Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                 
March 16, 1992
(Phone:  202/453-1547)

Jim Doyle
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)
 
 
RELEASE:  92-36

MAGELLAN TO ORBIT JUST ABOVE VENUS' DENSE ATMOSPHERE 

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

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

   The stereo mapping phase, which began Jan. 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 later 
that month.  Magellan is in its third mapping cycle around 
Venus.

   The spacecraft's closest approach to Venus will be moved 
downward from 186 miles to 111 miles above the surface for 
maximum sensitivity to  variations in the gravity field, he 
said.

   Gravity is mapped by analyzing slight variations in 
Magellan's 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, providing a near-global view.  This map revealed 
wind patterns from the distribution of wind-blown deposits of 
sand and dust.

                           - more -
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   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 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. 
                            - end -

NOTE:  To illustrate this story, three photographs are 
available to media representatives through NASA's Audio/Visual 
Branch, (202/453-8375).  

Photo Numbers:
         Color                                    B&W
     92-HC-31                                92-H-34
                                                    92-H-79
                                                    92-H-141