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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 253 records · Page 14
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NASA NTRS1990

X-rays from superbubbles in the Large Magellanic Cloud

Diffuse X-ray emission not associated with known supernova remnants (SNRs) are found in seven Large Magellanic Cloud H II complexes encompassing 10 OB associations: N44, N51D, N57A, N70, N154, N157 (30 Dor), and N158. Their X-ray luminosities range from 7 x 10 to the 34th ergs/s in N57A to 7 x 10 to the 36th ergs/s in 30 Dor. All, except 30 Dor, have simple ring morphologies, indicating shell structures. Modeling these as superbubbles, it is found that the X-ray luminosities expected from their hot interiors fall an order of magnitude below the observed values. SNRs close to the center of a superbubble add very little emission, but it is calculated that off-center SNRs hitting the ionized shell could explain the observed emission.

Chu, You-Hua↗
NASA NTRS1991

Photoionization modeling of Magellanic Cloud planetary nebulae. I

The results of self-consistent photoionization modeling of 38 Magellanic Cloud PNe are presented and used to construct an H-R diagram for the central stars and to obtain both the nebular chemical abundances and the physical parameters of the nebulae. T(eff)s derived from nebular excitation analysis are in agreement with temperatures derived by the classical Zanstra method. There is a linear correlation between log T(eff) and the excitation class. The majority of the central stars in the sample with optically thick nebulae have masses between 0.55 and 0.7 solar mass and are observed during their hydrogen-burning excursion toward high temperatures. Optically thin objects are found scattered throughout the H-R diagram, but tend to have a somewhat smaller mean mass. Type I PN are found to have high core masses and to lie on the descending branch of the evolutionary tracks. The nebular mass of the optically thick objects is closely related to the nebular radius, and PN with nebular masses over one solar are observed.

Dopita, M. A.↗
NASA NTRS1991

Magellan in-flight gyro/star scanner misalignment calibration

Techniques are described for the in-flight calibration of gyro/star scanner misalignments for the Magellan spacecraft. The poor observability of one of the six components of misalignment is discussed in the context of a simple least-squares estimation model. The assumptions that lead to singularity in the information matrix are explicitly stated and it is shown that the singularity persists for all scanner slit configurations using only two slits, regardless of slit geometry or separation. A set of misalignment error state variables, a configuration of three stars, and a maneuver/scan sequence is described which yields a well-conditioned information matrix for least-squares estimation of five of the six misalignments. Finally, it is shown by convariance simulation that ground-based optimal estimation can satisfactorily resolve all six components of the misalignment error when more than two star scanner slits are used.

Boone, Jack N.↗
NASA NTRS1991

Magellan - Radar performance and data products

The Magellan Venus orbiter carries only one scientific instrument: a 12.6-centimeter-wavelength radar system shared among three data-taking modes. The synthetic-aperture mode images radar echoes from the Venus surface at a resolution of between 120 and 300 meters, depending on spacecraft altitude. In the altimetric mode, relative height measurement accuracies may approach 5 meters, depending on the terrain's roughness, although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty. In areas of extremely rough topography, accuracy is limited by the inherent line-of-sight radar resolution of about 88 meters. The maximum elevation observed to date, corresponding to a planetary radius of 6062 kilometers, lies within Maxwell Mons. When used as a thermal emission radiometer, the system can determine surface emissivities to an absolute accuracy of about 0.02. Mosaicked and archival digital data products will be released in compact disk (CDROM) format.

Pettengill, Gordon H.↗
NASA NTRS1991

Magellan - Electrical and physical properties of Venus' surface

Magellan probes Venus'surface by 12.6-cm-wavelength vertical and oblique radar scattering and measures microwave thermal emission. Emissivity and root-mean-square slope maps between 330 deg and 30 deg E and 90 deg N and 80 deg S are dissimilar, although some local features are exceptions. Inferred surface emissivities typically are 0.85, but vary from 0.35 at Maxwell to 0.95 northeast of Gula Mons and other locations. Lowest emissivities appear in topographically high areas; this relation suggests that a phase change or differences in chemical weathering occur at about 6055-kilometer radius. Initial results indicate that there are significant variations in the surface scattering function.

Tyler, G. Leonard↗
NASA NTRS1991

Venus volcanism - Initial analysis from Magellan data

Magellan images confirm that volcanism is widespread and has been fundamentally important in the formation and evolution of the crust of Venus. High-resolution imaging data reveal evidence for intrusion (dike formation and cryptodomes) and extrusion (a wide range of lava flows). Also observed are thousands of small shield volcanoes, larger edifices up to several hundred kilometers in diameter, massive outpourings of lavas, and local pyroclastic deposits. Although most features are consistent with basaltic compositions, a number of large pancake-like domes are morphologically similar to rhyolite-dacite domes on earth. Flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas (perhaps komantiites) may be present. Volcanism is evident in various tectonic settings (coronae, linear extensional and compressional zones, mountain belts, upland rises, highland plateaus, and tesserae). Volcanic resurfacing rates appear to be low (less than 2 cu km/yr) but the significance of dike formation and intrusions, and the mode of crustal formation and loss remain to be established.

Head, James W.↗
NASA NTRS1991

Impact craters on Venus - Initial analysis from Magellan

The general features of impact craters are described emphasizing two aspects: the effect of the atmosphere on crater and ejecta morphology and the implications of the distribution and appearance of the craters for the volcanic and tectonic resurfacing history of Venus. Magellan radar images reveal 135 craters about 15 km in diameter containing central peaks, multiple central peaks, and peak rings. Craters smaller than 15 km exhibit multiple floors or appear in clusters. Surface flows of material initially entrained in the atmosphere are characterized. Zones of low radar albedo originated from deformation of the surface by the shock or pressure wave associated with the incoming meteoroid surround many craters. A spectrum of surface ages on Venus ranging from 0 to 800 million years indicates that Venus must be a geologically active planet.

Phillips, Roger J.↗
NASA NTRS1991

Venus tectonics - Initial analysis from Magellan

The styles of lithospheric deformation, the inferred mechanical properties of the lithosphere, and their implications for the tectonic history of Venus are discussed on the basis of radar imaging and altimetry data from Magellan. Observations of the planet plains reveal a superposition of different episodes of deformation and volcanism, strain both distributed and concentrated into zones of extension and shortening, and features reflecting a crustal response to mantle dynamic processes. Lithospheric shortening and crustal thickening are represented by ridge belts and mountain belts. The latter show the evidence for extension and collapse both during and following crustal compression. Venus displays quasi-circular coronae and broad rises with linear rift zones, associated with significant volcanism. Large-offset strike-slip faults have not been observed, although horizontal shear is accommodated across broad zones of crustal shortening. On Venus strain is distributed across zones that are one to a few hundred kilometers wide, and separated by stronger and less deformed blocks hundreds of kilometers in width, as in actively deforming continental regions on earth.

Solomon, Sean C.↗
NASA NTRS1991

DD 13 - A very young and heavily reddened early O star in the Large Magellanic Cloud

This paper investigates the Large Magellanic Cloud star DD 13, which is likely the major ionizing source of the nebula N159A. New optical spectroscopy and new estimates of the broadband photometric properties of DD 13 are obtained. A spectral type of O3-O6 V, E(B-V) = 0.64, and M(V) = -6.93 is found. The spectral type cannot be more precisely defined due to contamination of the spectral data by nebular emission, obliterating the important He I classification lines. These results, plus a published estimate of the Lyman continuum photon injection rate into N159A, suggest that DD 13 actually consists of about 2-4 young, early O stars still enshrouded by their natal dust cloud. The star DD 13 may be a younger example of the type of tight cluster represented by the LMC 'star' Sk-66 deg 41, recently revealed to be composed of six or more components.

Conti, Peter S.↗
NASA NTRS1991

The detection of X-ray emission from the OB associations of the Large Magellanic Cloud

A systematic study of the X-ray properties of OB associations in the Large Magellanic Cloud has been carried out using data from the Einstein Observatory. An excess of young, X-ray-bright supernova remnants is found in the vicinity of the associations. In addition, diffuse X-ray emission is detected from over two dozen other associations; luminosities in the 0.16-3.5 keV band range from 2 x 10 to the 34th (the detection threshold) to 10 to the 36th ergs/s. For several of the more luminous examples, it is shown that emission from interstellar bubbles created by the OB stellar winds alone is insufficient to explain the emission. It is concluded that transient heating of the bubble cavities by recent supernovae may be required to explain the observed X-rays and that such a scenario is consistent with the number of X-ray-bright associations and the expected supernova rate from the young stars they contain.

Wang, Q.↗
NASA NTRS1991

NASA's Magellan mission to Venus

At the end of its planned 243-day mission, by which time Venus will have completed a full rotation beneath Magellan's orbit, over 70 percent of the Venus surface shall have been mapped to high resolution; the volume of data thus obtained will exceed that from all previous U.S. planetary missions combined. An innovative SAR instrument will map Venus to an order-of-magnitude higher resolution than that possible with previous probes. Parameters controlling the spacing of SAR S-band pulses' transmission and reception, their number and frequency of repetition, etc., are changed as many as 300 times per mapping pass in order to accommodate the spacecraft's constantly varying orbital geometry.

Doody, Dave↗
NASA NTRS1991

Magellan imaging radar mission to Venus

The Magellan imaging-radar mapping mission has collected and processed data from the spacecraft in an elliptical orbit around Venus. A brief description is given of the mission and the spacecraft, followed by a more detailed description of the radar system design, which used earth-orbiting SAR experience and several innovations in its design to operate from an orbit around another planet. The radar sensor, ground processing, and data products are described. This multimode radar is the only science instrument on the mission and has the objective of mapping at least 70 percent of the planet surface. It has three modes: SAR, altimetry, and passive radiometry. The radar system has produced maps of almost all of the Venusian surface with a resolution better than 600-m equivalent optical line pair, and the best resolution obtained is equivalent to less than 300 m. Some of the early radar images are shown.

Johnson, William T. K.↗
NASA NTRS1991

Magellan

The Magellan Mission consists of a single spacecraft to be placed in an elliptical orbit around Venus. The main objective of the mission is to perform radar imaging, altimetry, and reflectivity of 90 percent of the planet for one Venusian year (243 days). A flight profile is given, and information is presented in tabular form on the following topics: Deep Space Network support, frequency assignments, telemetry, command, and tracking support responsibilities.

Scott, J.↗
NASA NTRS1991

Absorption of the x ray background by the large magellanic cloud

Soft x ray flux of extragalactic origin should be partially absorbed during passage through the Large Magellanic Cloud. Past attempts to measure this absorption have been frustrated by the LMC's intrinsic x ray emission. New calibration techniques were developed to enable precise (within 5 pct.) measurement of the flux and spectrum of diffuse x rays detected by the Einstein Observatory IPC. These techniques were used to measure the absorption of extragalactic flux by neutral gas in the LMC, and to determine that at least 50 pct. of the observed flux in the 0.16 to 2 keV band is of extragalactic origin.

Hamilton, Thomas T.↗
NASA NTRS1991

Magellan battery operations: An overview

The Magellan Spacecraft's mission to map Venus's surface provides a unique application for Nickel-Cadmium batteries. An overview of the spacecraft, power subsystem, battery, and mission requirements are presented. The reliability and performance of the batteries were extensively studied with a comprehensive test and analysis program. Actual data for cruise and a limited number of mapping orbits is also presented.

Timmerman, Paul J.↗
NASA NTRS1992

Global scale concentrations of volcanic activity on Venus: A summary of three 23rd Lunar and Planetary Science Conference abstracts. 1: Venus volcanism: Global distribution and classification from Magellan data. 2: A major global-scale concentration of volcanic activity in the Beta-Atla-Themis region of Venus. 3: Two global concentrations of volcanism on Venus: Geologic associations and implications for global pattern of upwelling and downwelling

As part of the analysis of data from the Magellan Mission, we have compiled a global survey of the location, dimensions, and subsidiary notes of all identified volcanic features on Venus. More than 90 percent of the surface area was examined and the final catalog comprehensively identifies 1548 individual volcanic features larger than approximately 20 km in diameter. Volcanic features included are large volcanoes, intermediate volcanoes, fields of small shield volcanoes, calderas, large lava channels, and lava floods as well as unusual features first noted on Venus such as coronae, arachnoids, and novae.

Crumpler, L. S.↗
NASA NTRS1991

A spectroscopic survey of B supergiants in the Large Magellanic Cloud

The results of a low-dispersion digital optical spectral survey of about 100 B-type supergiants in the Large Magellanic Cloud are presented. The MK spectral classification framework for B supergiants has been transferred to the metal-weak LMC stars, and recommended classification standards have been designated. Variations among the metal line strengths are examined. The most extreme variations are found for the nitrogen lines, for which a range of a factor of 3 or more may be seen in the equivalent widths within some spectral subclasses. It is suggested that these variations indicate a range of nitrogen surface abundances among the B supergiants, resulting from contamination of some of the stellar surfaces by processed material from the original H-burning core.

Fitzpatrick, Edward L.↗
NASA NTRS1991

Magellan - Mission summary

The surface morphology, electrical properties, and related processes of Venus are the major areas of investigation of the Magellan Mission. The spacecraft employs a SAR for imaging, altimetry, and radiometry, and the mapping of 83 percent of the surface was complete after one of Venus' sidereal days. The strips of image data can provide a global map of Venus to permit the determination of such geologic features as volcanic and tectonic patterns.

Saunders, R. S.↗