NASA Administrator Michael Griffin said goodbye Friday to the space agency's employees, thanking them for their hard work during his four years on the job and urging them to support his successor.

Griffin spoke to employees around the country in a live televised address from NASA headquarters in Washington.

He became NASA's 11th administrator in 2005, two years after the Columbia disaster and right as space shuttles were gearing up to return to flight.

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He was appointed by President George W. Bush, and like other agency heads, offered his letter of resignation as the federal government changes hands. Griffin said in recent months that he would be willing to stay on, but, in the end, was not asked.

The incoming Obama administration has not yet named a replacement, but is floating the name of retired Air Force Gen. J. Scott Gration, who has almost no space experience but was a military adviser to the president-elect during the campaign.

His lack of space experience has raised some concerns with Senate Space Subcommittee Chairman Bill Nelson, D-Fla.

Griffin, by contrast, holds a doctorate in aerospace engineer with multiple other degrees. He worked earlier in his career at NASA and was serving as space department head at Johns Hopkins University's applied physics lab in Laurel, Md., when asked to take over NASA.

In his 45-minute address, Griffin urged employees to support the next NASA administrator, whoever it is, and support the new president's space policy, whatever that turns out to be.

Griffin has been instrumental in guiding Bush's plan to retire the space shuttles by 2010 and return astronauts to the moon by 2020 with a new rocketship. The Constellation program as it's called has been criticized more and more; some of the naysayers are inside NASA.

"NASA will look great whether we're asked to return to the moon and establish permanent presence there and go to Mars, as I think we ought to be asked to do, or whether we're asked to carry out some other task," Griffin said.

In a time of transition, the No. 1 job is to cooperate and support the new leaders, Griffin told employees.

"If you can't support the agenda, then the proper thing to do is to leave," he said. "There are many different things that you could do with a $17.5 billion NASA civil space program. But what we can't do is squabble and fight."

Griffin, who will be on duty until noon on Inauguration Day, said associate administrator Christopher Scolese will serve as acting administrator until a new NASA chief is picked.

His greatest accomplishment while leading NASA, he said, was getting space shuttles flying again after the Columbia

accident and coming close to finishing the international space station.

"Nothing — nothing in the world — is harder than picking yourself up after a cataclysm like that and moving forward, and we've done it," he said.

The 59-year-old Griffin told employees he has no idea what he will do next.