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At least 109 records · Page 6

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NASA NTRS1986

High-resolution imaging studies of the near-nucleus regions of comets

High-resolution, calibrated images of the near-nucleus regions of Halley, Giacobini-Zinner (for ICE encounter) and additional comets displaying widely differing levels of activity were acquired with a CCD sensor at the Cassegrain focus of the 3.6 m Canada-France-Hawaii Telescope (CFHT). The International Halley Watch designated interference filters were used. Successful correlation of ICE magnetic field data with the optical images obtained with the CFHT on the night of encounter (11 Sept. 1985) primarily through the H2O(+) and neighboring continuum passbands and with 300 km spatial resolution provokes reinterpretation of the observed structure of cometary ion tails. The observations of Halley, limited by bad weather, were made at significant periods of the apparition: the first, in December 1984 with the comet at 5.3 AU during the period of the onset of activity; and the last, in November 1985 with the comet near opposition. The December observations show evidence of residual activity. The 18 November data show a highly-condensed inner coma with an extreme falloff in brightness from the region of the nucleus. These data provide the basis for characterizing the gas and dust environment in the immediate vicinity of the active nucleus.

Goldberg, B. A.↗

NASA NTRS1986

Comet Halley near-nucleus phenomena in 1986

Computer enhanced high resolution groundbased images taken during Halley's most active phases in 1985-86 exhibit all of the coma structures identified in 1910 photographs. The structures result mostly from discrete dust jet activity producing spirals, arcs, and jagged-edged envelopes. Multiple nucleus and antisunward jet phenomena are also observed. The curvature of the jets is consistent with the location of the Earth with respect to the nucleus equatorial plane. The variation in jet activity during the Vega and Giotto flybys is easily seen on the groundbased images. Images of CN jets show that they usually do not correlate with dust jets. Wide field, medium resolution images of faint outer envelopes are required to augment the smaller field, high resolution images of near-nucleus jets in modeling the jets.

Larson, S.↗

NASA NTRS1986

A three-dimensional model of the nucleus of Comet Halley

The nucleus of comet Halley was observed by the Halley Multicolor Camera (HMC) during the last minutes of the flyby sequence before reaching the point of closest approach. The phase angle change during the observational period was less than 17 deg, so HMC data can only define a two-dimensional contour of the nucleus at a certain time. The overall geometry during the HMC observations is outlined and the findings are related to information available from other sources at different times. The nucleus of the comet is described as a triaxial ellipsoid with major axes of 16, 10, and 9 km. The rotation axis is directed towards delta 1950 = -(40 + or - 5) deg and alpha 1950 = (50 + or - 10) deg. The rotation with a period of P = (54 + or - 1) is in a prograde sense with respect to the orbital motion.

Wilhelm, K.↗

NASA NTRS1987

Near-nucleus optical observations of P/Halley

The Near-Nucleus Studies Net of the International Halley Watch has obtained an extensive series of high resolution optical images of P/Halley during its most active phases in 1985-86 which may be useful in interpreting radio observations of Comet Halley. They often show coma structure resulting from anisotropic emission of dust and gas from the inhomogeneous nucleus. Images were obtained in broadband spectral regions to study dust coma morphology, and in medium to narrow spectral bands to isolate the principal emissions of CN, C3, C2, CO+ and H2O+. The goals and methods of near-nucleus studies are discussed and recent studies of 1910 images are briefly reviewed. The role of dust jets and cometary activity in P/Halley is discussed and several examples of anisotropic emission of dust during the current apparition are shown.

Larson, Stephen M.↗

NASA NTRS1988

An investigation of the nucleus and coma of Comet P/Arend-Rigaux

Narrow-band photometry of Comet P/Arend-Rigaux in the visible and near-ultraviolet are presented for six nights during the 1884/1985 apparition, including three nights of simultaneous observations in the thermal infrared made with the NASA IRTF. The results indicate a period of rotation of 13.47 hr, dimensions for the nucleus of 13 x 8 x 8 km (under the assumption of prolate spheroid shape), and a steady decrease in the comet's gas production from a month before perihelion to more that two months after perihelion. Results for the nucleus and the coma suggest that the nucleus is totally covered with a nonvolatile mantle.

Millis, Robert L.↗

NASA NTRS1988

Nucleus of Comet IRAS-Araki-Alcock (1983 VII)

Optical, radar, infrared, UV, and microwave-continuum observations of Comet IRAS-Araki-Alcok were obtained in May 1983, the week of the comet's close approach to earth. The comet has a nucleus dimension and a rotation period which are similar to those of Comet Halley, but a different morphological signature (a persisting sunward fan-shaped coma). Time variations are noted in the projected nucleus cross section. Results suggest significant limb-darkening effects in the relevant domains of radio waves, and that the comet's interior must be extremely cold. It is found that the thermal-infrared fluxes from the inner coma of the comet are dominated by the nucleus.

Sekanina, Z.↗

NASA NTRS1988

Low- to high-grade metamorphic transition in the Southern part of Karnataka Nucleus, India

The southern part of Karnataka Nucleus has a strong imprint of 2.6 Ga metamorphism. This has affected the schist belts of Karnataka Nucleus from greenschist to upper amphibolite facies. The higher grades of metamorphism are in the Holenarasipur, Nuggihalli, Krishnarajpet, Hadnur and Melkote schist belts. In the high grade transition zone, around Sargur only keels of schist belts are preserved and occur as highly dismembered, disconnected belts with the top and bottom of the stratigraphic column obliterated due to high grade metamorphism and accompanying migmatization. Absence of high-grade metamorphic minerals in the sediments of the Dharwar schist belts supports the contention that high grade metamorphism post-dated the Dharwar sedimentation and occurred around 2.6 Ga ago. Sargur type metamorphism occurred at upper crustal levels and charnockite type metamorphism occurred in lower crustal levels. The P-T conditions for the mineral assemblage in metapelites of Sargur Group indicate burial depths up to at least 15 km suggesting that they were subducted and later obducted during the development of Early Proterozoic Mobile Belt along the southern border of the Karnataka Nucleus.

Naqvi, S. M.↗

NASA NTRS1988

Monte Carlo modeling of the net effects of coma scattering and thermal reradiation on the energy input to cometary nucleus

A Monte Carlo simulation method is presented that can, to an accuracy of a few percent, calculate the effects of a dusty coma on the total energy input to the cometary nucleus. This method treats nonconservative nonisotropic scattering, as well as the reflection from the nucleus surface. Results are presented as a function of the optical thickness of the dust column in the sun-comet axis. The total energy input to the nucleus appears to be only weakly dependent on the opacity of the coma, the radial distribution of the dust, or the details of the extinction processes.

Salo, H.↗

NASA NTRS1989

The double-nucleus galactic merger Mkn 463

Deep CCD imaging in broadband and forbidden O III wavelengths of the twin-nucleus active galaxy Markarian 463 is presented. These data show that the system has triple curved tails indicative of a strong tidal interaction and probable merger. Colors, morphology, and their implications are discussed. The forbidden O III line emission is seen to be extended and linear, perhaps filling twin cones with apex at the eastern optical nucleus. It is argued that this indicates nonisotropic radiation from an obscured AGN. Spatially resolved spectroscopy on and off the nuclei reveals that the radial velocities of the gas lie in a small range, unlike some other multiple nucleus systems. The nature of the emission-line regions is discussed, along with the evolutionary state of the system indicated by the present data and IR and radio data.

Hutchings, J. B.↗

NASA NTRS1989

Non-destructive trace element microanalysis of as-received cometary nucleus samples using synchrotron x ray fluorescence

The Synchrotron X ray Fluorescence (SXRF) microprobe at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, will be an excellent instrument for non-destructive trace element analyses of cometary nucleus samples. Trace element analyses of as-received cometary nucleus material will also be possible with this technique. Bulk analysis of relatively volatile elements will be important in establishing comet formation conditions. However, as demonstrated for meteorites, microanalyses of individual phases in their petrographic context are crucial in defining the histories of particular components in unequilibrated specimens. Perhaps most informative in comparing cometary material with meteorites will be the halogens and trace metals. In-situ, high spatial resolution microanalyses will be essential in establishing host phases for these elements and identifying terrestrial (collection/processing) overprints. The present SXRF microprobe is a simple, yet powerful, instrument in which specimens are excited with filtered, continuum synchrotron radiation from a bending magnet on a 2.5 GeV electron storage ring. A refrigerated cell will be constructed to permit analyses at low temperatures. The cell will consist essentially of an air tight housing with a cold stage. Kapton windows will be used to allow the incident synchrotron beam to enter the cell and fluorescent x rays to exit it. The cell will be either under vacuum or continuous purge by ultrapure helium during analyses. Several other improvements of the NSLS microprobe will be made prior to the cometary nucleus sample return mission that will greatly enhance the sensitivity of the technique.

Sutton, S. R.↗

NASA NTRS1989

Electron Spin Resonance (ESR) studies of returned comet nucleus samples

The most important objective of the Comet Nucleus Sample Returm Mission is to return samples which could reflect formation conditions and evolutionary processes in the early solar nebula. It is expected that the returned samples will consist of fine-grained silicate materials mixed with ices composed of simple molecules such as H2O, NH3, CH4 as well as organics and/or more complex compounds. Because of the exposure to ionizing radiation from cosmic-ray, gamma-ray, and solar wind protons at low temperature, free radicals are expected to be formed and trapped in the solid ice matrices. The kind of trapped radical species together with their concentration and thermal stability can be used as a dosimeter as well as a geothermometer to determine thermal and radiation histories as well as outgassing and other possible alternation effects since the nucleus material was formed. Since free radicals that are known to contain unpaired electrons are all paramagnetic in nature, they can be readily detected and characterized in their native form by the Electron Spin Resonance (ESR) method. In fact, ESR has been shown to be a non-destructive, highly sensitive tool for the detection and characterization of paramagnetic, ferromagnetic, and radiation damage centers in terrestrial and extraterrestrial geological samples. The potential use of ESR as an effective method in the study of returned comet nucleus samples, in particular, in the analysis of fine-grained solid state icy samples is discussed.

Tsay, Fun-Dow↗

NASA NTRS1989

The 100 and 160 micron maps of the dust reemission from the nucleus and inner-arm regions of NGC 6946

Dust reemission from the Scd galaxy NGC 6946 has been measured at 100 and 160 microns with the 32-channel University of Chicago Far-Infrared Camera. Researchers present fully sampled maps of the nucleus and inner spiral arms at 45 seconds resolution. The far-infrared morphology of the galaxy is a bright peak centered on a diffuse disk, where the peak occurs about 24 seconds NE of the Dressel and Condon optical center. The 100/160 micron color temperature is correlated with the H alpha surface brightness. Assuming the distance from Earth to the galaxy is 10.1 Mpc, researchers determine that Tc is 32 K at the nucleus and at radius 5.4 kpc, where there is a concentration of H II regions. In the intermediate annulus of relatively low H alpha surface brightness, the temperature drops to a local minimum of 25 K at radius 3 kpc. The ratio of reradiated to transmitted stellar luminosity is approx. 3.0 at the nucleus and approx. 0.9 for the disk. The optical depth at 100 micron increases from .0005 at the edges of our map to .0035 at the far infrared radiation (FIR) peak. Combining our observations with a fully sampled map of similar spatial extent in CO(1 greater than 0), researchers determine that the ratio F sub IR/I sub CO at the center of the galaxy is almost twice that for the disk, where the value is more or less constant.

Engargiola, Gregory↗

NASA NTRS1990

Models of the spin state of the comet Halley nucleus

Eight rotation precession models of the comet Halley nucleus have been proposed by eight authors. The eight models were evaluated in relation to the constraints imposed by: (1) the observed long axis directions at the Vega 1, Vega 2, and Giotto encounters; (2) the ground based emission periods harmonically related to 7.4 days; (3) the need for a two day spin period in the analysis of the jet morphology; (4) the Smith et al. constraint on the net long axis roll between Vega 2 and Giotto; (5) the resistance of the spin state of the nucleus to change due to the torque from the jets; and (6) the 7.4 day repetition of the spatial orientation of the nucleus. The eight constraints are briefly described.

Julian, William H.↗

NASA NTRS1992

The ionization cone, obscured nucleus, and gaseous outflow in NGC 3281 - A prototypical Seyfert 2 galaxy?

Narrow-band images and long-slit spectroscopy of the central region of the highly inclined Seyfert galaxy NGC 3281 are presented. The image of the continuum-subtracted forbidden 4959 emission line shows a very clear conical morphology for the high-excitation gas. A possible similar structure can also be seen on the other side of the nucleus, but is dimmed by patchy obscuration in the dusk. The continuum images and long-slit spectroscopy are used to derive and map the extinction in the inner regions of NGC 3281; heavy obscuration is found along the present line of sight to the apex of the cone, suggesting that the true nucleus is located at the apex and is obscured. Low-resolution long-slit spectra are used to study the stellar population, which is found to be old, uniform within 2.5 kpc of the nucleus, and typical of the bulges of early-type galaxies. It is suggested that NGC3281 may be another example of a 'hidden' Seyfert 1, even though there is no direct evidence for a broad-line region in this particular galaxy.

Storchi-Bergmann, Thaisa↗

NASA NTRS1992

Comet nucleus and asteroid sample return missions

Three Advanced Design Projects have been completed this academic year at Penn State. At the beginning of the fall semester the students were organized into eight groups and given their choice of either a comet nucleus or an asteroid sample return mission. Once a mission had been chosen, the students developed conceptual designs. These were evaluated at the end of the fall semester and combined into three separate mission plans, including a comet nucleus same return (CNSR), a single asteroid sample return (SASR), and a multiple asteroid sample return (MASR). To facilitate the work required for each mission, the class was reorganized in the spring semester by combining groups to form three mission teams. An integration team consisting of two members from each group was formed for each mission so that communication and information exchange would be easier among the groups. The types of projects designed by the students evolved from numerous discussions with Penn State faculty and mission planners at the Johnson Space Center Human/Robotic Spacecraft Office. Robotic sample return missions are widely considered valuable precursors to manned missions in that they can provide details about a site's environment and scientific value. For example, a sample return from an asteroid might reveal valuable resources that, once mined, could be utilized for propulsion. These missions are also more adaptable when considering the risk to humans visiting unknown and potentially dangerous locations, such as a comet nucleus.

Source record↗

NASA NTRS1992

Forced precession of the cometary nucleus with randomly placed active regions

The cometary nucleus is assumed to be triaxial or axisymmetric spheroid rotating about its axis of maximum moment of inertia and is forced to precess due to jets of ejected material. Randomly placed regions of exposed ice on the surface of the nucleus are assumed to produce gas and dust. The solution of the heat conduction equation for each active region is used to find the gas sublimation rate and the jet acceleration. Precession of the comet nucleus is followed numerically using a phase-averaged system of equations. The gas production curves and the variation of the spin axis during the orbital motion of the comet are presented.

Szutowicz, Slawomira↗

NASA NTRS1988

Effects of systemic L-tyrosine on dopamine release from rat corpus striatum and nucleus accumbens

Intracerebral dialysis was used to monitor extracellular fluid from rat striatum and nucleus accumbens following the intraperitoneal administration of tyrosine. Dopamine concentrations in dialysates from both the striatum and the nucleus accumbens increased significantly in response to the tyrosine. The magnitude of the tyrosine effect was greater in the nucleus accumbens than in the striatum. Hence, mesolimbic dopaminergic neurons may be especially responsive to precursor availability.

During, Matthew J.↗

NASA NTRS1993

Theoretical antideuteron-nucleus absorptive cross sections

Antideuteron-nucleus absorptive cross sections for intermediate to high energies are calculated using an ion-ion optical model. Good agreement with experiment (within 15 percent) is obtained in this same model for (bar p)-nucleus cross sections at laboratory energies up to 15 GeV. We describe a technique for estimating antinucleus-nucleus cross sections from NN data and suggest that further cosmic ray studies to search for antideuterons and other antinuclei be undertaken.

Buck, W. W.↗