NASA has replaced the head of its human space exploration directorate in a major shake-up, US media reported Wednesday, as the agency scrambles to meet President Donald Trump's ambitious deadline to return astronauts to the moon by 2024.
The project -- named Artemis -- would be the first attempt to return humans to the lunar surface since the last Apollo landing in 1972, but some experts doubt if the deadline is realistic given budgetary constraints and delays in developing the next-generation rockets and equipment needed for the journey.
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 US operations on the International Space Station before becoming head of HEO.
"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 of Gerstenmaier in his email, according to CBS News.
Former astronaut Ken Bowersox will be the acting head of the section, according to Bridenstine's email.
The American plan to return humans to the moon -- including the first woman -- is beset with delays and cost overruns, according to an official audit released last month.
The cost of Boeing's giant Space Launch System (SLS) rocket -- at the core of the Artemis project -- has risen by nearly 30 percent to $8 billion, and its already-delayed first flight is unlikely to take place by June 2020 as planned.
Costs for the Orion capsule being built by Lockheed Martin to transport astronauts have also grown.
But Vice President Mike Pence, who announced the accelerated 2024 target in March, has criticized NASA for "bureaucratic inertia" and asked for a new mindset.
Critics have said such tight deadlines may lead to serious risk-taking to save time, including reducing the amount of testing done on rockets.
Bridenstine has described the 2024 deadline as "aggressive" but doable.