PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011

Contact: Franklin O'Donnell

FOR IMMEDIATE RELEASE                    September 7, 1994

     NASA's Magellan spacecraft has begun a unique
experiment designed to return data about the upper
atmosphere of Venus and the behavior of a spacecraft
entering it.  The experiment marks the beginning of final
activities for the spacecraft, which is expected to burn up
in the atmosphere of Venus by October 14.

     "This is the next to last act of a truly magnificent
performance by Magellan and its science and operations
teams," said Dr. Wesley T. Huntress, associate administrator
for space science at NASA Headquarters, Washington, D.C.
"Magellan has far surpassed all of its original mission
goals and, in the process, revolutionized our understanding
of a planet that represents what Earth might be like with a
runaway greenhouse effect."

     With its primary mission of mapping the surface of
Venus successfully accomplished, Magellan has been used for
a series of experiments that were unanticipated before its
launch.  In the latest maneuver, known as the "windmill"
experiment, the spacecraft's wing-like solar arrays are
turned in opposite directions -- like windmill sails -- to
encounter pressure from molecules in the upper atmosphere of
Venus.

     The experiment is measuring how much torque will be
needed to keep the spacecraft from spinning on its axis,
said Project Manager Doug Griffith at NASA's Jet Propulsion
Laboratory.  These data will allow engineers and scientists
to better understand basic gas-surface interactions and to
gain additional aerodynamic and atmospheric data on Venus
for future mission designs.

     The windmill experiment is scheduled to last until
September 14.  Two weeks later, more orbit trim maneuvers
are scheduled to lower the spacecraft's altitude to prepare
for the final termination experiment.  Three further trim
maneuvers will change the altitude by 8 to 9 kilometers (5
to 6 miles) each on October 10, placing the altitude of
periapsis -- or closest approach to the planet -- at 155
kilometers (96 miles).  The spacecraft's orbit will be
lowered finally to 136 kilometers (85 miles) on October 12,
with Magellan again put in a windmill attitude to collect
more atmospheric data during its final entry.  Gravity data
acquisition will continue during all these periods up until
October 10.

     "After October 12, Magellan will permanently enter the
atmosphere in about two days, possibly in one day," Griffith
said.  The atmosphere will drag the spacecraft toward the
surface of the planet, but it will burn up high in the skies
over Venus, he said.

     There are two primary possibilities leading to NASA's
final loss of contact with Magellan, Griffith said.  Either
the spacecraft will overheat and its communications systems
will be damaged, or Magellan's control thrusters will be
unable to maintain pointing control toward ground-based
receiving dishes on Earth as the spacecraft spins to its
demise.

     In recent weeks, the performance of the spacecraft's
solar arrays has begun to degrade due to the extreme
temperature changes as the spacecraft passes from direct
sunlight to shadow during its orbit.  The thermal stress
after more than five years in space and several weeks in low
orbit has caused degeneration of its solar cell connections
and has brought the spacecraft near the end of its useful
life, Griffith said.

     "It is a race to the finish," said Betsy Beyer,
Magellan program manager at NASA Headquarters.  With the
continuing loss of power due to the solar cell degeneration,
the spacecraft may shut down even earlier than projected,
before a planned entry experiment.  "Magellan has done more
than its duty," Beyer said.  "If it goes in its own way,
instead of how we planned to end it, it is still a winner."

     Controllers sent commands to Magellan in late August
for orbital trim maneuvers that reduced its altitude from a
near-circular orbit of 197 by 541 kilometers (123 by 338
miles) to an orbit of 172 by 390 kilometers (107 by 242
miles).  These altitude reductions were required to set up
conditions for the final experiment phases.

     Magellan was launched in May 1989, and began mapping
the surface of the planet using synthetic aperture -- or
side-looking -- radar in September 1990.  In three cycles,
each comprising one Venus day or 243 Earth days, the
spacecraft mapped 98 percent of the planet's surface,
providing unprecedented views of its unique pancake domes of
lava, strange volcanic structures, craters and high
mountains.

     In three subsequent cycles, it has measured Venusian
gravity over 95 percent of the planet, gathering data so
that scientists can map the planet's interior.  Magellan
also has contributed to ongoing study of the planet's
massive atmosphere of carbon dioxide and high sulfuric acid
clouds.  This period included the first-ever "aerobraking"
of a spacecraft into a near-circular planetary orbit, for
Magellan's final operations.

     The data which have streamed back from Magellan's radar
imager, its atmospheric studies and its gravity data
acquisition maneuvers have built a vast data base of new
knowledge about Venus and the formation of the solar system
that will be studied by scientists for decades to come,
project officials said.

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

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9/7/94 JJD
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