Astronauts Piers Sellers and Mike Fossum have completed the second of three spacewalks on the Discovery shuttle's latest mission into orbit.
The spacewalk was the second of three planned for the mission
The pair carried out repairs to cabling on the exterior of the International Space Station (ISS).
The work was necessary to ensure the proper functioning of a mobile transporter used to move a robotic arm around the outside of the platform.
The crane will be a vital tool in the final construction effort on the ISS.
It will grapple heavy station components and take them to their proper locations for installation.
The system has been out of action since December, when its power and data cable was inadvertently severed.
Sellers and Fossum also used the walk to attach a spare pump module for the space station's cooling system.
In total, the men spent more than six and a half hours working through a long list of tasks, grabbing only brief moments to admire their extraordinary surroundings and gaze at Planet Earth some 350km (220 miles) below.
"It's like standing in an all-around Imax (3D movie theatre). It's just beautiful," said UK-born astronaut Dr Sellers.
DISCOVERY SHUTTLE FLIGHT
Mission known as STS-121
Discovery's 32nd flight
18th orbiter flight to ISS
Landing: 0914 EDT (1314 GMT), Monday 17 July
Location: Kennedy Space Center
Objective: To test new safety equipment and procedures
Payload: Cargo bay has 12.75t of equipment and supplies
Crew: Lindsey, Kelly, Fossum, Nowak, Wilson, Sellers, Reiter
The Briton's backpack came loose on two occasions during the spacewalk. Colleague Fossum had to come to his aid, bypassing a faulty clamp on the backpack with a short tether.
The Discovery orbiter docked with the ISS on Thursday. Its mission is just the second shuttle flight since the Columbia disaster in 2003.
Sellers and Fossum will put on their spacesuits once more on Wednesday, to try out shuttle repair techniques considered important to the US space agency's (Nasa) plans to improve flight safety.
The duo will "fix" pre-damaged samples of reinforced carbon-carbon (RCC), a composite material used on the shuttle's wing leading edges as a heat shield.
It was RCC panels on Columbia that were pierced by flyaway foam from the vehicle's external tank during launch - damage that led directly to the ship's destruction when it later tried to re-enter the Earth's atmosphere.
Discovery's own surface tiles were subjected to a thorough examination following the vehicle's launch from the Kennedy Space Center in Florida last week; and although the investigation found some defects in the heat shield they are not considered serious enough to prevent the orbiter's safe return to Earth.
A landing at Kennedy has been scheduled for 0914 (EDT) on 17 July.