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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Time-dependent dusty gasdynamical flow near cometary nuclei

This paper presents time-dependent solutions to the coupled dusty hydrodynamics equations describing the spherically symmetric expansion of cometary neutral gas in the vicinity of a cometary nucleus. The sublimation process is repressented by gas outflow from a dust-covered reservoir containing stationary gas whose pressure and density values are determined by the sublimating (Ts) and surface (T0) temperatures. The model resolves earlier ambiguities in determining gas production rates and provides analytic relations between Ts, T0, and the gas parameters at the sonic point. The time evolution of a cometary outburst was modeled. It was found that, as a result of the strong gas-dust interaction in the inner coma region, a 'slow' disturbance in both the dust and gas parameters will be created in addition to the familiar gas blast-wave solution. This new 'slow' disturbance, which propagates with a velocity of about 0.2 km/s, might be responsible for some of the observed slowly expanding cometary halos, such as the one which was recently identified using 1910 Mount Wilson high-resolution comet Halley photographs.

Gombosi, T. I.↗
NASA NTRS1984

Predictions of occultations of stars by Comet Crommelin

Predictions of occultation of stars are presented on the basis of star and radio-source catalogs; these are: the AGK3 catalog, the SAO catalog south of the celestial equator, and Perth 70 catalog, as well as the Master List of Radio Sources. The predictions are formulated under the assumption of the coma radius of 22,500 km and a very large cometary radius of 675,000 km for cometary ion tails that may be involved in radio-source occultations. The predictions are expected to change with time.

Bowell, E.↗
NASA NTRS1986

Studies of radiative transfer in planetary atmospheres

The research emphasis during the period of this report has been on radio observations of comets, related to the International Halley Watch. Observations of the 18 cm lambda-doublet of OH have been carried out monthly. Both of the 1667 and 1665 MHz lines have been repeatedly detected for Comets Halley, Giacobini-Zinner, Thiele, and Hartley-Good. The first astronomical detection of the weak satellite line at 1720 MHz was made for P/Halley. These data promise to supply the highest signal-to-noise-ratio data over an extended period ever obtained for cometary radio observations. Analysis will provide gas production rates as a function of heliocentric distance and allow for detailed comparisons with the OH excitation model involving ultraviolet pumping. In the case of Halley, the lines appear quite symmetric, usually being centered within 0.1 km/s with respect to the nominal nuclear velocity. Line widths have been typically within 10% of 2 km/s. There is thus little evidence for asymmetric emission in the data obtained to date. It seems likely that non-steady-state models of the coma will be required to interpret the data. Evidence for significant departures from the LTE hyperfine ratios was found for Comet Giacobini-Zinner. Smaller, but nonetheless significant, deviations have been found for Comet Halley.

Irvine, W. M.↗
NASA NTRS1986

Studies of radiative transfer in planetary atmospheres

A set of high signal-to-noise spectra of the 18 cm OH emission from the comet P/Halley was obtained. During the course of the Halley OH campaign, high signal-to-noise spectra were also obtained on the comets P/Giacobini-Zinner, C/Thiele, and C/Hartley-Good. The HCN observations of Comet Halley were performed during a total of 56 individual observing sessions between 18 November 1985 and 11 May 1986. These observations represent the largest monitoring program of HCN emission from any comet, and moreover, represent the first time that a cometary parent molecule was so extensively observed. In addition to offering a direct means to monitor the production of a parent molecule from the nucleus, these data also provide fundamental information about the kinematics of parent molecules in the coma. Finally, in addition to monitoring HCN from the comet, attempts were also made to detect other molecues via millimeter-wave spectroscopy. Future plans are descussed.

Irvine, W. M.↗
NASA NTRS1986

International Ultraviolet Explorer (IUE) observation of Halley's Comet

The main objectives of the IUE Halley's Comet program are: study of the physical and chemical properties of a comet clearly different from those observed previously with IUE; study of the long-term evolution of the gas and dust coma and the ion and dust tails, by combining IUE data with UV and in-situ observations obtained by other spacecraft; and assessment of the gas and dust environment of Halley in terms of its possible impact on the trajectories of the flyby spacecraft. Another program investigates the CS scale length, and the rotational profile of CS in order to test and calibrate dynamic-flow models of comets. These observations will begin in August 1985 when the heliocentric distance of the comet is 2.8 AU and its total visual magnitude + 13. They will continue into the ninth IUE observing period, through mid-1986.

Kondo, Y.↗
NASA NTRS1986

Observations of Halley's Comet by the Solar Maximum Mission (SMM)

Solar Maximum Mission coronagraph/polarimeter observations of large scale phenomena in Halley's Comet are discussed. Observations of the hydrogen coma with the UV spectrometer are considered. It is concluded that coronograph/polarimeter observations of the disconnection event, in which the entire plasma tail uproots itself from the head of the comet, is convected away in the solar wind at speeds in the 50 to 100 km/sec range (relative to the head), and is replaced by a plasma tail constructed from folding ion-tail rays, are the most interesting.

Niedner, M. B.↗
NASA NTRS1986

The Giotto radio-science experiment

The scientific objectives of the Giotto Radio Science Experiment (GRE) are to determine the columnar electron content of Comet Halley/s ionosphere and the cometary mass fluence from atmospheric drag by using the radio signals from Giotto during the Halley encounter. The radio science data (S and X-band Doppler and range measurements) will be collected at NASA/s deep-space 64 m tracking antenna at Tidbinbilla near Canberra, in Australia. In order to separate the effects of the terrestrial ionosphere and the interplanetary plasma, S-band Doppler measurements will also be taken at Tidbinbilla along the line-of-sight of Japan/s cometary probe Sakigake during the Giotto-Halley Encounter. The measurements of cometary electron content and mass fluence will be inverted to derive the spatial distribution of the electron and mass (dust and gas) density within Halley/s coma. The GRE is the only experiment on Giotto capable of measuring the low-energy (10 eV) electron bulk population of Halley/s ionosphere and the total cometary mass flow impacting upon the spacecraft.

Edenhofer, P.↗
NASA NTRS1985

Visibility of Halley's nucleus from the Giotto spacecraft

An effort is made to estimate the visibility of the patterns on the nucleus of Comet Halley from the Giotto spacecraft through the comet's dusty coma. The visibility computation is based on the theory of contrast reduction by a scattering and absorbing medium (a planetary atmosphere). It is shown that some details may be seen on the surface of the cometary nucleus even for an optical thickness as high as 2. It is noted that the surprisingly large apparent contrast for such large optical thicknesses can be explained by the strong forward scattering by the dust.

Mekler, Y.↗
NASA NTRS1985

Amorphous and porous ices in cometary nuclei

The heat flux into the cometary nucleus before and after perihelion and its influence on the nucleus are discussed. The surface temperature and the associated rate of evaporation are lower than usually calculated, and the effect is strongly dependent on the porosity of the nucleus. The surface temperature reaches a maximum after perihelion, in agreement with statistical studies of comae and tails. Delayed heating of the CO2-rich inclusions deep in the nucleus leads to outbursts and splittings as far as 9 AU after perihelion, also in agreement with observation. At sufficiently high temperatures, heat transport in porous cometary nuclei is controlled by vapor diffusion. The presence of CO2 vapor in the pores significantly increases the heat flow in mixed nuclei at temperatures above 140-150 C. Closed pores move up the radial thermal gradient so that the outside layer of a cometary nucleus should become denser.

Smoluchowski, R.↗
NASA NTRS1985

What we do not know about cometary ices - A review of the incomplete evidence

Only two features seem rather well established about the cometary nucleus: it is an icy conglomerate that contains a large amount of volatile ices, and its major constituent seems to be water ice. The evidence on all the minor constituents is incomplete and fragmentary and there are still major difficulties in establishing a complete list of the parent molecules from the numerous radicals, ions, and atoms observed in the coma. In particular, the case of the forbidden lines of oxygen is discussed here in detail; new arguments are shown to contradict the need for CO2 as a major source of O(1D).

Delsemme, A. H.↗
NASA NTRS1985

The atmospheres of icy bodies

Ices are considered as the major volatile-carrying solid in the outer solar system. As such, they play a major role in forming atmospheres of icy satellites and the comas of comets and they have probably contributed to the enrichment of heavy elements in outer planet atmospheres. A possible role for clathrate hydrates in causing the activity of distant comets and sub-surface processes on icy satellites seems worthy of further study.

Owen, T.↗
NASA NTRS1986

Effects of the law for nongravitational forces on the precession model of comet Encke

The precession model for P/Encke formulated by Whipple and Sekanina (1979) is reexamined in the light of the recent measurements of the OH and H production rates. An optimized precession solution derived using the production law of A'Hearn et al. (1985) is shown to require unacceptably small nucleus dimensions and to offer an inferior fit to observed variations in the orientation of the comet's perihelion fan-shaped coma. Observed changes in the emission fan suggest that both the thrust on the nucleus exerted by unit mass of sublimating ice and the average lag in outgassing are substantially greater before than after perihelion. It is concluded that at this time the Whipple-Sekanina model remains the best available precession solution for this comet.

Sekanina, Z.↗
NASA NTRS1986

A survey of chromospherically active stars

Photometric and/or spectroscopic observations have been obtained of 52 late-type stars which are suspected or known to be chromospherically active. Although not all types of observations were obtained for each star, these observations include all-sky BVRI Johnson photometry, ultraviolet spectrograms, low-dispersion blue-wavelength spectrograms, and high-dispersion red-wavelength spectrograms. From the spectroscopic observations v sin i's, radial velocities, and the appearance of the Ca II H and K emission lines have been determined as well as the H-alpha line. The photometric observations indicate that chromospherically active stars have V - R and V - I color excesses. Such excesses will affect the surface fluxes determined with the surface brightness-color relationship. On the other hand all-sky BVRI photometry appears to be an excellent way to identify chromospherically active stars. A small group of moderately rapidly rotating, possibly single G8-K2 giants has been found. These stars have very modest chromospheric activity and so are not FK comae stars. A number of other unusual chromospherically active stars are identified.

Fekel, F. C.↗
NASA NTRS1986

Comet Kopff - A case for mantle development

Comet Kopff was observed during its 1983 apparition. Intensified Dissector Scanner (IDS) spectra were obtained mapping the distribution of the gases in the coma. The comet appears to have a preperihelion brightness maximum in monochromatic light. This result is shown to be model independent. These data are consistent with a partial mantle developing due to the high obliquity of the comet.

Cochran, A. L.↗
NASA NTRS1986

A slice of the universe

A preliminary discussion is presented of recent results obtained as part of the extension of the Center of Astrophysics redshift survey. Several features of the results are striking. The distribution of galaxies in the sample, which contains 1100 galaxies in a 6 deg x 117 deg strip going through the Coma cluster, looks like a slice through the suds in the kitchen sink. It appears that the galaxies are on the surfaces of bubble-like structures with diameters of 25-50/h-Mpc. The largest bubble in the survey has a diameter comparable with the most recent estimates of the diameter of the void in Bootes. This topology poses serious challenges for current models for the formation of large-scale structure. The best available model for generating these structures is the explosive galaxy formation theory of Ostriker and Cowie (1981).

De Lapparent, V.↗
NASA NTRS1986

Ion distribution functions in the vicinity of Comet Giacobini-Zinner

Photoionization of neutral molecules in the coma surrounding a comet produces heavy ions which contaminate and mass-load the solar wind. Cometary ion distribution functions in the vicinity of Comet Giacobini-Zinner (GZ) are calculated using a Monte Carlo method. The distribution function calculated behind the shock has both cold (approximately 2 keV) and hot (approximately 40 keV) components.

Cravens, T. E.↗
NASA NTRS1986

Groundbased infrared imaging of Comet Giacobini-Zinner - The distribution of dust during the ICE flyby

A thermal-infrared image of Comet Giacobini-Zinner has been used to determine the spatial distribution of dust grains in the Comet's coma. In the present paper the results of this imaging which bear on the expected impact rate of dust grains on the International Cometary Explorer (ICE) are emphasized. The expected impact rate is calculated for a range of grain radii. For particle sizes comparable to those inferred for most comets, the impact rate is on the order of several hundred per second. This rate is in agreement with the predictions by Divine (1985), but it is more than two orders of magnitude larger than the rate observed by ICE if the observed impacts were on the spacecraft body. However, the observed impact rate is roughly comparable to that inferred from the infrared imaging if, as suggested by Gurnett et al. (1986), the impacts were on the ICE antennae, which had a much smaller cross-sectional area than the spacecraft body.

Campins, H.↗
NASA NTRS1986

Modeling of dust halo formation following comet outbursts Preliminary results

Evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic - hydrodynamic method. It was found that a comet outburst resulted in significantly increased dust terminal velocities. As these higher terminal velocity values result in larger apex distances, therefore the outburst generates distinct dust envelopes in front of the regular dust coma. Such envelopes were observed at several comets (cf. comet Donati).

Gombosi, T. I.↗