Reaction wheels are used by spacecraft for stabilization and to adjust attitude - the direction a spacecraft points in relation to the sun, Earth or Mars. Should attitude be lost, solar panels may point away from the solar energy source (draining batteries) or communication antennae may point in the wrong direction (causing loss of contact), jeopardizing the mission.
Reaction wheels, positioned at right angles to each other inside the spacecraft, are spun-up or spun-down so a gyroscopic force can be applied to the spacecraft's body, allowing it to turn. As the reaction wheels use electricity harvested from solar panels, valuable fuel is not wasted on orienting the spacecraft.
But for a few minutes on June 8, for reasons unknown, one of Odyssey's reaction wheels became stuck, prompting NASA mission controllers to switch the satellite into an "Earth-directed safe mode." This is the first time the spacecraft's reaction wheel configuration has suffered a glitch since launch in 2001.
Controllers have now enlisted the help of a spare reaction wheel to allow control of the spacecraft's orientation when facing down. Odyssey should commence normal operations some time next week.
HOWSTUFFWORKS: How Mars Odyssey Works
Odyssey is one of three spacecraft currently in orbit around Mars. The European Mars Express and NASA's Mars Reconnaissance Orbiter (MRO) joined Odyssey in Mars orbit in 2003 and 2006, respectively.
It continues to be the longest-lived Mars observation satellite in history and acts as a critical communications relay between NASA's tenacious rover Opportunity and Earth. With help from its younger sibling, the MRO, Odyssey will also be used as a relay for communications with the Mars Science Laboratory (MSL) Curiosity which touches down on the Martian surface Aug. 6.
Image credit: NASA