NASA has been without a permanent leader for more than a year. Now the agency’s temporary leader is leaving, too.

In an email to the space agency’s employees on Monday, Robert M. Lightfoot Jr., the acting administrator, announced that he would retire on April 30.

Mr. Lightfoot said he was leaving “with bittersweet feelings,” but did not say why he was retiring.

“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,” Mr. Lightfoot wrote.