WASHINGTON — As a series of NASA planetary spacecraft begin extended missions, a recent National Academies report recommends that the agency work with Congress to stretch out the review process that determines whether science missions should continue.

Among the missions that formally started two-year extended missions Oct. 1 was NASA’s flagship Mars mission, the rover Curiosity. The rover landed on Mars in August 2012 and, after completing a prime mission of nearly two years, NASA approved a two-year extended mission in 2014. The agency earlier this year approved a second two-year extension that runs through September 2018.

Mission scientists plan to use the two-year extension to climb further up Mount Sharp, the mountain in the center of Gale Crater that Curiosity has been gradually ascending since landing. Those studies will help scientists understand the early history of the planet, probing younger layers the higher the rover ascends the mountain.

Curiosity was one of several NASA planetary science missions that won extensions in July after what’s known as a “senior review” of those spacecraft that have completed their primary missions but continue to function. Similar senior reviews are carried out every two years for NASA’s astrophysics, heliophysics and earth science missions as well.

Most NASA science spacecraft are in some form of an extended mission. A report on the senior review process, released last month by the National Academies, noted that about three-quarters of the 60 active science missions are in some form of extended mission. Those extended missions, though, account for only 12 percent of the overall budget of NASA’s Science Mission Directorate (SMD), far less than those under development.

The study largely supported NASA’s implementation of the senior review process, where missions submit proposals on what they plan to do over the next two years, and how much funding they require to do so. “Overall, the committee was impressed with the way NASA SMD conducts its mission-extension review process and how much the four SMD divisions communicate amongst themselves regarding the reviews,” the report concluded.

One major recommendation from the report, though, was to extend the time between senior reviews. “Regular reviews of operating missions are essential. However, the current 2-year cadence creates an excessive burden on NASA, mission teams, and the Senior Review panels,” the report states. That burden comes from the time spent by mission teams preparing proposals and by NASA to recruit other scientists to review those proposals and then act on the recommendations of the reviewers.

The report instead recommends holding senior reviews every three years. “A 3-year cadence would ease this burden, while enabling timely assessment of the quality of the data returned from these missions and their potential for continued productivity,” the report stated. It added that NASA should conduct regular technical reviews of extended missions to ensure they continue to be able to carry out the science planned for their extensions.

That change, however, requires congressional action. The current senior review process, calling for reviews every two years, was set in place by NASA authorization legislation in 2005 and 2010.

“The committee recognizes that NASA alone cannot change this cadence and that it ultimately requires a change of language in NASA authorization bills,” the report states. “The committee believes that NASA can work with Congress to seek a change in the authorization language to allow for a 3-year cadence and that this will have a significant impact on reducing the burden and improving the overall efficiency of NASA’s mission extension process.”