Mission managers "cleared Endeavour's thermal protection system for reentry after analysis of data and images from an early-morning focused robotic inspection of the tiles," a NASA update said.

"Ultimately, their recommendation... was that we clear this area," said NASA's LeRoy Cain in a briefing.

"In accepting that recommendation, we're essentially clearing the vehicle for re-entry at this point."

Earlier Saturday the International Space Station had the historic chance to speak with Roman Catholic leader from his armchair in the Vatican library.

"Welcome aboard the Space Station your Holiness," said Dmitry Kondratyev, Russian commander of the 26th long-duration mission to the International Space Station.

The crews of the ISS and the linked US space shuttle Endeavour excitedly waved to the pope, who smiled and waved back, saying he admired the astronauts' courage and commitment and described their mission as "an adventure to discover the origins of humanity."

After mission managers closely examined the seven chipped tiles on the underside of the orbiter on Friday, they decided that just one needed a focused inspection, according to deputy shuttle program manager Leroy Cain.

The shuttle Columbia disintegrated in 2003 during its fiery re-entry toward Earth after its heat shield was damaged by a piece of foam that broke off the external fuel tank during launch, weakening the shuttle's protective cover.

NASA has taken care to closely examine the shuttle's heat shield after liftoff ever since.

Endeavour blasted off on its final mission Monday with six astronauts on board -- five Americans and one Italian -- and docked at the ISS on Wednesday in the second to last mission ever by an American space shuttle.

The US program is set to end after the launch of Atlantis, set for July 8.

The Endeavour mission is being commanded by astronaut Mark Kelly, the husband of US Representative Gabrielle Giffords, who is recovering after being shot in the head at a January political meeting with local voters.

The shuttle will remain at the space station until May 30, returning to the United States on June 1.

Spacewalkers this week have installed an ammonia jumper cable that will connect the cooling loops of two of the station's segments, part of a larger effort to fix a leak in the photovoltaic thermal control system cooling system.

They also affixed two antennas for an external wireless communications system at the station's Destiny laboratory.

The excursion will be followed by three more space walks over the course of the 16-day mission.

NASA said a total of 980 spacewalk hours and 12 minutes have now been spent building, maintaining and repairing the orbiting space lab.

On Thursday, Endeavour's astronauts installed a massive physics experiment, part of a 16-nation collaboration that aims to discover how the universe began.

The Alpha Magnetic Spectrometer-2 is a $2-billion, 15,000-pound (7,000-kilogram) particle detector that will remain at the ISS to scour the universe for hints of dark matter and antimatter over the next decade.

It is expected to send data to scientists on Earth for the next 10 years.

The 30-year US space shuttle program formally ends later this year with the flight of Atlantis, leaving Russia's space capsules as the sole option for world astronauts heading to and from the orbiting research lab.

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