WASHINGTON — The head of the organization that manages the national laboratory portion of the International Space Station will leave next month as it deals with an uncertain long-term future for the station.

In a statement late Feb. 16, the Center for the Advancement of Science in Space (CASIS) announced that Greg Johnson, the former NASA astronaut who serves as president and executive director of the organization, will leave March 10. Johnson had led CASIS since August 2013.

The statement didn’t give any reason for Johnson’s departure, other than he is seeking new opportunities elsewhere. “I am very proud of the team’s progress to date and am ready to transition to the next phase of my professional career,” Johnson said in the statement.

Johnson foreshadowed the announcement with a pair of tweets late Feb. 15 reflecting on the progress the organization has made in using the station for a wide range of research. “Thanks go to our Board of Directors & NASA for their strategic help and guidance,” he said in one tweet.

Reflecting on lSS NL and CASIS last year: the addition of new users, commercial facilities, and the expansion of our portfolio to new areas of research and technology, is a testament to our culture. Thanks go to our Board of Directors & NASA for their strategic help and guidance. — Gregory H. Johnson (@Astro_Box) February 16, 2018

Proud of team! In the beginning our focus was to build a strong cohesive team with right skills & core values to execute the mission. We’ve gained traction building new demand by facilitating non-traditional innovation and augmenting the NASA ISS portfolio. Kudos to NASA as well! — Gregory H. Johnson (@Astro_Box) February 16, 2018

The CASIS board, at its annual meeting Jan. 30 in League City, Texas, praised Johnson for his work. “Greg Johnson is our terrific executive director,” retired Air Force Lt. Gen. James Abrahamson, chairman of the CASIS Board of Directors, said in opening remarks at the meeting. “He’s built the organization in a way that’s very special, and we really appreciate all that he’s done.”

In the Feb. 16 statement, CASIS said that Abrahamson will step down from the board upon Johnson’s departure and serve as interim president and executive director. The organization is beginning a “national search” for a permanent successor.

Whoever accepts the job on a full-time basis will take over an organization that, despite the successes it frequently touts, faces its share of challenges. A Jan. 11 report by the NASA Office of Inspector General (OIG) concluded that CASIS had met expectations established in a 2011 agreement between NASA and CASIS in only two of nine areas, development of research pathways on the ISS and education.

CASIS, that report noted, has fallen far short of making full use of the crew time allocated to it as part of the agreement. In the last period analyzed by OIG, from September 2016 to April 2017, CASIS used 68 percent of the crew hours provided by NASA. In some prior six-month periods, dating back to September 2013, CASIS used as little as 33 percent of its allotted crew hours.

At the board meeting, the organization emphasized more recent trends that showed CASIS using all its crew time allocation for the full 2017 fiscal year. However, CASIS officials noted that it had not been able to take advantage of the additional time now available with four astronauts in the U.S. segment of the station versus the usual three.

CASIS also faces the long-term uncertainty of the future of the ISS itself. NASA’s fiscal year 2019 budget proposal, released Feb. 12, calls for ending direct federal funding of the station by 2025. That proposal opens the door for transferring some or all of the station to a commercial operator, but could also mean the end of the station itself. That proposal may face strong opposition in Congress, where several key members have already expressed opposition to it.

Rumors of those plans existed prior to the CASIS board meeting, but there was little discussion of the topic during its three-and-a-half-hour session, which focused more on near-term research activities CASIS is supporting on the station. “The best way that we can plan for anything in the future is to really work effectively to create this market and to be successful,” Abrahamson said when asked at the board meeting about any plans by CASIS to deal with a potential end of the station in the mid-2020s.

The CASIS statement announcing Johnson’s departure also made no reference to the administration’s proposal. “It’s an exciting time in the history of space exploration and development, full of opportunities and possibilities, as our nation and NASA seek to maximize the benefits and returns of our investments in space,” Abrahamson said in the statement.