Astronauts from space shuttle Endeavour began the third of five spacewalks on Wednesday, a week into their 16-day mission to help complete construction of the International Space Station.
Astronauts Dave Wolf and Chris Cassidy began their spacewalk about 30 minutes ahead of schedule, exiting the Endeavour's decompression chamber at 1432 GMT, NASA said.
The two spacewalkers plan to remove multi-layered insulation from the Japanese Kibo science laboratory and also will replace batteries in the space station's photovoltaic power system.
NASA said each ISS battery measures 40 inches by 36 inches by 18 inches (101 centimeters by 90 centimeters by 45 centimeters), weighs 375 pounds (170 kilos), and is designed to last 6.5 years.
On the second Endeavour spacewalk Sunday, astronauts moved equipment from Endeavour onto the ISS and repaired a malfunctioning toilet on the orbiting station.
During a first spacewalk on Saturday, astronauts needed five-and-a-half hours to complete construction of the Kibo lab.
Endeavour's crew of six Americans and one Canadian hope to complete construction of "an out-of-this-world space laboratory," delivering state-of-the-art equipment to conduct experiments in the vacuum of space, according to NASA.
The ISS should be completed in 2010, which also is the target date for the retirement of the US fleet of three space shuttles.