the ecliptic in the astrometric JPL Very Long Baseline Interferometry (VLBI) catalog has been increased by over 50 percent since 1985. This density increase has been driven by the need for more sources for the VLBI navigation of the Magellan and Galileo spacecraft, but the sources also will be usable for Mars Observer and other future missions. Since the last catalog, including observations made through 1985, was published in 1988, a total of 21 radio sources has been added that fulfill the following criteria: (1) they lie within 10 deg of the ecliptic plane; (2) their correlated flux densities are above 0.2 Jy on at least one of the Deep Space Network intercontinental baselines at both 2.3 and 8.4 GHz; and (3) the source positions are known to better than 5 milliarcseconds (25 nanoradians). The density of such sources in the catalog has been increased from 15.6 per steradian to 25.2 per steradian. Ten more sources have been added that fulfill the last two criteria given above and lie between 10 deg and 20 deg from the ecliptic plane. Analysis shows that there may be approx. 70 more sources with correlated flux densities above 0.2Jy that are within approx. 20 deg of the ecliptic. However, VLBI navigation observations of the new and prospective sources with the 250-kHz bandwidth of the current operational system will require the use of two 70-m antennas in most cases. Including both old and new sources, if two 34-m antennas are used, there will be usable navigation sources within 10 deg of a spacecraft in only 30 percent of the ecliptic, and sources within 20 deg of a spacecraft over 70 percent of the ecliptic.

Ulvestad, J. S.↗
NASA NTRS1989

Model atmospheres for SN 1987A and the distance to the Large Magellanic Cloud

A code which simultaneously solves the non-LTE equations of statistical equilibrium and the time-independent equation of radiation transport in an expanding atmosphere is applied to analyze the spectrum of SN 1987A. The behavior of a model atmosphere in which the density structure is a power-law function of radius has been studied and compared closely with SN 1987A data. This model atmosphere is used to derive corrections to the expanding photosphere method for determining distances to supernovae. The distance to the Large Magellanic Cloud derived from SN 1987A observations by this 'custom yardstick' is 49 + or - 6 kpc in excellent accord with results for RR Lyrae stars and for Cepheids. This empirical test of the expanding photosphere method provides the basis for hope that it may make a significant contribution to the extragalactic distance scale at cosmologically interesting distances.

Eastman, Ronald G.↗
NASA NTRS1990

Cepheids in Magellanic Cloud star clusters - Fundamental and overtone pulsators in NGC 2157

CCD survey data are employed to examine Cepheids in young Magellanic Cloud star clusters. The properties of three Cepheids observed in NGC 2157 are described. It is detected that the two short-period (3 days) Cepheids have photometric properties that correspond to overtone pulsators and the long-period (7.7 days) Cepheid pulses in the fundamental mode. The pulsational masses for the three Cepheids are calculated to be about 5 solar masses. This mass value does not correlate with the average pulsational mass for Cepheids of 3.0 + or - 0.4 solar masses. The potential cause of this deviation in evolutionary/pulsational mass is investigated.

Mateo, Mario↗
NASA NTRS1990

The Magellan Venus radar mapping mission

The NASA Magellan Venus Radar Mapper spacecraft, which will be placed into orbit around Venus on August 10, 1990, is described and its mission is discussed. The orbiter's 12-cm wavelength, multimode radar system is examined and the applications of its modes are addressed. In the SAR mode, it can image most of the Venus surface at a resolution of better than 300 m, approaching 120 m over more than half the planet. In the altimeter mode, the radar will determine topographic relief to a vertical accuracy of better than 50 m averaged over a surface resolution cell approximately 10 km in diameter. In the radiometer mode, the radar receiver can determine the surface radio emission brightness temperature with an absolute accuracy of 20 K, at a resolution of 2 K. The nature of the data products and the archiving plans are also considered.

Saunders, R. S.↗
NASA NTRS1990

Venus geology and tectonics - Hotspot and crustal spreading models and questions for the Magellan mission

Spacecraft and ground-based observations of Venus have revealed a geologically young and active surface - with volcanoes, rift zones, orogenic belts and evidence for hotspots and crustal spreading - yet the processes responsible for these features cannot be identified from the available data. The Magellan spacecraft will acquire an unprecedented global data set which will provide a comprehensive and well resolved view of the planet. This will permit global geological mapping, an assessment of the style and relative importance of geological processes, and will help in the understanding of links between the surface geology and mantle dynamics of this earth-like planet.

Head, James W.↗
NASA NTRS1990

Tectonics of Lakshmi Planum, Venus - Tests for Magellan

The origin of Lakshmi Planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of Venus. From the perspective of gravity signature and potential driving forces, the mantle upwelling model is the simplest, and its principal failure, that it cannot produce radial shortening on the uplift periphery, may be resolved if the lithosphere is laterally heterogeneous. The preferred model consists of a hot mantle plume rising beneath a preexisting block of tessera. The lithosphere is weakened at this hotter and presumably thicker crust, and the outward near-surface flow is attenuated at the peripheral discontinuity in lithospheric strength. Crustal thickening and mountain belt formation occur there. Several criteria are proposed to test this 'tessera-plume' model together with its competitors at the higher resolution in both imaging and gravity afforded by the Magellan mission.

Grimm, Robert E.↗
NASA NTRS1990

Large-scale patterns of eolian sediment transport on Venus - Predictions for Magellan

The atmosphere of Venus is likely to exert a major influence on the surface. Arecibo and Venera 15/16 observations of the population of impact craters with bright halos give direct evidence of surface modification, on the scale of centimeters, to smooth the surface on time scales of 50 - 250 my. Both chemical and mechanical modifications probably occur. Diurnal winds in the planetary boundary layer can transport particles. Using boundary layer theory, including the effects of topography, surface stresses and resulting transport and deposition of sand-size particles are calculated. Regional slopes are sites of largest surface stresses. Sand will be generally transported downhill, although there is a preferential net transport from east to west, in the same direction as the atmospheric superrotation. It is predicted here that streaks may be seen in the Magellan radar images which will indicate directions of net eolian transport on the surface of Venus.

Saunders, R. S.↗
NASA NTRS1990

Geologic evidence of hotspot activity of Venus - Predictions for Magellan

A number of distinctive types of geologic features have been identified on Venus that are interpreted to be related to thermal plumes including domal rises, coronae, and major composite shield volcanoes. The basic characteristics of these features as well as their distribution are documented. The three types of features have related morphologies and are interpreted to represent a continuum of features formed by mantle plumes at scales from 100s to over 1000 km. The Artemis structure, located in Aphrodite Terra, is proposed to be a large corona. If crustal spreading processes are operating on Venus, hotspot features should form chains on the surface as seen in terrestrial ocean basins. On the basis of current data on hotspot-related feature distribution on Venus, no clear evidence exists for hotspot chains. The complete distribution of hotspot features in Magellan data will be used to understand better the relationship between interior processes and surface features, as well as to provide a test for the crustal spreading hypothesis.

Stofan, Ellen R.↗
NASA NTRS1990

Consequences of a hydrous mantle for Venus tectonics - Predictions for Magellan

The consequences of a Venus mantle enriched in water relative to the earth are considered. If the moon-forming impact hypothesis is correct, earth may have lost substantial volatiles. Venus, however, experienced no such impact and is therefore likely to be relatively more water-rich. It is demonstrated that a higher abundance of water in the Venus mantle has the effect of inhibiting the rise of crust-forming melts and volatiles to the surface, which results in a thin stable crust, a dry, stiff upper mantle, and restricted magmatism. Conduction through a shallow thermal lithosphere dominates the removal of heat from the interior, and flow in the mantle is strongly coupled to the surface deformation. Current observations of Venus are consistent with this model. A review of the consequences of this model makes it possible to make predictions for the Magellan radar images.

Williams, David R.↗
NASA NTRS1990

Magellan radar to reveal secrets of enshrouded Venus

Imaging Venus with a synthetic aperture radar (SAR) with 70 percent global coverage at 1-km optical line-pair resolution to provide a detailed global characterization of the volcanic land-forms on Venus by an integration of image data with altimetry is discussed. The Magellan radar system uses navigation predictions to preset the radar data collection parameters. The data are collected in such a way as to preserve the Doppler signature of surface elements and later they are transmitted to the earth for processing into high-resolution radar images. To maintain high accuracy, a complex on-board filter algorithm allows the altitude control logic to respond only to a narrow range of expected photon intensity levels and only to signals that occur within a small predicted interval of time. Each mapping pass images a swath of the planet that varies in width from 20 to 25 km. Since the orbital plane of the spacecraft remains fixed in the inertial space, the slow rotation of Venus continually brings new areas into view of the spacecraft.

Saunders, R. Stephen↗
NASA NTRS1989

Multicolor optical polarimetry of reddened stars in the small Magellanic cloud

First results of an on-going program to determine the wavelength dependence of the interstellar optical polarization of reddened stars in the Small Magellanic Cloud (SMC) are presented. IUE observations of reddened stars in the SMC (Bouchet et al. 1985) generally show marked differences in the extinction law as compared to both the Galaxy and the Large Megallanic Cloud. The aim here is to determine the wavelength dependence of the optical linear polarization in the direction of several such stars in the SMC in order to further constrain the dust composition and size distribution in that galaxy.

Magalhaes, Antonio M.↗
NASA NTRS1990

Emission line nebulae in the Magellanic clouds

H II regions, planetary nebulae, and supernova remnants in the large and small Magellanic clouds have been favored targets among all galactic and extragalactic emission nebulae for study with the IUE satellite due to their known distances and low line of sight extinction. Several of the major results arising from successful IUE (International Ultraviolet Explorer) observations of these three types of emission nebulae in the clouds are presented. Carbon abundances in the H II regions from UV emission lines with implications regarding the chemical evolution of the clouds and stellar origins of these elements are discussed. The UV spectral characteristics of several evolved and young SNRs in the clouds with comparisons made to similar objects in the Galaxy and other nearby galaxies are discussed. CNO abundance variations among planetary nebulae and characteristics of their central stars with emphasis on the new insights provided regarding PN formation and C-N nucleosynthesis during the various AGB dredge-up phases are discussed.

Dufour, Reginald J.↗
NASA NTRS1991

Magellan: Preliminary description of Venus surface geologic units

Observations from approximately one-half of the Magellan nominal eight-month mission to map Venus are summarized. Preliminary compilation of initial geologic observations of the planet reveals a surface dominated by plains that are characterized by extensive and intensive volcanism and tectonic deformation. Four broad categories of units have been identified: plains units, linear belts, surficial units, and terrain units.

Saunders, R. S.↗
NASA NTRS1990

Magellan orbits Venus

Various events surrounding Magellan's orbit of Venus are recounted. Significant events include the successful firing of a solid rocket motor while the spacecraft was behind Venus to transfer it from a solar-centered trajectory to an orbit around the planet. The spacecraft orbits Venus every 3.26 hours at a maximum altitude of 8500 km and minimum altitude of 291 km in an elliptical orbit. The successful August 16 test of the synthetic-aperture radar system is discussed, noting that it produced two strips, each about 20 km x 16,000 km, revealing details as small as 120 m. Two anomalies causing a delay in the start of mapping operations and subsequent breaks in the communication link with earth for 14.5 hours and 17.7 hours are discussed. Protective measures directed from the spacecraft's ROM during breach of contact are listed, and possible causes of the anomalies are suggested, such as solar activity or hardware or software elements, although the actual cause is not yet known.

Mclaughlin, W. I.↗
NASA NTRS1990

The effects of stellar age and metallicity on the infrared emission in the Magellanic Clouds

Infrared Astronomical Satellite (IRAS) maps of the Magellanic Clouds smoothed to a 5-min angular resolution and young star catalogs are used to study the effects of stellar age and metallicity on the infrared emission of these galaxies. The construction of the stellar catalog is described, along with the conversion from the observational to the theoretical temperature-luminosity diagram and the derivation of stellar ages. The infrared color-color diagram of the Clouds is investigated, and the effect of stellar age on their infrared emission is investigated. The effects of the UV radiation field and of the metallicity on the infrared emission are also considered with emphasis on the influence of such parameters as gas-to-dust ratio on the infrared colors. It is shown that the far-infrared fluxes of individual pixels in the Clouds' maps follow the same behavior as the integrated emission of normal galaxies in the IRAS color-color diagram and that the precise position of a particular pixel depends on the age of the youngest star in that pixel.

Sauvage, M.↗
NASA NTRS1989

Magellan SAR processing algorithm and H/W design

The SAR (synthetic-aperture radar) data-processing algorithm to be used for the Magellan mission is described. Radar system design, SAR data characteristics, and hardware (H/W) constraints, which are critical to the processing algorithm design, are highlighted. Data flow and the H/W architecture are given to show the real-time data processing capability. Simulation results obtained from processing the synthetic point-target echos are presented to demonstrate the performance of the processing algorithm.

Chen, M.↗