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At least 1,333 records · Page 74
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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.↗
NASA NTRS1987

Plasma structures in comets P/Halley and Giacobini-Zinner

An overview of large-scale plasma phenomena is presented based on results of spacecraft probing of comets Halley and Giacobini-Zinner and on worldwide submissions to the Large-Scale Phenomena Discipline Specialist Team of the International Halley Watch. Examples of tail phenomena and science are presented with emphasis on observed disconnection events. The archive of this material will clearly be very valuable for studying the comet/solar-wind interaction during the 1985-1986 apparition of Halley's comet.

Brandt, J. C.↗
NASA NTRS1987

Activity of comet P/Halley 23-25 March, 1986 - IUE observations

A large but gradual increase in the brightness of the emission bands of comet Halley and, thus, in the coma abundance of OH, CS, CO2(+), and dust was observed with the International Ultraviolet Explorer (IUE) satellite from Mar. 23 to 25, 1986. This brightness change was also monitored by the Fine Error Sensor (FES) tracking and acquisition camera at a higher temporal resolution than that of the spectrophotometric measurements. The amplitude of the increase varied among the species, depending on the lifetime of the parent molecule. By comparing the FES light curve with the optical light curve measured in the light of C2 by Millis and Schleicher (1986), it can be shown that this increase in brightness corresponds to part of the periodic light curve of the comet. None of the IUE observations show evidence of the large increase in H2O reported to have occurred on March 24.7 by Weaver et al. (1986). It is concluded that the activity reported here is controlled primarily by photodissociation and ionization of molecules ejected form an active area of the nucleus that rotates into the field of view of the sun approximately every 7.4 days.

Mcfadden, L. A.↗
NASA NTRS1987

A rotational-state population analysis of the high-resolution IUE observation of CS emission in comet P/Halley

The high resolution spectrum of comet P/Halley has been measured in the region of the CS emission. CS in comet Halley is produced by photodissociation of CS2 and subsequently reexcited by broad band solar radiation. An inversion procedure is introduced to extract the rotational population distribution from the dispersed fluorescence spectrum. Intensity anomalies in the wavelength region corresponding to P and Q branch emission are considered.

Prisant, M. G.↗
NASA NTRS1988

An investigation of the nucleus and coma of Comet P/Arend-Rigaux

Narrow-band photometry of Comet P/Arend-Rigaux in the visible and near-ultraviolet are presented for six nights during the 1884/1985 apparition, including three nights of simultaneous observations in the thermal infrared made with the NASA IRTF. The results indicate a period of rotation of 13.47 hr, dimensions for the nucleus of 13 x 8 x 8 km (under the assumption of prolate spheroid shape), and a steady decrease in the comet's gas production from a month before perihelion to more that two months after perihelion. Results for the nucleus and the coma suggest that the nucleus is totally covered with a nonvolatile mantle.

Millis, Robert L.↗
NASA NTRS1987

Comet Rendezvous Asteroid Flyby - First Mariner Mark II

The Mariner Mark II, a novel, low-cost series of spacecraft, will find its initial use during the Comet Rendezvous Asteroid Flyby (CRAF) Project; the spacecraft is expected to obtain the first observations of asteroids, and to return over three years' worth of detailed science data from the vicinity of Comet Tempel 2. The second Mariner Mark II spacecraft, designated 'Cassini' will incorporate an ESA Titan probe. Attention is given to a possible solar/electric earth gravity-assist trajectory for a mission to Tempel 2.

Draper, Ron↗
NASA NTRS1987

Rocket ultraviolet observations of Comet Halley

Ultraviolet observations of Comet Halley have been obtained in February and March, 1986 with two instrument payloads, one with the Faint Object Telescope and one with a direct imaging electrographic Schmidt camera and an objective grating spectrograph. The observations include spectroscopic imagery in the 1200-200 A wavelength range and imagery of the comet in hydrogen Lyman-alpha (1216 A) radiation. The present observations have been reduced to intensity contour plots in the different emission wavelengths, and production rates are given for the emitting species H, C, O, S, and CO.

Carruthers, George R.↗
NASA NTRS1988

A multicomponent model of the infrared emission from Comet Halley

The 3-200-micron emission of Comet Halley is characterized theoretically by means of numerical simulations. Models with different populations of refractory dust grains are developed, applying compositional and spectral constraints based on ground-based and spacecraft observations. The results are presented in extensive graphs and discussed in detail. Best agreement between predicted and observed Halley emission is obtained for models with 8-40 times more crystalline silicate grains (by mass) than amorphous carbon grains. For the silicate grains, the predominance of large grains and thin carbonaceous coatings are found to account for the lack of a 10-micron silicate feature and the dark appearance of the comet, respectively.

Swamy, Krishna K. S.↗
NASA NTRS1988

Nature of dust grains in comets

The observed reddening in the wavelength range 0.26-2.25 microns for comae of several comets can be represented quite well with model grains having real and imaginary parts of refractive index, m = m-prime - im-double prime, given by m-prime of about 1.38 and m-double prime of about 0.039, respectively. It is also possible to explain the observations of linear polarization and geometric albedo of comet comae.

Swamy, K. S. Krishna↗