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  • Other Sources  (16)
  • ASTROPHYSICS  (16)
  • General Chemistry
  • Inorganic Chemistry
  • 1
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    Intensity profiles of dust near extended sources on Comet Halley (1988)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The dust particle intensity decreases proportional to 1/R from the Comet Halley, as expected for free outflow. Flattening of the profile is evident within several tens of km from the nucleus. It is shown here that these phenomena can be explained by a model that considers the extended size and nonunifomity of the active region on the surface of the cometary nucleus. A critical scale length, defined by the opening angle of a jetlike feature and the size of the source, can be introduced to describe the flattening. The model is consistent with the observations and provides the basis for studying other mechanisms such as particle fragmentation.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 332; 51
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  • 2
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    High resolution Lyman alpha observations of comet Kohoutek /1973f/ near perihelion (1975)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: The hydrogen Lyman-alpha emission line of comet Kohoutek (1973f) was observed and resolved by the high-dispersion Skylab/ATM S082B spectrograph shortly after the comet's perihelion passage (29-31 December 1973). The Lyman-alpha line width was determined from three different exposures of the comet nuclear region. A simplified analysis of the optical thickness effects showed that this line width is consistent with the established hydrogen outflow velocity of 8 to 10 km/sec.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics; 38; 3, Fe; Feb. 197
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  • 3
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    A three-dimensional model of the nucleus of Comet Halley (1986)
    In:  Other Sources
    Details
    Publication Date: 2013-08-29
    Description: The nucleus of comet Halley was observed by the Halley Multicolor Camera (HMC) during the last minutes of the flyby sequence before reaching the point of closest approach. The phase angle change during the observational period was less than 17 deg, so HMC data can only define a two-dimensional contour of the nucleus at a certain time. The overall geometry during the HMC observations is outlined and the findings are related to information available from other sources at different times. The nucleus of the comet is described as a triaxial ellipsoid with major axes of 16, 10, and 9 km. The rotation axis is directed towards delta 1950 = -(40 + or - 5) deg and alpha 1950 = (50 + or - 10) deg. The rotation with a period of P = (54 + or - 1) is in a prograde sense with respect to the orbital motion.
    Keywords: ASTROPHYSICS
    Type: ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus; p 367-369
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  • 4
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    Dust-gas interaction deduced from Halley multicolor camera observations (1986)
    In:  Other Sources
    Details
    Publication Date: 2013-08-29
    Description: The dust and gas productions of comet Halley were measured by the dust counter and the mass spectrometers on the Giotto spacecraft. These instruments give only little information about the spatial asymmetry of the activity. The asymmetry in the dust production is clearly evident from the dust jets seen in the Halley Multicolor Camera images. Since the dust is entrained by the gas, gas production must be similarly asymmetric. The intensity profiles along and across several dust jets are related to their source regions on the nucleus. Properties of the dust jets are investigated. A few compact, but highly active source regions on the nucleus produce most of the visible dust and can account for most of the gas produced by the comet.
    Keywords: ASTROPHYSICS
    Type: ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's comet. Volume 2: Dust and Nucleus; p 363-364
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  • 5
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    Dust activity of Comet Halley's nucleus (1986)
    In:  Other Sources
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    Publication Date: 2013-08-29
    Description: Images obtained by the Halley multicolor camera using the clear filter with a pass band from 300 to 1000 nm were used to study dust activity in the comet nucleus. Comparisons with ground based observations confirm that dust production towards the Sun increases in activity relative to the southern background source while the Giotto spacecraft was approaching. This is in agreement with the assumption that the sunward activity becomes stronger when the source rotates towards the Sun. Estimated dust column density is 90 billion/sqm, with optical thickness less than or = 0.3. Surface reflectivity is less than 1%, indicating a very rough surface with large fractions of shadowed areas.
    Keywords: ASTROPHYSICS
    Type: ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus; p 359-362
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  • 6
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    Modifications of comet materials by the sublimation process: Results from simulation experiments (1989)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: An active comet like comet Halley loses by sublimation a surface layer of the order of 1 m thickness per perihelion passage. In situ measurements show that water ice is the main constituent which contributes to the gas emission although even more volatile species (CO, NH3, CH4, CO2 etc.) have been identified. Dust particles which were embedded in the ices are carried by the sublimating gases. Measurements of the chemical composition of cometary grains indicate that they are composed of silicates of approximate chondritic composition and refractory carbonaceous material. Comet simulation experiments show that significant modifications of cometary materials occur due to sublimation process in near surface layers which have to be taken into account in order to derive the original state of the material.
    Keywords: ASTROPHYSICS
    Type: Lunar and Planetary Inst., Workshop on Analysis of Returned Comet Nucleus Samples; p 24-25
    Format: application/pdf
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  • 7
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    Radiometric observations of the nucleus of Comet Halley (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Images obtained by the Halley multicolor camera (HMC) were used to determine the surface brightness of the nucleus. Radiometric values of jet-free areas of the surface are presented and a range of possible surface brightness values are derived. These direct measures are compared with brightnesses derived from the size of the nucleus, as determined from HMC images, and ground-based observations obtained before the onset of coma activity.
    Keywords: ASTROPHYSICS
    Type: ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus; p 355-357
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  • 8
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    Nucleus morphology of Comet Halley (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Images obtained by the Halley multicolor camera were used to determine the projected size and shape of the nucleus. The location of the terminator and numerous surface features were determined. There is good correlation between the brightest surface features and the dust jets; however, many bright features are seen which are not associated with jets. Most of the observed features are circular and appear to be related to surface elevation. The angularity of the terminator gives an indication of the three-dimensional structure of the face which was observed.
    Keywords: ASTROPHYSICS
    Type: ESA Proceedings of the 20th ESLAB Symposium on the Exploration of Halley's Comet. Volume 2: Dust and Nucleus; p 351-354
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  • 9
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    Multiple scattering of solar resonance radiation in the nearby interstellar medium. I (1979)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The paper presents approximate solutions obtained for a radiative transfer problem that represents a highly-idealized description of the multiple scattering of solar resonance radiation in the nearby interstellar medium. The problem of a point source in the center of a spherically symmetric cavity imbedded in an infinite uniform medium is solved for a range of cavity radii. First and second-order scattering contributions are calculated, and the Eddington approximation is used to estimate the higher order components of the radiation field. It is shown that for coherent scattering at very large cavity radii, the backscattered intensity from the cavity approaches three times the value deduced from the optically thin solution, in agreement with previous work. It is concluded that an accurate analysis of sky background will require including not only the frequency redistribution, but also the correct spatial distribution of density.
    Keywords: ASTROPHYSICS
    Type: Astronomy and Astrophysics; 80; 2, De; Dec. 197
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  • 10
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    The interpretations of ultraviolet observations of comets (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The paper summarizes recent cometary UV observations, most of which were made in Ly-alpha light with instruments aboard earth-orbiting satellites. These include OAO-2 observations of comets Bennett and Tago-Sato-Kosaka, OGO-5 observations of comets Bennett and Encke, and numerous observations of comet Kohoutek. Models for the production of cometary hydrogen atoms are described, including the fountain, syndyname, and parent-daughter models. Calculations of emission line profiles and multiple-scattering effects are also discussed. Results of observations and interpretations are reviewed for each cited comet, far-UV observations in other emission lines are noted, and the use of comets as solar-wind probes is considered. It is concluded that the results of the present cometary Ly-alpha observations strongly support the concept of an icy conglomerate solid cometary nucleus and suggest water to be one of the most abundant molecules in comets.
    Keywords: ASTROPHYSICS
    Type: Space Science Reviews; 18; Mar
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