A NASA astronaut who spent more hours walking in space than any other American and also set a record for the longest spaceflight mission has retired to join the private sector, the US space agency said Monday.

Michael Lopez-Alegria, 53, flew three space shuttle missions and spent seven months aboard the International Space Station as commander of the Expedition 14 mission in 2006-2007, arriving aboard a Russian Soyuz spacecraft.

The 215-day mission, the longest single spaceflight by an American, made up a significant chunk of his total 257 days in space.

He logged over 67 hours during 10 spacewalks, more than any other American, ranking him second in the world after record-holder Russian cosmonaut Anatoly Solovyev.

He also served as NASA's director of operations at the Yuri Gagarin Cosmonaut Training Center in Star City, Russia, where he led training for American astronauts heading to the Russian space station Mir and the ISS.

"Mike has been a huge asset to the astronaut office during the course of his career," said Peggy Whitson, chief of the Astronaut Office at Johnson Space Center.

"His contributions in spacewalking, shuttle, space station and Soyuz operations are notable and very distinguished."

Lopez-Alegria will as of March 19 take over the presidency of the Commercial Spaceflight Federation from Rear Admiral Craig Steidle.

The 40-member federation includes providers of commercial orbital and suborbital spaceflight, spaceports and launch facilities, suppliers, and educational and research institutions.

"I have been impressed with all that the commercial spaceflight industry has accomplished and I look forward to joining the team as it continues to take important strides that are fundamental in maintaining our nation's preeminence in space," he said in a statement.

Lopez-Alegria was born in Madrid, Spain and grew up in California. He obtained advanced degrees in aeronautical engineering from the US Naval Postgraduate School and Harvard University, and speaks Russian, French and Spanish.

With the 30-year space shuttle program over since last year, the US astronaut corps has shrunk in size to about five dozen, down from its peak of 150 in 1999 at the height of the shuttle era and the building of the ISS.

Numerous former astronauts have joined the private sector to help develop the successor to the space shuttle -- a commercially built spacecraft that will once again give the United States access to the ISS perhaps by 2015.

For now, Russia is the sole nation capable of carrying the world's astronauts into space.