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Contact: Jim Doyle, JPL

FOR IMMEDIATE RELEASE                                                March 16, 1992

       The Magellan spacecraft's orbit around Venus will be changed later this year to just above the planet's dense atmosphere for gravity studies, it was announced today at the Lunar and Planetary Science Conference in Houston, Texas.

       Magellan continues to map a region near the equator of Venus to provide three-dimensional views of the highlands of Aphrodite Terra in high-resolution.

       The stereo mapping phase, which began January 26 near Maxwell Montes, the highest mountain on Venus, will end in mid-September, said Project Scientist Dr. Stephen Saunders. The orbit at its closest point to the planet will be lowered. Magellan is in its third mapping cycle around Venus.

       The spacecraft's orbit will be moved downward from 300 kilometers (186 miles) to 180 kilometers (111 miles) above the surface for maximum sensitivity to variations in the gravity field, he said.

       Gravity is mapped by analyzing slight variations in the radio signal sent back to Earth. Gravity mapping is planned for a complete 243-day cycle, or one Venus rotation, during the fourth cycle around Venus.

       In the first 243-day mapping cycle, Magellan mapped 84 percent of Venus and at the end of the second cycle had mapped 94 percent. Magellan has provided a near-global view. This map revealed wind patterns from the distribution of wind-blown deposits of sand and dust.

       Although no Earth-like plate tectonic features have been identified, Saunders said, the global distribution of fractures and various terrain types can be determined.

       In the second cycle, Magellan pointed its imaging radar to the opposite side, mapping to the right. Most of the features look similar, but some appear very different from the opposite angle, he said. The right-pointing imaging also allowed the first views of Venus' south polar region.

       A thick volcanic lava field has been measured from opposite sides revealing a thickness of about 1,000 meters (3,000 feet) of congealed lava. The lava was once a thick, viscous molten rock that poured from the subsurface of Venus.

       Impact craters on Venus also can produce flows that resemble volcanic flows. One of the puzzles is why some large impact craters appear to have produced large volumes of lava-like flows, and others do not produce flows at all, Saunders said.

       Detailed shapes of impact craters also are being determined from opposite side images and stereo.

       JPL manages the Magellan Project for NASA's Office of Space Science and Applications.


3/16/92 JJD