NASA’s acting administrator is retiring in April.

Robert M. Lightfoot Jr., who’s served in the position since January 2017, sent out an agency-wide memo on Monday announcing his “bittersweet” decision. It’s unclear who will replace him.

“The grit and determination you all demonstrate every day in achieving our missions of discovery and exploration are simply awe inspiring,” he wrote. “I leave NASA blessed with a career full of memories of stunning missions, cherished friendships, and an incredible hope for what is yet to come.”

Lightfoot was named acting administrator after Charles Bolden, a former astronaut and retired Marine Corps aviator, resigned on January 20, 2017 — following Trump’s inauguration. NASA’s deputy administrator, Dava Newman, also left NASA after Trump took office, which led to Lightfoot’s appointment.

Lightfoot joined NASA in 1989 as a test engineer working on the space shuttle program. In the memo, he wrote that he would “work with the White House on a smooth transition to the new administrator.”

This is the longest NASA has gone without a permanent administrator.

In September Trump nominated Rep. Jim Bridenstine (R-Okla.) as NASA administrator but the contentious nomination has yet to be confirmed as many in the Senate disagree with a politician leading the apolitical space agency.

During a confirmation hearing in November, Sen. Bill Nelson, of Florida, said Bridenstine lacked the leadership skills to unite “scientists, engineers, commercial space interests, policymakers and the public.”

“Your recent public service career does not instill great confidence about your leadership skills or ability to bring people together,” Nelson said. “In fact, your record and behavior in Congress is as divisive and extreme as any in Washington.”

Lighthouse did not give a reason for his departure, only stating that he was looking forward to “more time with my family and closest friends, and cheering the NASA team on from the outside.”