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NASA NTRS1980

Evidence for hot gaseous coronae around the Magellanic Clouds

High-dispersion IUE far-UV spectra were obtained for five stars in the LMC and two stars in the SMC. At Magellanic Cloud velocities, all the spectra exhibit very strong interstellar lines of Al III, Si IV, and C IV, in addition to the usual lower ionization stage lines associated with H I region absorption. In two stars interstellar N V is also detected at the 50 mA level. From a consideration of the strength, width, and radial velocity of these high ionization stage lines, it is argued that regions of hot halo gas have probably been detected in front of the Magellanic Clouds. The regions detected exhibit absorption characteristics very similar to that of the Milky Way corona described previously by Savage and de Boer (1979). A cloud detected in the line of sight to HD 5980 in the SMC at a radial velocity of 300 km/s is of unknown origin.

De Boer, K. S.↗

NASA NTRS1980

On the abundance of carbon monoxide in galaxies - A comparison of spiral and Magellanic irregular galaxies

CO emission has been sought in 52 directions toward six irregular, Magellanic-type galaxies, with null results at a typical detection limit of 0.04 K for a line width of 10 km/s. The positions searched correspond to those of bright OB star clusters, H II regions, H I peaks, and dark clouds. In two cases, one-third of the optical galaxy was covered by the observations. CO has been detected at 2.6 mm toward the nucleus of the low-mass SA(s)dm galaxy NGC 7793. The results imply that the mean surface brightness of CO emission in Magellanic irregulars is less than that of the Galaxy and other spirals which have been studied. Two categories of explanations for this are discussed, including (1) those based on a lower heating rate for interstellar clouds in irregulars, and thus a lower CO temperature, and (2) those based on a lower CO abundance.

Elmegreen, B. G.↗

NASA NTRS1982

CNO abundances in H II regions of the Magellanic clouds and the galaxy with implications regarding the nucleosynthesis of the CNO element group

Final abundance results of IUE observations of the UV spectra of three H II regions in the Small Magellanic Cloud and four H II regions in the Large Magellanic Cloud are presented. Calculated yields of carbon and oxygen derived are y(C)=.00063 and y(O)=.0016. The nucleosynthetic origin of nitrogen was evaluated as being predominantly a secondary element produced from carbon as its seed. Plotting log N/C versus log C/H yielded the rather unexpected result that log N/C decreases with lo C/H over the SMC-LMC-Orion range. The cause of this relationship is discussed.

Dufour, R. J.↗

NASA NTRS1981

Discovery of a protostar in the Large Magellanic Cloud

A near infrared search of the H II region/molecular cloud complex N159 in the Large Magellanic Cloud has revealed a very red (H-K = 2.1, K-L-prime = 2.7) compact object. The location, brightness, color and 2.1-2.4 micron spectrum of this source suggest that it is very young, and similar to the galactic infrared protostars. This is the first identification of an infrared protostar in an external galaxy. Its discovery provides direct evidence of current star formation in the Large Magellanic Cloud, and suggests that regions of star formation in external galaxies will appear similar to those in the Milky Way.

Gatley, I.↗

NASA NTRS1982

Tidal stability of gas clouds in the Large Magellanic Cloud and M101

The tidal stability of the giant atomic gas clouds in the Large Magellanic Cloud (LMC) and in M101 is examined. The giant atomic clouds in the Large Magellanic Cloud are found to be unstable against tidal disruption by the gravity of the LMC, but the clouds in M101 are in approximate tidal balance. It is unlikely that there is sufficient unobserved molecular gas to stabilize the LMC clouds, although they may have substantial molecular cores which are tidally stable. The time scale for tidal disruption of the LMC clouds is of the order of 20-30 million years. Because most of the clouds have associated H II regions, the onset of star formation in these clouds must occur rapidly. In order to balance the loss due to tidal disruption, the clouds would have to be forming at a rate of approximately 0.09 solar masses/year per sq kpc, a value comparable to that derived for the Milky Way.

Blitz, L.↗

NASA NTRS1984

Far IR Observations of the Magellanic Clouds

Several Large Magellanic and Small Magellanic Cloud H II regions were observed at 50 and 100 microns. Observations were made on three flights of the Kuiper Airborne Observatory using a six channel detector system. Fully sampled maps were made simultaneously at both wavelengths. The integrated properties of the H II regions are listed and interpretations of the results regarding stellar formation are presented.

Jones, T. J.↗

NASA NTRS1984

X-ray surveys of the Magellanic Clouds

The proximity, well-determined distance, and low foregound obscuration of the Magellanic Clouds makes them an ideal target for the study of galactic X-ray source populations. Steady progress since the initial detection of X-rays from the LMC in 1968 culminated in the recent Einstein Observatory surveys from which over three dozen supernova remnants and about 10 compact binaries have been identified. This review records the 15-year history of Magellanic Cloud X-ray research, summarizes current knowledge, and offers a brief prospectus of what the next 15 years may hold.

Helfand, D. J.↗

NASA NTRS1987

Star formation in the Magellanic clouds

Because of their proximity, the Magellanic Clouds provide the opportunity to conduct a detailed study of the history and current state of star formation in dwarf irregular galaxies. There is considerable evidence that star formation in the Clouds was and is proceeding in a manner different from that found in a typical well-ordered spiral galaxy. Star formation in both Clouds appears to have undergone a number of relatively intense bursts. There exist a number of similarities and differences in the current state of star formation in the Magellanic Clouds and the Milky Way. Examination of Infrared Astronomy Satellite (IRAS) sources with ground based telescopes allows identification of highly evolved massive stars with circumstellar shells as well as several types of compact emission line objects.

Frogel, Jay A.↗

NASA NTRS1987

The 1989 Magellan mission to Venus

The primary objective of the Magellan mission is to provide the needed topographical data at resolutions and coverages adequate to shed light on the geological history and formation of Venus. Altimetry and gravity experiments are also planned. The Magellan mission consists of the launch phase, the cruise phase, the Venus orbit insertion, and mapping. The prospective launch vehicle is the Shuttle 'Atlantis' with an IUS upper stage.

Brown, Charles D.↗

NASA NTRS1987

The Magellan mission to Venus

The Magellan mission will be the next NASA mission to Venus. This paper describes the mission as it is currently planned, showing how the design of the science payload, the spacecraft, and the mission satisfies the science objectives and requirements as well as other programmatic constraints. The Magellan mission is dedicated to obtaining SAR images of at least 70 percent of the surface of Venus at a resolution of 1 km per line-pair, or better, which is comparable to the coverage and resolution of the Mars Mariner 9 mission. Other investigations will study the geophysical characteristics of the planet using altimetric data and gravity field measurements, and measurements to determine global surface emissivity.

Dallas, S. S.↗

NASA NTRS1988

Effects of variation in look angle and wavelength in radar images for geologic applications: Implications for SIR-C and Magellan

Implications for SIR-C and Magellan of Seasat, SIR-A/B, and aircraft radar images which show that a variety of geologic targets can have similar backscatter and be indistinguishable under given observing wavelengths and/or look angles are discussed. The SIR-C/X-SAR combination provides three wavelengths, each of which may provide significant information not provided by another, and HH, VV, and one cross polarization each provide different and significant information. For study of unvegetated geological targets in relatively flat terrain, images made at smaller look angles probably provide more information than images from larger look angles. For Magellan, the possible existence of aeolian landforms such as sand dunes or sand sheets on Venus cannot be ruled out simply because they are not seen on radar images. The problem of dissimilar features having identical backscatter must also be considered in the Venusian context, where perhaps lava flows and impact ejecta could have similar high backscatter, or conversely, smooth aeolian materials and smooth volcanic ash (if either exist) could have a similar backscatter.

Blom, R. G.↗

NASA NTRS1988

Construction and analysis of simulated Venera and Magellan images of Venus

The 1-3 km resolution Venera 15 and 16 images of Venus and the expected 120-300 m resolution Magellan mission image data are presently simulated through a digital processing of Seasat radar images covering a desert dune complex in the Gran Desierto of Sonora, accreted terranes in the central interior of Alaska, and the Appalachian Valley and Ridge Province. The simulations suggest that the nature and extent of terrain modification on Venus by such exogenic processes as atmosphere-surface weathering, erosion, and deposition, will remain uncertain, since the length scale of features indicative of such processes may be too small to be discerned from Venera data; Magellan data may provide this critical fine-scale morphological data, however, and thereby allow the testing of the two competing resurfacing scenarios.

Arvidson, R. E.↗

NASA NTRS1989

Radar scattering from desert terrains, Pisgah/Lavic Region, California: Implications for Magellan

A major component of the 1988 Mojave Field Experiment involved the simultaneous acquisition of quad-polarization multifrequency airborne Synthetic Aperture Radar (SAR) imaging radar data and ground measurements thought to be relevant to the radar scattering behavior of a variety of desert surfaces. In preparation for the Magellan mission to Venus, the experiment was designed to explore the ability of SAR to distinguish types of geological surfaces, and the effects of varying incidence angles on the appearance of such surfaces. The airborne SAR system acquired images at approx. 10 m resolution, at 3 incidence angles (30, 40, 50 degs) and at 3 wavelengths (P:68 cm, L:24 cm, C:5.6 cm). The polarimetric capabilities of the instrument allow the simulation of any combination of transmit and receive polarizations during data reduction. Calibrated trihedral corner reflectors were deployed within each scene to permit absolute radiometric calibration of the image data. Initial analyses of this comprehensive radar data set is reported, with emphasis on implications for interpretation of Magellan data.

Plaut, J. J.↗

NASA NTRS1989

Spaceborne radar observations: A guide for Magellan radar-image analysis

Geologic analyses of spaceborne radar images of Earth are reviewed and summarized with respect to detecting, mapping, and interpreting impact craters, volcanic landforms, eolian and subsurface features, and tectonic landforms. Interpretations are illustrated mostly with Seasat synthetic aperture radar and shuttle-imaging-radar images. Analogies are drawn for the potential interpretation of radar images of Venus, with emphasis on the effects of variation in Magellan look angle with Venusian latitude. In each landform category, differences in feature perception and interpretive capability are related to variations in imaging geometry, spatial resolution, and wavelength of the imaging radar systems. Impact craters and other radially symmetrical features may show apparent bilateral symmetry parallel to the illumination vector at low look angles. The styles of eruption and the emplacement of major and minor volcanic constructs can be interpreted from morphological features observed in images. Radar responses that are governed by small-scale surface roughness may serve to distinguish flow types, but do not provide unambiguous information. Imaging of sand dunes is rigorously constrained by specific angular relations between the illumination vector and the orientation and angle of repose of the dune faces, but is independent of radar wavelength. With a single look angle, conditions that enable shallow subsurface imaging to occur do not provide the information necessary to determine whether the radar has recorded surface or subsurface features. The topographic linearity of many tectonic landforms is enhanced on images at regional and local scales, but the detection of structural detail is a strong function of illumination direction. Nontopographic tectonic lineaments may appear in response to contrasts in small-surface roughness or dielectric constant. The breakpoint for rough surfaces will vary by about 25 percent through the Magellan viewing geometries from low to high Venusian latitudes. Examples of anomalies and system artifacts that can affect image interpretation are described.

Ford, J. P.↗

NASA NTRS1990

Incidence angle and resolution - Potential effects on interpreting Venusian impact craters in Magellan radar images

The effects of incidence angle and spatial resolution are analyzed in order to evaluate the types of information that Magellan imaging radar may provide about impact craters on Venus. Seasat synthetic-aperture radar images of small terrestrial craters at high resolution (about 25 m) and Soviet Venera images of comparatively large Venusian craters at low resolution (1000 to 2000 m) and shorter wavelength show comparable radar responses to crater morphology. At low incidence angles, it is difficult to locate the precise position of crater rims on images, while at higher incidence angles, images show less distortion of crater morphology. Magellan will obtain images through a range of incidence angles varying from about 45 deg at low latitudes to 16 deg at polar latitudes. Radially symmetrical landforms such as impact craters are expected to show outlines that vary with the incidence angle and to appear bilaterally symmetrical and increasingly compressed at higher latitude.

Ford, J. P.↗

NASA NTRS1989

The Magellan Venus mapping mission

The April 1989 Magellan (MGN) Mission to Venus will initiate a new phase in the exploration of the solar system. In addition to being the first U.S. planetary mission in 10.5 years, it will also be the first such mission to use the Space Shuttle and IUS 2-Stage as launch vehicles. Upon arrival at Venus the spacecraft will begin a systematic mapping of the surface of that planet using side-looking SAR. This paper discusses some of the interesting trade-offs in mission design for this mission in the areas of launch and injection, interplanetary cruise, Venus Orbit insertion, and mapping. The Magellan mapping strategy is discussed briefly along with a few special experiments being considered for the MGN extended mission.

Cutting, E.↗

NASA NTRS1990

An accuracy assessment of Magellan Very Long Baseline Interferometry (VLBI)

Very Long Baseline Interferometry (VLBI) measurements of the Magellan spacecraft's angular position and velocity were made during July through September, 1989, during the spacecraft's heliocentric flight to Venus. The purpose of this data acquisition and reduction was to verify this data type for operational use before Magellan is inserted into Venus orbit, in August, 1990. The accuracy of these measurements are shown to be within 20 nanoradians in angular position, and within 5 picoradians/sec in angular velocity. The media effects and their calibrations are quantified; the wet fluctuating troposphere is the dominant source of measurement error for angular velocity. The charged particle effect is completely calibrated with S- and X-Band dual-frequency calibrations. Increasing the accuracy of the Earth platform model parameters, by using VLBI-derived tracking station locations consistent with the planetary ephemeris frame, and by including high frequency Earth tidal terms in the Earth rotation model, add a few nanoradians improvement to the angular position measurements. Angular velocity measurements were insensitive to these Earth platform modelling improvements.

Engelhardt, D. B.↗

NASA NTRS1990

Magellan space flight operations

This paper describes the Magellan spacecraft designed to collect information from the surface of Venus while orbiting this planet. Special attention is given to the attitude and articulation control of the spacecraft; the command, data, and data storage; the thermal control; and instrumentation. The operations at Venus, which will include Venus orbit insertion and in-orbit checkout and to both routine and nonroutine (at a period when Venus will be in superior conjunction, and the solar plasma will drive Magellan's telecommunications signal-to-noise ratio to a very high value) mapping of the surface, are discussed. Also discussed are operations on earth, carried out by the new Space Flight Operations Control Center.

Doody, Dave↗