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At least 847 records · Page 47
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NASA NTRS1986

On the brightness variations of Comet Halley at large heliocentric distances

Sporadic variations of its intrinsic brightness of up to 500 percent, with time scales as short as a few hours, has been exhibited by Halley's comet at large heliocentric distances (11-8 AU). It is shown that many of these brightness enhancements are closely correlated to the encounter of high-speed solar wind streams by the comet. It is proposed that during such periods, the night side of the comet gets charged to numerically large negative electrostatic potentials, with consequent electrostatic levitation and blow-off of fine charged dust grains lying on it. This gives rise to the observed brightness variations.

Flammer, K. R.↗
NASA NTRS1986

Comment on the paper 'On the influx of small comets into the earth's upper atmosphere. II - Interpretation'

Frank et al. (1986) argue that, on the basis of dayglow observations, about 20 small comets of mass roughly 100,000 g hit the earth each minute. The proposed comets appear to require dust mantles; for otherwise they would evaporate quickly and contribute to the solar wind. Such an enrichment is not observed. An order-of-magnitude argument based on comet sizes, densities, and meteoritic dust influx is used here to show that the dust mantles can have a maximum thickness of about 200 microns. A reasonable estimate gives a thickness of about 0.2 micron. The mantles are thus too thin to suppress evaporation.

Frank, L. A.↗
NASA NTRS1986

The flight and impact dispersion of the Comet Nucleus Penetrator - A six-degree-of-freedom simulation

The flight dynamics of a Penetrator probe which, as part of a Comet Rendezvous and Asteroid Flyby mission will impact a known periodic comet, possibly Hempel 2, and penetrate beneath the surface to obtain measurements, is discussed. The mission is described, including the penetrator-separation dynamics, the dynamics during initial coast and rocket thrust phase, and the final coast to impact. The effects on the results that may be caused by the comet nucleus gravity field variations, uncertainty in the spacecraft state vector, the spacecraft pointing accuracy, initial separation errors, and rocket motor thrust errors are examined.

Mascy, A. C.↗
NASA NTRS1986

Coma morphology and dust-emission pattern of periodic Comet Halley. IV - Spin vector refinement and map of discrete dust sources for 1910

A hypothetical continuous ejection of dust from discrete emission sources on the sunlit hemisphere of a rotating comet nucleus is tested against 34 digitally processed plates of Comet Halley over the May 5-June 6, 1910 period, as well as the reported projected lengths of six antisunward jets observed between May 14 and June 6, 1910. A map of the comet's nucleus is presented of whose 13 discrete dust sources only two can be modeled as point-like regions. Attention is given to curious cases of 'chain' activation in which the sources that intersect one another on the surface emit dust sequentially, in successive rotations.

Sekanina, Z.↗
NASA NTRS1986

CCD scanning for asteroids and comets

A change coupled device (CCD) is used in a scanning mode to find new asteroids and recover known asteroids and comet nuclei. Current scientific programs include recovery of asteroids and comet nuclei requested by the Minor Planet Center (MPC), discovery of new asteroids in the main belt and of unusual orbital types, and follow-up astrometry of selected new asteroids discovered. The routine six sigma limiting visual magnitude is 19.6 and slightly more than a square degree is scanned three times every 90 minutes of observing time during the fortnight centered on New Moon. Semiautomatic software for detection of moving objects is in routine use; angular speeds as low as 11.0 arcseconds per hour were distinguished from the effects of the Earth's atmosphere on the field of view. A typical set of three 29-minute scans near the opposition point along the ecliptic typically nets at least 5 new main-belt asteroids down to magnitude 19.6. In 18 observing runs (months) 43 asteroids were recovered, astrometric and photometric data on 59 new asteroids were reported, 10 new asteroids with orbital elements were consolidated, and photometry and positions of 22 comets were reported.

Gehrels, T.↗
NASA NTRS1986

Study of comets at wavelengths between 0.5 and 18 microns

Since 1969 every bright comet was measured which has reached a distance of one astronomical unit from the sun. Observing techniques were developed which make it possible to observe as close as elongation 3 deg., and to find, track and measure comets at all wavelengths in full daylight. Comet Halley was measured on twenty four occasions between December 12, 1985 and March 25, 1986. Clear weather seems to come on holidays, and days on which are observed including Winter Solstice, Christmas, New Years, Valentines day, Super Bowl Sunday and the Vernal Equinox. Halley is observed within hours of perihelion passage.

Net, E. P.↗
NASA NTRS1986

IUE observations of Comet Halley: Evolution of the UV spectrum between September 1985 and June 1986

The UV spectrum of comet P/Halley (1982 i) was monitored with the IUE at regular time intervals. A complete characterization of the UV spectrum of the comet was obtained to derive coma abundances and to study the light emission mechanisms of the observed species. The Fine Error Sensor (FES) camera of the IUE was used to investigate the coma brightness variability on short time scales. Spectroscopic observations as well as FES measurements show that the comet nucleus activity was highly variable.

Feldman, P. D.↗
NASA NTRS1987

Crater production on Venus and Earth by asteroid and comet impact

New calculations of the collision probabilities of asteroids and comets with Venus were carried out based on the orbits of the known Venus-crossing asteroids and comets. For comparison, asteroid and comet collision probabilities and cratering rates on the Earth and Moon were recalculated and the estimated cratering rates on Venus were normalized to those of the Earth.

Shoemaker, Eugene M.↗
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

Model ion abundances for Comet Halley

A computer model for comet comae was applied to predict ion abundances for comet Halley, assuming that the volatile component of the nucleus is 85% water and that the remaining volatile molecules are composed of carbon, nitrogen, oxygen, and sulfur. Model parameters such as heliocentric distance, size, and albedo are chosen to be consistent with the 13 to 14 March 1986 encounter of Giotto with the comet. Photoprocesses, gas-phase chemical kinetics, coma energy balance including a separate electron temperature, multifluid hydrodynamics with a transition to free molecular flow, fast streaming atomic and molecular hydrogen, and counter and cross streaming of species in the coma-solar wind interaction are taken into consideration. A comparison of the model results with data from the ion mass spectrometer at 6000 and 1500 km from the nucleus shows good agreement. Implications of the coma physics and chemistry are discussed.

Huebner, W. F.↗
NASA NTRS1986

HCN production from Comet Halley

Observations of the HCN J=1-0 rotational transition at 3.4 mm wavelength in comet P/Halley with a 14m antenna were obtained during 56 individual observing sessions between Nov. 1985 and May 1986. The HCN production rate is well correlated with the total visual magnitude of the comet, indicating that HCN follows the overall gas and dust production. However, comparison of HCN production to the total gas production of the comet indicates that it is a relatively minor consitiuent with 0.1% of the abundance of H2O. Spectra obtained by binning the HCN data with heliocentric distance show that the HCN line width, and thus the parent outflow velocity, increases with decreasing heliocentric distance, and that there is a tendency for the lines to be blue shifted as expected from the observed anisotropic outgassing from the nucleus. There is evidence of day-to-day time variability in the total HCN emission and variations in the HCN hyperfine ratios from their LTE values.

Schloerb, F. Peter↗
NASA NTRS1986

Observational factors affecting studies of Comet P/Halley's visual light curve

Over 5000 visual magnitude estimates of comet Halley were used as a data base for studying the factors which affect the distribution of visually estimated total magnitudes of comets. Observations made on eight selected days were studied. It is concluded that scatter in the magnitude data is a function of (in order of importance) observer experience, comet morphology, and instrumentation used. Experienced observers generally report the brightest magnitude estimates.

Edberg, Stephen J.↗
NASA NTRS1986

Dust environment of Comet Halley

Advances in the understanding of comet Halley's dust environment are highlighted. Spacecraft results on the dust composition, the detection of subfemtogram grains, and the particle-mass distribution; jets and the dust-emission pattern; the existence of discrete sources on the nucleus surface and the determination of their locations from day-to-day motions of coma features; CN jets; large-scale phenomena, including an antitail, streamers, and a sunward spike; infrared observations, such as the 3.4 microns emission feature, applications of array detectors, and rare far-infrared data; the chemical modification of low-temperature ices by UV and cosmic-ray irradiation processes, their laboratory simulations, and applications to the evolution of comets; the interstellar-connection models of comet origin; and the formation of a mantle made of refractory organic material on the nucleus surface.

Sekanina, Z.↗
NASA NTRS1986

Dynamic pressure model derived from an observation by Sakigake for Comet Halley on 31 December, 1985

An outstanding disconnection event (DE)-like knot was observed on 31 Dec. 1985 in P/Halley's tail. Analysis of the Sakigake/IMF data reveals that comet Halley did not encounter the heliospheric neutral sheet on the day, demanding a new explanation of the DE-like event. During this event, the comet encountered a high-speed solar wind from a coronal hole tongue of the Sun. The event can be explained by a dynamic pressure model, according to which the DE-like plasmoid was caused by a sudden increase in the dynamic pressure of the solar wind. A simulation result is found to support this interpretation. The dynamic pressure model for a comet can be compared with the mechanism of a possible geotail disturbance during a spacecraft triggered auroral substorm.

Saito, Takao↗
NASA NTRS1986

Measurements of thermal ion energy spectra from the Giotto encounter with Comet Halley

Ram ion measurements made by the Giotto neutral mass spectrometer show the spatial evolution of the composition and energy spectrum of coma ions. Near the nucleus the plasma is cold and has many constituents, but at distances of the order of 200,000 km the main ions are the terminal products of water dissociation, H(+) and O(+). The ion velocity distribution at the time of the Giotto encounter is highly anisotropic. Ion flow is generally away from the comet with energies increasing with distance to the order of 100 eV in the comet frame of reference. Similarities in the proton and oxygen ion energy spectra in the comet frame suggest an acceleration mechanism other than v x B pickup.

Hodges, R. R.↗
NASA NTRS1986

A comparison of the continuum colors of P/Halley, other comets, and asteroids

Early observations (1985 Feb.) of P/Halley were made with broadband BVJK filters when a substantial fraction of the light came from the nucleus; the resulting colors appear red, but with signal-to-noise ratios less than or = 5. More recent observations were made with the narrower International Halley Watch continuum filters when the light from the comet is dominated by the coma. The colors over the wavelength range 0.3 to 0.7 microns are more neutral. Continuum colors were also observed for other comets. All show moderately red to very red colors that can be best matched by asteroids of the P and D spectral classes, suggesting that the refractory component of comets is similar to that of the more distant asteroids.

Tholen, D. J.↗
NASA NTRS1987

A magnetohydrodynamic model of the interaction of the solar wind with the Jovian magnetosphere and a magnetohydrodynamic simulation of the interaction of the solar wind with the out flowing plasma from a comet

A three-dimensional code for a rapidly rotating magnetosphere in which the MHD equations and the Maxwell equations were solved by using the two step Lax Endroff scheme, was developed. Preliminary results were presented at the Fall AGU meeting in San Francisco. The basic simulation model to study the solar wind interactions was adapted to other bodies in addition to Jupiter. Because of the recent comet flybys, a comet was chosen as the first model. The aim was to model the formation of the contact surface and the plasma tail. Later, work was begun on a three-dimensional model which would include the effects of mass loading. This model was designed to study the weak cometary bow shocks observed by the probes to comets Halley and Giacobini-Zinner. The model was successful in reproducing the position and shape of the bow shock which was determined by using observations from the Suisei spacecraft.

Walker, R. J.↗
NASA NTRS1986

Brightness asymmetry in comet Kopff

Observations of comet Kopff, obtained at 350-650 nm with resolution 1.15 nm using an IDS spectrograph on the 2.7-m telescope at McDonald Observatory during the 1983 apparition, are reported, summarizing the results of Cochran (1985). The data are presented in tables and graphs and analyzed using two models: the one-dimensional nonequilibrium-chemistry code developed by Cochran (1982 and 1985) for comet Stephan-Oterma, or the simpler model of Haser (1957). A brightness asymmetry in comet Kopff is attributed to preperihelion gas excess and postperihelion gas depletion.

Cochran, A. L.↗