Space missions often fit a pattern. They either fail right away, or they run and run.

NASA’s Mars rovers are clearly in the running category of space probes.

Rongxing Li and a team of Ohio State University mapping specialists have tracked the Mars rover Spirit since its landing on the planet three years ago today.

The rover and its twin, Opportunity, which arrived in an identical controlled crash landing three weeks later on the opposite side of Mars, were designed to work for three months. They have defied the most optimistic expectations.

"No one wants to make a prediction any more of how long they will go. Everyone has been wrong," said Li, a member of NASA’s rover science team.

The 384-pound, solar-powered vehicles are the size of a go-cart and have six wheels. They are equipped with high-tech cameras, tools and sensors. They have discovered signs of water, evidence of volcanoes, new rock types and meteorite impacts: everything but Martians.

Spirit has returned more than 88,800 images. Opportunity, more than 81,100.

The OSU scientists have produced detailed maps of the rovers’ separate routes. Spirit has traveled 4.3 miles. And Opportunity has gone 6.1 miles, surviving sandstorms, blistering days and freezing nights, dipping into Martian craters and climbing into the planet’s hills.

Li and the other mission scientists are now planning a fourth year of exploration for Spirit in the Gusev Crater and for Opportunity, 6,000 miles away, in the Meridiani Planum, where the rover discovered unmistakable signs that the area was once covered by a large, shallow body of water.

"We’re not trying to better that (discovery). We’re trying to add to our understanding," said Steve Squyres, a Cornell University planetary scientist and the science director of the mission. "How much water? What is its chemistry, the minerals formed in it and how frequently did it come to the surface?"

Scientists are planning to steer Opportunity into a large crater named Victoria in Meridiani Planum.

After spending the winter on the side of a hill to charge its solar batteries, Spirit started moving last week, only to have to stop to weather a dust storm.

"We saw a dip in energy. We had to drive to a nearby north-facing slope to get more energy and ride out the storm," Li said.

Geoff Landis, a physicist at NASA’s Glenn Research Center in Cleveland, helped design the rovers’ solar panels.

"Sooner or later, the motors are going to wear out. It’s hard to guess," Landis said. "The rovers are both getting a little bit arthritic."

A motor in one of Spirit’s wheels no longer works. Opportunity has a sticky steering actuator on one wheel, and its robotic arm has a sticky joint.

The scientists are learning just by keeping the rovers going, said Jamie Dyk, an engineer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. That knowledge will help in the design and construction of the Mars Science Laboratory, a minivan-size lab that is designed to roam the planet for two years. That mission is set to launch in 2009.

"I think every day, ‘How long can it last?’ " said Squyres, who won’t celebrate the third anniversary until Jan. 24, the date of Opportunity’s landing.

"Each day could be our last. We’re so far past warranty," he said. "But they haven’t disappointed me yet. That day will come, but I have no idea when it’s going to be."

mlafferty@dispatch.com