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

"There is nothing alarming here and we are really not concerned. We are doing what we understand and know that we need to go do in these scenarios," Cain said.

Astronauts will maneuver the shuttle's robotic arm to get a better view of the area with a digital camera and laser.

The chunks were likely gouged out by errant foam or ice, and may be repaired by astronauts during a spacewalk if needed.

"This is one that we feel pretty confident that we are going to be able to clear it once we get some higher fidelity data," Cain said.

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.

Earlier Friday, a malfunction in a US astronaut's spacesuit caused NASA to slightly shorten his spacewalk outside the orbiting International Space Station.

A carbon dioxide sensor failed in the spacesuit worn by American astronaut Greg Chamitoff after he embarked on his first-ever spacewalk along with fellow astronaut and veteran spacewalker Drew Feustel.

The glitch gave no indication that harmful CO2 levels would rise, but NASA shaved about 10 minutes off the excursion as a precaution.

"Because of a carbon dioxide sensor failure in Chamitoff's spacesuit, flight controllers limited his spacewalk time to about 6 hrs 20 minutes, 10 minutes less than the planned six hours and 30 minutes," NASA said in a statement.

The pair floated out of the station at 0710 GMT to work on repairs and additions to the orbiting lab and returned at 1330 GMT.

They 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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