Two astronauts on Friday embarked on the first space walk of the
Endeavour shuttle's final mission to the International Space Station (ISS), according to NASA.
Andrew Feustel and Greg Chamitoff, wearing spacesuits with solid and broken red stripes, respectively, floated out of the station at 0710 GMT to retrieve two science experiments that had been delivered in November 2009.
Feustel, who is on his fourth lifetime space walk, will then retrieve a similar experiment brought up aboard Endeavour and install it.
The "excursion" was to last six and a half hours, according to NASA, and will be followed by three more space walks over the course of the 16-day mission, the penultimate journey of the decades-old US shuttle program.
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.
NASA managers at mission control in Houston were meanwhile inspecting the shuttle's heat shield after seven tiles appeared to have been damaged during its ascent, but officials have said there is no reason to be alarmed.
Endeavour blasted off on its final mission on Monday with six astronauts on board -- five Americans and one Italian -- and docked at the ISS on Wednesday.
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.
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.