Eventually, the problem fixed itself, leaving scientists with a little mystery for their trouble.

The rovers, which do look a bit like six-wheeled dune buggies, landed on opposite sides of Mars last January and promptly set off on their missions to explore the Red Planet. In addition to the equipment used to gather information about the environment, the vehicles carry an array of sensors used to monitor their operating condition. These include circuits that measure the electrical current needed to turn each wheel. By summer, they were showing a problem: The readings for Spirits right front wheel began to climb above the other five.

When things go the way they should, the torque for all the wheels should stay in family, said Henry W. Stone, chief of the Spacecraft Rover Engineering Team at the lab. He spoke to Lube Report during a telephone interview last week. What we were seeing was that readings for one wheelweretrending out of sync with the others. When you see that, you become concerned that there is some kind of degradation process affecting it.

Stones team went to work developing theories about potential causes for the aberrant readings. Their first thought was that some type of internal or external contamination may have been interfering with normal operations. But when engineers examined current profiles for the problem wheel, they were unable tocorrelate the fluctuations with degradation ofany particular element of the gear train.

At that point, we began to look into loss of lubricant, Stone said, explaining that the rovers drive trains are lubricated with a very thin specialty grease capable of performing in Red Planets frigid temperatures, which are often around minus 100 degree C. Perhaps extended exposure to such coldhad caused the grease to thicken and become stranded away from contact surfaces. We said, If we assume it is a lubricant migration problem, what could we do to ameliorate it?

The answer was to raise the temperature of thegrease by activating heaters and turning the vehicle so that the troubled wheel was exposed to the sun.

We did that and then tested the wheel again, Stone said. There was some improvement but not much, so that led us to think that something else must be causing the problem.

Team members decided that the problem was more likely due to mechanical wear somewhere on the drivetrain and therefore not something that could be fixed. Their focus shifted then to finding ways to minimize the pace at which it worsened, in hopes of prolonging Spirits motoring life. Instead of continuing to operate the problem wheel, the lab had the rover drag it most of the time, activating it only when challenging terrain made it absolutely necessary.

Then, in early September, while Spirit was climbing a hill, the torque readings for the problem wheel suddenly dropped so that it was more closely in line with the others. The labs engineers were surprised but pleased.

At the present time, the problem has significantly subsided and we are experiencing no constraints on the use of the rover, Stone said.

His teams best guess is still that the problem was caused by mechanical wear along the drivetrain. Stone said NASA will probably never definitively answer this tribological riddle. Overall, the agency is thrilled that Spirit and Opportunity have lasted as long as they have. The rovers original missions were for just 90 days, so they have already exceeded that timeline three times over and continue gathering information.

Spirit has traveled almost 2.5 miles, so it still has quite a distance to go before being due for its 3,000-mile oil change. Thats good, because the Jet Propulsion Lab can’t do that from Earth – and there dont appear to be any quick lubes in the neighborhood.