NASA's Mars Curiosity rover uses its mast cam to take a selfie.

WASHINGTON — Shaking off the criticism of a senior review panel that said the Mars Science Laboratory mission was at risk of underperforming, a pair of NASA officials said Sept. 11 that the mission team has a solid science plan that will ensure a good return on the $2.5 billion Curiosity rover that has been exploring the red planet the past two years.

Jim Green, NASA’s director of planetary science, and John Grotzinger, project scientist for Curiosity, said the rover’s productivity is not defined solely by the number of surface samples it collects with its drill bit and scoop — a metric scientists honed in on in a report detailing the results of a senior review of planetary science missions, which despite harsh words for Curiosity recommended a two-year, $115 million extension for the flagship rover that landed on Mars in August 2012.

“We have to have a good reason to drill something,” Grotzinger said during a conference call. “We can’t just roll the dice and hope something good is going to happen. The notion that you have to drill dozens and dozens of times … is not the way we prefer to do it.”

In its two-year primary mission, Curiosity collected five Mars surface samples. The rover’s science team has proposed collecting eight more during the course of the two-year mission extension just recommended by senior planetary scientists in their senior review. In a report released Sept. 3, the review panel said 13 samples in four years was a “poor science return for such a large investment in a flagship mission.”

Grotzinger said the five samples Curiosity has collected so far resulted in “millions” of data points.

Green said that even if the rover stopped taking samples altogether, it would not matter: Curiosity has already completed its primary science mission to search Mars for signs that the planet was once habitable.

“Within weeks, Curiosity fulfilled its goal,” Green told reporters, noting that the rover discovered “all of the key ingredients of life and a chemical source for microbes” when a sample drilled near the region known as Yellowknife Bay revealed evidence that the now-desolate area may once have been the bottom of a freshwater lake.

Curiosity “has made a stirring set of measurements,” Green said.

Green also addressed the senior review panel’s knock on Grotzinger, who during the May meeting convened to evaluate the rover’s extended mission science plan spoke with the reviewers by phone rather than in person. That made reviewers think the Curiosity team was acting like it was “too big to fail,” according to the Sept. 3 report.

Green said it was all a misunderstanding.

“We’d already approved for John to be at [another] particular activity [and] for whatever reason, the senior review panel didn’t get that information, or didn’t understand that,” Green said.

Meanwhile, Grotzinger shared news that Curiosity has reached the base of Mount Sharp, where the rover plans to collect the eight samples that are the focus of its extended mission. The rover is set to arrive at the first such site, the area known as the Pahrump Hills, in another week or two, the senior NASA scientist said.

All in all, prospects for Curiosity going forward look good, Grotzinger told reporters. The rover’s damaged wheels, which forced the team at the Jet Propulsion Laboratory to plot a new course to Mount Sharp, should be sturdy enough to hold up through the extended mission phase underway and a further extension that would begin in 2016 — assuming the rover fares well in the next planetary senior review.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.