The science team behind NASA's Curiosity rover on Mars says it has chosen not to drill into a rock being considered as the rover's next science target out of concerns it is not sufficiently stable to allow safe drilling.

The pale rock, which the team had dubbed "Bonanza King," moved perceptibly during a test operation when a percussion drill was used to make a preliminary indentation, the space agency reported.

"We have decided that the rocks under consideration for drilling, based on the tests we did, are not good candidates for drilling," said Curiosity Project Manager Jim Erickson of NASA's Jet Propulsion Laboratory, Pasadena, California.

Three previous successful drillings were conducted on rocks that formed a part of large outcrops that provided stability, but Bonanza King and some other potential targets at Curiosity's current location are simply too unstable for safe drilling, he said.

"Instead of drilling here, we will resume driving toward Mount Sharp," Erickson said.

The mountain in the center of the rover's Gale Crater landing site remains the rover's ultimate destination as it continues its examination of the surface of the Red Planet.

The rover has traveled around 5.5 miles since its August 2012 landing inside the crater, and will need to traverse an additional 2 miles to reach a point where it can enter the layered slopes of Mount Sharp, JPL said.

On its voyage toward Bonanza King, Curiosity was driving through an area dubbed "Hidden Valley" but found the going difficult when its wheels began slipping in very loose sand, leading controllers to instruct the rover to back out to a safe location while another route was considered.

Mars Science Laboratory mission managers had first thought the sand would be gentler on the rover's wheels, which have suffered some damage from sharp rocks on the planet's surface.

"After further analysis of the sand, Hidden Valley does not appear to be navigable with the desired degree of confidence," Erickson said. "We will use a route avoiding the worst of the sharp rocks as we drive slightly to the north of Hidden Valley."

Curiosity has already yielded significant findings in its year on Mars including evidence suggesting ancient river and lake environments that may have been favorable conditions that microbes could have habited in the planet's past.

Scientists are hopeful the layered geography of Mount Sharp will allow the rover to investigate different ages in the early environmental evolution of the Red Planet.