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At least 1,189 records · Page 66
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Martian meteorology

Viking mission results concerning the structure and dynamics of the Martian atmosphere are reviewed. The large-scale variability of pressure, temperature, and zonally averaged wind at the Lander sites is described, and small-scale phenomena involving the boundary layer and internal gravity waves are discussed. Observational evidence for the existence of transient planetary-scale waves, forced quasi-stationary waves, and thermal tides on Mars is examined. Atmospheric dynamics and properties of dust storms are considered, with emphasis on dust properties and distribution, the time evolution of planetwide dust storms, and dust-storm genesis and decay. Attention is also given to the nature of past Martian climates, the sizes and composition of volatile reservoirs, and volatile escape from the atmosphere.

Leovy, C. B.↗
NASA NTRS1979

Middle- and far-ultraviolet observations of hot white dwarfs

Observations of HD 149499 B, the previously unrecognized white dwarf star found by the Skylab ultraviolet objective-prism survey, are discussed. Spectra of the binary HD 149499 AB were obtained by the short wavelength and long wavelength cameras of the IUE, and converted into fluxes. The absorption line strengths of the white dwarf secondary indicate a He/H composition ratio greater than or equal to 1. Comparison with model atmospheres of the IUE data suggests a temperature of around 93,000 K for HD 149499 B, while comparison with visual magnitude data indicates a temperature of 82,000 K. Either temperature value would make the star the hottest white dwarf for which reasonably reliable temperature estimates are available. Future work intended includes the repetition of the Skylab ultraviolet experiment on Spacelab for the detection of a complete sample of hot white dwarfs and subdwarfs.

Parsons, S. B.↗
NASA NTRS1982

Long term frequency stability analysis of the GPS NAVSTAR 6 Cesium clock

Time domain measurements, taken between the NAVSTAR 6 Spacecraft Vehicle (SV) and the Vandenberg Global Positioning System (GPS) Monitor Site, by a pseudo random noise receiver, were collected over an extended period of time and analyzed to estimate the long term frequency stability of the NAVSTAR 6 onboard frequency standard, referenced to the Vandenberg MS frequency standard. The technique employed separates the clock offset from the composite signal by first applying corrections for equipment delays, ionospheric delay, tropospheric delay, Earth rotation and the relativistic effect. The data are edited and smoothed using the predicted SV ephemeris to calculate the geometric delay. Then all available passes from each of the four GPS monitor stations, are collected at 1-week intervals and used to calculate the NAVSTAR orbital elements. The procedure is then completed by subtracting the corrections and the geometric delay, using the final orbital elements, from the composite signal, thus leaving the clock offset and random error.

Mccaskill, T. B.↗
NASA NTRS1982

Mars - Near-infrared spectral reflectance of surface regions and compositional implications

Both morphological and compositional information are needed to define and characterize surface geologic units on Mars. A description is presented of new, near-infrared spectra (0.65 to 2.50 micrometers) for 11 regions on the Martian surface observed in 1978. The high photometric quality of these data combined with increased near-infrared spectral coverage provide new information about the spectral behavior and, therefore, the composition and physical nature of Martian surface materials. The spectral reflectances were obtained with the aid of a 2.2-m telescope located on Mauna Kea, Hawaii. A cooled (to 77 K) circular variable filter spectrometer with an InSb detector was used to measure alternatively Mars and the standard star Beta Geminorum. Attention is given to general spectral characteristics, the dark region composition, spectral evidence for water, and the 2.3 micrometer absorption.

Mccord, T. B.↗
NASA NTRS1983

Worldwide photometry and lightcurve observations of 16 Psyche during the 1975-1976 apparition

Twenty-six lightcurves of Psyche are presented together with UBV photometry and phase functions from 1975 and 1976. Combining photometric data from this opposition with those from previous apparitions resulted in a mean phase coefficient in V of 0.026 + or - 0.002 mag/deg. No significant phase-dependent variation in the U-B color could be determined from the data; the B-V color, however, displayed a reddening with phase of 0.0010 + or - 0.0004 mag/deg. It is concluded that compositional variations over Psyche's surface are minor, and that Psyche's opposition effect is typical of that for other well-observed asteroids. Psyche's behavior is accounted for if, to the first order, its shape is that of a triaxial ellipsoid with axial ratios near 5:4:3.

Tedesco, E. F.↗
NASA NTRS1983

Cleaning chemistry of GaAs(100) and InSb(100) substrates for molecular beam epitaxy

Ploog (1980) and Bachrach and Krusor (1981) have pointed out the importance of substrate preparation and surface cleaning for obtaining high quality films with the aid of molecular beam epitaxial growth techniques. In the present investigation, high resolution X-ray photoemission (XPS) is used to determine the oxide removal mechanism for GaAs(100) substrates which have undergone a standardized cleaning procedure. Other objectives of the investigation are related to a comparison of different cleaning procedures in order to minimize carbon contamination, the extension of these cleaning techniques to other III-V compound semiconductors such as InSb, and the evaluation of the sensitivity of the compositional results to electron-induced damage effects.

Vasquez, R. P.↗
NASA NTRS1986

Characterizing Semiconductor Alloys for Infrared Sensors

Report presents results of continuing program aimed at characterizing mercury/cadmium/tellurium alloys and eventually developing improved methods of preparing alloys for use as infrared sensors. Work covered by report includes series of differential thermal analysis (DTA) measurements of alloy compositions with x varied from 9 to 1 in 0.1 increments.

Lehoczky, B. S. L.↗
NASA NTRS1985

Geology of the Ganymede Quadrangle Jg7

The western half of Ganymede quadrangle Jg7 is covered by Voyager 2 images with resolution of .5 to 5 km/pixel. The imaged area includes dark terrain in the north underlying the southeastern part of Galileo Regio, and light terrain in the south underlying the southeastern extension of Uruk Sulcus. Several slivers and wedges of dark terrain occur within the light areas. Numerous craters are present, as well as three large palimsests (crater scars with little topographic expression) in the dark terrain and a basin in the light terrain. The boundary between dark and light materials on the south side of Galileo Regio is locally marked by a scarp that faces the light terrain. The scarp truncates several craters. The truncation and rotation of structures suggest that light material grew at the expense of dark material, and that the destruction, except for minor pivoting, was essentially in situ. The composition and structural properties of the palimsests and craters are examined.

Lucchitta, B. K.↗
NASA NTRS1986

Process for Making Tris(N-methylamino) Methylsilane

Efficient process aids production of silicon carbide/silicon nitride fibers. Fibers 10 to the sixth power times as electrically resistive as carbon fibers having similar mechanical properties; promising replacements for carbon fibers in composite materials in which high conductivity poses hazard.

Clemons, J. M.↗
NASA NTRS1987

Physical and chemical modification of the surface of Venus by windblown particles

The results of simulations of the Venusian surface environment involving windblown grains are presented which show that significant chemical and physical changes may occur even in the slow-moving winds recorded on Venus. The edges of grains beome worn and shed comminuted debris, which collects on weathered surfaces and grains alike. The resulting transfer of material from loose grains to bedrock surfaces (and vice versa) could yield misleading results on rock composition; moreover, the generation of comminuted debris would enhance chemical reactions that could affect the composition of the atmosphere. The results are thus relevant in assessing rates of surface degradation, the evolution of small-scale surface features as seen in images returned from the Soviet Venera missions, and in the interpretation of compositional data for surface materials.

Greeley, Ronald↗
NASA NTRS1991

Mapping compositional and particle size variations across Silver Lake Playa: Relevance to analyses of Mars TIR data

The high spectral and spatial resolution thermal infrared (TIR) data to be acquired from the upcoming Mars Observer-Thermal Emission Spectra (TES) mission will map the composition and texture of the Martian sediments. To prepare for these data, portions of two remote sensing experiments were conducted to test procedures for extracting surface property information from TIR data. Reported here is the continuing analysis of Thermal Infrared Multispectral Scanner (TIMS) data, field emission spectra, laboratory Fourier Transform Infrared (FTIR) reflectance spectra, and field observations with respect to the physical characteristics (composition, emissivity, etc.) of Silver Lake playa in southern California.

Petroy, S. B.↗
NASA NTRS1992

Short- and long-term climate changes on Mars

The present climate of Mars is dominated by the seasonal cycles of dust, water vapor, and carbon dioxide. Understanding these cycles represents a first step towards interpreting climate changes in the past. Past climates on Mars were probably different from the present one due to astronomical variations of orbital and axial properties, to major changes in atmospheric pressure and/or composition, and to long-term changes in solar luminosity.

Pollack, James B.↗
NASA NTRS1992

Effectiveness of metal matrix and ceramic matrix composites as orbital debris shield materials

The effectiveness of two metal matrix composites and one ceramic matrix material in defeating hypervelocity impacts at about 3.8 km/s are evaluated to determine the potential of these composites as spacecraft shield materials. The metal matrix composites investigated consist of SiC particles (70 percent by volume) in an aluminum matrix and Al2O3 particles (50 percent by volume) in an Al matrix. The ceramic composite consists of ZrB2 platelets in a ZrC matrix. Both the metal matrix and ceramic matrix composites are found to perform as well or better than 6061-T6 aluminum, which is presently used in the Whipple type bumper shield of Space Station Freedom. Test results indicate that the composites tested may have applications as micrometeoroid/orbital debris shield materials.

Mcgill, Preston B.↗
NASA NTRS1993

Ultrasonic Inspection Of Thick Sections

Ultrasonics used to inspect large, relatively thick vessels for hidden defects. Report based on experiments in through-the-thickness transmission of ultrasonic waves in both steel and filament-wound composite cases of solid-fuel rocket motors.

Friant, C. L.↗
NASA NTRS1992

Properties of five toughened matrix composite materials

The use of toughened matrix composite materials offers an attractive solution to the problem of poor damage tolerance associated with advanced composite materials. In this study, the unidirectional laminate strengths and moduli, notched (open-hole) and unnotched tension and compression properties of quasi-isotropic laminates, and compression-after-impact strengths of five carbon fiber/toughened matrix composites, IM7/E7T1-2, IM7/X1845, G40-800X/5255-3, IM7/5255-3, and IM7/5260 have been evaluated. The compression-after-impact (CAI) strengths were determined primarily by impacting quasi-isotropic laminates with the NASA Langley air gun. A few CAI tests were also made with a drop-weight impactor. For a given impact energy, compression after impact strengths were determined to be dependent on impactor velocity. Properties and strengths for the five materials tested are compared with NASA data on other toughened matrix materials (IM7/8551-7, IM6/1808I, IM7/F655, and T800/F3900). This investigation found that all five materials were stronger and more impact damage tolerant than more brittle carbon/epoxy composite materials currently used in aircraft structures.

Cano, Roberto J.↗
NASA NTRS1994

Flight service evaluation of composite helicopter components

This paper presents the results of a NASA funded contract and Sikorsky research and development programs to evaluate structural composite components in flight service on Sikorsky Model S-76 helicopters. Selected components were removed and tested at prescribed intervals over a nine year time frame. Four horizontal stabilizers and thirteen tail rotor spars were returned from commercial service in West Palm Beach, Florida and in the Gulf Coast region of Louisiana to determine the long term effects of operations in hot and humid climates on component performance. Concurrent with the flight component evaluation, panels of materials used in their fabrication were exposed to the environment in ground racks. Selected panels were tested annually to determine the effects of exposure on physical and mechanical properties. The results of 55,741 component flight hours and 911 months of field exposure are reported and compared with initial Federal Aviation Administration (FAA) certification data. The findings of this program have provided increased confidence in the long term durability of advanced composite materials used in helicopter structural applications.

Mardoian, George H.↗
NASA NTRS1995

A two dimensional interface element for coupling of independently modeled three dimensional finite element meshes and extensions to dynamic and non-linear regimes

The work reported here pertains only to the first year of research for a three year proposal period. As a prelude to this two dimensional interface element, the one dimensional element was tested and errors were discovered in the code for built-up structures and curved interfaces. These errors were corrected and the benchmark Boeing composite crown panel was analyzed successfully. A study of various splines led to the conclusion that cubic B-splines best suit this interface element application. A least squares approach combined with cubic B-splines was constructed to make a smooth function from the noisy data obtained with random error in the coordinate data points of the Boeing crown panel analysis. Preliminary investigations for the formulation of discontinuous 2-D shell and 3-D solid elements were conducted.

Aminpour, Mohammad↗
NASA NTRS1997

NASA DC-8 Airborne Scanning Lidar Cloud and Contrail Observations

An angular scanning backscatter lidar has been developed and operated from the NASA DC-8 aircraft; the lidar viewing direction could be scanned from vertically upward to forward in the direction of aircraft travel to vertically downward. The scanning lidar was used to generate real-time video displays of clouds and contrails above, below, and ahead of the aircraft to aid in positioning the aircraft for achieving optimum cloud/contrail sampling by onboard in situ samplers. Data examples show that the lidar provides unique information for the interpretation of the other data records and that combined data analyses provides enhanced evaluations of contrail/cloud structure, dynamics, composition, and optical/radiative properties.

Uthe, Edward E.↗