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At least 1,333 records · Page 74
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Solar wind mass-loading at Comet Halley - A lesson from Venus?

Recent observations at Comet Halley show that the region within which cometary ions become the dominant component lies outside of the magnetic field-free cavity. This behavior resembles that found at Venus under conditions where the incident solar wind dynamic pressure exceeds the ionospheric pressure. On these occasions the magnetosheath magnetic field is found well inside of the region where planetary ions are observed. Although scaling and the details of formation of the inner boundary of the magnetic field are different for these two objects, the processes by which the interplanetary magnetic field penetrates into the ionospheres at Venus and at Comet Halley are in many ways analogous.

Breus, T. K.↗
NASA NTRS1987

Surface brightness profiles of 10 comets

CCD photometric observations of the comae of 10 comets, obtained at the 4-m and 2.1-m telescopes at KPNO during 1985-1986 using filters centered at 700.5, 650.0, or 546.0 nm, are reported. The data are presented in extensive tables and graphs and characterized in detail. The radial surface brightness profiles are shown to be steeper than predicted by an idealized spherically symmetric steady-state comet model, the steepness increasing with the projected distance from the nucleus. These profiles are attributed, on the basis of Monte Carlo simulations, to imperfect coupling between the sublimated gas and the optically dominant grains of the coma.

Jewitt, D. C.↗
NASA NTRS1987

The nucleus of comet P/Arend-Rigaux

Simultaneous broadband visual and infrared photometry of comet P/Arend-Rigaux is reported. Its VJHK colors are similar to other periodic comets and the class D asteroids. The visual flux decreased by 0.5 mag within 2 hr, while the thermal flux at 10 microns continued to decrease by 0.7 mag over 3 hr. These observations are consistent with thermal models of a nucleus with a geometric albedo of 0.03 and equivalent radii of 5.1 and 3.8 km.

Veeder, Glenn J.↗
NASA NTRS1987

Evidence for chain molecules enriched in carbon, hydrogen, and oxygen in comet Halley

In situ measurements of the composition and spatial distribution of heavy thermal positive ions in the coma of comet Halley were made with the heavy-ion analyzer RPA2-PICCA aboard the Giotto spacecraft. Above 50 atomic mass units an ordered series of mass peaks centered at 61, 75, 91, and 105 atomic mass units were observed. Each peak appears to be composed of three or more closely spaced masses. The abundances decrease and the dissociation rates increase smoothly with increasing mass. These observations suggest the presence of chain molecules that are enriched in carbon, oxygen, and hydrogen, such as polyoxymethylene (polymerized formaldehyde), in comet Halley.

Mitchell, D. L.↗
NASA NTRS1987

Large-scale interaction of the solar wind with comets Halley and Giacobini-Zinner

In-situ measurements of comets Halley and Giacobini-Zinner have confirmed the accepted basic physics of comet/solar wind interaction. The solar wind magnetic field is captured by the comet through the mechanism of field-line loading by cometary ions and the field lines drape around the cometary ionosphere. With this basic model in hand, the large-scale structure of the plasma tail as revealed by submissions to the Large Scale Phenomena Network of the International Halley Watch is reviewed. The turn-on and turn-off of plasma activity seem consistent with theory. Some 16 obvious disconnection events (DEs) have been recorded. Preliminary results showed agreement with the sector-boundary model; a detailed analysis of all DEs will be required in order to make a definitive statement. A study of plasma activity around the time of the VEGA encounters provides strong support for the sector-boundary model and illustrates once again the power of simultaneous remote and in-situ measurements.

Brandt, John C.↗
NASA NTRS1987

Occultation of compact radio sources by the ion tail of Halley's Comet

Enhancements of scintillations of the compact radio sources PKS 2314+03 and 1827-360 were observed at 103 MHz and 408 MHz during 18-21 December 1985 and on 29 March 1986, respectively, when the plasma tail of Halley's Comet swept across them. At 103 MHz the RMS plasma density variation along the tail was 10 and 3.3/cu cm at 0.12 AU and 0.18 AU, respectively, as measured from the comet's position. At 408 MHz it was 1.9/cu cm at 0.036 AU. Comparison of results of these two sets of observations is presented.

Alurkar, S. K.↗
NASA NTRS1987

Spatial distribution of O(supra 1D) from Comet Halley

Images of Comet Halley in (OI)6300A emission obtained using a 150 mm Fabry-Perot spectrometer in the imaging mode were combined with spectra taken in the scanning mode to deduce the distribution of cometary O(1D) within a 10 arc min field of view centered on the comet head. The 10 km/sec bandpass of the system allows the distribution to be measured with little contamination from airglow O(1D) and cometary NH2 lines in the nearby spectrum. The results are modeled to provide photodestruction scale lengths for cometary H2O and OH, the predominant parents of O(1D). Results are compared with other experimental and theoretical results.

Roesler, F. L.↗
NASA NTRS1987

The comet rendezvous asteroid flyby mission

The Comet Rendezvous Asteroid Flyby Mission (CRAF) is described. After gravity assists from Venus and Earth, the spacecraft will fly by the asteroid 46 Hestia en route to a rendezvous with P/Tempel 2 in Nov. 1996, when the comet is near aphelion. The scientific experiments for the CRAF mission are: an imaging system; a visual and infrared mapping spectrometer; an infrared radiometer; a penetrator carrying a gamma-ray spectrometer, a scanning differential calorimeter and evolved gas analyzer, accelerometers, and temperature probes; a neutral gas and thermal ion mass spectrometer; a secondary ion mass spectrometer for analyzing dust, gas, and thermal ions; a scanning electron microscope and particle analyzer; an X-ray fluorescence and gas chromatographic analyzer for collected dust and ice samples; a dust counter and velocity analyzer; a retarding potential ion mass spectrometer; a supra-thermal ion mass spectrometer and electron analyzer; a magnetometer; and a coordinated radio, electron, and plasma wave analyzer.

Neugebauer, M.↗
NASA NTRS1988

Extended atmospheres of outer planet satellites and comets

In the third year of this 3-year project, research accomplishments are discussed and related to the overall objective. In the area of the distribution of hydrogen in the Saturn system, new Voyager UVS data have been discovered and are discussed. The data suggest that both Titan's hydrogen torus and Saturn's hydrogen corona play a major role in the circumplanetary gas source. Modeling analysis of this new data establishes a strong basis for continuing studies to be undertaken in a new NASA-sponsored project. In the area of the cometary atmospheres, observational data for H, O, C, and OH acquired with the Pioneer Venus Orbiter are evaluated and preliminary modeling analysis for some of the hydrogen Lyman-alpha data is presented. In addition, the importance of collisional thermalization in spatial properties and structure of the inner and extended comae of comets has been demonstrated using the recently developed particle trajectory model. The successful simulation by this model of the hydrogen Lyman-alpha image for Comet Kohoutec near perihelion, an extreme case for collisional thermalization, is particularly noteworthy.

Smyth, William H.↗
NASA NTRS1987

Polyoxymethylene in Comet Halley

It is shown that polyoxymethylene (POM) with n about four or five explains the mass spectrum obtained with the PICCA instrument during the Giotto encounter with Comet Halley. A sequence of processes is presented showing the likelihood that POM will form under interstellar conditions. A preliminary comet coma model that includes POM is presented which predicts a qualitatively correct spectral intensity behavior when compared with observations.

Huebner, W. F.↗
NASA NTRS1987

Steepened magnetosonic waves at Comet Giacobini-Zinner

Intense MHD waves at Comet Giacobini-Zinner were examined to investigate the mode and direction of wave propagation and thereby to provide important constraints on potential mechanisms for the wave origin in the vicinity of the comet. From observations of steepened wave forms, it is found that the waves must be propagating toward the sun but are blown back across the ICE spacecraft. The correlation between magnetic field magnitude and electron density enhancements indicates that these waves are fast magnetosonic mode emissions. The sense of rotation of the partial rotations are left-hand circularly polarized in the spacecraft frame, consistent with anomalously Doppler-shifted right-hand waves.

Tsurutani, Bruce T.↗
NASA NTRS1987

The Comet Rendezvous Asteroid Flyby project

The first Mariner Mark II (MMII) mission, the Comet Rendezvous Asteroid Flyby (CRAF), is described in detail. The comet Tempe 2 has been selected as the rendezvous target. The CRAF payload will include a cometary retarding ion mass spectrometer, a cometary ice and dust experiment, a penetrator/lander, a thermal IR radiometer, and a visual IR mapping spectrometer.

Draper, Ronald F.↗
NASA NTRS1987

An examination of possible solar wind sources for a sudden brightening of Comet IRAS-Araki-Alcock

Terrestrial and near-earth measurements are examined in order to investigate possible solar wind sources for the sudden global brightenings noted in Comet IRAS-Araki-Alcock. These brightenings are not found to be associated with increases in the solar wind momentum flux, quantity of solar energetic particles, or solar activity, in contradiction to the proposal of Lutz and Wagner (1986). If a radial alignment of the IMF seen at IMP-8 after about 0800 UT was not responsible for the cometary brightening, then it is suggested that the brightening must have been intrinsic to the comet.

Russell, C. T.↗
NASA NTRS1987

Observations of HCN in Comet Halley

Observations of the HCN J = 1-0 rotational transition at 3.4 mm wavelength in comet P/Halley are discussed. The data were obtained during a total of 56 individual observing sessions between Nov. 18, 1985 and May 11, 1986, and represent the first time that a cometary parent molecule has been so extensively monitored. The HCN production rate is well correlated with the total visual magnitude of the comet, indicating that HCN follows the overall gas production. There is also evidence of time variability and variations in the HCN hyperfine ratios from their LTE values. 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.

Schloerb, F. Peter↗
NASA NTRS1987

The nature of comets

The large-scale structure, dynamics, and chemical composition of comets are examined on the basis of preliminary ground-based and space-probe data from the Halley return of 1986. Diagrams, graphs, photographs, and false-color images are provided, and data on Comet Giacobini-Zinner are included for comparison. Features noted include solar-wind interaction with magnetic-field draping around the head region, dramatic disconnection events, an irregularly shaped monolithic primarily water-ice nucleus and a dark surface, and dust with chemical composition like that of (H, C, N, O)-enriched carbonaceous chondrites.

Brandt, J. C.↗
NASA NTRS1987

Comets

Observations of comets obtained with the IUE satellite since its launch in 1978 are reviewed. The status of UV observation of comets prior to IUE is discussed, and particular attention is given to low-resolution UV spectroscopy of cometary comae, the detection of new species in the UV emission, high-dispersion spectroscopy, spatial mapping of the emissions, abundance determinations, and short-term variability. Diagrams, graphs, sample spectra, and tables of numerical data are provided.

Festou, M. C.↗
NASA NTRS1987

The composition and radial dependence of cometary ions in the coma of comet P/Halley

The heavy ion analyzer, RPA2-PICCA, on board the Giotto spacecraft detected an increase in the densities of cometary ions within a cometocentric distance of 150,000 km. The composition of cometary ions changed dramatically as the comet's nucleus was approached, but it was clearly dominated by the water group. The second and third most abundant ions identified were associated with the CO- or S-group and the CO2-group, respectively. Ions of larger atomic mass units were also present closer to the comet, and they possibly correspond to sulphur compounds and/or various hydrocarbons. Radial profiles of various groups of heavy ions and certain abundance ratios are presented. The peak density for all mass groups was detected at a cometocentric distance of about 11,000 km. A distinct boundary, where the ion velocity and temperature dropped significantly, was identified at about 27,000 km.

Korth, A.↗
NASA NTRS1987

Pick-up ions at Comet P/Halley's bow shock - Observations with the IIS spectrometer on Giotto

Gaseous material expanding from the nucleus of Comet Halley into space forms the neutral coma around the comet. Ionization in the solar UV radiation removes particles from the coma and injects them into the solar wind plasma. These freshly created ions are accelerated by the interplanetary electric field on cycloidal trajectories with gyrocenters moving with the speed of the magnetic field lines. In the solar wind frame of reference these particles move along the magnetic field lines with a fixed pitch angle. Pitch-angle scattering and energy diffusion reduce quickly the initial energy anisotropy which is associated with the narrow pick-up structures. First observations of heavy cometary pick-up ions (water group ions) at the bow shock are presented. The evolution of the distribution function in the vicinity of the shock and radial density profiles are discussed.

Wilken, B.↗