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At least 1,441 records · Page 80
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NASA NTRS1991

Descriptions of the neutral gas outflow in Comets P/Halley and Wilson (1987 VII) from analyses of velocity-resolved H2O line profiles

The spatial distribution and expansion velocity of the Comets Wilson (1987 VII) and pre- and postperihelion P/Halley are derived on the bases of velocity-resolved H2O spectral line profiles, using a kinematic model which synthesizes line profiles for comparison with observed line shapes. The results thus obtained demonstrate that the spherically symmetric outflow at constant velocity is a poor characterization of cometary neutral-gas outflow. While the radial dependence of the H2O expansion velocity is noted to be consistent with theoretically envisioned trends, the high H2O outflow velocity observed in Comet Wilson resists reconciliation with any existing kinematic model.

Larson, Harold P.↗
NASA NTRS1991

Comprehensive model for the nucleus of Periodic Comet Tempel 2 and its activity

A comprehensive synergistic physical model for the nucleus of Periodic Comet Tempel 2 was developed on the basis of observations carried out in 1988. The model includes the best possible estimates of the comet's bulk properties (including the dimensions and the approximate shape), information on its state of rotation, and the characterization of its activity. The model is shown to be consistent with all lines of evidence that are currently available, including relevant information from earlier apparitions.

Sekanina, Zdenek↗
NASA NTRS1991

The development of the CO(+) coma of Comet P/Schwassmann-Wachmann 1

The development of the CO(+) coma of Comet P/Schwassmann-Wachmann 1 is studied in spectroscopic observations indicating that the maximum rise time of the CO(+) gas is 1.2 days. This result implies a maximum creation time-scale for the CO(+) gas of the order of 100,000 sec at 5.8 AU. Evidence is adduced that dust events and gas events observed in this comet are not necessarily connected.

Cochran, Anita L.↗
NASA NTRS1991

Cometary ray closing rates - Comet Kobayashi-Berger-Milon

Cometary ray closing rates are measured for a sequence of a dozen plates of Comet Kobayashi-Berger-Milon (1975 IX) on July 31, 1975. The angular velocity is a factor of three to six higher than previously published in the literature for other comets. A deceleration of one to two deg/hr-squared, with no secular trend, is measured. Comparisons are made between these observations and some existing models.

Moore, E. P.↗
NASA NTRS1991

Observations of solar wind ion charge exchange in the Comet Halley coma

Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) observations of solar wind ions show charge exchange effects and solar wind compositional changes in the coma of Comet Halley. As the comet was approached, the He(2+) to proton density ratio increased from 2.5 percent in the solar wind to about 4 percent about 1 hr before closest approach after which time it decreased to about 1 percent. Abrupt increases in this ratio from 2.5 to 4.5 percent were also observed in the beginning and near the end of the so-called Mystery Region. These abrupt increases in the density ratio were well correlated with enhanced fluxes of keV electrons as measured by the Giotto plasma electron spectrometer. The general increase and then decrease of the He(2+) to proton density ratio is quantitatively consistent with a combination of the addition of protons of Cometary origin to the plasma and loss of plasma through charge exchange of protons and He(2+).

Fuselier, S. A.↗
NASA NTRS1991

Implications of C2H photochemistry on the modeling of C2 distributions in comets

Laboratory studies of the secondary photolysis of the C2H radical are summarized and used to explain some discrepancies between models of C2 emission in comets. These studies show that several states of the C2 radicals produced in the photolysis of C2H2 at 193 nm have bimodal rotational distributions when plotted as a Boltzmann diagram. They also establish that the C2 radicals are formed with varying degrees of vibrational excitation, so that if they are formed in a similar manner in comets, the C2 radicals must start out with this initial vibrational excitation.

Jackson, William M.↗
NASA NTRS1991

Evolution on large-scale plasma structures in comets: Kinematics and physics

The disconnection event (DE) consists of the periodic loss of a comet's entire plasma tail and the growth of a new one. This spectacular phenomenon is not understood. The strategy was to assemble a data base of specific events studied in detail, determine the solar wind conditions responsible for DEs, and develop a consistent physical model. Analysis is complete for the sequence of DEs that took place during 13 to 18 April 1986. The first DE correlates well with a sector boundary crossing for the comet and a group of DEs that occurred approximately a day later could have produced by polarity reversals seen in the IMP-8 data. Thus, these DEs are consistent with the frontside, magnetic reconnection mechanism.

Brandt, John C.↗
NASA NTRS1991

Radar interferometric studies of comets

Our objectives are to use radio interferometry to study composition, velocity distribution, maser excitation, and plasma interactions of cometary gas. Two new cometary chemistry programs were started with radio interferometers: (1) the VLA used to search for HC3N emission from Comet Brorsen-Metcalf at 3.3 cm wavelength; and (2) the BIMA millimeter array used to observe Comet Austin in HCN.

Snyder, Lewis E.↗
NASA NTRS1991

Comet and asteroid dynamics

In order to provide the ground-based observing community and NASA flight projects with accurate comet and asteroids ephemerides, improvements are being made to the existing dynamic models and new data type are being investigated. For active comets, non-gravitational forces must be taken into account; these forces are assumed due to the rocket-like thrusting of outgassing cometary ices.

Yeomans, Donald K.↗
NASA NTRS1991

Radiative transfer in planetary atmospheres: Submillimeter-wave spectroscopy of comets

During 1990, a significant step was made in asertaining the molecular composition of comets by our Planetary Astronomy group at the University of Massachusetts. We obtained several exciting new detections of submillimeter wave spectral lines from molecules in the coma of Comet Levy (1990c) using the 10m telescope of the Caltech Submillimeter Observatory, located on Mauna Kea. The molecules HCN, formaldahyde, and methanol were all detected in abundances that make them important minor constituents of the nucleus. Moreover, the emission was so strong that, for the first time, it was possible to map the distribution of these species in the coma and study their behavior as they flow outwards from the nucleus.

Schloerb, F. Peter↗
NASA NTRS1991

Observations of formaldehyde and search for cyanoacetylene in Comet Brorsen-Metcalf (1989o)

The Very Large Array (VLA) was used in September 1989 to search Comet P/Brorsen-Metcalf (1989o) for the 1 sub 11-1 sub 10 transition of formaldehyde (H2CO) at 4,829.659 MHz and for the J equals 1-0, F equals 2-1 rotational transition of cyanoacetylene (HC3N) at 9098.3321 MHz. A new technique was used in reducing the data. Data blocks which were either 3x3 pixels, 5x5 pixels, or 9x9 pixels were examined for a signal from H2CO. Using this approach, different pixel clusters within the field of view can be sampled to optimize the coupling of the synthesized beam to the gas distribution. HC3N is of immediate interest as a cometary molecule because it may be a reservoir of carbon and a source of cometary CN. Our search for HC3N emission at 3.3 cm wavelength demonstrated that for this molecule the VLA can be expected to reach significant levels of sensitivity in many comets.

Snyder, Lewis E.↗
NASA NTRS1992

Mass loading at Titan and comets

Present understanding of the cometary mass loading process (the addition of newborn ions to high speed magnetized plasma flows) is explored and the cometary lesson is applied to the environment of Titan. Mass loading is the dominant physical process controlling the interaction of the solar wind with comets, and it might play an important role in the interaction of Titan's neutral torus and exosphere with Saturn's magnetosphere. Freshly born ions follow cycloidal trajectories, resulting from the superposition of gyration and E/B drift. In the plasma frame, the resulting velocity space distribution is a ring beam distribution at comets and a ring distribution at Titan. This difference in free energy is expected to generate significantly different levels of low frequency broad band electromagnetic turbulence and, thus, result in important differences in the implanted ion distributions. It is predicted that mass loading creates a hot, equatorially trapped plasma population at Titan.

Gombosi, Tamas I.↗
NASA NTRS1991

Energetic water-group ions at Comet Giacobini-Zinner - An overview of observations by the EPAS instrument

An overview is presented of the observations of energetic (45-480 keV) cometary water group ions made by the EPAS instrument on the ICE spacecraft during the fly-by of Comet Giacobini-Zinner in September 1985. The discussion covers ion observations in the large-scale ion pick-up region surrounding the comet on about 10 exp 6 km spatial scales, observations of the cometary shock at about 100,000 km and the region of slowed mass-loaded flows which occur downstream, and the finite gyroradius effects which occur near the closest approach.

Cowley, S. W. H.↗
NASA NTRS1991

Comet P/Halley - Spatial distributions and scale lengths for C2, CN, NH2, and H2O

Long-slit spectroscopic observations of Comet P/Halley on 12 dates between October 1985 and May 1986 were used to extract spatial profiles for C2, CN, NH2, and O I (1D) (which effectively map out the comet's H2O distribution), and Haser model scale lengths were fitted to these data. The preperihelion analysis yielded Haser model scale lengths of sufficient consistency that they could be used for the production rate determinations. It was found that the average parent scale lengths found for NH2 were consistent with photodissociation lifetimes for NH3 and confirm the dissociation chain NH3-NH2-NH.

Fink, Uwe↗
NASA NTRS1991

Quasi-linear pitch angle and energy diffusion of pickup ions near Comet Halley

The process of ion pickup in the environment of Halley's comet is studied in order to see if velocity diffusion driven by the observed level of turbulence can explain the observed development of the implanted ion distribution. The theoretical description used is based on a quasi-linear approach and considers the implantation and transport of cometary ions along solar wind flow lines. To make such a study requires some way of extrapolating the measurements on the Giotto trajectory into the upstream region; models for mass loading and turbulence are used. A simplified kinetic equation describing the source, convection, and quasi-linear velocity diffusion of the heavy cometary ions is solved numerically along flow lines parallel to the sun-comet line. Full two-dimensional (pitch angle and velocity) distributions are obtained at positions along the Giotto trajectory, which are compared with measurements. This study finds that quasi-linear theory, with the empirical model for the observed turbulence level, produces the right order of pitch angle diffusion.

Huddleston, D. E.↗
NASA NTRS1992

The contribution of methanol to the 3.4 micron emission feature in comets

A fluorescence model for cometary emission in the nu2, nu3, and nu9 bands of methanol is described. This model is used in conjunction with observed spectra of several comets to determine the possible contribution of CH3OH to the frequently seen 3.4-micron 'cometary organic' feature. For Comets P/Halley and Wilson, methanol production rates are retrieved from the flux in the nu3 band centered at about 3.52 microns.

Reuter, Dennis C.↗
NASA NTRS1992

CN jet velocity in Comet P/Halley

The projected expansion velocity of Comet Halley's coma is presently determined in CN jet images that were processed with a radial profile subtraction technique. It is established that the flow is approximately radial, and that the velocity can also be determined by measuring specific radial features in jets. The largest projected velocity is 1.7 +/- 0.3 km/sec, at 50,000 km from the nucleus. This is the largest expansion velocity found for this region of the Comet Halley coma. Comparisons are made with other observations and theoretical models.

Klavetter, James J.↗
NASA NTRS1992

Helium and argon abundance constraints and the thermal evolution of Comet Austin (1989c1)

Far-UV rocketborne spectrometer observations have established upper Ar and He abundance limits on Comet Austin. These limits indicate, relative to solar abundance, that Comet Austin is depleted in He/O and no more than 30-times enriched in Ar/O. Implications of these values for thermal conditions experienced during formation and evolution are discussed.

Stern, S. A.↗