NASA acting director Robert Lightfoot announced his retirement from the space agency Monday.

In a letter to NASA employees, Lightfoot said he had "bittersweet" feelings about the retirement and he would work with the White House on a smooth transition to the new administrator.

"I cannot express enough my gratitude to the entire NASA team for the support during my career and especially the last 14 months as your acting administrator," Lightfoot said in the letter.

Robert Lightfoot has announced that he will retire on April 30. He has been Acting Administrator since January 2017. pic.twitter.com/DaN7wQIfWw

— Marcia Smith (@SpcPlcyOnline) March 12, 2018

Lightfoot began his career with NASA at Marshall Space Flight Center in Huntsville in 1989 as a test engineer. He moved up through the ranks and was named director of Marshall in August 2009 after serving several months as acting director.

Lightfoot was named an associate administrator for NASA in 2012. In January 2017 he was named acting director of the space agency when Charles Bolden left.

Lightfoot's retirement date is April 30.