The worst of the Martian winter is over for the Mars Exploration Rovers, but the robots' own dark days appear to be looming as Spirit hits a 'bump' that's kept her at a standstill for a week now.
While Opportunity has been continuing her investigation of the stadium-sized Endurance Crater at Meridiani Planum, Spirit has been climbing into the Columbia Hills uplands in Gusev Crater, and in recent weeks, has been inching her way up the West Spur towards the top of Husband Hill, a peak named after Columbia Commander Rick Husband.
All things had been steady as she goes until last Friday when the relay for steering actuators on Spirit's right-front and left-rear wheels failed to respond to driver commands. The robot field geologist has not moved since and MER engineers are currently investigating possible causes and solutions.
Each of the front and rear wheels on the rovers has a steering actuator, or motor, that adjusts the direction in which the wheels are headed independently from the motor that makes the wheels roll. When the actuators are not in use, the electric relays are closed and the motor acts as a brake to prevent any unintended turning of the wheels. Spirit's right front wheel and left rear wheel are, in essence, now locked, much like the wheels on a car when the emergency brake is in place.
Spirit has been having trouble with her right front wheel since last July when she experienced so much increased internal resistance that she wound up having to drag that wheel. This was the "first sign of wear in the actuator to Spirit's right front drive," Joe Melko, rover engineer at the Jet Propulsion Laboratory (JPL), acknowledged then. The engineers attempted to mitigate that problem throughout the summer by redistributing the wheel's lubricant through rest and heating, but those efforts were only partially successful, so they ordered the intrepid little rover to drive backward more than forward as part of a creative plan to accommodate her 'arthritic' front wheel. The rovers are equally comfortable driving forward from either end so Spirit has carried on without much whining.
The steering actuator is a different motor, however, so the MER engineers don't believe the steering-brake issue is related to the excessive friction Spirit experienced in her right front wheel last summer.
Yesterday, the engineering team received results from the first set of diagnostic tests on the most recent anomaly Spirit has encountered and immediately began interpreting the data and planning additional tests, according to Rick Welch, rover mission manager at JPL. "We hope to determine the best work-around if the problem does persist," he said.
"If we do not identify other remedies, the brakes could be released by a command to blow the fuse controlling the relay, though that would make those two brakes unavailable for the rest of the mission," explained Jim Erickson, rover project manager. Without the steering-actuator brakes, small bumps, dips, or potholes that a wheel hits during a drive would probably cause the wheel to twist and turn away from the intended drive direction. "If we do need to disable the brakes, errors in drive direction could increase. However, the errors might be minimized by continuing to use the brakes on the left-front and right-rear wheels, by driving in smaller segments, and by adding a software patch to reset the direction periodically during a drive," Erickson added.
In the meantime, Spirit continues to study and collect data from what she can, using her robotic arm and camera mast to examine rocks and soils in the immediate area. Until the cause of the anomaly is understood and corrective measures can be implemented, the team will keep the rover parked right where she is now. To date, she has driven more than 2.2 miles [3.6 kilometers], six times the distance set as a goal for mission success.
On the other side of the Red Planet, Opportunity is slowly making tracks that will eventually take her to Burn's Cliff, named after the late Mars scientist Roger Burns. She is currently examining an odd, lumpy rock that the team has dubbed Wopmay. The rock caught the MER science's team members' eyes some weeks ago because it is so strange looking and different from the other rocks in the area.