Mission specialists Mike Fossum and Ron Garan left the safety of the U.S. space shuttle Discovery Tuesday to embark on the first spacewalk of this 14-day shuttle mission.
The main job for the astronauts was to unlock the largest component of the $1 billion Japanese "Kibo" laboratory from the shuttle and prepare it to be attached to the International Space Station.
The 6.5 hours set aside for the spacewalk also included time to perform repairs on a jammed joint on a solar panel.
The solar joint started showing increased vibration and power usage late last year.
During the spacewalk, U.S. mission specialist Karen Nyberg and Japanese astronaut Akihiko Hoshide were scheduled to use the station's robotic arm to remove the largest component of the lab from the shuttle's payload bay and install it on the Space Station.
"Kibo" is Japanese for "hope." In the laboratory, astronauts will be able to conduct experiments over several months, far longer than currently possible on the space shuttle, which only stays in space for about two weeks.
The first of about 100 experiments planned for Kibo are expected to start in August, including planned experiments on the influence of zero gravity and cosmic rays on organic cells over an extended period.
The U.S. space agency says the start of Tuesday's spacewalk was delayed by 50 minutes while Fossum repaired a faulty cable on his communications cap.
NASA officials also say the lift-off of Discovery on Saturday caused unprecedented damage to the launch pad, mostly to the flame trench, which helps deflect heat from the shuttle's rockets. Officials say they do not believe the shuttle was damaged by falling debris.
This spacewalk is taking place on the 43rd anniversary of the first U.S. spacewalk, by astronaut Ed White on the Gemini Four mission in 1965.