In March, US Vice President Mike Pence called on NASA to hasten its lunar initiatives and put people on the surface of the Moon within the next five years - a decision in part spurred by President Donald Trump, who has repeatedly pushed NASA to return to the lunar surface.
The widely-respected Gerstenmaier is a NASA veteran who joined the agency in 1977, rising to become one of its top managers, overseeing the space shuttle program and U.S. operations on the International Space Station before becoming head of HEO. The move signifies something of a demotion for Gerstenmaier at the agency.
Effective immediately, Ken Bowersox - a five-flight shuttle veteran, space station astronaut and Gerstenmaier's deputy - will take over on an acting basis while Gerstenmaier serves as "special adviser" to NASA Deputy Administrator Jim Morhard, according to Bridenstine's letter sent to agency employees. Effectively, NASA's first and second in command of humans in space have been replaced at the same time.
Gerstenmaier was ousted from his role hours after he testified before Congress on the future of the International Space Station and plans for low-Earth orbit. "NASA's Artemis program will build a sustainable, open architecture that returns humanity to our nearest neighbor", Gerstenmaier wrote in his witness testimony for the hearing.
The high-profile executive changes at NASA come at a time when the agency is working on its new moonshot program named Artemis. The White House asked for the money to be taken out of a surplus in the Pell Grant program, which provides financial aid to college students.
"400,000 people in 2,500 companies made five and a half million bits that went together to make the rocket, each one of which had to be absolutely flawless and ideal and wouldn't break down, and that kind of management is something that Nasa invented and I think has changed the world more than anything else", he said.
NASA administrator Jim Bridenstine said there was nothing wrong with what Gerstenmaier was doing.
"NASA has all the time been lucky to have nice talent that has served our nation properly", Bridenstine wrote.
He said "if NASA's not now capable of landing American astronauts on the moon in five years, we need to change the organization, not the mission". "As we work to fill these key positions within HEO, we will stay mission focused knowing that exploration will go forward".