Astronauts on the space shuttle Discovery are unloading tons of supplies and equipment for the crew of the International Space Station.
Experts are confident the shuttle will return safely to Earth
The transfer has been given added urgency by Nasa's decision to ground the shuttle fleet after this mission.
The astronauts had planned to give the station 13.6 metric tons (15 tons) of items stored in a cargo unit.
They are now also transferring computers, office supplies, food, water and other items from Discovery.
The US space agency has extended Discovery's mission by a day to allow time to unload the extra cargo, with the shuttle's return to Earth now set for 8 August.
The Discovery mission is the first shuttle trip to the ISS since 2002, and the first mission since the Columbia disaster.
Nasa announced it would ground the shuttle fleet once again last week after videos showed insulating foam from Discovery's external fuel tank breaking loose as the shuttle launched from Florida.
Loose foam from the fuel tank was blamed for the damage which caused the loss of the Columbia and its seven crew on re-entry to the Earth's atmosphere in February 2003, and was not supposed to happen again after Nasa spent $1bn and two and a half years attempting to solve the problem.
Discovery suffered minor damage to its protective tiles at launch because of the flying debris, but Nasa says laser and television scans of the orbiter show no major problems.
The Discovery astronauts are working with space station astronauts Sergei Krikalev and John Phillips to unload the Italian-made cargo unit, named Raffaello.
The unit will be refilled with 13 tons of "space junk" that has accumulated on the station since it last linked up with a shuttle in November 2002.
The two astronauts spent nearly seven hours on the outside
"It is kind of just like working in your closets and your garage," station flight director Mark Ferring said. "It's a lot of work."
During the moratorium on shuttle flights, Russian spacecraft were used to ferry supplies to the space station but they cannot transport as much cargo - or the large modules used to piece together the unfinished outpost.
Discovery astronauts Steve Robinson and Soichi Noguchi made a spacewalk of nearly seven hours to test heat shield repair techniques developed since such damage led to the disintegration of Columbia.
The tests were made not on Discovery itself but on damaged heat shield tiles carried into space for that purpose.
They also carried out some overdue repairs to the space station, restoring power to a failed gyroscope and replacing a broken positioning antenna.
Robinson and Noguchi are due to make another spacewalk on Monday.