Engineers operating NASA's highly successful Mars Global Surveyor spacecraft, which concludes its primary mission at the red planet next week, report that one of the spacecraft's reaction wheels, which helps stabilize the spacecraft's orientation, has turned itself off. The spacecraft correctly responded by switching to a backup reaction wheel, allowing Global Surveyor to continue its mapping mission without interruption.
The mission proceeds as planned, and the spacecraft is operating normally.
Engineering data from the spacecraft indicate that an electrical short led to the automatic shutdown of the X-axis reaction wheel on Jan. 18. Spacecraft orientation is stabilized by the rotation of three such wheels (X,Y and Z) during normal operations.
Subsequent attempts to restart the X-axis reaction wheel on Jan. 24 lead mission engineers to believe that a short circuit caused the reaction wheel subsystem's 7 amp fuse to become permanently open, permanently disabling the reaction wheel. Should another wheel fail, the spacecraft would use its attitude control thrusters to maintain proper orientation much like the previous Mariner and Viking Orbiters.
On Jan. 31, Mars Global Surveyor will conclude its mapping mission phase having accomplished all of the planned science objectives during a full Mars year (687 days). Over the next 14 months, continued operation of the science instruments will provide observations of potential future landing sites, make inter-annual seasonal comparisons and obtain measurements of targets selected by guest observers. Following this period, operations personnel at JPL in Pasadena, Calif., and Lockheed Martin Astronautics in Denver, Colo., expect to coax the aging spacecraft to perform telemetry relay during the landing of the Mars Exploration Rovers in January of 2004.
The Global Surveyor mission is managed by JPL for NASA's Office of Space Science, Washington, D.C. Lockheed Martin Astronautics developed and operates the spacecraft. JPL is a division of the California Institute of Technology.
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