Douglas Isbell
Headquarters, Washington, DC          April 30, 1997
(Phone: 202/358-1753)

Diane Ainsworth

Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)


RELEASE: 97-85

MARS GLOBAL SURVEYOR TO
AEROBRAKE IN MODIFIED CONFIGURATION

     NASA's Mars Global Surveyor spacecraft can safely
and successfully aerobrake into its final orbit around
Mars this fall with its one partially deployed solar
panel in a modified configuration, mission managers
have decided.

     No special maneuvers will be conducted to attempt
to force the array to latch, and the focus of the
Surveyor engineering team now will turn to minor
modifications to the critical aerobraking phase that
will circularize the spacecraft's orbit for the
beginning of two years of science operations.

     "After careful analysis of the situation, we've
determined that the solar panel on Mars Global
Surveyor that is not fully deployed presents very
little risk to the mission," said Glenn E. Cunningham,
Mars Global Surveyor project manager at NASA's Jet
Propulsion Laboratory (JPL), Pasadena, CA.

     The decision by NASA's flight team at JPL and its
partners at Lockheed Martin Astronautics, Denver, CO,
was reached after several months of extensive analysis
of spacecraft data, ground-based computer simulations
and a series of very slight spacecraft maneuvers that
were carried out in January and February to
characterize the situation.

     "Thanks to an early launch that gave us an
advantageous trajectory, we will not have to aerobrake
into the Martian atmosphere as fast as we had
originally planned to reach the mapping orbit, and
that will reduce the amount of heating that the solar
panels undergo during this gradual descent,"
Cunningham explained.

     "We will rotate the solar-cell side of the panel
that is not fully deployed by 180 degrees, so that it
faces into the direction of the air flow that exerts
drag force on the spacecraft as it dips repeatedly
into the atmosphere," he said. "This way, the
unlatched panel will not be in danger of folding up
onto the spacecraft's main structure, nor will the
panel be at any greater risk of heating up too much."

     The solar panel in question is one of two 11-foot
wings that were unfolded shortly after Surveyor's Nov.
7, 1996, launch from Cape Canaveral Air Station, FL.
Data suggest that a piece of metal called the "damper
arm," which is part of the solar array deployment
mechanism located at the "elbow" joint where the
entire panel is attached to the spacecraft body,
probably was sheared off during deployment in the
first day of flight.  The lever that turns the shaft
became wedged in a two-inch space between the shoulder
joint and the edge of the solar panel, leaving the
panel tilted at 20.5 degrees from its fully deployed
and latched position.

     Although the situation was never considered a
serious threat to accomplishing the science objectives
of the mission, the tilted array caused the
JPL/Lockheed Martin flight team to re-evaluate the
aerobraking phase, in which the spacecraft must rely
almost solely on its solar panels for the drag needed
to lower it into a nearly circular mapping orbit over
the poles of the planet.  This phase of the mission
will begin a week after Mars Global Surveyor is
captured in orbit around Mars on Sept. 11, and will
last approximately four months.

     Aerobraking was first tested in the final days of
the Magellan mission to Venus in October 1994. The
technique is an innovative method of braking which
allows a spacecraft to carry less fuel to a planet and
take advantage of the planet's atmospheric drag to
descend into a low-altitude orbit.

     Mars Global Surveyor will use an aerobraking
phase much like that used to circularize Magellan's
orbit.  The solar wings -- which feature a Kapton flap
at the tip of each wing for added drag -- supply most
of the surface area that will slow the spacecraft by a
total of more than 2,684 miles per hour during the
four-month phase.  Surveyor's orbit around Mars will
shrink during this phase from an initial, highly
elliptical orbit of 45 hours to a nearly circular
orbit taking less than two hours to complete.

     Engineers determined that the deployment springs
currently holding the tilted solar panel in its nearly
deployed position will not be strong enough to
withstand the forces of aerobraking. To solve that
problem, they designed a new configuration in which
the tilted solar panel, along with the deployment
springs, will be rotated 180 degrees, using a motor-
driven inner gimbal actuator, and held in position
with force applied by an outer gimbal actuator.
Sequencing software will be modified to turn the
gimbal actuators on before each closest approach to
the planet and off at the conclusion of each drag
pass.

     As a consequence of the new aerobraking
configuration, the more sensitive cell-side of the
unlatched wing will be exposed directly to the wind
flow of atmospheric entry, requiring that aerobraking
be done in a more gradual, gentle manner.  Ground
tests have demonstrated that the unlatched solar panel
will have more than adequate thermal margin to
withstand additional heating as the spacecraft
circularizes its orbit for the beginning of science
mapping in March 1998.

     Meanwhile, Mars Global Surveyor continues to
perform very well on its arcing flight path toward the
red planet and its arrival in orbit.  A third, very
minor trajectory correction maneuver, planned for
April 21, was deemed unnecessary and canceled. In
addition, science instrument calibrations continue to
go well, and plans are being prepared to take an
approach image of Mars a few days before the July 4
landing of Mars Pathfinder, which passed Mars Global
Surveyor enroute to Mars on March 14, 1997.

         Mars Global Surveyor is the first mission in
a sustained program of robotic exploration of Mars,
managed by JPL for NASA's Office of Space Science,
Washington, DC.

                     -end-