As NASA scrambles to meet US President Donald Trump's mandate to return humans to the moon by 2024, two longtime heads of NASA's human exploration wing were demoted Wednesday in a slew of administrative shakeups, officials said in an internal memo.

The biggest change to rock the agency is the demotion of Bill Gerstenmaier, who was leading the efforts to return humans to the lunar surface.

He has been with the agency since 1977, led some of its most high-profile programs and was the head of the human exploration office in a 14-year tenure.

The agency's chief Jim Bridenstine announced the changes in an internal memo to employees.

"NASA has been given a bold challenge to put the first woman and the next man on the moon by 2024, with a focus on the ultimate goal of sending humans to Mars," Bridenstine said. "In an effort to meet this challenge, I have decided to make leadership changes."

Bridenstine placed Ken Bowersox, a former astronaut and vice president of Astronaut Safety and Mission Assurance for Elon Musk's SpaceX, as acting associate administrator for the human exploration wing.

Gertenmaier was reassigned as a special assistant to Bridenstine's deputy Jim Morhard.

The White House has shown frustration with the pace of NASA's efforts, especially with its premier workhorse rocket, which is years behind schedule and plagued with cost overruns.