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They've dealt with software glitches, a damaged and useless front wheel, and fierce dust storms that cut their electrical power production. Still, the two NASA Mars rovers continue to explore the Red Planet and beam back scientific data.

Now, for the fifth time since they landed in January 2004 for what were expected to be two separate 90-day missions, NASA has extended the rovers' exploratory tasks again.

There are no guarantees that the two rovers, named Spirit and Opportunity, will keep working as they have, but NASA said their missions will continue "possibly through 2009."

"We are extremely happy to be able to further the exploration of Mars," Alan Stern, associate administrator for NASA's Science Mission Directorate in Washington, said in a statement. "The rovers are amazing machines, and they continue to produce amazing scientific results, operating far beyond their design life."

Last month, the two rovers resumed work as summer dust storms began letting up, allowing them to produce needed electrical power by collecting sunlight with their solar panels. Opportunity has been driving into Victoria Crater in Mars' Meridiani Planum region, giving scientists an up-close view inside the half-mile-wide, 230-foot-deep crater. Meanwhile, Spirit has traversed a volcanic plateau near its landing site.

"We got a little bit of a scare with the dust storms," said Diana Blaney, deputy project scientist for the Mars rover projects. "I think this has given us a new lease on just how special...each day that we operate these vehicles is [to NASA]. It's a very energized period right now."

A team of about 14 specially trained Mars rover drivers have been putting the six-wheeled rolling geological vehicles through their paces since the missions began.

"After more than three and a half years, Spirit and Opportunity are showing some signs of aging, but they are in good health and capable of conducting great science," John Callas, rover project manager at NASA's Jet Propulsion Laboratory said in a statement. Since landing, the rovers have had to surmount a host of technical issues. Just a few weeks after landing, the Spirit rover had an out-of-memory problem that almost ended its mission before it began, but scientists were able to get the rover back into operation. In April 2004, both needed software updates to correct problems and improve their performance.

The Spirit rover has a right front wheel that no longer can rotate on its axle and has to be driven around gingerly as it drags its broken wheel, hampering its directional control.

The rovers' missions include researching the planet's geology and past environmental conditions. Opportunity has already found evidence in its landing zone of wetness on Mars that could have sustained microbial life, while Spirit found evidence that some form of water altered the mineral composition of some rocks, NASA said.

So far, NASA said, Spirit has driven 4.51 miles and sent back more than 102,000 photos of its exploration, while Opportunity has driven about 7.19 miles and sent back more than 94,000 images.