Engineering topics

Vilas, Faith

Publications and source records attributed to Vilas, Faith.

At least 37 records · Page 2

NASA NTRS1993

Ferric iron in primitive asteroids - A 0.43-micron absorption feature

A search of reflectance spectra of C- P-, D- and S-class asteroids to hunt for the Soret band near 0.4 micron that is indicative of porphyrins yielded an identification of an 0.43 micron absorption feature in 11 primitive asteroids of the C, P, and G classes and in one S-class asteroid. It is proposed that the feature is an Fe(3+) spin-forbidden transition in aqueously altered material, possibly located near 0.43 micron due to an enhancement effect similar to the mechanism operating in jarosite. The significance of the feature for the aqueous alteration history of these asteroids is addressed.

Vilas, Faith↗

NASA NTRS1993

Iron oxide bands in the visible and near-infrared reflectance spectra of primitive asteroids

High resolution reflectance spectra of primitive asteroids (C, P, and D class and associated subclasses) have commonly revealed an absorption feature centered at 0.7 microns attributed to an Fe(2+)-Fe(3+) charge transfer transition in iron oxides and/or oxidized iron in phyllosilicates. A smaller feature identified at 0.43 microns has been attributed to an Fe(3+) spin-forbidden transition in iron oxides. In the spectra of the two main-belt primitive asteroids 368 Haidea (D) and 877 Walkure (F), weak absorption features which were centered near the location of 0.60-0.65 microns and 0.80-0.90 microns prompted a search for features at these wavelengths and an attempt to identify their origin(s). The CCD reflectance spectra obtained between 1982-1992 were reviewed for similar absorption features located near these wavelengths. The spectra of asteroids in which these absorption features have been identified are shown. These spectra are plotted in order of increasing heliocentric distance. No division of the asteroids by class has been attempted here (although the absence of these features in the anhydrous S-class asteroids, many of which have presumably undergone full heating and differentiation should be noted). For this study, each spectrum was treated as a continuum with discrete absorption features superimposed on it. For each object, a linear least squares fit to the data points defined a simple linear continuum. The linear continuum was then divided into each spectrum, thus removing the sloped continuum and permitting the intercomparison of residual spectral features.

Jarvis, Kandy S.↗

NASA NTRS1992

CCD reflectance spectra of selected asteroids. I - Presentation and data analysis considerations

Narrowband reflectance spectra have been acquired which contribute to the library of asteroid data in the visible and near-IR spectral regions. The spectra support the existence of aqueous alteration products on asteroids located in the outer part of the main asteroid belt out to at least 4 AU. No evidence for features similar to the spectral features of ordinary chondrite meteorites was found in the spectra of asteroids located near the 3:1 Kirkwood Gap chaotic zone.

Vilas, Faith↗

NASA NTRS1991

Compositional studies of primitive asteroids

The aqueous alteration history in the solar system are studied through acquiring additional CCD reflectance spectra in the blue-UV through near-IR spectral region and analyzing these spectra for information about iron oxides in phyllosilicates identified in the CM and CI carbonaceous chondrites. Emphasis is on the main-belt and Cybele primitive asteroids, as these asteroids show spectral diversity and are also spectral analogues for known meteorite samples. The porphyrin bands found in organics near 0.4 micron is also sought.

Vilas, Faith↗

NASA NTRS1991

Compositional studies of primitive asteroids

Primitive asteroids in the solar system (C, P, D class and associated subclasses) are believed to have undergone less thermal processing compared with the differential (S class) asteroids. Telescopic spectra of C class asteroids show effects of aqueous alteration products produced when heating of the asteroids was sufficient to melt surface water, but not strong enough to produce differentiation. Spectrum analysis of P and D class asteroids suggests that aqueous alteration terminated in the outer belt and did not operate at the distance of Jupiter's orbit.

Vilas, Faith↗

NASA NTRS1991

Near-infrared reflectance spectra-applications to problems in asteroid-meteorite relationships

Near-infrared spectral reflectance data were collected at the Infrared Telescope Facility (IRTF) at Mauna Kea Observatories in 1985 and 1986 for the purpose of searching the region near the 3:1 Kirkwood gap for asteroids with the spectral signatures of ordinary chondrite parent bodies. Twelve reflectance spectra are observed. The presence of ordinary chondrite parent bodies among this specific set of observed asteroids is not obvious, though the sample is biased towards the larger asteroids in the region due to limitations imposed by detector sensitivity. The data set, which was acquired with the same instrumentation used for the 52-color asteroid survey (Bell et al., 1987), also presents some additional findings. The range of spectral characteristics that exist among asteroids of the same taxonomic type is noted. Conclusions based on the findings are discussed.

Mcfadden, Lucy A.↗

NASA NTRS1990

A space-based concept for a collision warning sensor

This paper describes a concept for a space-based collision warning sensor experiment, the Debris Collision Warning Sensor (DCWS) experiment, in which the sensor will rely on passive sensing of debris in optical and IR passband. The DCWS experiment will be carried out under various conditions of solar phase angle and pass geometry; debris from 1.5 m to 1 mm diam will be observable. The mission characteristics include inclination in the 55-60 deg range and an altitude of about 500 km. The results of the DCWS experiment will be used to generate collision warning scenarios for the Space Station Freedom.

Talent, David L.↗

NASA NTRS1990

Collision warning and avoidance considerations for the Space Shuttle and Space Station Freedom

The increasing hazard of manmade debris in low earth orbit (LEO) has focused attention on the requirement for collision detection, warning and avoidance systems to be developed in order to protect manned (and unmanned) spacecraft. With the number of debris objects expected to be increasing with time, the impact hazard will also be increasing. The safety of the Space Shuttle and the Space Station Freedom from destructive or catastrophic collision resulting from the hypervelocity impact of a LEO object is of increasing concern to NASA. A number of approaches to this problem are in effect or under development. The collision avoidance procedures now in effect for the Shuttle are described, and detection and avoidance procedures presently being developed at the Johnson Space Center for the Space Station Freedom are discussed.

Vilas, Faith↗

NASA NTRS1990

Thermal models applicable for visual and infrared studies of orbital debris

Over the past decade, thermal models have been developed for the determination of asteroid diameters and albedos. As a first step to understanding the size/frequency distribution of the debris population in earth orbit, these thermal models have been modified to determine the sizes of orbiting debris. When possible, the model results have been compared to spherical satellites of known diameter.

Lebofsky, Larry A.↗

NASA NTRS1990

The detection of earth orbiting objects by IRAS

A systematic examination of 1836 images of the sky constructed from scans made by the Infrared Astronomical Satellite has resulted in the detection of 466 objects which are shown to be in earth orbit. Analysis of the spatial and size distribution and thermal properties of these objets, which may include payloads, rocket bodies and debris particles, is being conducted as one step in a feasibility study for space-based debris detection technologies.

Dow, Kimberly L.↗

NASA NTRS1989

Phyllosilicate absorption features in main-belt and outer-belt asteroid reflectance spectra

Absorption features having depths up to 5 percent are identified in high-quality, high-resolution reflectance spectra of 16 dark asteroids in the main belt and in the Cybele and Hilda groups. Analogs among the CM2 carbonaceous chondrite meteorites exist for some of these asteroids, suggesting that these absorptions are due to iron oxides in phyllosilicates formed on the asteroidal surfaces by aqueous alteration processes. Spectra of ten additional asteroids, located beyond the outer edge of the main belt, show no discernible absorption features, suggesting that aqueous alteration did not always operate at these heliocentric distances.

Vilas, Faith↗

NASA NTRS1989

Observing orbital debris using space-based telescopes. I - Mission orbit considerations

In this paper, mission orbit considerations are addressed for using the Space Shuttle as a telescope platform for observing man-made orbital debris. Computer modeling of various electrooptical systems predicts that such a space-borne system will be able to detect particles as small as 1-mm diameter. The research is meant to support the development of debris- collision warning sensors through the acquisition of spatial distribution and spectral characteristics for debris and testing of detector combinations on a shuttle-borne telescopic experiment. The technique can also be applied to low-earth-orbit-debris environment monitoring systems. It is shown how the choice of mission orbit, season of launch, and time of day of launch may be employed to provide extended periods of favorable observing conditions.

Reynolds, Robert C.↗

NASA NTRS1989

Distant asteroids and Chiron

Knowledge of the physical properties of distant asteroids (a greater than 3.3 AU) has grown dramatically over the past five years, due to systematic compositional and lighcurve studies. Most of these objects have red, dark surfaces, and their spectra show a reddening in spectral slope with heliocentric distance, implying a change in surface composition. Trojans for which near-opposition phase curve information is available appear to show little or no opposition effect, unlike any other dark solar system objects. The lightcurve amplitudes of Trojan and Hilda asteroids imply significantly more elongated shapes for these groups than for main-belt asteroids of comparable size. These recent observations are reviewed in the context of their implications for the formationan and subsequent evolution of the distant asteroids, and their interrelations with the main belt, Chiron, and comets.

French, Linda M.↗

NASA NTRS1988

Structure of scintillations in Neptune's occultation shadow

An exceptionally high-quality data set from a Neptune occultation is used here to derive a number of new results about the statistical properties of the fluctuations of the intensity distribution in various parts of Neptune's occultation shadow. An approximate numerical ray-tracing model which successfully accounts for many of the qualitative aspects of the observed intensity fluctuation distribution is introduced. Strong refractive scintillation is simulated by including the effects of 'turbulence' with projected atmospheric properties allowed to vary in both the direction perpendicular and parallel to the limb, and an explicit two-dimensional picture of a typical intensity distribution throughout an occulting planet's shadow is presented. The results confirm the existence of highly anisotropic turbulence.

Hubbard, W. B.↗

NASA NTRS1988

Mercury

Papers are presented on future observations of and missions to Mercury, the photometry and polarimetry of Mercury, the surface composition of Mercury from reflectance spectrophotometry, the Goldstone radar observations of Mercury, the radar observations of Mercury, the stratigraphy and geologic history of Mercury, the geomorphology of impact craters on Mercury, and the cratering record on Mercury and the origin of impacting objects. Consideration is also given to the tectonics of Mercury, the tectonic history of Mercury, Mercury's thermal history and the generation of its magnetic field, the rotational dynamics of Mercury and the state of its core, Mercury's magnetic field and interior, the magnetosphere of Mercury, and the Mercury atmosphere. Other papers are on the present bounds on the bulk composition of Mercury and the implications for planetary formation processes, the building stones of the planets, the origin and composition of Mercury, the formation of Mercury from planetesimals, and theoretical considerations on the strange density of Mercury.

Vilas, Faith↗

NASA NTRS1988

Future observations of and missions to Mercury

Key scientific objectives of Mercury explorations are discussed, and the methods by which remote observations of Mercury can be carried out from earth and from space are examined. Attention is also given to the scientific rationale and technical concepts for missions to Mercury. It is pointed out that multiple Venus-Mercury encounter trajectories exist which, through successive gravity assists, reduce mission performance requirements to levels deliverable by available systems, such as Titan-Centaur, Atlas-Centaur, and Shuttle/TOS. It is shown that a single launch in July of 1994, using a Titan-Centaur combination, could place a 1477-kg payload into orbit around Meercury. The components of a Mercury-orbiter payload designed to study surface geology and geochemistry, atmospheric composition and structure, the local particle and fields environment, and solid-body rotation dynamics are listed.

Stern, Alan S.↗

NASA NTRS1988

Surface composition of Mercury from reflectance spectrophotometry

The controversies surrounding the existing spectra of Mercury are discussed together with the various implications for interpretations of Mercury's surface composition. Special attention is given to the basic procedure used for reducing reflectance spectrophotometry data, the factors that must be accounted for in the reduction of these data, and the methodology for defining the portion of the surface contributing the greatest amount of light to an individual spectrum. The application of these methodologies to Mercury's spectra is presented.

Vilas, Faith↗

NASA NTRS1987

Oblateness, radius, and mean stratospheric temperature of Neptune from the 1985 August 20 occultation

The oblateness and equatorial radius of Neptune at the 1-microbar pressure level, together with the position angle of the projected spin axis, are the goals of a general oblate atmosphere model for Neptune employing a data ensemble obtained from the occultation of a bright IR star that provided accurate measurements of the limb position at these and several other stations. The observed reduction in central flash intensity is explained by a 150-135 K temperature decrease as pressure rises from 1 to 400 microbar. Attention is given to the implications of these oblateness results for models of the Neptune interior.

Hubbard, W. B.↗