共查询到20条相似文献，搜索用时 500 毫秒
1.
The problem of visibility of a cometary nucleus discussed in general terms for single scattering by dust grains. The ratio of radiatio scattered in the dust column above the surface and that reflected from the nucleus determines the visibility of features on the nuclear surface. A contrast parameter characterizing the ration of radiation foming from the nuclear surface and that of the nuclear vicinity describes the visibility of the full nucleus against the dust fore- and background. These quantities and the intensity distribution of scattered solar radiation across the nucleus and its vicinity are calculated for the case of comet Halley at a heliocentric distance of 0.9 AU after perihelion (Giotto encounter). The scattering calculations are based on an isotropic dust distribution derived from hydrodynamics gas-dust interactions resulting in a steep densiity increase right above the surface. For Newburn's nominal model of comet Halley, an optical depth of about 0.5 impairs the visibility of the nucleus somewhat. 相似文献
2.
Hanner MS Tokunaga AT Golisch WF Griep DM Kaminski CD 《Astronomy and Astrophysics》1987,187(1-2):653-660
2 to 20 micrometers photometry of the inner dust coma of comet Halley was obtained at the NASA IRTF on Mar 6.85, 12.8, 13.75, 17.7, and 24.8. Positions offset 10" were measured as well as the central brightness. The strength of the 10 micrometers emission feature was observed to vary with location in the coma. The infrared emission is in general agreement with the dust size distribution measured from the Vega and Giotto spacecraft. Mar 6.8, 17.7, and 24.8 corresponded to strong dust jet activity. The strength of the 10 micrometers silicate emission is shown to be a sensitive indicator of grain size and thus of jet activity. Dust production rate on March 13.75, 6 h before Giotto encounter, was approximately 10(7) gm s-1. 相似文献
3.
《Planetary and Space Science》2007,55(9):1031-1043
A new three-dimensional magnetohydrodynamic model of the coma of a comet has been developed and applied to simulations of a Halley-class coma using the solar-wind conditions of the Giotto flyby of Halley in 1986. The code developed for high-performance parallel processing computers, combines the high spatial resolution of smaller than 1 km grid spacing near the nucleus, with a large computational domain that enables structures nearly 10 million km down the comet tail to be modeled. Ions, neutrals, and electrons are considered as separate interacting fluids. Significant physical processes treated by the model include both photo and electron impact ionization of neutrals, recombination of ions, charge exchange between solar-wind ions and cometary neutrals, and frictional interactions between the three fluids considered in the model. A variety of plasma structures and physical parameters that are the output of this model are compared with relevant Giotto data from the 1986 Halley flyby. 相似文献
4.
Allen M Delitsky M Huntress W Yung Y Ip WH Schwenn R Rosenbauer H Shelley E Balsiger H Geiss J 《Astronomy and Astrophysics》1987,187(1-2):502-512
Methane and ammonia abundances in the coma of Halley are derived from Giotto IMS data using an Eulerian model of chemical and physical processes inside the contact surface to simulate Giotto HIS ion mass spectral data for mass-to-charge ratios (m/q) from 15 to 19. The ratio m/q = 19/18 as a function of distance from the nucleus is not reproduced by a model for a pure water coma. It is necessary to include the presence of NH3, and uniquely NH3, in coma gases in order to explain the data. A ratio of production rates Q(NH3)/Q(H2O) = 0.01-0.02 results in model values approximating the Giotto data. Methane is identified as the most probable source of the distinct peak at m/q = 15. The observations are fit best with Q(CH4)/Q(H2O) = 0.02. The chemical composition of the comet nucleus implied by these production rate ratios is unlike that of the outer planets. On the other hand, there are also significant differences from observations of gas phase interstellar material. 相似文献
5.
Negative ions (anions) were identified in the coma of comet 1P/Halley during in situ Electron Electrostatic Analyzer measurements performed by the Giotto spacecraft in 1986. These anions were detected with masses in the range 7–110 amu, but with insufficient mass resolution to permit unambiguous identification. We present details of a new chemical‐hydrodynamic model for the coma of comet Halley that includes—for the first time—atomic and molecular anions, in addition to a comprehensive hydrocarbon chemistry. Anion number densities are calculated as a function of radius in the coma, and compared with the Giotto results. Important anion production mechanisms are found to include radiative electron attachment, polar photodissociation, dissociative electron attachment, and proton transfer. The polyyne anions C4H? and C6H? are found to be likely candidates to explain the Giotto anion mass spectrum in the range 49–73 amu. The CN? anion probably makes a significant contribution to the mass spectrum at 26 amu. Larger carbon‐chain anions such as C8H? can explain the peak near 100 amu provided there is a source of large carbon‐chain‐bearing molecules from the cometary nucleus. 相似文献
6.
H. Boehnhardt 《Earth, Moon, and Planets》1989,46(3):221-226
Clusters and packets of dust grains, detected by the VEGA spacecrafts in the coma of comet Halley, are interpreted either as due to microjets arising directly from the nucleus or as due to fragmentation processes of larger conglomerates. After a summary of the characteristics of the particle swarms observed and of the conclusion from their interpretation as microjets, some aspects from the fragmentation scenario of this phenomenon are outlined. From the spatial widths and the total number of grains in a particle swarm the mass of the parent conglomerate can be estimated. For the clusters and packets parent bodies of about 1 to several 100 g can be expected. Their travel time in the coma after release from the nucleus range from 10 to about 100 days before the spacecraft encounters. Finally, possible fragmentation processes are discussed and some comments for further investigations, both in theory and for in-situ data, are given. 相似文献
7.
《Planetary and Space Science》1999,47(8-9):1029-1050
We predict the amount of cometary, interplanetary, and interstellar cosmic dust that is to be measured by the Cometary and Interstellar Dust Analyzer (CIDA) and the aerogel collector on board the Stardust spacecraft during its fly-by of comet P⧸Wild 2 and during the interplanetary cruise phase. We give the dust flux on the spacecraft during the encounter with the comet using both, a radially symmetric and an axially symmetric coma model. At closest approach, we predict a total dust flux of 1060 m−2 s−1 for the radially symmetric case and 1065 m−2 s−1 for the axially symmetric case. This prediction is based on an observation of the comet at a heliocentric distance of 1.7 AU. We reproduce the measurements of the Giotto and VEGA missions to comet P⧸Halley using the same model as for the Stardust predictions. The planned measurements of interstellar dust by Stardust have been triggered by the discovery of interstellar dust impacts in the data collected by the Ulysses and Galileo dust detector. Using the Ulysses and Galileo measurements we predict that 25 interstellar particles, mainly with masses of about 10−12 g, will hit the target of the CIDA experiment. The interstellar side of the aerogel collector will contain 120 interstellar particles, 40 of which with sizes greater than 1 μm. Furthermore, we investigate the contamination of the CIDA and collector measurements by interplanetary particles during the cruise phase. 相似文献
8.
D.T Young F.J Crary J.E Nordholt D Boice A Eviatar J.J Hanley D.J McComas D Reisenfeld R.C Wiens 《Icarus》2004,167(1):80-88
The Plasma Experiment for Planetary Exploration (PEPE) made detailed observations of the plasma environment of Comet 19P/Borrelly during the Deep Space 1 (DS1) flyby on September 22, 2001. Several distinct regions and boundaries have been identified on both inbound and outbound trajectories, including an upstream region of decelerated solar wind plasma and cometary ion pickup, the cometary bow shock, a sheath of heated and mixed solar wind and cometary ions, and a collisional inner coma dominated by cometary ions. All of these features were significantly offset to the north of the nucleus-Sun line, suggesting that the coma itself produces this offset, possibly because of well-collimated large dayside jets directed 8°-10° northward from the nucleus as observed by the DS1 MICAS camera. The maximum observed ion density was 1640 ion/cm3 at a distance of 2650 km from the nucleus while the flow speed dropped from 360 km/s in the solar wind to 8 km/s at closest approach. Preliminary analysis of PEPE mass spectra suggest that the ratio of CO+/H2O+ is lower than that observed with Giotto at 1P/Halley. 相似文献
9.
Images of comet Halley's nucleus taken by the HMC camera during the GIOT-TO encounter in 1986 show that a major part of the total dust production is localized in a few active areas which are the sources of gas-dust jets. The global dust distribution in the inner coma is dominated by two main jets roughly directed to the sun. A combination of a 1D thermal nucleus model with an axisymmetric continuum model of the jet outflow was used to investigate the properties of the inner coma. Detailed investigations show that the characteristics of the observed jets can be reproduced by outgassing from free sublimating active areas of a few km in diameter, a dust to gas ratio of 1–2.5 and a size distribution dominated by the larger grains. It is further shown that most of the observational constraints provided by the HMC data can be met simultaneously by a model of three jets superimposed on a weak background. 相似文献
10.
Images of comet Halley's nucleus taken by the HMC camera during the GIOT-TO encounter in 1986 show that a major part of the total dust production is localized in a few active areas which are the sources of gas-dust jets. The global dust distribution in the inner coma is dominated by two main jets roughly directed to the sun. A combination of a 1D thermal nucleus model with an axisymmetric continuum model of the jet outflow was used to investigate the properties of the inner coma. Detailed investigations show that the characteristics of the observed jets can be reproduced by outgassing from free sublimating active areas of a few km in diameter, a dust to gas ratio of 1–2.5 and a size distribution dominated by the larger grains. It is further shown that most of the observational constraints provided by the HMC data can be met simultaneously by a model of three jets superimposed on a weak background. 相似文献
11.
Shi Zhi-dong Li Zhong-yuan 《紫金山天文台台刊》1998,(2)
The equilibrium potential of fluffy dust grains in plasma environments is modeled numerically for comet P/Halley. It is found that the dust grain acquires its largest negative potential and probability of disruption at about 50, 000km from the comet where the mass spectrum measured by the spacecraft took a great change. 相似文献
12.
Max K. Wallis 《Earth, Moon, and Planets》1984,30(1):31-37
Hypervelocity impact sputter causes impulses substantially greater than the initial momenta of micro-grains of comet Halley's dust coma, the effective factor being discontinuous at the dust mass (0.2 g) that just penetrates the spacecraft bumper shield. Marginally non-penetrating grains determine the net drag and torque, calculated here for the Giotto shield and exposed components. The torque due to asymmetries induces a precession of the spacecraft axis, whose amplitude is solved for passage through the model dust coma, to find slowly damped oscillations of significant (1°) amplitude. 相似文献
13.
We use Mie scattering theory to determine the expected thermal emission from dust grains in cometary comae and apply these results to mid-infrared images of comet Hyakutake (C/1996 B2) obtained preperihelion in 1996 March. Calculations were performed for dust grains in the size range from 0.1 to 10 micrometers for two different compositions: amorphous olivine (a silicate glass) and an organic residue mixture. The resulting emission efficiencies are complicated functions of wavelength and particle size and are significantly different for the two materials in question. The Hyakutake data set consists of three nights of high-resolution imaging (100-150 km pixel-1 at the comet) of the inner coma at 8.7, 11.7, 12.5, and 19.7 micrometers. Attempts to fit the observed colors (ratios of fluxes at different wavelengths) using a single grain composition failed. However, fits to the data were achieved for all three nights using a mixture of approximately 1 micrometer olivine grains and approximately 7 micrometers organic grains. The resulting olivine mass fraction was between 8% and 16% of the total dust mass-loss rate. We also estimate the radius of the nucleus to be r = 2.1 +/- 0.4 km. 相似文献
14.
J. Romstedt A. Jäckel W. KlöckK. Nakamura H. ArendsK. Torkar W. Riedler 《Planetary and Space Science》2002,50(3):347-352
The micro-imaging dust analysis system (MIDAS) is an essential element among the scientific payload on the international Rosetta mission to comet 46P/Wirtanen. The MIDAS instrument based on an atomic force microscope (AFM) collects small particles drifting outwards from the nucleus surface. AFM is able to image small structures in 3D at nanometer-scale resolution. These images provide morphological and statistical information like grain size distribution on the dust population. In order to support the development of the flight hardware, optimisation of the control functions and consolidation of a proper scheme of data interpretation, laboratory studies with similar instruments were carried out. The obtained data demonstrate the capabilities of this technique. For the first time an instrument is able to observe the smallest (nm-sized) grains which are predicted by models and were to a certain extent deduced from previous measurements on the Giotto and Vega missions to comet 1P/Halley. On larger (μm-sized) particles the complex morphology will be visualised with high precision in 3D, and if present, within these aggregates crystalline materials with defined crystal faces can be identified. 相似文献
15.
We define a procedure which allows estimation of the optical thickness of a cometarydust coma and the ambient illumination
of the nucleus for any given comet, if estimatesof the nucleus radius and the dust activity (Afρ) are available. The calculation
isperformed for a singly scattering coma with a cos(ϑ) distribution of dust overits day side. We find that the ambient illumination
is of the same order as the incidentsunlight if the optical thickness is of order one. The optical thickness increases, all
elseequal, linearly with the nucleus radius. Therefore the effect of the presence of the comamay be neglected for small (≈
1 km diameter) comets, but is important forcomets such as 1P/Halley and Hale–Bopp. 相似文献
16.
《Planetary and Space Science》1999,47(3-4):557-576
A significant flux enhancement in energetic particles (E ∼ 60–⩾260 keV),showing internal fine structure interpreted to represent signatures produced during the traversalof various cometary boundaries in P⧸Grigg-Skjellerup, was recorded by the EPONA instrumentaboard spacecraft Giotto on 10 July 1992. A further internally structured flux enhancement withabout the same amplitude, recorded by EPONA in the energy range ∼60–100 keV but detected90×103 km further on along the Giotto trajectory, is herein compared with theP⧸Grigg-Skjellerup record. Possible explanations for the second flux enhancement areindividually considered and it is suggested, on the basis of the available evidence, that itconstituted the signature of another smaller comet, either having a separate genesis from, ororiginating in a splitting of, the P⧸Grigg-Skjellerup nucleus. 相似文献
17.
We briefly describe an advanced 3D gas dynamical model developed for the simulation of theenvironment of active cometary nuclei.
The model canhandle realistic nucleus shapes and alternative physical models for the gas and dust production mechanism.The
inner gas coma structure is computed by solving self-consistently(a) near to the surface the Boltzman Equation(b) outside
of it, Euler or Navier-Stokes equations.The dust distribution is computed from multifluid ``zero-temperature' Euler equations,extrapolated
with the help of a Keplerian fountain model.The evolution of the coma during the nucleus orbital and spin motion,is computed
as a succession of quasi-steady solutions. Earlier versions of the model using simple,``paedagogic' nuclei have demonstrated
that the surface orographyand the surface inhomogeneity contribute similarly to structuring the near-nucleusgas and dust coma,casting
a shadow on the automatic attribution of such structures to ``active areas'.The model was recently applied to comet P/Halley,
for whichthe nucleus shape is available. In the companion paper of this volume,we show that most near-nucleus dust structuresobserved
during the 1986 Halley flybys are reproduced, assuming that the nucleus is strictly homogeneous. Here, we investigate the
effect of shape perturbations and homogeneityperturbations. We show that the near nucleus gas coma structure is robust vis-a-vissuch
effects. In particular, a random distribution of active and inactive areaswould not affect considerably this structure, suggesting that such areas,even if present, could not be easily identified on images
of the coma. 相似文献
18.
An analysis of the spectra from the PUMA dust-impact mass spectrometers onboard the Vega-1 and Vega-2 spacecraft shows that a large number of the observed, unidentified small-amplitude peaks are produced by impacts of very-low-mass (from 10?17 to 10?20 g) particles. The mass flux of very fine particles accounts for a few percent of the total dust mass flux from comet Halley. The elemental composition of the finest cometary particles is identical to the composition of large particles (10?12–10?16 g), in agreement with present views about the nucleus of comet Halley as an aggregate of interstellar dust. 相似文献
19.
Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ～ 105 g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ～ 10?5 g/cm2/sec. 相似文献
20.
Photographs of Comet Bennett 1969i taken in the dust-scattered continuum reveal that the dust particles, leading to the formation of the type II tail, leave the vicinity of the nucleus only within a certain cone with the aperture in the direction to the Sun. Three parabolic envelopes embracing the nucleus are formed by the dust (vertex always about on the radius vector) reaching distances from the nucleus of 30 000, 60 000 and 100 000 km.There exists no relation between the production and motion of this dust and the production and motion of the neutral coma gases. The cone of expulsion of the dust is identical with the cone of expulsion for the ions leading to the formation of the type I tail. Dust- and ion envelopes have, however, different kinematical properties. The cone of expulsion is identical with Bessel's Ausströmungskegel of visible matter observed by him in Comet Halley 1835.Comet Bennett is compared with Comet Halley 1910; they are related in many respects although Comet Halley had a lower dust production than the Comet Bennett.We ascribe to the dust particles of the tail II from the beginning of the expulsion an electrical charge. 相似文献