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At least 379 records · Page 21
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NASA NTRS1986

Cometary nuclei and asteroids - Has a link been found?

Data from recent spectrophotometric observations of cometary dust are summarized and compared with similar data on asteroids. A number of systematic similarities are noted between comet materials and dark reddish-black asteroids such as the Trojans. It is suggested that Trojans and comet nuclei are members of a class of outer-solar-system planetesimals with colors due to opaque carbonaceous dust containing reddish low-temperature organic condensates.

Hartmann, W. K.↗
NASA NTRS1986

Carbon monoxide in cometary ice

Carbon monoxide has been positively identified by means of ultraviolet fluorescence in the Fourth Positive System in two comets, West (1976 VI) and Bradfield (1979 X). In both cases, the spatial distribution of the emission is consistent with the CO being a 'parent' molecule, present in the cometary ice, rather than a dissociation product of a polyatomic species such as CO2 or HCOH. However, the abundance of CO relative to that of H2O is more than an order of magnitude smaller for comet Bradfield than for comet West. Further data on CO are expected with the use of more sensitive ultraviolet instruments such as the ASTRO spectrographs which will observe P/Halley in March 1986.

Feldman, P. D.↗
NASA NTRS1986

The International Cometary Explorer mission to comets Giacobini-Zinner and Halley - An update

Aspects of the International Cometary Explorer (ICE) flight to the comet Giacobini-Zinner (GZ) are discussed. The most important experiments to be performed by ICE are reviewed, and the orbital parameters of GZ are described. The dust characteristics of GZ that pose a hazard to the spacecraft are addressed, and the ICE targeting strategy toward the comet is discussed. Requested ground-based coverage of GZ is indicated, and the complementarity of the GZ coverage with that given to the Halley mission is shown.

Brandt, J. C.↗
NASA NTRS1986

Rapid pickup of cometary ions due to strong magnetic turbulence

Magnetic turbulence observed with the ICE spacecraft near Giacobini-Zinner indicates that the energy density of the fluctuating field is higher than the thermal energy density of the solar wind plasma. It is shown that in the presence of such strong turbulence the newly created ions are assimilated into the solar wind very rapidly. The time scale for the assimilation process is estimated and shown to be on the order of 100 sec which is consistent with the ICE results for the H2O(+) cometary ions.

Wu, C. S.↗
NASA NTRS1985

Charging effects in the cometary environment of Halley

Electrostatic charging of the Giotto spacecraft in different impact induced charged particle environments was studied with 3D numerical particle-in-cell models. The simulation results are assessed according to first experimental results of Giotto and Vega instruments. Decreasing and even negative spacecraft potentials measured on Vega near closest approach to the comet suggest the influence of the cometary plasma, which is also confirmed by corresponding numerical simulations.

Thiemann, H.↗
NASA NTRS1986

Comet Halley - Chapter I in cometary exploration

The information gained on the Comet Halley by the international probe studies is presented. The new information includes data on the true size and shape of the cometary nucleus and the mass of its dust grains, the chemical composition of the nucleus, and the characteristics of the 'bow wave' of the comet. The requirements of future missions for solving the many questions that are still open are discussed.

Newburn, Ray L., Jr.↗
NASA NTRS1986

A survey of bimolecular ion-molecule reactions for use in modeling the chemistry of planetary atmospheres, cometary comae, and interstellar clouds

All bimolecular positive ion-molecule reactions reported from 1965 to 1985 for temperatures below 1000 K are included in the present survey of those ion-molecule reactions pertinent to the chemistries of planetary atmospheres, cometary comae, and interstellar clouds. This survey is intended as an update of the first, by Huntress (1977). The tabular presentation is organized according to reactant ion, with cross-references for both the ionic and the neutral reactants as well as the ionic and neutral products.

Anicich, V. G.↗
NASA NTRS1986

Solar flare track densities in interplanetary dust particles The determination of an asteroidal versus cometary source of the zodiacal dust cloud

The possibility is explored whether an IDP (interplanetary dust particle) is cometary or asteroidal from measurements of the solar flare track density within its constituent mineral grains. Dust particles that are larger than 1 micron, when injected into the Solar System from comets and asteroids, will spiral into the sun due to the Poynting-Robertson effect. During the process of spiraling in, such dust particles accumulate solar flare tracks. The accumulated track density for a given dust grain is a function of the duration of its space exposure and its distance from the sun. Using a computer model, it was determined that the expected track density distributions from grains produced by comets are very different from those produced by asteroids. Individual asteroids produce populations of particles that arrive at 1 AU with scaled track density distributions containing 'spikes,' while comets supply particles with a flatter and wider distribution of track densities.

Sandford, Scott A.↗
NASA NTRS1986

Stability of the sunlit cometary ionopause

The stability of the sheared MHD flow at the sunlit cometary ionopause at several heliocentric distances is examined, taking into account the effects of ion-neutral drag, sources, curvature, and compressibility. If the ionosphere is regarded as a magnetic field-free cavity, whether it is stable or unstable to the Kelvin-Helmholtz (K-H) mode depends on the value of the velocity shear assumed. Also, for any assumed value of the shear the instability is greater for smaller wavelengths (about 100 km) than for larger (at least 1000 km). It is also shown that even if the ionopause is initially unstable, a modest penetration of the interplanetary magnetic field into the ionosphere could lead to its restabilization. Which of the three instabilities, K-H, flute, or drag, dominates depends on the relative magnitude of the shear, the rather uncertain ion-neutral drag coefficient, and the density ratio across the ionopause. The apparent marginal instability of the ionopause of comet Giacobini-Zinner and the stability of the ionopause of comet Halley are explained essentially in terms of the different solar wind conditions encountered by the two comets.

Ershkovich, A. I.↗
NASA NTRS1987

The effect of a sector boundary crossing on the cometary dust tail

Recognizing that gains in the cometary dust tail are electrically charged, the effect of an interplanetary sector boundary crossing on grain distribution is studied. Specifically, Halley's comet around the time of encounter by the GIOTTO and VEGA 1 and 2 spacecraft in March 1986 is considered. The smallest dust particles are strongly effected, and the projection of their distributions in a plane containing the sun-comet axis and normal to the orbital plane shows a wavy appearance. Also, since reversals in the interplanetary magnetic field occur with a periodicity of 5 to 10 days, the spacecraft, which follow 3 to 4 days apart, are likely to encounter entirely different dust distributions at the lower end of the mass spectrum.

Horanyi, M.↗
NASA NTRS1986

Navigating the International Cometary Explorer for encounter with Comet Giacobini-Zinner

The navigation of the International Cometary Explorer for its encounter with Comet Giacobini-Zinner, provided by orbit determination using radio metric data from the 64-m Deep Space Network antennas in California and Spain, is discussed. Orbital solutions providing predictions of 50-km geocentric delivery accuracy in the target plane were achieved using 6-12 week data arcs between periodic attitude change maneuvers. The one-sigma two-way range and range rate were found to be 40 m and 0.2 mm/s or better, respectively. Post-flight analysis shows tail centerline passage to be achieved within 10 s of the predicted time, and a geocentric position uncertainty at the encounter of better than 40 km is found.

Efron, L.↗
NASA NTRS1986

Cometary evidence for a solar companion?

It is demonstrated that a large anisotropy exists in a set of 126 cometary orbits that is manifested in a plane almost perpendicular to the ecliptic. This anisotropy would dissipate by orbital diffusion in 10 to 20 Myr, and thus must be due to a recent impulsive event in the Oort cloud. It is shown that this anisotropy cannot be due to gravitational perturbations from fast-moving stars or molecular clouds. A massive body slow enough to be bound to the solar system is the probable cause. The strip of sky centered on its presumed orbit reveals large anomalies in the ratio of retrograde to prograde comets which suggest the position of the perihelion of an eccentric orbit. It is proposed that the massive body is the solar companion Nemesis; other possibilities are discussed.

Delsemme, A. H.↗
NASA NTRS1987

Tunable ultraviolet laser studies of photon-molecular interactions of cometary interest

Most of the cometary radicals are thought to be formed in the coma by photodissociation processes. Successful modeling of the coma requires a detailed knowledge of the products that are formed from the various parent molecules and the energy partitioning among these products. This information has to be obtained as a function of wavelength, because the sun is not a monochromatic source. In this review, the status of the experimental knowledge of some key molecules is discussed, along with the prospects of some new laboratory techniques that can fill in the gaps in the present knowledge.

Jackson, William M.↗
NASA NTRS1987

Light scattering in cometary dust comae

Single and multiple scattering calculations were performed for a spherically symmetric cometary atmosphere irradiated by a plane parallel source. It is suggested that the increased flux found for anisotropic phase functions is due to the effect of directional scattering in the forward sun-comet axis. The isotropic multiply scattered flux at the surface is shown to be an increasing function of the opacity (tau) for tau of less than about 2.5. At large tau values, the maximum in the downward directed scattered flux still increases, but occurs at a height of several radii above the nucleus, resulting in a reduction at the surface. Results suggest that, except in the vicinity of the sun-comet axis, the plane parallel geometry tends to underestimate the degree of scattering.

Herman, Gary↗
NASA NTRS1987

Cometary MHD and chemistry: Application to Halley

An MHD and chemical comet coma model was applied to the plasma flow, the magnetic field, and the ion abundances in Comet Halley. By alternating iterations between axisymmetric models with detailed chemistry and three dimensional models with rigorous account of the Lorentz forces, a consistent description of the plasma flow and chemical evolution in the ionized cometary coma is derived. Models appropriate to Comet Halley in which the magnetized plasma flow, the bow shock, the magnetic cavity of size 5000 km, and the model ion abundances at 1500 km and 6000 km from the nucleus are in qualitative agreement with the Giotto data were derived. The axisymmetric model correctly shows the three groupings of ions at each distance. The model abundances for the light ions, up to 21 amu, are in very good agreement with the 1500 km observations. The comparison becomes worse at higher molecular masses and greater distances from the nucleus.

Wegmann, R.↗
NASA NTRS1987

Alternative cometary targets for the Giotto extended mission

The plan to use an Earth gravity-assist maneuver to redirect the Giotto spacecraft toward Comet Grigg-Skjellerup (GS) for an encounter in July 1992, and 2 additional cometary opportunities are discussed. The first is an encounter with Comet Hartley-2 (H2) in Aug., 1991. The H2 is intrinsically brighter than GS and the encounter would take place much closer to the Earth. The other potential target is Comet du Toit-Hartley (DTH), which is of considerable scientific interest because it split into 2 pieces during its 1979 apparition. Ths GS and H2 missions are the most viable; DTH is eliminated because its aphelion of transfer orbit and Sun distance at encounter (both 1.2 AU) are incompatible with Giotto's degraded thermal insulation.

Farquhar, R. W.↗
NASA NTRS1987

Sulphur compounds in cometary IUE spectra

From a study of cometary IUE spectra, a tentative identification of bands of the B-X systems of the SO molecule in the comet IRAS-Araki-Alcock in the 2200-2400 A region is reported, and it is suggested that sulphur compounds are more prevalent in comets than has been previously realized. S(+) and possibly SH are evident in comet Halley spectra, and new bands in the (A-X) system of CS and the (B-X) system of S2 are identified. The present results throw doubts on previous assessments of dust abundances from scattered continua in the 2900-3000 A region. The relatively strong abundance of S2 compared to OH in the Cernis and Bowell comets suggests that S2 is not well mixed and is not bound in H2O-ice.

Krishna Swamy, K. S.↗
NASA NTRS1987

Cometary plasma - A review

Results of spacecraft probing of Comet Halley and Comet Giacobini-Zinner are used to investigate cometary plasma processes and large-scale structure. Examples of tail phenomena are discussed, with emphasis on the observed disconnection events (DEs). The sector boundary/magnetic reconnection model for DEs of Niedner and Brandt, (1978) is shown to be consistent with the times of known DEs in Comet Halley as well as the magnetic polarity data available from interplanetary and near-earth satellites, and from solar data. Results of the present review have application to understanding of the solar-wind/comet interaction during the 1985-1986 apparition of Comet Halley.

Brandt, John C.↗