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  • LUNAR AND PLANETARY EXPLORATION  (3)
  • ASTRONOMY  (2)
  • 1990-1994  (5)
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Year
  • 1
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    Venus gravity and topography: 60th degree and order model (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978-1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Geophysical Research Letters (ISSN 0094-8276); 20; 21; p. 2403-2406
    Format: text
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    NASA TECHNICAL REPORTS
  • 2
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    Targeting an asteroid - The Galileo spacecraft's encounter with 951 Gaspra (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: On 1991 October 29, the Galileo spacecraft provided the first in situ observations of an asteroid. To a significant degree, the successful encounter with asteroid 951 Gaspra was made possible by the high accuracy of the ground-based Gaspra ephemeris and the refinement to this initial ephemeris provided by the use of optical navigation images taken on board the spacecraft itself. The success of these celestial navigation efforts was dramatically demonstrated when Gaspra appeared in the central frame of the large mosaic of CCD images-less than one asteroid diameter from Gaspra's expected location in the camera's field of view. The corrections to Gaspra's ground-based ephemeris, based upon the optical on-board navigation images, were less than 80 km. The ground-based observation program that went into this ephemeris development is discussed and the accuracy of the ephemeris is used to validate the assumptions that went into the error analysis studies.
    Keywords: ASTRONOMY
    Type: Astronomical Journal (ISSN 0004-6256); 105; 4; p. 1547-1552.
    Format: text
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  • 3
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    Asteroid and comet orbits using radar data (1992)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: For the 30 asteroids and 4 comets for which radar astrometric data were given by Ostro (1991), orbits have been computed using both the radar and the existing optical measurements. The techniques required to process radar data in orbit determination solutions are outlined, and future radar observation opportunities for asteroids and comets are identified. For asteroids and comets that have only short intervals of optical astrometric data, the additional use of only a few radar observations allows a far more accurate extrapolation of their future motions. The use of radar data can often ensure an object's successful recovery at future earth returns and greatly assist efforts in monitoring the motions of the rapidly growing population of known near-earth objects, including their future close-earth approaches.
    Keywords: ASTRONOMY
    Type: Astronomical Journal (ISSN 0004-6256); 103; 303-317
    Format: text
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  • 4
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    The spin vector of Venus determined from Magellan data (1992)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: A control network of the north polar region of Venus has been established by selecting and measuring control points on full-resolution radar strips. The measurements were incorporated into a least-squares adjustment program that improved initial estimates of the coordinates of the control points, pole direction, and rotation rate of Venus. The current dataset contains 4206 measurements of 606 points on 619 radar strips. The accuracy of the determination is driven by spacecraft ephemeris errors. An accurate estimate of the rotation period of Venus was obtained by applying an ephemeris improvement technique. The second cycle closure orbits improved ephemeris solutions for 40 orbits (376-384, 520-528, 588-592, 658-668, 1002-1010, 1408-1412, 1746-1764, and 2166-2170) are included and fixed in the geodetic control computations, thus trying the network to the J2000 coordinate system.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Lunar and Planetary Inst., Papers Presented to the International Colloquium on Venus; p 27
    Format: application/pdf
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  • 5
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    The rotation period, direction of the north pole, and geodetic control network of Venus (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Three related activities that use Magellan data to derive improved estimates of the rotation period and direction of the spin axis of Venus are discussed. These are the computation of the Magellan geodetic control network, the use of measurement landmarks identified in overlapping image strips to improve the spacecraft ephemeris, and the use of common points identified on both the Venera 15/16 and the Magellan images. Since seven years separate the acquisition of the Magellan and Venera images, it should be possible to compute an accurate rotation period and possibly the spin vector of Venus.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; E8 A; 13
    Format: text
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