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At least 1,243 records · Page 69
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NASA NTRS1985

CCD observations of comet Tuttle 1980 XIII - The H2O(+) ionosphere

A CCD spectrum of comet Tuttle 1980h has been analyzed with emphasis on the emission of H2O(+) ions. The fine angular resolution (1.5 arcsec) and the capability of absolute brightness calibration of the CCD instrument made it possible to determine the spatial concentration of the H2O(+) ions of this faint comet and the total number of these ions confined within a spherical region. Solar photoionization of the H2O atmosphere can account for the production of the H2O(+) ions observed in the confined region. The dimension of the H2O(+) ionosphere at the time of observation was found to be comparatively small, implying the possible existence of an ionopause sharply delineating the extension of the cometary ionosphere.

Ip, W.-H.↗
NASA NTRS1985

The nondetection of continuum radiation from Comet IRAS-Araki-Alcock (1983d) at 2- to 6-cm wavelengths and its implication on the icy-grain halo theory

The Comet IRAS-Araki-Alcock was observed with the VLA at 6 and 2 cm, when the comet was at geocentric distances of 0.08 and 0.035 AU, respectively, and the results are discussed. The three sigma upper limits to the flux density are 90 and 750 micro-Jy at the two wavelengths, respectively, values fully two orders of magnitude below the flux densities predicted by the icy-grain halo theory as initially developed. The corrected theory also indicates that the icy grain halo theory does not give an adequate description of the cometary environment. It is shown that the halo is either very optically thin, with a filling factor of the order of 10 to the -5th, or that the maximum size of the grain that can be lifted off the nuclear surface is only of the order of 10-100 microns.

De Pater, I.↗
NASA NTRS1985

Archive of observations of periodic comet Crommelin made during its 1983-84 apparition

This is an archive of 680 reduced observations of Periodic Comet Crommelin made during its 1984 apparition. The archive integrates reports by members of the eight networks of the International Halley Watch (IHW) and presents the results of a trial run designed to test the preparedness of the IHW organization for the current apparition of Periodic Comet Halley.

Sekanina, Z.↗
NASA NTRS1986

The hydrogen coma of Comet Halley before perihelion: Preliminary observations with dynamics Explorer 1

The hydrogen coma of Comet Halley has been observed in resonantly scattered solar Lyman-alpha radiation during the period 1-29 January 1986 as the comet approached perihelion. These observations were obtained with the imaging photometer for vacuum-ultraviolet wavelengths on the spacecraft Dynamics Explorer 1. For the initial analysis of observations available in 17 orbits distributed throughout the period, least-squares fits are computed for the observed exponential decrease in brightness with radial distance from the nucleus. Brightness at the nucleus increased from approx. 3 to 17 kR during the observing period. Preliminary analysis yields water production rates of approx. 3.6 x 10 to the 29th power and 1.9 x 10 to the 30th power molecules/sec on 1 and 29 January, respectively.

Craven, J. D.↗
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 Comet Halley ephemeris development effort

Nongravitational forces affecting Comet Halley's motion are discussed. Analysis of the comet's orbit since 1982 is described. The astrometry network of the International Halley Watch is introduced. Ephemeris (with perturbations) from 1 January 1985 through 30 June 1986 are listed.

Yeomans, D. K.↗
NASA NTRS1985

The origin of comets - Implications for planetary formation

Primordial and episodic theories for the origin of comets are discussed. The implications of the former type for the origin of the solar system are considered. Candidate sites for the formation of comets are compared. The possible existence of a massive inner Oort cloud is discussed.

Weissman, P. R.↗
NASA NTRS1985

Orbital motion, nucleus precession, and splitting of periodic Comet Brooks 2

Results of ten nongravitational orbital solutions are given for Comet Brooks 2, based on observations from the apparitions 1889-1980. The time history of the nongravitational parameters, including the transverse component A-sub-2, is vital for modeling the nucleus precession. The comet is obviously a fragment of a once larger object that split during a close encounter with Jupiter in 1886. Factors possibly responsible for the breakup are examined. A similarity between the orbital histories of P/Brooks 2 and P/Wild 2 is noted.

Sekanina, Z.↗
NASA NTRS1985

Comet Encke - Gas production and lightcurve

A comprehensive set of observations, both from the ground and with the IUE, was planned for the 1984 apparition of Comet Encke. The observations were intended to confirm the behavior seen in 1980 and to study the behavior of the comet after perihelion. The results of the observations indicate that all the measured trace species display an asymmetry around the perihelion that is consistent with the visual light curve (VLC). But the total gas production as monitored by OH (the dominant species) displays a behavior that has no relation to the VLC.

Ahearn, M. F.↗
NASA NTRS1985

Spectrophotometry of seventeen comets. II - The continuum

One-hundred-twenty IDS scans of the continua in 17 comets are analyzed to determine dust production rates and color as a function of heliocentric distance. Improved theory indicates that the dust loading of gas typically varies between 0.05 and 0.3 by mass (assuming a geometric albedo of 0.05 and oxygen expansion at 1 km/s) except during outbursts, when it rises much higher. P/Encke near perihelion falls much lower yet, to 0.004 or less. Dust loading is not always constant as a function of time in a given comet. Dust color is typically reddish, as has often been noted before.

Newburn, R. L., Jr.↗
NASA NTRS1985

Radar observations of asteroids and comets

Radar techniques for the observation of asteroids and comets are reviewed, emphasizing the logical basis for inferring physical properties from radar measurements. Results to date are reviewed, focusing on some recent highlights of the research to demonstrate the synergism between radar and other ground-based techniques. Particular attention is given to the asteroids 2 Pallas, 16 Psyche, 2101 Adonis, and the comet IRAS-Araki-Alcock.

Ostro, S. J.↗
NASA NTRS1985

Dark grains in comet Crommelin

Comet P/Crommelin (1983n) was observed at 1-20 microns at the NASA IRTF March 29-April 1, 1984. The thermal emission spectrum fits a black body of about 290 K, cooler than other comets observed at 1 AU. The ratio of thermal to scattered radiation indicates that these grains were extremely dark with a geometric albedo of about 0.015 at J(1.25 microns) and 0.02 at K(2.25 microns). The 10 micron flux distribution on April 1 was very asymmetric, decreasing rapidly on the sunward side, less than 6000 km from the nucleus, and extended in the north and east. The spatial distribution and photometric data suggest that large grains emitted from the nucleus at very low velocity were viewed.

Hanner, M. S.↗
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

CO(+) fluorescence in comets

Calculations of the fluorescent equilibrium of the CO(+) molecule to determine the Swings and Greenstein effects as a function of heliocentric radial velocity have been carried out. The previously ignored Baldet-Johnson (B-A) bands were included in addition to the comet tail (A-X) and first negative (B-X) bands, thus allowing a determination of the absolute intensities of all bands. As with certain other molecules such as OH and CN, the Swings effect on individual lines is large enough that integrated band fluxes in the comet tail system also exhibit the Swings effect. The flux variations for velocities in the range - 50 to + 350 km/s have been calculated and may be used to map the acceleration of CO(+) ions into the tail, even with low dispersion spectra.

Magnani, L.↗
NASA NTRS1986

The bow wave of Comet Giacobini-Zinner - ICE magnetic field observations

Fitting of a Mach 2 shock surface to the ICE magnetic field data obtained near Comet Giacobini-Zinner has provided subsolar bow wave distances that infer neutral gas outflow rates comparable to previous measurements, and orientations of the bow wave symmetry axis that are consistent with the plasma measurements and motion of the comet relative to the solar wind. Mach values of 1.5-2 and transition thicknesses of the order of 10,000 km are inferred when the field magnitude and variance data are compared. Cross spectra of the transverse field components in and near the bow wave exhibit a peak near 0.01 Hz, or near the cyclotron frequency of ions from the water group. However, the level of turbulence is not consistent with that observed for similar configurations at planetary bow shocks.

Jones, D. E.↗
NASA NTRS1986

Large amplitude, low frequency plasma fluctuations at Comet Giacobini-Zinner

Very large amplitude fluctuations in electron density, temperature, and flow velocity were a prominent aspect of the solar wind interaction with Comet Giacobini-Zinner during the ICE encounter in September 1985. These fluctuations were detected at a distance of at least 900,000 km and grew in amplitude as ICE approached the comet, peaking in amplitude 60,000 km from closest approach. A typical period associated with the fluctuations was about 2 min, which corresponds to a scale length in the solar wind frame of about 50,000 km. For the most part these fluctuations appear to be a product of one or more plasma instabilities associated with the solar wind pick up of cometary ions.

Gosling, J. T.↗
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

The Comet Halley dust and gas environment

Quantitative descriptions of environments near the nucleus of comet P/Halley have been developed to support spacecraft and mission design for the flyby encounters in March, 1986. To summarize these models as they exist just before the encounters, the relevant data from prior Halley apparitions and from recent cometary research are reviewed. Orbital elements, visual magnitudes, and parameter values and analysis for the nucleus, gas and dust are combined to predict Halley's position, production rates, gas and dust distributions, and electromagnetic radiation field for the current perihelion passage. The predicted numerical results have been useful for estimating likely spacecraft effects, such as impact damage and attitude perturbations. Sample applications are cited, including design of a dust shield for spacecraft structure, and threshold and dynamic range selection for flight experiments. It is expected that the comet's activity may be more irregular than these smoothly varying models predict, and that comparison with the flyby data will be instructive.

Divine, N.↗