Deputy NASA Administrator Lori Garver, who has been a policy lightning rod at agency headquarters as the Obama administration worked to shift U.S. human spaceflight from a government-run operation to a commercial venture, has resigned to take a job as general manager of the Air Line Pilots Association (ALPA).

Associate Administrator Robert Lightfoot, the agency’s No. 3 manager and top-ranking civil servant, is a likely possibility to fill Garver’s post on an acting basis until the White House can nominate another political appointee.

Garver, a former aerospace consultant, joined President Barack Obama’s 2008 campaign after initially supporting Hillary Clinton. She helped run the space-policy transition team after Obama was elected, forming a close alliance with John Holdren, Obama’s science adviser, and other newcomers at the White House Office of Science and Technology Policy.

Since then she has been at the forefront of the administration’s “New Space” initiatives, including an acceleration of efforts to commercialize human transport to low Earth orbit initiated under the administration of President George W. Bush, and emphasis on open-ended technology development for deep-space exploration.

Those plans ran into stiff opposition on Capitol Hill and within the agency, and led to NASA’s current, two-pronged approach of seeding development of commercial crew vehicles for access to the International Space Station, while developing the Orion crew vehicle and heavy-lift Space Launch System (SLS) in-house for deep-space human exploration.

In recent weeks Garver has been an advocate for the administration’s plan to capture a small asteroid and nudge it into a high-retrograde lunar orbit for study by astronauts arriving in an Orion launched on an SLS. That scheme has run into strong Republican opposition in Congress, and NASA has said it wants to send an Orion crew to that particular orbit even if the asteroid-relocation mission doesn’t materialize.

Garver’s departure will come on the heels of Elizabeth Robinson, the agency’s chief financial officer, who has been named under secretary of energy. Robinson and Garver were staunch allies in the often-heated management policy debates that pitted them against more traditional NASA managers, including Administrator Charles Bolden.

“While I am sorry to be losing such a talented and passionate co-pilot, I am happy that Lori is continuing to pursue her dreams and make her mark in the aerospace industry,” Bolden stated in a message to NASA staff in which he praised Garver for being “an extraordinary role model for young girls, inspiring them to study science, technology, engineering and mathematics. ... Her last day at NASA will be Sept. 6, and she assumes her new role at ALPA on Sept. 9. I will personally miss her candid and sage advice and good humor.”