Engineering topics
Publications and source records attributed to Davies, M. E..
We are undertaking, in support of the Mars Digital Image Mosaic (MDIM) 2.1, many improvements in the RAND-USGS photogrammetric control network for Mars, primarily involving the use of Mars Orbiter Laser Altimeter (MOLA)-derived radii and DIMs to improve control point absolute radii and horizontal positions. Additional information is contained in the original extended abstract.
A revised global image mosaic of Mars (MDIM 2.0) was recently completed by USGS. Comparison with high-resolution gridded Mars Orbiter Laser Altimeter (MOLA) digital image mosaics will allow us to quantify its geodetic errors; linking the next MDIM to the MOLA data will help eliminate those errors. Additional information is contained in the original extended abstract.
A stellar-occultation light curve for Triton shows asymmetry that can be understood if Triton's middle atmosphere is distorted from spherical symmetry. Although a globally oblate model can explain the data, the inferred atmospheric flattening is so large that it could be caused only by an unrealistic internal mass distribution or highly supersonic zonal winds. Cyclostrophic winds confined to a jet near Triton's northern or southern limbs (or both) could also be responsible for the details of the light curve, but such winds are required to be slightly supersonic. Hazes and clouds in the atmosphere are unlikely to have caused the asymmetry in the light curve.
A local control network is being developed using Galileo images that cover the region north of the Apollo area and lie between 10 and 100 degrees east longitude. This network is tied to the Apollo control network and will have a positional accuracy of approximately 500-1500 m. This region has been photographed by Earth-based telescopes and the Mariner 10 and Lunar Orbiter spacecraft, but the Gallileo images are preferred for control because of their superior viewing angles, resolution, and Galileo's geometrically stable sensor. Based on Davies et al., 1987, the potential accuracy of the near-side Apollo network is estimated to be between 50 and 300 m. The laser ranging retroreflector locations at the Apollo 11, 14, and 15 sites and the Lunikhod 2 site have been determined with an accuracy of about 10 m. The locations of the ALSEP transmitters relative to the retroreflectors have also been measured. These known coordinates are used to estimate errors in the Apollo network. The telescopic control is approximately bounded by 75 degrees north and south latitude and 75 degrees east and west longitudes. The accuracy of the telescopic network is thought to be about one to two km. Thus, the Galileo network might improve coordinates in the telescopic area as well as north and east of it.
The Clementine Mission is being built and flown by the Naval Research Laboratory under the sponsorship of the Strategic Defense Initiative Organization of the United States Department of Defense in joint-cooperation with NASA, and will explore the Moon and the near-Earth asteroid (NEA) 1620 Geographos with lightweight sensors developed by the Lawrence Livermore National Laboratory. A NASA Science Team for this mission will be selected by way of a NRA in April 1993. The instrument suite includes imaging cameras that cover a spectral range from the near-ultraviolet to the mid-infrared, a laser ranger, and, potentially, a charged particle telescope. To be launched in early 1994, Clementine will be in lunar orbit from February through May 1994, at which time it will depart the Moon for a flyby of 1620 Geographos in August 1994. This mission represents an outstanding opportunity for scientists interested in the Moon and asteroids. It is anticipated that the data returned from this mission will permit: an assessment of global lunar crustal heterogeneity and a resolution of less than 1 km; an assessment of the lithologic heterogeneity of Geographos at a scale of 100 m or better; and an assessment of surface processes on Geographos on the order of 10 m. The basic mission of Clementine and some of the key scientific questions that will be addressed are described. Additional material on the Clementine mission, its data handling and processing, and its instrument suite is presented elsewhere.
Galileo images of Gaspra reveal it to be an irregularly shaped object (19 by 12 by 11 kilometers) that appears to have been created by a catastrophic collisional disruption of a precursor parent body. The cratering age of the surface is about 200 million years. Subtle albedo and color variations appear to correlate with morphological features: brighter materials are associated with craters especially along the crests of ridges, have a stronger 1-micrometer absorption, and may represent freshly excavated mafic materials; darker materials exhibiting a significantly weaker 1-micrometer absorption appear concentrated in interridge areas. One explanation of these patterns is that Gaspra is covered with a thin regolith and that some of this material has migrated downslope in some areas.
Three images of Venus have been returned so far by the Galileo spacecraft following an encounter with the planet on UT February 10, 1990. The images, taken at effective wavelengths of 4200 and 9900 A, characterize the global motions and distribution of haze near the Venus cloud tops and, at the latter wavelength, deep within the main cloud. Previously undetected markings are clearly seen in the near-infrared image. The global distribution of these features, which have maximum contrasts of 3 percent, is different from that recorded at short wavelengths. In particular, the 'polar collar', which is omnipresent in short wavelength images, is absent at 9900 A. The maximum contrast in the features at 4200 A is about 20 percent. The optical performance of the camera is described and is judged to be nominal.
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.
The several thousand asteroids constitute the second largest population of substantial solid objects in the solar system (there are probably more comets), and have been studied from the Earth for nearly two centuries. Asteroids are important members of the solar system for several reasons, among them: (1) they contain the materials that reflect the processes going on early in the time of planet formation; (2) their impacts have been the major external influences on the surface of some terrestrial planets; and (3) their impacts on Earth have probably had major effects on biologic evolution. This paper examines and describes the asteroid Gaspra, and compares it to small satellites.
Revised values are presented for the directions of the north poles of rotation, the prime meridians, and for the sizes and shapes of the planets and satellites. Also presented are definitions of rotational elements and the cartographic coordinate systems. These revised values and definitions are the results of a report provided every three years by an international working group with members from IAU, IAG, and COSPAR.
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.
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A unified network program can tie independent regional networks into a single consistent planetwide control network. The Apollo 15, 16, and 17 ALSEP stations were identified on Apollo panorama photography and their locations transferred to Apollo mapping frames. Two primary control networks were computed based on the Apollo mapping pictures. Although these systems were computed relative to the center-of-mass, it was necessary to translate their origins to best-fit the ALSEP locations. The DMAAC/A15 system was translated 299 + or - 165 m and the NOS/USGS system was translated 2033 + or - 575 m. Many control networks have been computed based on pictures of the Moon taken through telescopes; these cover the Earth-facing region. Points common to the Apollo networks and to a telescopic network (Meyer, 1980) were selected. Using these points in the overlapping region of the two networks, best-fit translation, rotation, and scale parameters are computed to adjust the telescopic net to the Apollo net. As a start, ten well distributed points have been selected and parameters determined to adjust the telescopic network to the translated DMAAC/A15 system and the NOS/USGS system. The goodness of fit has not yet been examined.
Strips of Viking mapping pictures are being added to the planetwide control networks of Mars. These high resolution strips run from the Viking 1 lander site east to Airy-O, north along the 0 deg meridian to 60 deg latitude, southwest through the Viking 1 lander site to the equator, and along the equator encircling the planet. Everywhere along these strips, old points are incorporated in the measurements, thus assuring that the strips and planetwide net make a single large data set. The control points are much denser in the areas covered by the strips than in those regions not covered by strips and their coordinates of the control points is estimated to be less than 3 km and the error in longitude of a few points near Airy-O is less than 40 m. The horizontal coordinates of the control points on Mars have been updated with a single-block planetwide analytical triangulation. The standard error of measurement was 18.06 micron m. The longitude of the Viking 1 lander site was 47 deg.962 and the latitude 22 deg. 480. The latitude of Airy-O was -5 deg. 152.
Geodetic control networks are being computed photogrammetrically for the large satellites of Jupiter and many of the satellites of Saturn using pictures from the Voyager 1 and 2 encounters. Control points have been identified on the satellites and their coordinates computed by single-block analytical triangulations. Contents of data sets for Io, Europa, Ganymede, Callisto, and Rhea are summarized.
A control network of the Saturnian satellite Iapetus has been established photogrammetrically from pictures taken by the two Voyager spacecraft. Coordinates of 62 control points have been computed and listed; pixel measurements of these points were made on 14 Voyager 1 and 66 Voyager 2 pictures. Some of these points are identified on the preliminary U.S. Geological Survey map of Iapetus and many are identified by name. The Voyager 1 and Voyager 2 pictures covered limited regions of the satellite's surface and contained no overlapping areas. The longitude system on Iapetus is defined by the crater Almeric; the 276 deg meridian passes through the center of this crater. The obliquity of Iapetus has been measured as 0.4 deg + or - 1.6 deg. The mean radius of Iapetus has been determined at 718 + or - 8 km.
The rotation period of Hyperion is determined here on the basis of a light-curve derived from low-resolution Voyager images taken over a two-month span. The data indicate a coherent 13-day spin period over 61 days, with the spin axis nearly parallel to the orbital plane. Since Hyperion is not in synchronous rotation, it cannot be used to test the hypothesis that dark dust spiralling inward on retrograde orbits causes the pronounced leading side/trailing side albedo asymmetry observed on the surface of Iapetus.