Two Discovery shuttle astronauts began the first spacewalk of the current mission at the International Space Station at 0531 GMT on Friday, with a list of outside chores to complete.
The duo Clay Anderson and Rick Mastracchio stepped into the vacuum of space, intending to change out an ammonia tank and replace a malfunctioning gyroscope.
The walk was slated to last about 6.5 hours. Two more spacewalks are planned on Sunday and Tuesday before Discovery undocks on April 16 to begin its homeward journey.
On Thursday, crew members of Discovery began unloading the Italian-built Leonardo container, which was packed full of 8 tons of cargo, including experiments, a small fitness studio for the station crew, an observation module with cameras and sensors and a ream of scientific instruments.
Discovery took off Monday from Cape Canaveral, Florida, into a stunning sunrise, providing a picturesque launch watched by many on the beach.
The planned 13-day mission is to be followed by only three more shuttle missions before the ageing fleet of spacecraft is retired for good by year's end or early next year. That will leave only the Russia Soyuz craft for human transport into space.