Illustration of Mars Global Surveyor

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 Propulsion Laboratory.

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 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 Pathfinder.

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, Washington, D.C.

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