Robert Lightfoot, who has been serving as NASA’s Acting Admistrator for more than a year, today announced that he will retire from the agency on April 30. In a memo to employees, he said he would work with the White House on the transition to “the new Administrator.” President Trump nominated Rep. Jim Bridenstine (R-Oklahoma) to be NASA’s new Administrator in November, but it is controversial and the Senate has not confirmed him yet.

Lightfoot has been Associate Administrator (AA) — the third highest ranking position in the agency and usually the highest ranking civil servant position — since 2012. He became Acting Administrator on January 20, 2017 when the Trump Administration took office and Charlie Bolden, who held the position during the Obama Administration, left.

Prior to his appointment as AA, Lightfoot was Director of NASA’s Marshall Space Flight Center in Huntsville, AL. where he began his NASA career in 1989. A mechanical engineer, he served in many capacities at Marshall, Stennis Space Center and Headquarters, including assistant associate administrator for the space shuttle program (2003-2005) at headquarters and manager of the space shuttle propulsion office at MSFC (2005-2007). He was named MSFC Deputy Director in 2007 and Director in 2009.

Lightfoot said he will depart on April 30 with “bittersweet feelings.” No successor was announced. Steve Jurczyk was just appointed to serve as Acting AA. Typically the AA fills the role of Acting Administrator when the Administrator and Deputy Administrator positions are vacant as they are now, but whether that will hold true in this case remains to be seen.

Bridenstine’s nomination is controversial because he is a politician rather than a technical expert and because of his positions on climate change, LGBTQ rights, and other matters. He was approved by the Senate Commerce, Science, and Transportation Committee twice on party-line votes: in November 2017 and again in January 2018 because the nomination had to be resubmitted when the second session of the 115th Congress began. Along with Democratic opposition, the nomination is also publicly opposed by Republican Senator Marco Rubio of Florida who has said it would be “devastating to the space program” to have a politician leading NASA at a “critical juncture in history.”