National Aeronautics and Space Administration engineers said the standby mode was triggered by an unexpected June 3 rebooting of the spacecraft's computer.
Engineers brought the spacecraft out of the standby mode Saturday and cameras and other scientific instruments resumed operation Tuesday.
The Mars Reconnaissance Orbiter reached Mars in 2006 and has returned more data about the planet than all other Mars missions combined, NASA said.
"The June 3 rebooting resembled a Feb. 23 event on the spacecraft," the space agency said in a statement. "Engineers are re-investigating possible root causes for both events. The new investigation includes reconsidering the likelihood of erroneous voltage readings resulting from cosmic rays or solar particles hitting an electronic component."
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