The two moves are huge personnel changes within NASA's human exploration division coming at a critical time when the agency is trying to sell its ambitious lunar program, called Artemis, to Congress.
He 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.
Gerstenmaier began his career with NASA in 1977 doing aeronautical research.
"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", Rep. Eddie Bernice Johnson of Texas said in a statement. An extended-time NASA veteran, Hill helped manage the growth of the company's new heavy-lift rocket, the Space Launch System, or SLS wanted to hold astronauts again to the moon.
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. Pictured: An Apollo 11 astronaut's footprint in the lunar soil, photographed by a 70 mm lunar surface camera during the Apollo 11 lunar surface extravehicular activity.
"In an effort to meet this challenge, I have decided to make leadership changes to the Human Exploration and Operations (HEO) Mission Directorate".
The Democratic chairwoman of the House Committee on Science, Space and Technology said she was "baffled" by Bridenstine's decision to demote Gerstenmaier without a permanent replacement to fill the role.
NASA has shaken up it leadership ranks as the Trump administration pushes the agency to return to the moon.
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.
"You smile, but while we were out there on the moon, the world was growing closer together right here".