Test of Spare Wheel Puts Orbiter on Path to Recovery
Mission controllers test a spare reaction wheel on NASA's Odyssey for potential use with two other reaction wheels in adjusting and maintaining the spacecraft's orientation.
Mars Odyssey Mission  Status ReportPASADENA, Calif. -- In a step toward returning NASA's Mars  Odyssey orbiter to full service, mission controllers have tested a spare reaction  wheel on the spacecraft for potential use with two other reaction wheels in  adjusting and maintaining the spacecraft's orientation.After more than 11 years of non-operational storage, the  spare reaction wheel passed preliminary tests on Wednesday, June 12, spinning  at up to 5,000 rotations per minute forward and backward. Odyssey engineers  plan to substitute it for a reaction wheel they have assessed as no longer  reliable. That wheel stuck for a few minutes last week, causing Odyssey to put  itself into safe mode on June 8, Universal Time (June 7, Pacific Time). Safe  mode is a precautionary status with reduced activity."We are taking steps to assess the replacement of the troublesome  wheel with the spare that Odyssey has been carrying for exactly this  purpose," said Mars Odyssey Project Manager Gaylon McSmith of NASA's Jet  Propulsion Laboratory, Pasadena, Calif. "If the assessment results are  positive, this will put us on a path toward resuming full use of Odyssey."Like many other spacecraft, Odyssey uses a set of three  reaction wheels to control its attitude, or which way it is facing relative to  the sun, Earth or Mars. Increasing the rotation rate of a reaction wheel inside  the spacecraft causes the spacecraft itself to rotate in the opposite  direction. The configuration in use since launch combines the effects of three wheels  at right angles to each other to provide control in all directions. The orbiter  carries a fourth reaction wheel skewed at angles to all three others so that it  can be used as a substitute for any one of them. This spare wheel had not rotated  since before Odyssey's April 7, 2001, launch.Odyssey can also use thrusters to control its attitude. Reaction  wheels offer the advantage of running on renewable electricity from the  orbiter's solar array, rather than drawing on the finite supply of thruster  fuel. They also provide more precise control of pointing, which can enable  higher data-rate communications through the orbiter's directional antenna.Odyssey has worked at Mars for more than 10 years, which is longer  than any other Mars mission in history. Besides conducting its own scientific  observations, it serves as a communication relay for robots on the Martian  surface. NASA plans to use Odyssey and the newer Mars Reconnaissance Orbiter as  communication relays for the Mars Science Laboratory mission during the landing  and Mars-surface operations of that mission's Curiosity rover.Odyssey is managed by NASA's Jet Propulsion Laboratory,  Pasadena, for NASA's Science Mission Directorate in Washington. Lockheed Martin  Space Systems in Denver built the spacecraft. JPL and Lockheed Martin  collaborate on operating the spacecraft. For more about the Mars Odyssey  mission, visit:http://mars.jpl.nasa.gov/odyssey.