NEW YORK — After nearly a year rolling around inside an expansive crater on Mars, NASA’s trusty rover Opportunity is headed back out to explore the martian plains.

“The rover is back on flat ground,” said Paolo Belluta, engineer and rover driver at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

The golf cart-sized Opportunity climbed up and out of the 800-meter-wide Victoria Crater on Mars Aug. 28 with one last 6.8-meter push that sent it charging over the top of the crater’s rim and through a sand ripple on the other side. The maneuver brought to an end Opportunity’s studies of Victoria, which began in September 2007 when the rover made its first foray into the crater.

“We’re headed to the next adventure out on the plains of Meridiani,” said John Callas, NASA’s rover project manager for Opportunity and its robotic twin Spirit on the other side of Mars. “We safely got into the crater, we completed our exploration there and we safely got out.”

Opportunity and Spirit have been exploring different parts of Mars since they landed in January 2004. Since then, the rovers have found evidence that water once soaked the martian terrain in the ancient past among their other discoveries.

But Victoria Crater, a deep depression blasted into the martian surface with exposed bedrock that serves as a window into planet’s geological history, has dominated Opportunity’s attention. The rover spent more than half of the four years since it landed on Mars studying the giant crater.

Opportunity first headed for Victoria in late 2004 after visiting a smaller, stadium-sized crater, dubbed Endurance, earlier that year. The rover took 22 months to cross the few kilometers between Endurance and Victoria, and managed to escape a deep sand dune that held it fast for five weeks before engineers were able to work it back out.

After arriving at Victoria, Opportunity spent a year meticulously circling partway around the crater’s rim to find the best spot to drive into its interior. The rover spent so long at Victoria that NASA’s Mars Reconnaissance Orbiter circling the red planet managed to catch its path around the massive crater.

After spotting a power spike in Opportunity’s left front wheel, its handlers decided in July to order the rover to leave Victoria Crater. The rover then backtracked along its entry path to get back out, mission managers said.

“We were concerned that any wheel failure on our aging rover could have left us trapped inside the crater,” Callas said.

A similar spike preceded the full-blown failure of a wheel on the rover Spirit in 2006. Spirit is currently building a full-color panorama photograph as it awaits better sunlight conditions for its solar panels at its location on Mars, mission managers said.

A third NASA spacecraft, the Phoenix Mars Lander, also hit a milestone at the end of August. The stationary probe surpassed its initial 90-day mission and began extended operations while digging for buried martian ice in the planet’s arctic circle.

Since landing, Opportunity has spent 1,635 days exploring Mars and traversed more than 11 kilometers of martian terrain during its mission.

The rover is poised to begin hunting new targets: chunks of Mars rocks called cobbles that lie strewn across the planet’s surface. Researchers believe the cobbles, about the size of a human fist and larger, are chunks of material ejected from impacts that caused craters that are too far away from Opportunity to be fully explored.

“Our experience tells us there’s lots of diversity among the cobbles,” said rover mission planner Scott McLennan of the State University of New York, Stony Brook. “We want to get a better characterization of them. A statistical sampling from examining more of them will be important for understanding the geology of the area.”