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Contact: Franklin O'Donnell

FOR IMMEDIATE RELEASE                                                 May 26, 1993

       Having successfully completed its original mission of radar mapping the planet Venus, NASA's Magellan spacecraft is embarking on a new experiment that will give scientists glimpses into the planet's interior and a better understanding of its atmosphere.

       Magellan scientists and mission managers reported on the project's recent findings and a new experiment in "aerobraking" it started this week in a press conference today.

       On May 25 the spacecraft completed its fourth eight-month orbital cycle at Venus, during which it collected data on the planet's gravity field, particularly close to the equator.

       On that same day Magellan executed the first in a series of aerobraking maneuvers to be conducted over the next 70 days in which it dips into Venus' atmosphere, taking advantage of drag on the spacecraft to lower its orbit. The maneuvers are designed to place Magellan in a circular orbit, allowing it to get better gravity data at the planet's north and south poles.

       "This experiment is a scientific bonus for what is already a highly successful mission," said Dr. R. Stephen Saunders, Magellan project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

       According to Saunders, the gravity data that Magellan is collecting allow scientists to "see" into the interior of the planet because they can gauge how dense the material underlying various parts of the planet is.

       In recent weeks, for example, Magellan passed over a region dominated by three volcanoes -- Hathor, Innini and Ushas. "They occupy a broad swelling of the Venusian crust believed to result from upwelling of hot material from the deep interior, a phenomenon known on Earth as a `hot spot,'" Saunders added.

       In other ways, however, Venus seems to be distinctly different from Earth. While Earth's surface geology is largely created by tectonic motion -- enormous continental plates that move slowly over an underlying magma -- the Magellan team found little evidence of plate tectonics at Venus.

       One possible exception to that is the Ovda region at the western end of the equatorial highlands of Aphrodite Terra.

       "In this region we see what appear to be the closest thing on Venus to Earth's continents," said Saunders. "It has features that seem to have been formed by compression of the Venusian crust in a process that may resemble some plate tectonic regions on Earth."

       Saunders said that Ovda and similar terrains -- called tesserae, they are intensely fractured regions that are pushed upward compared with most of the rest of the planet -- may represent ancient crustal materials on Venus. "They could, in fact, be fragments of the oldest rocks on the planet," he said.

       During Magellan's fourth eight-month orbital cycle which ended May 25, flight controllers collected gravity data by monitoring the frequency of the signal sent to Earth from the spacecraft. Changes in the gravity field would make Magellan speed up or slow down slightly, causing the frequency of its signal to change by tiny fractions.

       During that cycle, however, Magellan was in a widely looping elliptical orbit, with a low point near 170 kilometers (105 miles) from Venus and high point of 8,500 kilometers (5,300 miles). Because of the varying distance, it could collect high resolution gravity data at the planet's equator but not near its poles.

       If successful, the aerobraking maneuver will put Magellan into an orbit 200 by 600 kilometers (125 by 375 miles) above Venus. The change will be made gradually over the course of about 70 days.

       The change in orbit will also provide important new data about Venus' atmosphere, which can be studied through its effect on the spacecraft. The upper atmosphere varies with the 11-year cycle of activity on the sun. "We are currently approaching a solar minimum, which means that the number of sunspots and solar storms will be at a minimum," said Saunders.

       Magellan has fulfilled all of its prime mission objectives, mapping 98 percent of the surface of Venus with many areas covered up to three times. "This provides us with stereo imaging," said Saunders, "as well as a long-time base so that we can search for surface changes in the high-resolution images."

       JPL manages the Magellan mission for NASA's Office of Space Science, Washington, D.C.


5/19/93 FOD