Mars Global Surveyor to Aerobrake in Modified Configuration

From: George Zack <>
Subject: Mars Global Surveyor to Aerobrake in Modified Configuration
Date: Wed, 30 Apr 1997 17:39:22 +0100

>Date: Wed, 30 Apr 1997 12:45:46 -0400 (EDT)
>Subject:  Mars Global Surveyor to Aerobrake in Modified Configuration
>To: undisclosed-recipients:;
>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
>     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
>     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-

George Zack		Gilpin County School District RE-1
MS/HS Science Educator	Home of the Eagles /Elevation: 9200' AMSL		10595 Highway 119, Black Hawk CO 80403
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