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NASA's International Space Station program manager is leaving the space agency, marking the second high-profile exit from NASA in recent weeks.

Kirk Shireman is retiring from NASA, the space agency confirmed Tuesday, on June 25. News of his exit comes just two weeks after NASA astronauts Robert Behnken and Douglas Hurley made history by riding to the space station aboard a SpaceX Crew Dragon capsule. Both men are still on board the ISS.

A source familiar with the matter told CNN Business that Shireman already planned to retire this year before SpaceX's historic mission, which took two astronauts to the ISS, took off last month.

Shireman could not be immediately reached for comment Tuesday.

News of his departure, which was first reported by NASAWatch.com, comes at a pivotal moment for the program he has overseen for the past five years. After nine years without having a way to get its own astronauts to and from the space station, NASA oversaw the first-ever crewed flight of a SpaceX capsule last month.

Shireman's retirement also comes after NASA's chief of human spaceflight, Doug Loverro, abruptly resigned from the space agency last month, citing a 'mistake' he had made earlier this year. CNN Business previously reported that Loverro's departure was related to contracts that were awarded for the development of lunar landers, or vehicles that can carry astronauts to the moon's surface, according to a source familiar with the matter. Kathy Lueders, who has spent 28 years at NASA, was named to lead the agency's Human Exploration and Operations Mission Directorate, or HEO, NASA announced last week.

Shireman joined NASA in 1985 and was deputy manager of the ISS program for seven years before he was named manager in 2015.

'It has been the honor of a lifetime to have worked through the development of the International Space Station from a fledgling facility to a world-class laboratory,' Shireman said on Wednesday after this article was initially published.

Joel Montalbano, who has served as NASA's deputy manager of the ISS program for eight years, will become acting manager after Shireman leaves the agency later this month. He'll report to Lueders, who praised Shireman and Montalbano in a statement and said she is 'confident' in the leadership handover.

The rate of recent top-brass departures is rare for the space agency's human spaceflight program. The shakeup comes as NASA is pursing an ambitious new directive to return astronauts to the moon by 2024 and as the space agency begins ramping up US activity on the ISS now that NASA can fly its own astronauts to the orbiting laboratory.

CNN Business' Rachel Crane conducted a remote interview with the astronauts Tuesday morning and asked about the leadership changes.

'One of the strengths of an organization like NASA is that we don't rely on a single individual to drive the entire assessment and evaluation and management effort,' Behnken said. 'And if the leader needs to move, then we'll get a new leader and continue to move forward. And the team is strong enough to recognize their role in assisting that new leader as they're coming in and take over the organization.'