Artist's concept of Mars Global Surveyor
Artist's concept of Mars Global Surveyor. Image credit: NASA/JPL
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Mission engineers studying a solar array on NASA's Mars Global Surveyor that did not fully deploy during the spacecraft's first day in space have concluded that the situation will not significantly impair Surveyor's ability to aerobrake into its mapping orbit or affect its performance during the cruise and science portions of the mission.

The solar panel under analysis is one of two 3.5-meter (11- foot) wings that were unfolded shortly after the Nov. 7 launch are used to power Global Surveyor. Currently the so-called -Y array is tilted 20.5 degrees away from its fully deployed and latched position.

"After extensive investigation with our industrial partner, Lockheed Martin Astronautics, using a variety of computer- simulated models and engineering tests, we believe the tilted array poses no extreme threat to the mission," said Glenn Cunningham, Mars Global Surveyor project manager at NASA's Jet Propulsion Laboratory. "We plan to carry out some activities in the next couple of months using the spacecraft's electrically driven solar array positioning actuators to try to gently manipulate the array so that it drops into place. Even if we are not able to fully deploy the array, we can orient it during aerobraking so that the panel will not be a significant problem."

Diagnosis of the solar array position emerged from two weeks of spacecraft telemetry and Global Surveyor's picture-perfect performance during the first trajectory maneuver, which was conducted on Nov. 21. The 43-second burn achieved a change in spacecraft velocity of about 27 meters per second (60 miles per hour), just as expected. The burn was performed to move the spacecraft on a track more directly aimed toward Mars, since it was launched at a slight angle to prevent its Delta third-stage booster from following a trajectory that would collide with the planet.

Both the telemetry data and ground-based computer models indicate that a piece of metal called the "damper arm," which is part of the solar array deployment mechanism at the joint where the entire panel is attached to the spacecraft, probably broke during the panel's initial rotation and was trapped in the 2-inch space between the shoulder joint and the edge of the solar panel, Cunningham said.

Engineers at JPL and Lockheed Martin Astronautics, Denver, CO, are working to develop a process to clear the obstruction by gently moving the solar panel. The damper arm connects the panel to a device called the "rate damper," which functions in much the same way as the hydraulic closer on a screen door acts to limit the speed at which the door closes. In Surveyor's case, the rate damper was used to slow the motion of the solar panel as it unfolded from its stowed position.

Engineers have been re-evaluating the aerobraking phase of the Global Surveyor mission, which begins in September 1997 after the spacecraft is captured into an elongated orbit around the planet using its onboard rocket engine. The solar arrays are essential to the aerobraking technique and will be used to drag the spacecraft into its final, circular mapping orbit. First tested on the Magellan spacecraft at Venus, aerobraking allows the spacecraft to carry less fuel to a planet and take advantage of its atmospheric drag to lower itself into the correct orbit.

"Because we launched early in our window of opportunity, we will not have to aerobrake as fast to reach the mapping orbit, and this reduces the amount of heating that the solar panels are exposed to," Cunningham said. "In the event that our efforts to latch the solar array properly in place are not successful, this reduced heating should allow us to tilt the array in such a way to prevent if from folding up and, yet, still provide enough useful aerobraking force." Additional analysis and testing will be performed over the next several months to verify this hypothesis.

Meanwhile, Mars Global Surveyor continues to perform very well as it completes its first two weeks in space, with ongoing science instrument calibrations being performed this week. At the same time, the Mars Relay radio transmitter has been turned on for a post-launch checkout. Radio amateurs around the world are gearing up to participate in a radio tracking experiment in which they will become receiving stations for the low-power beacon signal transmitted by the Mars Relay radio system.

Mars Global Surveyor is approximately 5.5 million kilometers (3.4 million miles) from Earth today, traveling at a speed of about 119,000 kilometers per hour (74,000 miles per hour) with respect to the Sun.

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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