Science observations by the Curiosity Mars rover are on hold pending tests to pin down the cause of an unexpected voltage change that was detected last Sunday, NASA said in a status report Wednesday. There is no evidence the anomaly is related to a computer reboot earlier this month that triggered protective "safe mode" software.

The Curiosity rover recorded images for this self-portrait mosaic on Feb. 3, 2013. Credit: NASA/JPL-Caltech/MSSS
 
In this case, the rover did not go into safe mode, a sort of electronic hibernation, and Jim Erickson, Mars Science Laboratory project manager at the Jet Propulsion Laboratory, said tCuriosity "is safe and stable, fully capable of operating in its present condition."

"But we are taking the precaution of investigating what may be a soft short," he said.

A "soft short," NASA explained in the update, is an electrical leak in a component that is "partially conductive of electricity rather than a hard short such as one electrical wire contacting another."

The voltage change was detected between Curiosity's chassis and a 32-volt circuit that routes power to systems throughout the rover. The voltage level had been around 11 volts, but it changed to just 4 volts last Sunday. The update said the rover's electrical system is designed to tolerate such differences.

A soft short was detected when Curiosity landed in August 2012, changing the voltage between the chassis and the 32-volt circuit, or bus, from 16 volts to 11 volts where it remained until Sunday,

"Soft shorts reduce the level of robustness for tolerating other shorts in the future, and they can indicate a possible problem in whichever component is the site of the short," NASA said in the update. "Operations planned for Curiosity for the next few days are designed to check some of the possible root causes for the voltage change. Analysis so far has determined that the change appeared intermittently three times during the hours before it became persistent."

On Nov. 7, Curiosity went into safe mode when an error in its on-board software caused a subsequent error in a computer catalog file. That error, in turn, triggered a reboot when the catalog file was checked during a planned software upgrade. After duplicating the problem on the ground, flight controllers uplinked commands that restored the rover to normal operations on Nov. 10.

Curiosity landed in Gale Crater on Aug. 6, 2012. The rover currently is making its way toward the base of a towering mound of layered rock known as Mount Sharp expected to shed light on the planet's habitability and environmental changes that may help explain the transition between a warmer, wetter climate in the distant past to the dry, cold climate seen today.

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