The head of the US space agency (Nasa), Sean O'Keefe, has resigned.
Sean O'Keefe has been Nasa's chief for three years
Mr O'Keefe's tenure lasted three years, a period that included the Columbia shuttle disaster in January 2003.
His letter of resignation was accepted by President Bush on Monday and he is now a candidate to become chancellor at Louisiana State University (LSU).
It is understood the White House is already weighing several candidates to replace Mr O'Keefe as the 11th administrator in the agency's history.
"[Sean O'Keefe] is someone the president believes has done a great job over at Nasa," said Scott McClellan, a White House spokesman.
In his resignation letter to the president, Mr O'Keefe wrote: "I will continue until you have named a successor and in the hope the Senate will act on your nomination by February."
Back to Moon
Administrator O'Keefe first joined the Bush administration
as the deputy director of the Office of Management and
Budget, working on the federal budget and government management initiatives.
It was with this expertise that he was asked to go to the National Aeronautics and Space Administration in 2001, an agency that was experiencing cost overruns.
But by far the biggest crisis he had to deal with was the aftermath of the Columbia shuttle disaster on 1 February 2003, when seven astronauts were killed on re-entry into the Earth's atmosphere.
The tragic event forced a complete rethink of many Nasa policies and of the entire US space programme.
This has included a major shift in emphasis towards human exploration, with President Bush setting out a vision to return astronauts to the Moon and, possibly, take them on to Mars.
"Nasa has a new direction that will push the boundaries of technology, science, space flight and knowledge, and will inspire new generations of explorers for years to come and secure this great nation's future," Mr O'Keefe said in his resignation letter.
Notable mission successes during his time in office have included the landing of the twin Mars Exploration Rovers Spirit and
Opportunity on the Red Planet; the arrival of the Cassini-Huygens spacecraft at Saturn; and the recent test flights of the X-43A jet plane which flew at 10 times the speed of sound.
However, Mr O'Keefe has been under fire of late for his insistence that no astronauts be sent to repair and upgrade the popular Hubble Space Telescope.
US newspapers suggest that Air Force Lieutenant General Ronald Kadish, who until September directed the Missile Defense Agency, is the leading candidate to replace Mr O'Keefe.
Other possible O'Keefe successors include former Pennsylvania Representative Robert Walker, a member of the president's Moon-Mars exploration commission this year, and former astronauts Charles Bolden, Robert Crippen and Ronald Sega, all with military ties.