PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE
April 30, 1997
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 will turn now 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
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
"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 3.5-meter (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" -- part of the
solar array deployment mechanism located at the "elbow" joint
where the entire panel is attached to the spacecraft body -- was
probably sheared off during deployment in the first day of
flight. The lever that turns the shaft became wedged in a five-
centimeter (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 September 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 1,200 meters per second (about
2,700 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
Mars Global Surveyor is the first mission in a sustained
program of robotic exploration of Mars, managed by the Jet
Propulsion Laboratory for NASA's Office of Space Science,