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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

The grains and gas in Comet Bowell 1980b

The importance of coordinating different types of observations of a comet is illustrated in terms of spectral data on Comet Bowell. Photometric data in 1980 indicated that OH emissions were occurring when the comet was still more than 4 AU from the sun. Further measurements were arranged with the IUE satellite, the photoelectric scanner at Cerro Tololo and the IR telescope on Mauna Kea. Several sizes of diaphragms were used in order to estimate variations with respect to aperture. Calculations were made to quantify the wavelengths observed as a function of the grains' cross-sections. The OH emission was confirmed by the spectrographic and photometric data. It is suggested that sufficient dark materials are present on the cometary surface to have enhanced the absorption of solar radiation to levels sufficient for heating and dissociating surface ice particles.

Dwek, E.↗
NASA NTRS1984

Comet Bowell (1980b) - Measurement of the optical thickness of the coma and particle albedo from a stellar occultation

Comet Bowell (1980b) was observed to pass within 0.25 + or 0.09 arc sec of a star (about 540 km at the comet), where the absorption of starlight by the dust coma was found to be 3 percent (+ or - 1 percent). The implied optical thickness of 0.03 differs greatly from other determinations and gives a mass of 3 x 10 to the 13th g for the coma within 10,000 km of the nucleus. Coupled with absolute continuum filter photometry, these results indicate a very low particle albedo consistent with fluffy carbonaceous material. This experiment indicates the need to observe nearly central occultations by several observers to measure the optical thickness profile of a comet. The advantages of using a charge-coupled device area photometer for such observations are discussed.

Larson, S. M.↗
NASA NTRS1984

Comet nucleus sample return

A comet nucleus sample return mission has been proposed for implementation near the end of this century. The objective of the mission is to collect a sample of undisturbed material from beneath the surface of an active comet and return it to earth in a minimally altered state. Potential targets include the short-period comets Encke, Tempel 2 and Wild 2. This article defines such a mission and describes its requirements with regard to science, sampling, thermal protection and performance. It has been determined that, with present launch capabilities, Solar Electric Propulsion (SEP) is an enabling technology for this mission.

Feingold, H.↗
NASA NTRS1983

On the unexplained outbursts of Comet P/Tuttle-Giacobini-Kresak

The difficulty in explaining comet outbursts is reviewed, with specific attention given to the case of Comet P/Tuttle-Giacobini-Kresak. A combination of molecules and Platt particles is considered as the source, using the triggering mechanism suggested by Niedner (1980), namely the crossing of a solar magnetic-field sector boundary by the comet at the time of the outbursts. Platt particles are small (radius about 10 A, mass about 10 to the -22nd g), solid particles with an albedo near unity.

Misconi, N. Y.↗
NASA NTRS1984

Giacobini-Zinner comet: Polarimetric and physical observations

The results of observations of the Giacobini-Zinner comet on 25 and 31 October 1959 are presented. The magnitude of the comet was measured photoelectrically in two spectral regions. The radius is on the order of one kilometer. The photoelectric measurements of comets 1959b and 1957c were used to measure the abundances of the CN and C2 radicals and of solid particles in the heads.

Martel, M. T.↗
NASA NTRS1984

Extended atmospheres of outer planet satellites and comets

The cometary hydrogen particle-trajectory model was used successfully to analyze observations of Comet P/Encke. The Pioneer Venus Orbiter Ultraviolet Spectrometer observed the comet at 1216A (hydrogen Lyman-alpha) on 15 April 1984, when the comet was .58 AU from the Sun and 1.02 AU from Venus. The analysis implies a production rate at .58 AU of 3.3 x 10 to the 28th power/sec of the water molecules which photodissociate to produce the observed hydrogen.

Smyth, W. H.↗
NASA NTRS1984

Current ideas on the nature of comets

The chemical composition, emission and line spectra, and structure of comet nuclei, cometary atmospheres, and comet tails are discussed. The role of ultraviolet and infrared astronomy in defining comets is examined.

Rahe, J.↗
NASA NTRS1984

Occultations of stars and radio sources by comets: Predictions and observing prospects

The results that might stem from observations of occultations of stars and radio sources by cometary comae and ion tails are outlined. Taking account of the current orbital accuracy of P/Halley, specific predictions of stellar appulses with that comet, including preliminary topocentric circumstances, are presented. A crude model of the absorption of starlight by grains in a dusty comet supports the notion that stellar extinction may be observable in practice. Some thoughts on observing techniques are set down, discussing the merits of a two-dimensional detector (a CCD camera) and a conventional aperture photometer. The need for accurate photometric observations with time resolution approx. 1 sec is stressed because the interval of greatest extinction will usually be brief. Finally, the efficacy of undertaking accurate relative astrometry of comets during predicted approaches to stars is explored.

Bowell, E.↗
NASA NTRS1985

Dust emission of Comet Halley at large heliocentric distances

Comet Halley is currently approaching the inner solar system. Four spacecrafts (NASA's spacecraft, two Russian Vega probes and the Japanese MS-T5 spacecraft) have already been launched to encounter the comet in March 1986. Two additional Halley probes (the European Giotto spacecraft and another Japanese Planet-A probe) will be launched in mid-85 to join the armada. Observations of dust emissions from Halley's Comet are discussed. The evaporation of cometary ices causes the emission of particulates from the nucleus. These observations will be used to determine the fly-by strategy of the Giotto spacecraft by taking into account the distribution of dust in the vicinity of the nucleus and the associated hazard for the space mission.

Gruen, E.↗
NASA NTRS1985

Extended atmospheres of outer planet satellites and comets

Model analysis of the extended atmospheres of outer planet satellites and comets are discussed. Understanding the neutral hydrogen distribution in the Saturn system concentrated on assessing the spatial dependence of the lifetime of hydrogen atoms and on obtaining appropriately sorted Lyman ALPHA data from the Voyager 1 UVS instrument. Progress in the area of the extended cometary atmospheres included analysis of Pioneer Venus Layman alpha observations of Comet P/Encke with the fully refined hydrogen cloud model, development of the basic carbon and oxygen models, and planning for the Pioneer Venus UVS observations of Comets P/Giacobini-Zinner and P/Halley.

Smyth, W. H.↗
NASA NTRS1984

Radioastronomical observations of comets IRAS-Araki-Alcock (1983d) and Sugano-Saigusa-Fujikawa (1983e)

Continuum and spectral-line radio observations of the comets IRAS-Araki-Alcock and Sugano-Saigusa-Fujikawa obtained at a number of observatories during May and June, 1983, are compiled in tables and briefly characterized. The species NH3, H2O, CO, and HCN are considered in detail; visible-range observations of CN are compared; and the implications of the results and the experience gained in performing and coordinating the observations for the detection of important species in other comets, especially Halley's Comet, are explored.

Irvine, W. M.↗
NASA NTRS1984

A radar study of Comet IRAS-Araki-Alcock 1983d

Radar echoes from Comet IRAS-Araki-Alcock at wavelengths 3.54 and 12.9 cm indicate that the comet's nucleus is very rough on a scale larger than the radar wavelengths; however, the low polarization ratio (25 percent at 3.54 cm) indicates that the scattering is not dominated by multiple reflections, internal reflections, or large abundances of sharp edges, cracks, and pits. The shape of the nucleus probably departs greatly from a sphere with average radii near 3-4 km. The nucleus does not appear to look significantly different from a number of Apollo and Amor asteroids except that: (1) there is a suggestion that minor structure moves rapidly across the spectra, and (2) the debris not gravitationally bound to the comet was detected, and contributes 25 percent of the total radar cross section at the 12.9-cm wavelength. Other considerations suggest that the pole was at least 45 deg away from the line of sight on two days of observation, and that the rotation period is approximately 1-2 days.

Goldstein, R. M.↗
NASA NTRS1984

The multiple modes of interaction of the solar wind with a comet as it approaches the sun

A quasi-steady one-dimensional hydrodynamic model is used to study the various modes of interaction between the solar wind and a medium-bright water-dominated comet approaching the sun. It is found through a series of calculations that at large heliocentric distances (greater than 5 AU) the solar wind penetrates to the surface of the comet. As mass loading reaches a critical value, a standing shock wave is created in the solar wind upstream from the cometary nucleus, and as distance decreases, the cometary atmosphere becomes dense enough to stand off the solar wind ahead of the nuclear surface. It is shown that the present scenario of solar wind/comet interaction is qualitatively valid for comets with different sizes, chemical compositions and optical properties.

Mendis, D. A.↗
NASA NTRS1984

Predictions of occultations of stars and radio sources by Comet Halley

Cataloged cometary ephemerides were compared with cataloged star and radio source positions to generate a comprehensive list of expected occultations by Halley's Comet. The comet path was integrated according of Yeoman's orbit no. 12 and account was taken of gravitational and nongravitational perturbations. A coma of 22,500 km was assumed for star occultation and a 675,000 km radius was projected for radio occultations, the latter to allow for effects of an ion tail. The ephemerides were projected back along the comet track and produced a good fit with observed fixes.

Bowell, E.↗
NASA NTRS1985

Optical observations of the AMPTE artificial comet and magnetotail barium releases

The first AMPTE artificial comet was observed with a low light level television camera operated aboard the NASA CV990 flying out of Moffett Field, California. The comet head, neutral cloud, and comet tail were all observed for four minutes with an unifiltered camera. Brief observations at T + 4 minutes through a 4554A Ba(+) filter confirmed the identification of the structures. The ion cloud expanded along with the neutral cloud at a rate of 2.3 km/sec (diameter) until it reached a final diameter of approx. 170 km at approx. T + 90 s. It also drifted with the neutral cloud until approx. 165 s. By T + 190 s it had reached a steady state velocity of 5.4 km/sec southward. A barium release in the magnetotail was observed from the CV990 in California, Eagle, Alaska, and Fairbanks, Alaska. Over a twenty-five minute period, the center of the barium streak drifted southward (approx. 500 m/sec), upward (24 km/sec) and eastward (approx 1 km/sec) in a nonrotating reference frame. An all-sky TV at Eagle showed a single auroral arc in the far North during this period.

Hallinan, T. J.↗
NASA NTRS1985

To catch a comet: Technical overview of CAN DO G-324

The primary photographing of aboard STS 61-E. INTRODUCTION objective of the C.E. Williams Middle School Get Away Special "CAN DO" is the Comet Halley. As it is currently scheduled, G-324 will fly in March of 1986 The project will involve middle school students, grades 6 through 8, in the study and interpretation of astronomical photographs and techniques. -The-objectives of these studies are detailed in another paper to be presented at this symposium. 6-324 is contained in a 5 cubic foot G.A.S. Canister with an opening door and pyrex window for photography. It will be pressurized with one atmosphere of dry nitrogen. Three 35mm still cameras with 250 exposure film backs and different focal length lenses will be fired by a combination of automatic timer and an active comet detector. A lightweight 35mm movie camera will shoot single exposures at about 4 minute intervals to give an overlapping "skymap" of the mission. The fifth camera is a solid state television camera specially constructed for detection of the comet by the microprocessor. In addition to the photographic goals of CAN DO, the canister also contains a separate package of middle school student designed passive experiments. These-projects will be described in detail in the accompanying paper on educational goals.

Thomas J. O'Brien↗
NASA NTRS1986

The International Cometary Explorer (ICE) mission to Comets Giacobini-Zinner and Halley

Use of the ISEE-3 satellite (renamed ICE) to study the interaction between the solar wind and a cometary atmosphere by passing through the plasma tail by intercepting Comet Giacobini-Zinner on 11 September 1985 is described. Details of the targeting strategy are discussed. Additional scientific objectives following the tail intercept of Comet Giacobini-Zinner include the support of Comet Halley studies through the measurement of solar-wind conditions upstream of P/Halley in October 1985 and March 1986.

Brandt, J. C.↗
NASA NTRS1985

Emission by OD in comets

A calculation has been made of the complete fluorescent equilibrium of OD in sunlight including ultraviolet (A-X) transitions, vibrational and rotational transitions (A-A and X-X), and Lambda-doubling transitions in the X state. The dependence on heliocentric radial velocity is strong. No lines of OD are visible in any cometary spectra taken with IUE, but the limits vary, considerably from one comet to another. In the most favorable case, comet Austin 1982 VI, the limit is OD/OH less than 0.004. Optimum conditions for detecting OD in comet Halley are also discussed.

Ahearn, M. F.↗