WASHINGTON — NASA’s leadership offered few details July 11 about the sudden reassignment of two top officials in its human spaceflight program the day before, a move that drew criticism from leading House members.

In a memo late July 10, NASA Administrator Jim Bridenstine announced he was reassigning Bill Gerstenmaier, NASA associate administrator for human exploration and operations, and Bill Hill, deputy associate administrator for exploration systems development, to “special advisor” positions. Gerstenmaier will be replaced, on an acting basis, by Ken Bowersox, a former astronaut who rejoined the agency earlier this year as Gerstenmaier’s deputy.

In a follow-up memo to NASA employees July 11, Bridenstine said little about why Gerstenmaier and Hill were replaced. He said he made the leadership changes “to better position the agency to meet the challenge to go forward to the Moon and on to Mars.”

In that memo, he said the agency would start a “nationwide search” to fill those two positions on a permanent basis. The agency is also creating a new deputy associate administrator position “to lead NASA’s Moon to Mars efforts,” including Gateway and lunar lander development.

NASA had sought to create a separate “Moon to Mars Mission Directorate” this spring, led by a new associate administrator. That reorganization, though, was rejected by Congress, and Mark Sirangelo, the former space industry executive brought in as a special assistant to the administrator to guide that effort, resigned in May.

Bridenstine said that the new associate administrator for human exploration and operations would be hired first, and then help hire the two deputy positions. One of the first jobs for the new associate administrator “will be to assess the costs and schedules for SLS, Orion, Commercial Crew, and other key programs.”

The reassignments also puzzled key members of Congress, including the chairs of the House Science Committee and its space subcommittee. “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,” said Rep. Eddie Bernice Johnson (D-Texas), chair of the House Science Committee, in a July 11 statement.

“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,” she said, calling on Bridenstine to provide an explanation for the reassignments and more details about the Artemis program and its cost.

“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,” said Rep. Kendra Horn (D-Okla.), chair of the space subcommittee, who added she was “surprised” by the personnel changes.

Gerstenmaier had been scheduled to speak July 11 at the American Astronautical Society’s Glenn Memorial Symposium in Cleveland. Instead, Bowersox, the new acting associate administrator, spoke in his place, but made no reference to the leadership changes.

Bowersox did hint at changes to come in NASA’s exploration programs. “You’ll probably see changes in the future as we iterate internally, and with all of our stakeholders in the government and all of our international and commercial partners,” he said at the beginning of a presentation providing an overview of the Artemis program.

Those changes could include the Gateway. His presentation included a slide NASA released earlier this year showing one configuration of the Gateway with various modules and components provided by international partners. “This changes as we discuss and look at our architecture and talk about our plans, so don’t be surprised if you see this change in the future,” he said.