HOUSTON — Two spacewalking astronauts floated out of a hatch on the international space station on Friday to help install a new live-in compartment on the orbiting lab.
Astronauts Scott Parazynski and Douglas Wheelock got started about 30 minutes ahead of schedule, as the space station passed over South America.
"Like kids on Christmas morning getting up early," one of the spacewalkers said just before he ventured outside.
Parazynski and Wheelock will help astronauts inside the station use a robotic arm to transfer the compartment - called Harmony - from Discovery's payload bay to its position on the space station.
Their first task, however, involved removing a broken antenna from the station and packing it aboard Discovery so it can be refurbished back on Earth.
Harmony will serve as the docking port for European and Japanese laboratories that will be delivered on the next three shuttle flights. The Italian-made module is about the size of a school bus, weighing nearly 16 tons.
Italian astronaut Paolo Nespoli joined Discovery's crew to personally deliver the pressurized chamber. He was coordinating the 6.5-hour spacewalk from inside the station.
A veteran spacewalker, Parazynski is set to participate in four of the record-tying five spacewalks scheduled for this jam-packed mission. This is Wheelock's first trip to space.
The 10 astronauts aboard Discovery and the space station face the most challenging construction tasks ever attempted in a single mission.
They may get a little more time to tackle their to-do list because engineers have not spotted any significant problems with the shuttle's thermal shield.
The crew has set aside several hours Saturday for a focused inspection of any trouble spots, but mission management team chairman John Shannon said that examination might not be necessary.
NASA has made damage inspections a priority since the disintegration of the shuttle Columbia in 2003.
A piece of foam broke off Columbia's external fuel tank during liftoff and gashed a wing, allowing hot gases to penetrate the spacecraft during its return to Earth. All seven of its astronauts were killed.
Further analysis is needed before NASA can say for sure that Discovery suffered no significant launch damage. But given all the construction work on this mission, "We are extremely lucky that we have a vehicle that is in such incredible shape," Shannon said.
During Friday's spacewalk, Parazynski and Wheelock are to remove a broken antenna from the station and pack it aboard Discovery for its return to Earth, and prepare a space station girder for relocation later in the flight.
"It's going to be a very interesting day. A lot of folks have been looking forward to this for a long time," Shannon said, referring to Harmony's installation.