More than two years after touching down inside Gale Crater, NASA’s Mars rover Curiosity has arrived at the place that drove scientists to select the landing site at the outset -- Mount Sharp.

The three-mile high mountain of layered rock rises from the center of the impact basin. Each strata holds clues about a different time in Martian history, including periods when warmer temperatures and a thicker, wetter atmosphere made the planet more hospitable to life, or at least life as we know it on Earth.

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The Curiosity science team took a gamble after the rover landed and headed off in the opposite direction of Mount Sharp to survey an area that from orbital imagery looked as though it not only had once been under water, but had chemical fingerprints of a water-rich past as well. Samples drilled out from an ancient mudstone showed that the area not only was once covered by a fresh water lake, but that it also had all the chemical ingredients necessary to support microbial life.

With that discovery meeting the major science goal of the mission, the stage was set for a more challenging quest to find places that also could have preserved organic carbon, a conundrum because the same processes that turn sediment into rock tend to destroy organics. Mount Sharp is the hunting ground.

The five-mile trek already has taken more than a year, with Curiosity stopping periodically to inventory and assess its surroundings and by unexpected wear-and-tear on the rover’s wheels.

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Using orbital imagery, scientists last month plotted a course that would be easier for Curiosity to navigate, a fortuitous decision that ended up shaving several months off the rover’s journey to the foot of the mountain.

“We have finally arrived at the far frontier that we have sought for so long,” California Institute of Technology geologist John Grotzinger told reporters on a conference call.

“The wheel damage ... drove us on a pathway further south to be safer to the wheels and once we got to the location ... we recognized that in fact this was an even better place to go across the boundary,” between the cratered plains of Gale Crater and the relatively smoother surface of the base of Mount Sharp, Grotzinger said.

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Within two weeks, Curiosity should reach an outcrop of rock called Pahrump Hills, where the first drill samples from Mount Sharp will be made.

The rover’s arrival at it prime science target comes weeks after a NASA oversight committee questioned the team’s penchant for driving versus drilling.

“We have to have a good reason to drill something,” Grotzinger said. “We can’t just roll the dice and hope something good is going to happen.”

During its two-year prime mission, Curiosity collected five samples, with eight more due to be analyzed during the recently approved two-year extension.