WASHINGTON — Now that NASA has picked out the seven scientific instruments the Mars 2020 rover will carry to the red planet in 2020, the agency is turning its attention to the sample-caching system the vehicle needs to contribute to the decade-spanning Mars sample-return campaign the planetary science community has anointed as its top priority.

A decision on the design of the Mars 2020 sample-caching mechanism is expected “at the end of the calendar year,” George Tahu, the Washington-based Mars 2020 program executive, said here at NASA headquarters during a Sept. 3 meeting of the NASA Advisory Council’s planetary science subcommittee.

That puts the decision about four years after the National Research Council (NRC) published a planetary science decadal survey endorsing a three-step Mars sample-return campaign that starts with the launch of a sample-caching rover. In the survey, scientists wrote that the caching rover should be NASA’s top-priority planetary science mission for the 10-year period that runs through 2022, and that the caching system itself would be the mission’s “greatest technology challenge.”

One of those scientists was surprised NASA has said so little about how it plans to meet that challenge.

“I’ve sort of been struck by the fact that sample caching tends to get very little discussion [because] that is the primary objective, at least from a decadal survey perspective,” Philip Christensen, co-chairman of the NRC’s astrobiology and planetary science committee, told the NASA Advisory Council via teleconference Sept. 3.

The decadal survey calls for a sample-caching rover able to collect almost half a kilogram worth of surface samples and preserve them for 20 years while NASA figures out how to retrieve them. Specifically, the decadal calls for caching 20 primary samples and 20 backup samples, each of which should weigh 10 grams. Back in 2011, scientists recommended that NASA proceed with a sample-caching rover only if the agency could build one for $2.5 billion. NASA has gone a step further with Mars 2020, capping development costs at around $2 billion.

NASA trotted out the Mars 2020 mission two years ago in response to the congressional and community backlash that followed the White House’s decision to withdraw NASA from Europe’s two-launch ExoMars campaign, which originally called for NASA to build a $3.5 billion Mars Astrobiology Explorer-Cacher rover and launch it in 2018 along with a smaller European rover. Europe is now counting on Russia to provide the launches and build the lander for a mostly European ExoMars rover.

NASA’s top planetary science official said there was no point rushing to design a sample-caching system for Mars 2020 before selecting science instruments to sniff out good samples.

“The first step in being able to design the coring, the caching system starts with the instruments,” Jim Green, NASA’s director of planetary science, told the NASA Advisory Council via video conference from Irvine, California, where he was discussing planetary science issues with Christensen and the rest of NRC’s astrobiology and planetary science committee. “I think you’ll hear much more about [caching] from now on, now that this big step has been made,” Green said.

NASA unveiled Mars 2020’s seven-instrument science payload in July. The instruments will cost NASA about $130 million and be carried to Mars aboard spacecraft that will closely resemble the lander and plutonium-powered Curiosity rover NASA built for the $2.5 billion Mars Science Laboratory mission. Curiosity completed its roughly two-year primary mission this year and won approval for two more years of roving in a senior review of planetary science missions completed in July.

However, the review was critical of Curiosity, which senior scientists said seemed to be falling short of its potential. Such underperformance, they cautioned, has ramifications for Curiosity’s yet-to-be launched sister rover.

“Given that Mars 2020 will be built on Curiosity heritage, maximizing the science return from Curiosity would go a long way to a smooth development of the next U.S. Mars rover mission,” the 15-person senior review team wrote in its report, which was published Sept. 3 in time for the NASA Advisory Council meeting the same day.

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.