Magellan Mission Description Extracted from the PDS3 CATALOG.CAT catalog file. Mission Description ==================== The Magellan spacecraft was launched from the Kennedy Space Center on May 4, 1989. The spacecraft was deployed from the Shuttle cargo bay after the Shuttle achieved parking orbit. Magellan, using an inertial upper stage rocket, was then placed into a Type IV transfer orbit to Venus. Magellan is powered by single degree of freedom, sun-tracking, solar panels. The spacecraft is 3-axis stabilized by reaction wheels using gyros and a star sensor for attitude reference. The spacecraft carried a solid rocket motor for Venus orbit insertion. A small hydrazine system provides trajectory corrections and certain attitude control functions. Earth communication with the Deep Space Network (DSN) is by means of S and X-band channels. The high-gain antenna also functions as the SAR mapping antenna during orbital operations. The interplanetary cruise phase lasted until August 10, 1990. During the cruise phase there were small trajectory correction maneuvers to insure proper approach geometry. Using the solid rocket motor, the spacecraft was placed in an elliptical orbit around the planet, with a periapsis latitude of approximately 10 degrees North, a periapsis altitude of 295 km, and a period of 3.263 hours. Apoapsis altitude is approximately 7762 km. After orbit insertion, the radar system acquired test data and then within days the signal from the spacecraft was lost twice. Placed in a 'Safe Mode', the spacecraft resumed mapping operations on September 15, 1990, after commands were relayed to avoid further communication losses. Each mapping cycle lasts 243 days, which is the time required for Venus to make one rotation under the spacecraft orbit. The first mapping cycle ended on May 15, 1991. Typical activities during a single mapping pass on cycle 1 were as follows. As the spacecraft neared the planet surface, it was oriented so the high-gain antenna pointed slightly to the side of the ground track. At a true anomaly of -59 degrees, the radar was commanded on. The radar continued to take data to a true anomaly of 80 degrees and then the radar was commanded off. On the next pass the swath started at -80 degrees and went to 59 degrees. Alternating north and south swaths were repeated during cycle 1. The range of latitude covered by the radar during cycle 1 was 67 degrees S to 90 degrees N. The range of incidence angles for the SAR was just under 20 to just over 40 degrees. The SAR data were taken at a data rate of 750 kb/s and were stored in the spacecraft tape recorder. Altimeter and radiometer data were also taken when SAR was acquired. The altimeter data were taken using the small fan beam antenna and a data rate of 30 kb/s. As the spacecraft moved away from the planet toward apoapsis, the spacecraft reoriented the high-gain antenna towards Earth and the stored radar data were transmitted to DSN stations on Earth. This data taking- and transmitting-cycle was repeated for every orbit. By May 15, 1991, the planet was completely mapped except for gaps and the area near the South Pole. Cycle 2 observations focused on filling gaps, mapping the south pole area, acquiring constant incidence angle (25 degrees) radar swaths, and conducting a suite of experiments, including high resolution imaging and acquisition of stereo data. To observe the south pole the spacecraft was rotated 180 degrees about its nadir-pointing axis to be able to conduct right-looking observations. Cycle 1 gaps were filled by rotating the spacecraft back to its initial left-looking direction. The principal objective of Cycle 3 was to perform stereo mapping of the Venusian surface. About 30 % of the Cycle 1 coverage was mapped in this Cycle. Gravity data, over Artemis Chasma, was also obtained. In addition, high resolution altimetry data were collected by pointing the high gain antenna straight down during orbits 4919 to 4921. Cycle 4 permitted 360 degree longitudinal coverage of gravity data due to the favorable planetary and spacecraft geometry. Prior to the start of Cycle 5, a circular orbit may be achieved, in preparation for acquisition of 360 degree in longitude of gravity data at constant altitude. Mission Objectives Summary ========================== VOLCANIC AND TECTONIC PROCESSES. Magellan images of the Venus surface show widespread evidence for volcanic activity. A major goal of the Magellan mission is to provide a detailed global characterization of volcanic landforms on Venus and an understanding of the mechanics of volcanism in the Venus context. Of particular interest is the role of volcanism in transporting heat through the lithosphere. While this goal will largely be accomplished by a careful analysis of images of volcanic features and of the geological relationships of these features to tectonic and impact structures, an essential aspect of characterization will be an integration of image data with altimetry and other measurements of surface properties. Explosive pyroclastic volcanism should not occur in the present Venus environment, unless the magma contains amounts of volatiles that are large by terrestrial experience. Thus, evidence for extensive pyroclastic deposits would imply the presence of large amounts of volatiles or, if the deposits are old, may suggest historic changes in atmospheric density. Such ideas will be tested using SAR and altimetry data, combined with knowledge of the local geopotential field and may shed light on magma dynamics. Measurements of longitudinal and transverse slope, flow margin relief, and flow surface relief will also provide powerful constraints on flow models, as well as on the rheological properties and physical state of the lava. A parallel goal is the global characterization of tectonic features on Venus and an appreciation of the tectonic evolution of the planet. This goal addresses issues on several scales. On the scale of individual tectonic features, we are interested in the mechanical nature of the faulting process, the documentation of geometry and sense of fault slip, and the relationship between mechanical and thermal properties of the lithosphere. On a somewhat broader scale, we are interested in linking groups of features to specific processes (e.g., uplift, orogeny, gravity sliding, flexure, compression or extension of the lithosphere) and in testing quantitative models for these processes with SAR images and supporting topographic, gravitational, and surface compositional data. On a global scale, we are interested in whether spatially coherent, large-scale patterns in tectonic behavior are discernible, patterns that might be related to an organized system of plates or to mantle convective flow. IMPACT PROCESSES. The final physical form of an impact crater has meaning only when the effects of the cratering event and any subsequent modification of the crater can be distinguished. To this end, a careful search of the SAR images will attempt to locate and document both relatively pristine and degraded impact craters, together with their ejecta deposits, in each size range, as well as to distinguish impact craters from those of volcanic origin. The topographic measures of depth-to-diameter ratio, ejecta thickness distribution as a function of distance from the crater, and the relief of central peaks will contribute to this documentation. It is expected that several time-dependent processes will influence the change in appearance of craters with increasing crater age, including continued bombardment of the surface, variations in the mechanical properties of the lithosphere (as a result of cooling or loss of near-surface volatiles), horizontal deformation of the lithosphere, possible variations in the mass of the atmosphere, volcanism, and finally, surface erosion and deposition. Distinguishing and understanding these processes constitute important components of the study of crater morphology. Beyond their intrinsic interest in providing a record of impact and deformational processes, craters provide a tool for the relative dating of surface geological units. Relative ages can be established from a comparison of the variations in the areal density of craters of a given size as well as from a comparison of the maximum extent to which different craters are degraded. Together with superpositional relationships (a lava flow that covers an older fault) and transectional relationships (a graben that cuts through an older volcano), the relative temporal evolution of large areas of the Venus surface can be reconstructed. EROSIONAL, DEPOSITIONAL, AND CHEMICAL PROCESSES. The nature of erosional and depositional processes on Venus is poorly known, primarily because of the diagnostic landforms typically occur at a scale too small to have been resolved in Earth-based or Venera 15/16 radar images. Magellan images show wind eroded terrains, landforms produced by deposition (dune fields), possible landslides and other down slope movements, as well as aeolian features such as radar bright or dark streaks 'downwind' from prominent topographic anomalies. One measure of weathering, erosion, and deposition is provided by the extent to which soil covers the surface (for Venus, the term soil is used for porous material, as implied by its relatively low value of bulk dielectric constant). The existence of such material, and its dependence on elevation and geologic setting, provide important insights into the interactions that have taken place between the atmosphere and the lithosphere. Because of the inference drawn from the deuterium-to-hydrogen ratio of the present atmosphere for the past existence of substantial amounts of water on Venus, the images continue to be searched for evidence of past episodes of fluvial activity (drainage systems) and for lake beds and coastal signatures (strandlines). The existence of a thick and cloudy atmosphere precludes infrared, visual, ultraviolet, x-ray, or gamma-ray observation of the Venus surfaces from orbit. Thus it is impossible to obtain information on a global basis about the surface composition of mineralogy using standard remote-sensing techniques. Magellan data have disclosed that very often the surfaces of elevated regions possess both anomalously high values of normal-incidence radar reflectivity, occasionally exceeding 0.43, and associated low values of radio emissivity, reaching as low as 0.50. In the absence of liquid water, which is known from a variety of evidence not to be present today on Venus, it is necessary to assume a surface composition that would be unusual in terrestrial experience to explain the large values of dielectric constant implied by these observations. The most acceptable of the current hypotheses requires a significant number of electrically conducting elements in surface materials. If these are iron sulfides, as some chemical evidence suggests, they may possibly be brought to the surface by volcanic activity. The good spatial resolution of the Magellan instrumentation, both in determining the surface reflectivity from the altimetric observations and in measuring the emissivity from radiometric observations, promises to outline the structure of these regions in far greater detail than is now available and may shed light on their origin. Results will be applied to testing hypotheses for regional and global buffering of atmospheric composition by reactions with crustal materials. ISOSTATIC AND CONVECTIVE PROCESSES. Topography and gravity are intimately and inextricably related, and must be jointly examined when undertaking geophysical investigations of the interior of a planet, where isostatic and convective processes dominate. Topography provides a surface boundary condition for modeling the interior density of Venus. Modeling of the interior density using gravity data is, of course, nonunique. Meaningful interpretation rests on integrating other data sets and/or incorporating specific mechanical models of the interior. For example, a single density interface underlying the known topography can be found that exactly matches any observed gravity field. The interface can be at any depth; the greater the depth, the larger the density contrast needed. The thickness of the elastic lithosphere of Venus, i.e., the outer region of the planet that behaves elastically over geologically long periods of time, is of special interest. The base of this zone is likely to be defined by a specific isotherm whose location depends on the particular temperature-dependent flow or creep properties of the material underneath. If this isotherm can be mapped in space and time, then models for the thermal evolution of the planet can be developed. The key to determining lithospheric thickness variations in space and time is through flexure studies. If a mass load, e.g., a shield volcano or a mascon, is placed on the planetary surface, then the elastic lithosphere will flex under the load. The controlling parameter is the flexural rigidity, which is dependent on the elastic constants and lithospheric thickness. Crucial to applying estimates of flexural rigidity to the task of unraveling the thermal history is an estimate of when the load was emplaced. Thus age determinations derived by various geologic techniques are essential to this scheme. Mission Phases ============== PRELAUNCH --------- The prelaunch phase extended from delivery of the spacecraft to Kennedy Space Center until the start of the launch countdown. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1988-09-01 Mission Phase Stop Time : 1989-05-04 Spacecraft Operations Type : ORBITER LAUNCH ------ The launch phase extended from the start of launch countdown until completion of the injection into the Earth-Venus trajectory. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1989-05-04 Mission Phase Stop Time : 1989-05-04 Spacecraft Operations Type : ORBITER CRUISE ------ The cruise phase extended from injection into the Earth-Venus trajectory until 10 days before Venus orbit insertion. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1989-05-04 Mission Phase Stop Time : 1990-08-01 Spacecraft Operations Type : ORBITER ORBIT INSERTION --------------- The Venus orbit insertion phase extended from 10 days before Venus orbit insertion until burnout of the solid rocket injection motor. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1990-08-01 Mission Phase Stop Time : 1990-08-10 Spacecraft Operations Type : ORBITER ORBIT CHECKOUT -------------- The orbit trim and checkout phase extended from burnout of the solid rocket injection motor until the beginning of radar mapping. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1990-08-10 Mission Phase Stop Time : 1990-09-15 Spacecraft Operations Type : ORBITER MAPPING CYCLE 1 --------------- The first mapping cycle extended from completion of the orbit trim and checkout phase until completion of one cycle of radar mapping (approximately 243 days). Mapping orbits included in the first cycle were 373 through 2165. Orbits 2159-2171 were used for an interferometry test, and orbits 2172-2175 were used to conduct an orbit trim maneuver (OTM). Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1990-09-15 Mission Phase Stop Time : 1991-05-15 Spacecraft Operations Type : ORBITER MAPPING CYCLE 2 --------------- The second mapping cycle extended from completion of the first mapping cycle through an additional cycle of mapping. Acquisition of 'right-looking' SAR data was emphasized. Orbits included in the second cycle were 2176 through 3976. Radio occultation measurements were first carried out on orbits 3212-3214. A period of battery reconditioning followed completion of Cycle 2. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1991-05-16 Mission Phase Stop Time : 1992-01-17 Spacecraft Operations Type : ORBITER MAPPING CYCLE 3 --------------- The third mapping cycle extended from completion of battery reconditioning through an additional cycle of mapping (approximately 243 days). Acquisition of 'stereo' SAR data was emphasized. Orbits included in the third cycle were 4031 through 5747. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1992-01-24 Mission Phase Stop Time : 1992-09-14 Spacecraft Operations Type : ORBITER MAPPING CYCLE 4 --------------- The fourth mapping cycle extended from completion of the third mapping cycle through an additional cycle of mapping. Acquisition of radio tracking data for gravity studies was emphasized. Radio occultation measurements were carried out on orbits 6369, 6370, 6471, and 6472. Because of poor observing geometry for gravity data collection at the beginning of the cycle, this cycle was extended 10 days beyond the nominal 243 days. Orbits included within the fourth cycle were 5748 through 7626. Periapsis was lowered on orbit 5752 to improve sensitivity to gravity features in Cycle 4. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1992-09-14 Mission Phase Stop Time : 1993-05-25 Spacecraft Operations Type : ORBITER AEROBRAKING ----------- The aerobraking phase extended from completion of the fourth mapping cycle through achievement of a near-circular orbit. Circularization was achieved more quickly than expected; the first gravity data collection in the circular orbit was not scheduled until 11 days later. Orbits included within the aerobraking phase were 7627 through 8392. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1993-05-26 Mission Phase Stop Time : 1993-08-05 Spacecraft Operations Type : ORBITER MAPPING CYCLE 5 --------------- The fifth mapping cycle extended from completion of the aerobraking phase through an additional cycle of mapping (approximately 243 days). Acquisition of radio tracking data for gravity studies was emphasized. The first orbit in the fifth cycle was orbit 8393, and the last was orbit 12248. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1993-08-16 Mission Phase Stop Time : 1994-04-15 Spacecraft Operations Type : ORBITER MAPPING CYCLE 6 --------------- The sixth mapping cycle extended from completion of the fifth mapping cycle through an additional cycle of mapping (approximately 180 days). Acquisition of radio tracking data for gravity studies was emphasized. The first orbit in the sixth cycle was orbit 12249, and the last was orbit 15032. The sixth cycle ended when radio contact was lost as the spacecraft entered the atmosphere and was destroyed in a 'terminal windmill' experiment. Spacecraft Id : MGN Target Name : VENUS Mission Phase Start Time : 1994-04-16 Mission Phase Stop Time : 1994-10-12 Spacecraft Operations Type : ORBITER References ==================== Saunders, R.S., G.H. Pettengill, R.E. Arvidson, W.L. Sjogren, W.T.K. Johnson, L. Pieri, The Magellan Venus Radar Mapping Mission, Journal of Geophysical Research, vol. 95, no. B6, pp. 8339-8355, June 10, 1990. Venus Radar Mapper Project Plan, Document 630-1, JPL D-814, 157 pp., Jet Propulsion Laboratory, Pasadena, Calif., 1983. Pettengill, G. H., Magellan Venus Radar Mapper Science Experiment Plan of the Radar Investigation Group (RADIG), MIT/JPL. Saunders, R. S., Pettengill, G. H., Magellan: Mission Summary, Science, V. 252, pp. 247 - 249, 1991. Saunders, R. S., Arvidson, R. E., Head III, J. W., Schaber, G. G., Stofan, E. R., Solomon, S. C., An Overview of Venus Geology, Science, V. 252, pp. 249 - 252, 1991. Solomon, S. C., Head, J. W., Fundamental Issues in the Geology of Venus, Science, V. 252, pp. 252 - 260, 1991. Pettengill, G. H., Ford, P. G., Johnson, W. T. K., Raney, R. K., Soderblom, L. A., Magellan: Radar Performance and Data Products, Science, V. 252, pp. 260 - 265, 1991. Tyler, G. L., Ford, P. G., Campbell, D. B., Elachi, C., Pettengill, G. H., Simpson, R. A., Magellan: Electrical and Physical Properties of Venus' Surface, Science, V. 252, pp. 265 - 270, 1991. Arvidson, R. E., Baker, V. R., Elachi, C., Saunders, R. S., Wood, J. A., Magellan: Initial Analysis of Venus Surface Modification, Science, V. 252, pp. 270 - 275, 1991. Head, J. W., Campbell, D. B., Elachi, C., Guest, J. E., McKenzie, D. P., Saunders, R. S., Schaber, G. G., Schubert, G., Venus Volcanism: Initial Analysis from Magellan Data, Science, V. 252, pp. 276 - 288, 1991. Phillips, R. J., Arvidson, R. E., Boyce, J. M., Campbell, D. B., Guest, J. E., Schaber, G. G., Soderblom, L. A., Impact craters on Venus: Initial Analysis from Magellan, Science, V. 252, pp. 288 - 297, 1991. Solomon, S. C., Head, J. W., Kaula, W. M., McKenzie, D., Parsons, B., Phillips, R. J., Schubert, G., Talwani, M., Venus Tectonics: Initial Analysis from Magellan, Science, V. 252, pp. 297 - 312, 1991.