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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Notes on the P/Halley nucleus

Indirect evidence and theory concerning the nucleus of P/Halley are summarized. Reasonable estimates place its diameter in the range 2.5 to 4.0 km, but the true value may well lie outside these bounds. The observed activity in its inner coma indicates that the nucleus is spotty and contains small activity areas as do other similar comets. Whether these active areas will show as dark or light regions on the nucleus is not known. Jets of dust will probably obscure parts of the nucleus when imaged by a nearby spacecraft.

Whipple, F. L.↗
NASA NTRS1981

Comets: Gases, ices, grains and plasma

The program and abstracts of the 97 papers delivered at the colloquium are presented. Cometary nuclei, comet dust, the coma, ion tails, several comet missions, and cometary origin and evolution were discussed.

Wilkening, L. L.↗
NASA NTRS1981

A simple radiation model of cometary dust for P/Halley

Algebra, parameter values, and results for a model of the radiation from the dust environment of Comet P/Halley are presented. The model includes absorption and single scattering of sunlight and IR emission, for the dust and nucleus. It employs dust particle photometric properties which are independent of wave length, particle size, and position within the coma, separately in the visible and IR regions. Values for these properties are obtained from published photometric data for several comets, and are applied to a numerical model for the dust distribution near Halley. A calculation for a spacecraft 1081 km sunward of Halley's nucleus (at 1.05 AU, postperihelion) leads to omnidirectional, integral flux values of 1235 W/sqm for direct sunlight, 0.24 W/sqm for dust IR emission, 0.095 W/sam for sunlight scattered from the dust, 0.0015 W/sqm for sunlight reflected from the nucleus, and 0.00033 W/sqm for nucleus IR emission.

Divine, N.↗
NASA NTRS1982

A new antenna concept for satellite communications

A novel antenna configuration of two reflecting surfaces and a phased array is examined for application to satellite communications and shown to be superior in every respect to earlier designs for service to the continental United States from synchronous orbit. The vignetting that afflicts other two reflector optical systems is eliminated by use of a reflecting field element. The remaining aberrations, predominantly coma, are isolated in the time delay distribution at the surface of the array and can be compensated by ordinary array techniques. The optics exhibits infinite bandwidth and the frequency range is limited only by the design of the array.

Skahill, G.↗
NASA NTRS1981

Infrared photometry of comets Bowell and P/Stephan-Oterma

Broadband IR photometry in the 1-20 micron range of the comets P/Stephan-Oterma and Bowell shows JHK colors similar to P/Meier and P/Tuttle, which are compatible with the scattering of sunlight by micron-sized grains. In addition, the P/Stephan-Oterma thermal emission was found to have an effective temperature higher than that expected from a blackbody in equilibrium. It is demonstrated that the thermal emission can be fit by models of the dust coma consisting of micron-sized grains, and that most of the flux at all observed wavelengths comes from the dust grains rather than the nucleus.

Veeder, G. J.↗
NASA NTRS1982

On the folding phenomenon of comet tail rays

It is shown that the folding phenomenon of the comet tail rays is compatible with the Ferraro isorotation law if the comet tail magnetic field has no azimuthal component, that is, B sub phi (the polar angle) equals zero. Considering electric drift due to convectional electric fields, a formula is obtained for the angular rate of a ray closure which reduces to that of Ness and Donn (1966) if the velocity profile across the tail is linear. The magnetic field B of approximately 20-40 gammas in the coma and less than about 10 gammas in the distant tail is estimated under typical solar wind conditions at 1 AU.

Ershkovich, A. I.↗
NASA NTRS1981

On the mechanism of ray closure in comet tails

The folding phenomenon of comet tail rays is explained by means of an electric drift due to convectional electric fields. This mechanism results in an angular rate of closure which reduces to that obtained by Ness and Donn (1966) if the velocity profile across the tail is linear and the plasma conductivity is ideal. Observations of both the ray closure and the disconnection events point to the phenomenon of anomalous resistivity. Magnetic fields of about 30-40 gammas in the coma and of 10 gammas in the distant tail (at 1 AU) are estimated from the MHD momentum equation.

Ershkovich, A. I.↗
NASA NTRS1981

The X-ray structure of a galaxy cluster at z = 0.54 - Implications for cluster evolution and cosmology

High-resolution X-ray observations of the rich cluster 0016+16 at a redshift of 0.541 are presented. The emitting gas in this cluster is hot and extremely luminous, and its structure resembles that seen in the brightest nearby cluster sources. In most of its properties, 0016+16 resembles a richer version of the Coma cluster, and it offers little support to the hypothesis that clusters at z greater than 0.5 differ fundamentally from nearer objects.

White, S. D. M.↗
NASA NTRS1982

Cometary NH - Ultraviolet and submillimeter emission

A description is presented of the results of computer calculations on the molecule NH, which is an important constituent of cometary comae, and a probable, but an as yet undetected, interstellar species. Attention is given to spectroscopic properties, the treatment of solar spectral data, and radiative transfer calculations. The results of calculations for the Einstein A-coefficients are given in an appendix, where the entries for the X-X and A-A transitions depend on the assumed permanent electric-dipole moment squared. Rotational-spin-Lambda-doublet energy levels used for statistical equilibrium population densities are listed in a table.

Litvak, M. M.↗
NASA NTRS1982

The cosmic distance scale - Methods for determining the distance to supernovae

It is established by an analysis of alternative methods for the determination of absolute distances to supernovae that there exists a kinematic method, superior to that of Baade (1926), which is illustrated for the case of the Type I supernova 1975a in NGC 2207. In addition, a dynamical method is developed which employs the recently-discovered analytic solutions for Type I supernovae. The requirement that the paradigm invoked in the present study be able to reproduce both fast and slow light curves has implied a dispersion in intrinsic brightness, whose size is approximately that observed in the Virgo and Coma clusters.

Arnett, W. D.↗
NASA NTRS1982

Imaging antenna arrays

Many millimeter and far-infrared imaging systems are limited in sensitivity and speed because they depend on a single scanned element. Because of recent advances in planar detectors such as Schottky diodes, superconducting tunnel junctions, and microbolometers, an attractive approach to this problem is a planar antenna array with integrated detectors. A planar line antenna array and optical system for imaging has been developed. The significant advances are a 'reverse-microscope' optical configuration and a modified bow-tie antenna design. In the 'reverse-microscope' configuration, a lens is attached to the bottom of the substrate containing the antennas. Imaging is done through the substrate. This configuration eliminates the troublesome effects of substrate surface waves. The substrate lens has only a single refracting surface, making possible a virtually aplanatic system, with little spherical aberration or coma. The array is characterized by an optical transfer function that is easily measured. An array with 19 dB crosstalk levels between adjacent antennas has been tested and it was found that the array captured 50 percent of the available power. This imaging system was diffraction limited.

Rutledge, D. B.↗
NASA NTRS1982

A morphological classification of clusters of galaxies from Einstein images

The earliest Uhuru observations showed that cluster X-ray sources were not associated with single individual galaxies but were extended sources. It has only been with the advent of the Einstein X-ray imaging observatory that a first look at cluster X-ray morphology and classification has been possible. The proposed classification system divides clusters into two families, including those with and those without X-ray dominant galaxies. Within each family, the dynamical indicators display a full range of values. One subgroup of clusters is those whose X-ray emission is not regular and which do not contain an X-ray dominant galaxy. One of the brighter, nearer and best studied members of this class is Al367. From the X-ray imaging observations, four clusters have been discovered to have double structure in their surface brightness distributions. The Coma cluster is the archtype of an evolved, fully relaxed cluster. Ellipticals in the Virgo core include M87, M86, and M84. It is concluded that the cluster images are useful in determining the cluster family and the cluster's dynamical state.

Jones, C.↗
NASA NTRS1983

Infrared molecular emissions from comets

The possibility of detecting IR molecular line emission from cometary parent molecules is explored. Due to the non-LTE conditions in the inner coma and the large amount of near IR solar flux, IR fluorescence will be a significant source of cometary emission and, in fact, will dominate the grain radiation in a sufficiently high resolution instrument. The detection of this line emission will be difficult due to absorption in the terrestrial atmosphere, but it appears possible to measure cometary H2O emission from airplane altitudes. As IR molecular line emission represents one of the few promising methods of detecting cometary parent molecules directly, further research on this problem should be vigorously pursued.

Weaver, H. A.↗
NASA NTRS1982

Spectrophotometry of comets at optical wavelengths

Techniques for spectrophotometry of comets are discussed, and results are reviewed for line and continuum spectrophotometry of comets at optical wavelengths. The techniques considered include photographic spectroscopy and spectrophotometry, photoelectric spectrophotometry, and methods based on the use of image dissector scanners, Fourier-transform instruments, and Fabry-Perot spectrometers. Results are summarized for the study of cometary emission features due to C2, CN, CH, OH, NH, C3, NH2, O, H, CO(plus), and H2O(plus). Relative abundances of various species in comets are examined, along with continuum spectrophotometry of the nuclei, comas, and tails of comets.

Ahearn, M. F.↗
NASA NTRS1982

Laboratory studies of photochemical and spectroscopic phenomena related to comets

The relationship between laboratory observations and cometary phenomena is reviewed, with particular attention to how laboratory studies must be done to obtain all data needed for astronomical purposes. It is pointed out how supersonic expansion of the gas evaporating from the nucleus can affect the photochemical lifetime and the energy partitioning among the fragments. The use of the CS radical as a remote sensing probe of conditions in the inner coma is also discussed.

Jackson, W. M.↗
NASA NTRS1982

Solar wind interaction with comets - Lessons from Venus

Data on the solar wind interaction with Venus are examined for the purpose of comparison with similar processes that may occur in comets. Attention is given to bow shock, magnetosheath, ionopause, ionosphere, and magnetotail of Venus. These features are compared with, respectively, the bow shock, magnetosheath, contact surface, coma, and plasma tail of a comet. It is concluded that observations of the solar wind interaction with Venus should provide new insight into the solar wind interaction with comets.

Russell, C. T.↗
NASA NTRS1982

Infrared photometry of periodic comets Encke, Chernykh, Kearns-Kwee, Stephan-Oterma, and Tuttle

Nearly simultaneous photometry of the reflected and thermal infrared spectra of periodic comets Encke, Chernykh, Kearns-Kwee, Stephan-Oterma, and Tuttle are presented. The 10-micron silicate emission feature has been detected for the first time in periodic comets and was observed in three of these objects. The albedo of the dust particles in the comae of these comets is calculated and compared to that of Comet Kohoutek. The peculiar behavior of the dust in Comet Encke is discussed.

Campins, H.↗
NASA NTRS1982

The photochemistry and dynamics of a dusty cometary atmosphere

The solving of a simultaneous set of differential equations representing conservation of number density, momentum, and energy together with solar radiation transfer in the streams which result in photolytic processes and the heating of the nucleus yields a self-consistent solution of the dynamical and thermal structure of an H2O-dominated two-phase dusty gas cometary atmosphere. Two models are considered for the transfer of solar radiation through the circumnuclear dust halo. The first considers only the direct extinction by the dust, and in the second the diffuse radiation field due to multiple scattering by the dust halo, which compensates for radiation removed by direct absorption when the optical depth is near unity, is approximated by neglecting the attenuation of radiation given off by the dust. It is shown that while dust attenuation has a strong effect on the H2O production rate, it also increases the electron density in the inner coma over the unattenuated case.

Marconi, M. L.↗