Engineering topics
Publications and source records attributed to Vilas, F..
We investigate Vesta's UV lightcurve and spectral reversal. We model the UV spectra using laboratory spectra of howardite and diogenite meteorites, as well as suing candidate materials based on knowledge of Vesta's composition from visible and near-infrared spectroscopy.
A reanalysis of IUE spectra of Vesta indicates longitudinal variations in a UV absorption feature that may be related to compositional and/or topographic variations, as revealed by recent HST maps.
The action of aqueous alteration of the near-subsurface material on asteroids and probably some satellites in the Solar System constitutes part of the formation history of the Solar System. The C-class asteroids (and subclasses B, G, and F) were initially believed to have undergone aqueous alteration based on their low albedos and neutral broadband visible and near-infrared colors. These spectra exhibit a sharp drop at wavelengths shorter than 0.55 microns due to a strong ferric oxide intervalence change transfer transition. This IVCT comprises multiple absorptions that are not uniquely indicative of phyllosilicates, but rather are present in the spectrum of any object containing Fe(2+) and Fe(3+) in its surface material. A definitive indication of aqueous alteration came when the broad IR absorption feature having a minimum near 3.0 microns indicative of structural hydroxyl (OH) and interlayer and and absorbed water in phyllosilicates was identified in the IR photometry of many C-class asteroids. Additional mineralogical compositional evidence of aqueously-altered asteroids has come as the result of high SNR narrowband spectrophotometry in the visible and near-infrared spectral regions taken using a CCD/spectrograph combination. An absorption feature centered at 0.7 microns indicative of an Fe(2+) - Fe(3+) charge transfer transition in oxidized iron in phyllosilicates in spectra of some low-albedo asteroids, especially C and G class, and CM2 carbonaceous chondrite meteorites was identified in the CCD spectra. The correlation and covariance of the 0.7-micron and 3.0-micron features were examined by comparing observations of asteroids common to both the CCD reflectance spectra and the 3.0-micron multicolor photometry data sets. Thirty-one pairs of observations were included in this training group. The results of the statistical study indicate that with a 95% confidence level, 84% of the objects observed either having or not having the 0.7-micron feature will correspondingly have the 3.0-micron water of hydration feature.
Minor absorption bands in the 0.55 to 0.7 micron wavelength range of reflectance spectra of 10 S asteroids have been found and compared with those of spinel-group minerals using the modified Gaussian model. Most of these S asteroids are consistently shown to have two absorption bands around 0.6 and 0.67 micron. Of the spinel-group minerals examined in this study, the 0.6 and 0.67 micron bands are most consistent with those seen in chromite. Recently, the existence of spinels has also been detected from the absorption-band features around 1 and 2 micron of two S-asteroid reflectance spectra, and chromite has been found in a primitive achondrite as its major phase. These new findings suggest a possible common existence of spinel-group minerals in the solar system.
Spectra of primitive asteroids (defined as C, P, and D classes and associated subclasses) were compared to the limited number of spectra of CM2 carbonaceous chondrites. An absorption feature located at 0.7 microns attributed to an Fe(+2) - Fe(+3) charge transfer absorption in iron oxides in phyllosilicates is apparent in some of the CM2 carbonaceous chondrite spectra and many of the asteroid spectra. Sawyer found a correlation between the area of the 0.7 micron feature and the mean semimajor axis of the asteroids. Spectra of a larger sample of carbonaceous chondrites, including 7 CM2 chondrites, covering a spectral interval of 0.30-2.5 microns were recently obtained using the Relab instrument at Brown University. These spectra were compared with spectrophotometric asteroid observations in a separate abstract. Those spectra of CM2 chondrites were isolated into the UV, visible and near-infrared spectral regions in order to compare them with high-quality narrowband reflectance spectra.
Although many similarities exist between meteorite spectra and 'primitive' asteroids, there are unexplained discrepancies. These discrepancies do not appear to arise from grain size effects. Assuming that primitive meteorites did in fact originate from the 'primitive' asteroids, we believe that there are two testable explanations for the observed spectral discrepancies: compositional or structural differences. We have begun to synthesize and collect reflectance and Mossbauer spectra of pertinent materials, beginning with olivine, pyroxene, and plagioclase (all found in primitive meteorites), and to assess the possible effects composition may have on spectral features. Our study focuses on the combination of composition and structural effects.
A 1.6-meter diameter f/0.95 all-reflecting telescope was designed to observe orbital debris particles as small as 1 mm from the shuttle payload bay. The telescope was specified to have a flat focal surface without the imposition of refractive elements. Two design configurations involving three mirrors were evaluated - a reflective Schmidt-Cassegrain and a modified Paul corrector. The Paul system was found to be more compact and appropriate for this application.
This is a report synthesizing discussions of the Astronomy/Astrophysics Definition Team at the Lunar Science Strategy Workshop for the Exploration and Development of the moon which was held at the NASA Johnson Space Center during August 15-16, 1989. The long-term goal of lunar based astronomy is envisioned as the development of a diverse facility covering all wavelengths of the electromagnetic spectrum. This facility will build on and eventually replace the Great Observatories. The development of optical and infrared interferometry should be a principal activity, optimized by a lunar site. However, lunar observatories should evolve from simple to complex systems and should not and need not make major demands on the limited amount of manpower envisioned for the first 20 years of the lunar outpost.
The composition of primitive asteroids and their relationship to satellites in the solar system will be studied by analyzing existing narrowband charge coupled device (CCD) reflectance spectra, acquiring additional spectra of asteroids and small satellites in the 0.5 to 1.0 micrometer spectral range, and exploring possibilities for obtaining compositional information in the blue-UV spectral region. Comparison with laboratory spectra of terrestrial chlorites and serpentines (phyllosilicates) and the clay minerals found in carbonaceous chondrite meteorites will continue. During 1987, narrowband CCD reflectance spectra of 17 additional asteroids were acquired. These spectra and spectra of 34 other asteroids have been used primarily for two studies: weak absorption features similar to those due to Fe2(+) and Fe2(+) - Fe3(+) transitions in iron oxides f ound in terrestrial chlorites and serpentines and carbonaceous chondrites have been identified in some primitive asteroid spectra. There is a first indication that asteroids grouped by heliocentric distance show similar weak absorption features. Nonparametric statistics are being applied to test the hypothesis of discrete remnants of a gradation in composition of outer-belt asteroids.
The occultation of a star by Neptune on August 20, 1985 was observed at 2.2 micron wavelength with telescopes at the European Southern Observatory (ESO) and the Cerro Tololo Inter-American Observatory (CTIO). The detection of a 'central flash' midway between immersion and emersion has allowed the determination of Neptune's oblateness and the atmospheric extinction at 2.2 microns, which is related to the stratospheric methane mixing ratio. An oblateness of (2.08 +0.19 or -0.18) x 10 to the 2nd is found and, assuming a stratospheric temperature of 120 K, a value of 0.6 percent is inferred (with an uncertainty of a factor of 10) for the methane mixing ratio (CH4/H2) at 0.3 mbar. The latter value may indicate supersaturation of methane in Neptune's stratosphere.
The apparent closest approach of the star SAO186001 to Neptune was observed photoelectrically on July 22, 1984 at Cerro Tololo Inter-American Observatory. A 32-percent signal drop, lasting about 1.2 s, was probably caused by a partially transparent arc of material at a distance of 67,000 km from Neptune. Neptune's arc(s) do not vary smoothly with azimuth, unlike the rings of other Jovian planets.
Hubbard et al. (1985) have observed the occultation of a star by a ring-like arc of Neptune. The semitransparent, incomplete ring was located at a distance of about 67,000 km from Neptune. In connnection with the program for the Voyager 2 deep space probe to fly past Neptune in 1989 at this distance from the planet, it is urgent to determine the nature of this ring arc or arcs. The best approach to obtain the required information is related to an observation of occultations of stars by Neptune. In this context, a study has been conducted of six stars which will be occulted by Neptune during 1986 and 1987, taking into account their JHK photometry. The obtained data are presented in a table.
The surface compositions of outer-belt asteroids were used to obtain information about the origin of these asteroids. High-resolution CCD reflectance spectra of 21 asteroids, primarily P class, were examined for compositional information. Distinct slope changes are observed that suggest that these asteroids are the remnants of a compositional gradation of planetesimals in the outer solar system, which were retained selectively in location when other material was ejected from the solar system. Other data suggest that this gradation could extend through the orbits of Uranus and Neptune.
A composite visible and near-IR (0.53-1.02 micron) reflectance spectrum of terrain on Mercury including smooth plains and intercrater plains was produced (with a theoretical resolution of 17 A) from 59 high-SNR spectra of the planet taken on November 24, 1984 at the 1.0-m telescope of the CTIO. No evidence was found in the spectrum for the proposed orthopyroxene absorption centered near 0.9 micron observed in older spectra of this terrain; the surface material is highly reduced, with any Fe present in metallic form.
An account is given of visual and infrared observations made of Amor asteroid 1982 DV during its discovery apparition. Broadband visual and near-infrared photometry shows it to be an S-class asteroid. Narrowband spectrophotometry shows an absorption feature due to olivine or pyroxene or both centered at 0.93 micrometer. The asteroid is either one of the most reflective S-class asteroids known, or a significant amount of bare rock is exposed on the asteroid's surface. For the nonrotating model considered, ellipsoidal dimensions are 3.5 x 1.4 x 1.4 km.
Observations of eight Neptune occultations from six sites in the southwestern Pacific on June 15, 1983. The data were used to search for evidence of rings around Neptune down to a distance of 0.03 Neptune radii from the planetary surface, but the results were negative. An astrometric analysis of the timings yielded solution for the equatorial radius a(0) of Neptune at 1 microbar pressure and the oblatness e at this level. The results are: a(0) = 25,295 + or - 50 km; e = 0.022 + or - 0.004, from which a value for a(1) (equatorial radius at 1 bar pressure) of 24,830 + or - 100 km; and a rotation period of P = 15 h (+3, -2 h) is derived. These results are based on a recent determination of Neptune's pole position and mass quadruple moment, and are consistent with the hypothesis that Neptune and Uranus have homologous mass distributions, although the constraint on interior structure is a weak one. If the probable uncertainty of the pole position is taken into account, error bars on e and derived quantities should be increased by about 50 percent. The present determinations of Neptune atmospheric temperatures at 1 microbar are consistent with earlier results and when combined with all previous data give an average value of 156 + or - 10 K. There is only slight evidence for any latitude dependence in the temperatures. Profiles with a high signal-to-noise ratio suggest the possible presence of an absorbing layer at altitudes higher than the 1-microbar level.
A stellar occulation event of Neptune revealed a ring segment that was previously undetected. It appears that the object is not a complete ring, but rather a localized swarm of particles which follows a ring orbit over a limited range of longitudes. To avoid confusion with the standard use of the word ring it is suggested that the feature be called an arc. The distance of the arc zone is not precisely known because so far there has been no confirmed occultation by both an arc and the planet.
Reflectance spectra of Mercury covering the spectral range of 0.3-1.1 micron, obtained during 1963-1976, were examined for any correlations with surface terrain. Mercury's 6.1385-deg/day rotational rate, the phases of the planet around maximum elongations, and bidirectional-reflectance-spectroscopy theory were used to identify the surface area associated with each spectrum. Data from 1974-1975, rereduced with improved standard-star-flux ratios, show a weak absorption band in the near infrared not seen in earlier analyses. Older spectra suggest that the western longitudes of the unimaged side of Mercury are similar to the rest of the planet. Spectra of the intercrater plains in the 0-90-deg quadrant suggest a possible absorption band. Spectra of areas dominated by Caloris Basin with the encompassing smooth plains may show Fe(2+) abundances in the soil comparable to lunar-highlands soil. No striking differences between spectra of intercrater plains and smooth plains are found. The absorption features seen in spectra of Mercury are generally weaker than features seen in lunar spectra.