Rover drivers are aided by the simulator, but they also must rely on their imaginations and experience to safely traverse the terrain, he said. "You have to make sure that the series of commands that you're sending to the rover can detect if something goes wrong and recover appropriately."

The commands are sent in text that the rovers can understand and interpret. "You can try and think of it as a sort of Pidgeon English," a simplified language with some numbers and measurements thrown into guide the rovers, he said.

When the rovers first began exploring Mars after landing in January 2004, it took the rover drivers nearly 19 hours to compile the commands for the next day's activities, Maxwell said. Now the driver teams can do the work in eight to 10 hours. "It's a good, full work day," he said.

"I get up and I go to work and I drive a rover around on another planet," he said. "It's the greatest job on two planets."

The six-wheeled rovers were designed to explore the planet for 90 days, but almost four years later, they are still working and collecting data. There have been software upgrades, hardware failures and other problems, but the missions continue.

The rover Spirit 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, Maxwell said.

"It's like driving a broken grocery cart by remote control from 100 million miles away," he said. "So it has its challenges."

Rover driver, Ashley Stroupe, 39, said the driver teams must work well together to be sure every command they send is correct and accomplishes the experiments planned for each day. But because the rovers are designed with deep safety features, they are able to "sense" dangers and can adjust their commands and paths without human intervention, she said.

"They're smart enough to monitor their safety, particularly in tricky terrain," Stroupe said. "If we [accidentally] tell it to drive into a rock and it's paying attention to what it's doing, it won't run into the rock" because it will sense the danger through its two "eyes," or cameras, that can see the obstacle. The rovers can compensate for such dangers using safety parameters that are programmed into them, she said.

"These are very complicated systems," Stroupe said. "It's a huge team effort, and we couldn't do it without everybody working together. Everybody on the project gives 120 percent because we're all so thrilled about being involved in such a special mission."

"Probably the most special day I've had as a rover [driver] was the day I built my first drive solo" on Mars, Stroupe said. "We were on the plateau on Husband Hill, and we were driving along the edge to get imagery of the valley below. I parked us right on the edge and got a spectacular view."

"I remember looking at those tracks and realizing what they meant -- my "first tracks on Mars, and the first tracks actually made by a woman driving on another planet," she said. "I am proud of it every time I see that panorama from the very top looking down at those tracks."