PASADENA, Calif. NASA is staunchly defending the science plans for its flagship Mars rover Curiosity in the wake of a recent senior-level review that at times harshly criticized the mission's science operations.

Mission scientists announced Thursday (Sept. 11) that the car-size Curiosity rover has reached the base of Mount Sharp, a 3-mile-high (5 kilometers) mountain rising from the center of the rover's Gale Crater landing site. Curiosity had been driving toward the mountain since it landed on Mars in 2012.

NASA officials lauded the success so far of Curiosity's $2.5 billion mission. However, they also responded to criticism raised in the recent NASA Planetary Senior Review Panel report, which NASA commissioned to help allocate financial resources for seven planetary missions, including Curiosity's Mars exploration. [Biggest Mars Discoveries by Curiosity So Far]

Jim Green, NASA's director of planetary science, was quick to point out Curiosity's early success, citing the rover's discovery that Mars was once a habitable world in the ancient past a key mission goal. The rover landed in August 2012 to begin a two-year primary mission.

"It immediately hit the jackpot," Green said with enthusiasm during the news conference here at NASA's Jet Propulsion Laboratory. "Curiosity explored Yellowknife Bay and found that it was in an ancient lake-bed environment that several billion years ago offered fresh water and all the key ingredients for life and a chemical source for microbes, if indeed any existed at that time."

But the NASA-commissioned report was less enthusiastic. The NASA Planetary Senior Review Panel, chaired by Clive Neal of the University of Notre Dame in Indiana, gave Curiosity's Mars Science Laboratory mission low marks for scientific return, placing the rover toward the bottom of the seven missions under review.

While all seven missions were ultimately approved for ongoing funding, the panel was not upbeat about Curiosity's prospects during the mission extension. The panel stated that the recently released plan for the next two years of Curiosity's operations on Mars "lacked specific scientific questions and testable hypotheses," and suggested that the mission should do more drilling and less driving to justify the continued funding of $59 million.

The panel's report also pointed out that the team intended to drill just eight samples in the upcoming funding period a plan that "the panel considered a poor science return for such a large investment." [Curiosity Quiz: Test Your Mars Rover Smarts]

The report went on to be specifically critical of Curiosity rover project scientist John Grotzinger, complaining that he was present only by phone for one round of discussion, and sent a deputy to respond to the next. "This left the panel with the impression that they were too big to fail," the report added.

Green responded to the latter comment by pointing out that Grotzinger had discussed the situation in advance with NASA management.

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NASA Defends Science Plan for Mars Rover Curiosity