WASHINGTON -- NASA's new administrator, Mike Griffin, told the U.S. space agency's workforce Thursday afternoon that carrying out President Bush's space exploration vision will require tough choices.

"There are going to be some difficult decisions to make and I cannot say otherwise and I would not say otherwise even if I could," Griffin said in his first address as NASA administrator to the agency's 18,000 employees. The 30-minute talk was carried live on NASA television.

"The day that we lost Columbia was probably the worst day that NASA has had," Griffin said. "But out of that accident came a new vision for NASA, a vision of exploration beyond Earth orbit."

Griffin called the vision "exactly the right strategy" for NASA. One that will call for the agency to change the way it does business, including making field centers compete for projects and doing more to involve commercial ventures in space exploration.

Describing NASA as an agency "in transition", Griffin said that more changes would be needed to "reshape" NASA to "head down a new path" that includes returning to the Moon by 2020 in preparation for eventual human expeditions to Mars.

"It's going to be difficult, and it is going to be hectic but we are going to do it together," he said.

Griffin, who was confirmed by the Senate Wednesday evening but has yet to be sworn in, spoke for less than 10 minutes before taking questions from the audience at NASA Headquarters here and via telephone from employees at NASA's regional field centers throughout the United States.

Asked by one employee about NASA's proposed aeronautics spending cuts that are a big enough concern on Capitol Hill that they momentarily held up what was otherwise a remarkably swift confirmation, Griffin said aeronautics research would always be "a core part of the NASA mission" but acknowledged that the program is "hard hit right now."

NASA field centers in California, Ohio, and Virginia are planning to shed a few thousand jobs in response to declining aeronautics spending. Griffin offered no false assurances that those jobs would be saved.

"I don't see a way to avoid some of the dislocation at present," he said. "We do live in a world of limited resources and we do have to set priorities"

Griffin said his schedule would be dominated in the weeks ahead by making sure the agency is ready to return safely to flight for the first time since the February 2003 loss of the Space Shuttle Columbia and her crew.

"All other commitments will flow around what I have to do to support the return to flight decision," Griffin said.

Griffin, 55, is replacing Sean O'Keefe, who led NASA for three years before stepping down in February to become chancellor of Louisiana State University in Baton Rouge.

Griffin most recently led the space department at Johns Hopkins University Applied Physics Laboratory and is a veteran aerospace executive who has held a variety of senior-level positions at the Pentagon, NASA and in industry.

Griffin said a tour of NASA's field centers is in the works, but assured employees that he is no stranger to the agency and what it does.

"I know the agency fairly well," he said." I have been to all of the centers each and every one of them on many occasions. I know the kind of talent that we have in this agency and I know what we can do when we set our minds to it."

Although much has been made of Griffin's half dozen advanced degrees by the press and lawmakers singing his praises, Griffin invited NASA employees Thursday to address him by his first name.

"I'm not 'sir', I'm not 'Dr. Griffin', I'm not anything besides Mike or Michael," Griffin said. "The NASA administrator is not royalty and I am certainly not. I'm just another person."

Griffin said during his Senate confirmation hearing Tuesday that seeing to the space shuttle's safe return to flight and accelerating the timetable for fielding the space shuttle's replacement would be his top two priorities as administrator.