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Contact: Jim Doyle, JPL
FOR IMMEDIATE RELEASE                                                January 28, 1992

       The Magellan spacecraft began its third radar mapping cycle of the surface of Venus Friday, Jan. 24, and controllers at NASA's Jet Propulsion Laboratory said today the mapping was successful despite earlier transmitter problems.

       The second cycle ended Jan. 15 and controllers then successfully performed a planned week of battery recharging.

       The spacecraft experienced difficulty with the primary transmitter earlier this month. Mission Director Doug Griffith reported that the project decided to use the faulty backup transmitter on a reduced basis. The backup transmitter had been turned off last year because of a spurious signal that degraded data transmission back to Earth.

       Mapping is now continuing, Griffith said, after a new operational mode was devised for the backup transmitter which required the transmission data rate to be reduced from 268 to 115 thousand bits per second (kbps).

       Also mission controllers had to contend with interference on the spacecraft's main radio signal caused by the spurious signal. That problem is being managed by choosing one of the two primary radio frequencies as necessary to avoid the spurious signal frequency.

       For the first five orbits starting on January 24, controllers tested various configurations of the radio frequencies to analyze the spurious signal. Finally the spurious signal settled at a frequency which allowed satisfactory radar data to be transmitted.

       The transmitter problem is, however, still under investigation by a special radio anomaly team. The team will try to determine the most probable failure mechanism in both transmitters and devise additional methods to optimize use of the backup transmitter.

       Now that mapping is continuing, stereo mapping of selected targets is underway as well as mapping of areas not previously imaged. The first stereo imaging target is Maxwell Montes, the highest mountain on Venus.

       Additionally, Project Scientist Steve Saunders said the science team has conducted a global survey based on the data acquired during the first two cycles and found that about 85 percent of the planet is covered by volcanic rocks, mostly lava flows that form the great plains.

       Much of the remaining 15 percent, he said, is high standing, chaotic material that he said was faulted and fractured. "Little can be determined about the composition or nature of the rocks that form these highlands," he said.

       One possibility, however, is that the highland rock represents crustal material that formed soon after the final accretion of the planet 4.6 billion years ago, he said.

       The high, ridged material is seen in the continent-like terrains such as Aphrodite Terra and Maxwell Montes. "These regions often appear as islands surrounded by the more recent lavas that form the volcanic plains," Saunders said.

       All of the terrestrial planets appear to have formed relatively low density crusts, Saunders said. "Little of the ancient crust remains on Earth because of the constant churning of plate tectonics."

       While Venus does not have Earth-like tectonic plates, it has a long, complex history of deformation with many episodes of tectonics and faulting.

       "The Magellan data will provide the key to understanding that complex history and perhaps to the understanding of many of Earth's geologic puzzles," Saunders said.

       Magellan has already mapped more than 95 percent of Venus. The primary objective for the Magellan mission called for one mapping cycle to obtain images of 70 percent of the planet.

       The Jet Propulsion Laboratory manages the Magellan Project for NASA's Office of Space Science and Applications.


1/29/92 JJD