Two astronauts stepped out of the International Space Stationon Saturday) for the first of three spacewalks.
British-born U.S. astronaut Piers Sellers and American colleague Mike Fossum repaired a transporter and tested the stability of the shuttle's 15-meter robotic arm. They also commented on the beauty of their view of Earth.
The spacewalk is expected to last six-and-one-half hours ending at about 1945 UTC.
In a lighthearted moment before venturing out of the space station, the astronauts asked mission control which teams were facing off in Sunday's World Cup championship.
Meanwhile, officials at the U.S. space agency, NASA, are assessing images of Discovery's heat shield.
NASA engineers on Friday took a closer look at six areas of concern on the shuttle's thermal skin. They determined three areas pose little risk to the shuttle and its crew, while three others warrant a closer look.
The heat shield protects the shuttle from the searing heat of re-entry into Earth's atmosphere.
NASA extended Discovery's 12-day mission to the International Space Station by a day to give astronauts time for a third spacewalk to test a heat shield repair technique.
There is a crew of three aboard the International Space Station for the first time in three years. Discovery brought a German astronaut, Thomas Reiter, who will stay behind to live at the station for the next six months.