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At least 1,261 records · Page 70
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NASA NTRS1986

Comet Giacobini-Zinner - In situ observations of energetic heavy ions

Conclusive evidence is presented for the existence of heavy energetic, singly charged cometary ions within 1.5 million km of comet Giacobini-Zinner. The most direct evidence for establishing the mass of these ions was obtained from an analysis of the energy signals in one of the solid state detector on board the International Cometary Explorer. Maximum fluxes were recorded approximately one hour before and after closest approach to the cometary nucleus. Transformation of the particle angular distribution observed at approximately 50,000 km radial distance from the comet, during the inbound pass into a rest frame in which the distributions are nearly isotropic, requires a transformation velocity that is consistent with the local solar wind velocity (assuming these particles are primarily singly ionized with a mass of 18 + or - 6 amu). It is implied that these water-group ions underwent strong pitch angle scattering subsequent to ionization and that they were accelerated or heated. The derived ion density, approximately 0.1/cu cm, is consistent with a crude model for the production and transport of pickup ions.

Ipavich, F. M.↗
NASA NTRS1986

A model of Comet P/Giacobini-Zinner

A computer model of Comet P/Giacobini-Zinner is presented which contains photoprocesses, gas-phase chemical kinetics, energy balance, multifluid hydrodynamics with a transition to free molecular flow, and solar wind interaction. Recently the physics for electrons in the model has been improved by including electron impact ionization and dissociation and separately accounting for electron energetics. Electron heating and cooling mechanisms include photoprocesses, recombination processes, inelastic and elastic collisions with heavy molecules, and expansion cooling. The model incorporates an internally consistent interaction of the solar wind with the coma gas using the axisymmetric ideal fluid dynamic equations. The nuclear size and composition have been chosen to make the calculations relevant to the September 11, 1985 International Cometary Explorer (ICE) encounter with Comet P/Giacobini-Zinner. Model profiles of the temperature, velocity, and number density of the electrons are in good agreement with measurements along ICE's trajectory. The results indicate the probe passed through a region of the coma at the onset of the plasma tail.

Boice, D. C.↗
NASA NTRS1986

Three component plasma electron distribution in the intermediate ionized coma of Comet Giacobini-Zinner

The observation of three distinct components of the electron distribution function measured in the intermediate ionized coma (IIC) and plasma tail of Comet Giacobini-Zinner is reported. It is believed that the cold component represents electrons produced close to the comet nucleus by ionization of cometary matter and subsequent cooling by Coulomb collisions. The second component also appears to be composed of electrons produced by photoionization of cometary neutrals, but sufficiently far from the nucleus that the distributions are largely unaffected by Coulomb interactions. The hot component is probably a population of electrons originating in the solar wind. Throughout the IIC, the electrostatic potential of the spacecraft was very low (less than 0.8 eV), implying that ICE generated very little impact-produced plasma during its passage.

Zwickl, R. D.↗
NASA NTRS1986

Broadening and occultation of radio sources by comet Giacobini-Zinner as observed from ICE

During the encounter of the International Cometary Explorer (ICE) with comet P/Giacobini-Zinner, the radio astronomy experiment observed occultation and propagation effects on several distant radio sources. The observations of the partial occultation of the galactic radio source between 360 and 1000 kHz show that the three-dimensional large scale structure of the coma is consistent with the structure determined from in-situ measurements made along the encounter trajectory. In addition, these observations remove a previously existing ambiguity in the position of the galactic source. Between 110 and 360 kHz the earth source is broadened by scattering by the comet random density inhomogeneities. The amount of broadening indicates that the electron density fluctuations measured in situ by ICE along its trajectory (100 pct fluctuations with a scale length of 1000-2000 km over a path length of 100,000 km) exist with the same properties in the perpendicular direction.

Steinberg, J. L.↗
NASA NTRS1986

The interaction of heavy ions from Comet P/Giacobini-Zinner with the solar wind

The encounter between the ICE spacecraft and Comet P/Giacobini-Zinner was characterized in the solar wind by intense fluxes of heavy ions, measurable over a region 6 x 10 to the 6th km in extent. The ions are observed with highly anisotropic angular distributions, steep energy spectra, and a change in the energy spectrum at around 80 keV, consistent with a composition predominantly of the water group. Flux versus time profiles follow a general fall off with increasing distance from the comet, but with a marked inbound/outbound asymmetry. This asymmetry is due to the higher solar wind velocity on the outbound pass, giving rise to an increased energy gain of the pick-up ions. The flux versus time profiles are strongly modulated by the rapid changes in the direction of interplanetary magnetic field. Correlated observations of energetic ions, the interplanetary magnetic field and the solar wind are presented, and these observations are compared with theoretical predictions of the ion pick-up process.

Sanderson, T. R.↗
NASA NTRS1985

The International Cometary Explorer (ICE) mission to comet Giacobini-Zinner (G/Z)

The primary objectives of the International Cometary Explorer (ICE) mission is to provide in situ data on the interaction between solar wind and the atmosphere of the P/Giacobini-Zinner comet (G/Z), making measurements of particles, fields, and waves while passing through the cometary tail of G/Z on September 11, 1985. Following the G/Z tail intercept, the ICE measurements will complement the later upstream measurements obtained by the Comet Halley probe. The major ICE payload includes a vector helium magnetometer, the plasma-wave experiment, the radio-wave experiment, the plasma-electron experiment, and the plasma ion experiment. Other experiments are intended to measure energetic protons, X-rays, low energy to high energy cosmic rays, cosmic ray electrons, and gamma-ray bursts. The ICE measurements of G/Z will be supplemented with ground-based measurements. Schematic diagrams are included.

Brandt, J. C.↗
NASA NTRS1986

Discovery of cometary kilometric radiations and plasma waves at Comet Halley

The plasma-wave probe carried by the spacecraft Sakigake discovered discrete spectra of emissions from Comet Halley in the frequency range 30-195 kHz. The observed cometary kilometric radiation appears to come from moving shocks in the coma region which are possibly associated with temporal variations of the solar wind. Waves due to plasma instabilities associated with the pick-up of cometary ions by the solar wind were observed within a region almost 10 million km from the comet nucleus.

Oya, H.↗
NASA NTRS1986

Pathfinder - Accuracy improvement of comet Halley trajectory for Giotto navigation

The 'Pathfinder' project uses the resources of NASA, ESA and Intercosmos to support the Giotto space probe flyby of comet Halley. The required 500-km approach to comet Halley on the sunward side was so uncertain as to preclude the trajectory's realization with sufficient accuracy on the basis of ground-based astronomical observations alone. Spaceborne observations by the two Vega satellites were accordingly conducted; these, in combination with a very accurate determination of the Vega trajectories using VLBI, allowed a flyby distance of 600 km to be achieved with only + or - 40 km uncertainty.

Muench, R. E.↗
NASA NTRS1986

Dust density and mass distribution near comet Halley from Giotto observations

The density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system. The dust spectrum obtained at 291,000 km from the comet nucleus show depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses. Most of the mass striking Giotto is noted to reside in the few large particles penetrating the dust shield. Momentum balances and energy considerations applied to an observed deceleration suggest that a large mass of the spacecraft was detached by an impact.

Mcdonnell, J. A. M.↗
NASA NTRS1986

Comet Halley-solar wind interaction from electron measurements aboard Giotto

The Giotto probe's three-dimensional electron spectrometer has obtained data that illustrate the complex interactions between comet Halley and the solar wind. As Giotto approached the comet, it traversed several regions exhibiting different plasma characteristics: a large upstream region, a foreshock, a bow wave whose apex lies at about 400,000 km, a transition region, a region permanently and strongly contaminated by cometary ions, a collision pause, and various inner regions up to the inner coma, including a region of depleted hot plasma.

Reme, H.↗
NASA NTRS1986

First results from the Giotto magnetometer experiment at comet Halley

The Giotto magnetometer experiment at comet Halley has for the first time provided magnetic field measurements in all the important spatial regions characterizing the front-side interaction between the solar-wind magnetoplasma and a cometary atmosphere. Upstream waves of cometary origin have been observed at distances greater than two million km from the comet, both inbound and outbound. A cometary bow shock has been identified at 1.15 million inbound on the dawn side and a thick quasi-parallel cometary bow shock outbound. A turbulent magnetosheath has been observed further inside. A magnetic pile-up region has been identified inside 135,000 km, inbound, and 263,000 km, outbound, with fields up to 57 and 65 nT, respectively. A cavity region with essentially zero magnetic field has been discovered, with a width of 8500 km along the trajectory around closest approach.

Neubauer, F. M.↗
NASA NTRS1986

Dust jets in comet Halley observed by Giotto and from the ground

A hypothesis for the short-lived, intermittent discrete sources of dust on the nucleus surface of comet Halley has been confirmed by closeup imaging by a camera aboard the Giotto probe. A previous conclusion for the nonexistence of jet emission from the dark side of the nucleus, reached on the basis of analysis of ground-based data, is also confirmed. Attention is given to such issues as possible spin vector variations for comet Halley over the 76-year period, the survival of dust sources, and the positions of the prime meridian at the two returns to the sun.

Sekanina, Z.↗
NASA NTRS1986

Space observations of comet Halley

Observations of comet Halley by the Solar Maximum Mission, the International Ultraviolet Explorer, the Pioneer Venus Orbiter, the Dynamics Explorer 1, and by sounding rockets are reviewed. Visual images taken through the wide-band blue filter showed an extended coma elongated approximately in the antisolar direction. Preperihelion observations show definite evidence for both a plasma and a dust tail. The ultraviolet observations indicate that an extensive hydrogen cloud extends to well over 10 billion km from the nucleus. The derived water production rate for the comet showed very large changes around the time of perihelion. The rocket observations showed strong H I, O I, and C I emissions, several CO fourth positive bands, and the C II emissions at 1335 A extending into the tail.

Brandt, J. C.↗
NASA NTRS1986

Interplanetary trajectory design for the Mariner Mark II Comet Rendezvous/Asteroid Flyby mission

The primary purpose of the first mission utilizing a Mariner Mark II spacecraft will be to rendezvous with the short-period comet Wild 2 and conduct a proximal examination during the comet's quiescent and active phases. The purpose of the present study is to elucidate, using an accurate dynamic model, some of the characteristics of the interplanetary trajectory that are relevant to the mission design. Optimal transfer trajectories are generated, constrained by the Shuttle/Centaur launch vehicle capability and the total spacecraft mass and performance. Asteroid flyby opportunities are examined, and the effects of selected asteroid encounters on mission performance are discussed.

Myers, M. R.↗
NASA NTRS1986

Early photometry of comet p/Halley - Development of the coma

The observation of comets over a large range of heliocentric distances can provide important information, taking into account, for instance, the appearance of true cometary characteristics. Photometric evidence is provided regarding coma formation at approximately 5.9 AU, giving attention to aspects of water-ice sublimation. New photometry of p/Halley was obtained when the comet was at 5.9 and 5.1 AU, during 1984 October and 1985 January, respectively. The new observations are compared with data obtained at larger heliocentric distances. It is argued that sustained mass loss from the nucleus began at about 5.9 AU, at a rate which is consistent with production by sublimation from a predominantly water-ice nucleus.

Meech, K. J.↗
NASA NTRS1986

Decametric radio emission from comets - An attempt at detection

In a brief discussion of radio emission of comets, it is pointed out that an occurrence of nonthermal radio emissions would be expected only at decametric or longer wavelengths. An attempt is reported to detect emission at decameter wavelengths during the International Halley Watch trial run on Comet P/Crommelin in March 1984. No plasma effects in the tail were observed either at visible or at radio wavelengths. Assuming free-free emission from an optically thin gas, the observations make it possible to place an upper limit in the range from 560 to 8900 electrons per cu cm on the electron density in the tail. The Teepee Tee array employed in the investigations has a frequency coverage of 15-125 MHz, and a sensitivity of approximately 1 Jy, (the highest currently available from the ground at low frequencies).

Gergely, T. E.↗
NASA NTRS1986

Approach strategy for the rendezvous with comet Tempel 2

The methods used to locate the comet nucleus and safety navigate through the debris surrounding the comet are discussed. The approach to the nucleus is in two parts: a high-speed (1.9 km/s) leg which ends with the spacecraft 200,000 km sunward of the nucleus, and a low-speed (25 m/s) leg which traverses the 200,000 km to the nucleus. The nucleus will be recovered by the narrow-angle camera 30 days prior to the completion of the high-speed inbound leg. The debris density is low enough to permit the safe passage of the spacecraft through the surrounding cometary debris.

Kakuda, R. Y.↗
NASA NTRS1986

Imaging studies of comets

Extensive photography was conducted on comet Giacobini-Zinner in support of the flyby of the International Cometary Explorer (ICE) through the plasma tail on 9/11/85, and a dense series of plates was secured of Halley's Comet during the 7-month interval approximately centered on perihelion, i.e., 1985 November - 1986 May. The hundreds of high quality plates of Halley contain copious numbers of dramatic plasma events, including DE's, helical waves, and turning tail rays. Initial study of the plate material lends support to the magnetic sector boundary model of DE's developed by Niedner and Brandt.

Brandt, J. C.↗