The Administrator and Deputy Administrator of NASA are the highest-ranked officials of NASA, the space agency of the United States. The Administrator serves as the senior space science adviser to the President of the United States. The first Administrator of NASA was Dr. T. Keith Glennan, who served from 1958 until 1961. Daniel Goldin (pictured) held the post for the longest term. The only person to hold the post twice is James C. Fletcher, who returned to NASA following the Challenger disaster. The longest-running acting Deputy Administrator was John R. Dailey, who held the post following his retirement from the United States Marine Corps. The longest-running full Deputy Administrator was Hugh Latimer Dryden, who was the first Deputy Administrator. William R. Graham has held the post of Deputy Administrator twice, and was the acting Administrator in between, as did Frederick D. Gregory. Dr. Daniel Mulville served as the acting Deputy Administrator twice, and was acting Administrator in between. (Full list...)