The head of NASA’s human exploration program has been replaced within the agency, just months after Vice President Mike Pence challenged NASA to send humans to the Moon within the next five years. The move is the latest in a couple of high-profile executive changes NASA has made in recent months as the agency strives to return humans to the lunar surface.

“As you know, 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,” NASA administrator Jim Bridenstine wrote to employees in a memo obtained by The Verge. “In an effort to meet this challenge, I have decided to make leadership changes to the Human Exploration and Operations (HEO) Mission Directorate.”

“In an effort to meet this challenge, I have decided to make leadership changes.”

William Gerstenmaier, the longtime associate administrator for human exploration at NASA, has been reassigned to a new role as special adviser to NASA’s deputy administrator. The move signifies something of a demotion for Gerstenmaier at the agency. As head of the human exploration division, Gerstenmaier has overseen many of NASA’s biggest projects for human spaceflight, such as the operations on the International Space Station; the development of NASA’s next big rocket, the Space Launch System; and the Commercial Crew program, an initiative to launch astronauts to the ISS using commercial spacecraft.

“We, as a nation, are thankful for his service in advancing America’s priorities and expanding the limits of science, technology, and exploration,” Bridenstine wrote in the memo. Ken Bowersox, a former NASA astronaut and agency official, will take Gerstenmaier’s place as the acting head of human exploration. Another high-profile NASA employee, Bill Hill, has also been reassigned from deputy associate administrator of human exploration to another special adviser position. Effectively, NASA’s first and second in command of humans in space have been replaced at the same time.

NASA did not immediately respond to a request for comment.

The news comes mere hours after Gerstenmaier testified at a House subcommittee meeting, focusing on the future of the International Space Station and NASA’s plans for exploring 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. “We are building for the long term, and this time are going to the Moon to stay. We are designing an open, durable, reusable architecture that will support deep space exploration for decades to come.”

The two moves are huge personnel changes within NASA’s human exploration division, and they at a critical time when the agency is trying to sell its ambitious lunar program, called Artemis, to Congress. The White House recently requested an additional $1.6 billion for NASA to help jump-start the Artemis program next year, on top of the $21 billion the administration already requested for the agency. So far, it’s unclear if Congress will appropriate the funds needed for the program, and some lawmakers have expressed concern over where the requested money is coming from. 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.

The news comes mere hours after Gerstenmaier testified at a House subcommittee meeting

The decision to reassign Gerstenmaier and Hill is already being criticized by lawmakers who help set NASA’s agenda. Rep. Eddie Bernice Johnson (D-TX), who chairs the House Science, Space, and Technology Committee, provided this statement to The Verge:

I am baffled by the NASA Administrator’s decision to abruptly remove the highly respected heads of NASA’s human spaceflight directorate and its Exploration Systems Development office with no permanent successors identified. The Trump Administration’s ill-defined crash program to land astronauts on the Moon in 2024 was going to be challenging enough to achieve under the best of circumstances. Removing experienced engineering leadership from that effort and the rest of the nation’s human spaceflight programs at such a crucial point in time seems misguided at best. The Administrator needs to explain this personnel action, as well as provide an executable program plan accompanied by a credible budget if Congress is to have any basis for supporting the President’s Moon initiative.

Rep. Kendra Horn (D-OK), chair of the House subcommittee on space and aeronautics, expressed similar criticism:

“I am concerned about the impacts that such abrupt leadership changes in our nation’s human space flight programs could have at a time when we are at the threshold of testing the integrated Space Launch System and Orion crew vehicle that will take humans into deep space and the commercial space flight systems that will take our astronauts to the International Space Station.”

These aren’t the only big personnel changes that NASA has made recently either. Mark Sirangelo, who was recently hired to helm a new “Moon to Mars” directorate at NASA, was resigned after just one month of working at the agency, and NASA decided to scrap plans for the new division altogether. “Given NASA is no longer pursuing the new mission directorate, Mark has opted to pursue other opportunities,” Bridenstine said in a memo to employees at the time.

“NASA has always been fortunate to have great talent that has served our country well,” Bridenstine wrote in today’s memo. “As we work to fill these key positions within HEO, we will stay mission focused knowing that exploration will go forward.”

Update July 11th, 3:50PM ET: This article was updated to include a new statement from Rep. Johnson and Rep. Horn.