In a prelude of more problems that are likely to arise, both of NASA's Mars rovers experienced glitches this week as they plow through unknown engineering territory, operating well beyond what the mission blueprints called for.

The twins are working at reduced capacity while project managers try to figure out what's wrong. Both rovers had 90-day primary missions and have more than doubled that time on the surface of the red planet.

Spirit is climbing the rocky Columbia Hills of Mars, examining bedrock for signs of past water.

While executing commands on Aug. 1, a semiconductor component on Spirit failed to power on as intended, according to a NASA statement issued today. The component, a programmable gate array, directly affects usability of the rover's three spectrometer instruments, which analyze light from various targets.

Fix likely

Subsequent commands for using the miniature thermal emission spectrometer in that day's sequence resulted in repeated error messages.

The most likely cause is a timing issue of one instruction reaching the gate array microseconds before another that was intended to precede it, engineers have determined. If that diagnosis is confirmed, a repeat could possibly be avoided by inserting a delay between commands that might reproduce the problem. Until then, Spirit cannot use the miniature thermal emission spectrometer, the Moessbauer spectrometer or the alpha particle X-ray spectrometer.

"While we're being very cautious in how we operate today and tomorrow, we expect to verify the problem and resolve this issue with a relatively easy workaround," said Jim Erickson, project manager for the twin rovers at NASA's Jet Propulsion Laboratory.

Already, Spirit has been driving backwards on just five of its six wheels, since the right front wheel developed extra resistance in July. Managers are saving the wheel for use only when it is truly needed.

Spirit has traveled more than 2.1 miles (3.5 kilometers) since landing in January.

Opportunity

On the other side of the planet, Opportunity has driven about 66 feet (20 meters) into Endurance Crater, looking at older bedrock with each step down. The rover's odometer is nearing 1 mile (1.5 kilometers) for its entire mission.

Four times in the past two weeks, Opportunity has sent error messages while successfully taking pictures with its microscopic imager, officials said. The problem might be related to degradation of flexible cabling that runs down the rover's robotic arm to the instrument.

As a precaution, the rover team is being cautious about using the arm while they try to diagnose the problem.

"We are being very conservative about this because we certainly don't want to do anything to jeopardize the instruments," said Ken Herkenhoff of the U.S. Geological Survey Astrogeology Team, Flagstaff, Ariz. And lead scientist for both rovers' microscopic imagers. "We are running more diagnostics that we hope will identify the problem."

There are potential explanations might lead to restoring full use of the arm. But mission managers know the rovers' days are ultimately numbered.

"We will no doubt have more issues with them in the future," Erickson said. "We'll do everything we can to milk the most value out of them while they are usable, but they won't last forever."