WASHINGTON — Three top-level NASA managers announced the week of July 25 plans to leave the U.S. space agency by early fall.

Laurie Leshin, deputy associate administrator of the Exploration Systems Mission Directorate, and Jon Morse, director of the Science Mission Directorate’s astrophysics division, are leaving the agency in September to return to academia.

Bryan O’Connor, NASA’s chief of safety and mission assurance since 2002, is also retiring from the agency effective Aug. 31, NASA said July 27 in a press release.

The agency had not named successors for the three managers at press time.

Leshin, who has been with NASA since 2005, will become dean of the School of Science at Rensselaer Polytechnic Institute in Troy, N.Y. Her appointment there is effective Oct. 1, according to a press release from the university. Morse will become Rensselaer’s associate vice president of research for physical and engineering sciences, effective Oct. 3, Rensselaer said. Leshin and Morse are married.

Leshin has been the exploration directorate’s second in command since 2010. She joined NASA in 2005 as the director of science and exploration at the Goddard Space Flight Center in Greenbelt, Md. Prior to that, she worked in academia at Arizona State University.

Morse has been in charge of the astrophysics division since 2007, when he joined the agency after a year with the White House Office of Science and Technology Policy. Morse also was previously with Arizona State University.

O’Connor’s retirement marks the third time the astronaut has left the agency. He has held prominent safety positions within NASA and taken part in the agency’s investigations into the losses of the Space Shuttle Columbia, destroyed on re-entry in 2003, and of the Challenger, destroyed shortly after liftoff in 1986.

Besides his safety-focused roles, O’Connor was acting space station program director in 1993 and director of the space shuttle program in 1994.

The three senior managers are leaving NASA at times of either upheaval or controversy within their respective divisions.

The Exploration Systems Mission Directorate is slated to be combined with what used to be the Space Operations Mission Directorate. NASA’s plan to consolidate those divisions was outlined in the agency’s 2011 operating plan. The combined entity will be called the Human Exploration and Operations Mission Directorate. William Gerstenmaier, currently associate administrator of the Space Operations Mission Directorate, will run the new directorate.

NASA had considered combining the directorates as soon as June, but as of July 28, Exploration and Space Operations remain separate, NASA spokesman Michael Braukus said.

Douglas Cooke, currently the Exploration Systems Mission Directorate’s top official, would have been deputy administrator of the new Human Exploration Directorate, according to an internal presentation circulated by NASA in February. However, Cooke plans to leave the agency once the two directorates are combined. With Leshin also departing, Gerstenmaier is deprived of his obvious choices for a deputy.

Activities to be managed by the planned Human Exploration Directorate will include commercial human spaceflight; the international space station; and the Space Launch System and Orion Multi-Purpose Crew Vehicle.

Meanwhile, NASA’s astrophysics division’s marquee program, the James Webb Space Telescope, is billions of dollars over budget and has been marked for cancellation in a spending bill approved by the House Appropriations Committee July 13 and now awaiting a vote on the floor.

O’Connor’s departure shakes up NASA’s Office of Safety and Mission Assurance at a time when the agency is trying to figure out the best way to certify that new, privately owned spacecraft — now in development — are safe to use for astronaut transportation.

The methods NASA should use to do so, and the depth of the agency’s involvement in the design and certification process for new spacecraft, have in some cases been a point of contention between NASA and industry.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.