NASA officials have been very pleased so far with the second flight since Columbia disintegrated over Texas in February 2003, as Discovery suffered no damage during liftoff on July 4.

The astronauts continued to empty a container brought by Discovery last week, filled with food, clothes, a lab freezer to store scientific samples, a large incubator to study plant growth in space and an oxygen generator system that will allow the ISS to accommodate six crew members.

"They're in excellent spirit," flight director Tony Ceccacci told reporters at the Johnson Space Center in Houston, Texas.

The six Discovery astronauts and their three ISS colleagues have completed 80 percent of the transfer so far, Ceccacci said.

"The crew is working very hard on that," he said.

Once emptied, the Italian-made Multi-Purpose Logistics Module, dubbed "Leonardo," will be filled with discarded material from the ISS that will be brought back to Earth on July 17.

The shuttle has been cleared of damage that would prevent it from coming home, although an unprecedented final inspection will take place at the end of the 13-day mission to make sure Discovery was not hit by micrometeorites.

Sellers and Fossum prepared for their third excursion out of the ISS on Wednesday to test material that would be used to fix the shuttle's heat shield.

Columbia's demise was caused by debris from its external fuel tank that pierced its heat shield during liftoff, leading to its destruction as it returned to Earth.

The spacewalkers will use cracked samples of reinforced carbon-carbon (RCC) used as a heat shield to test a sealant's effectiveness in orbit.

While the previous spacewalks required heavy lifting, "this seems more like kind of a careful, meticulous lab experiment," Sellers told the USA Today newspaper from the space station.

In their second spacewalk Monday, the pair replaced broken hardware needed to build the station.

NASA officials said the repair set the stage for resuming construction of the station in the next shuttle mission. The US space agency plans to launch Atlantis late next month.

"If we didn't get this successfully changed and checked out, then we couldn't proceed with the next mission, which is right on our heels," said Rick LaBrode, the lead ISS flight director for the current mission.

The astronauts also successfully tested Saturday a boom extension on the shuttle's robotic arm that would be used as a work platform for repairs in hard-to-reach places.

"Everything has been working great on this mission," Phil Engelauf, the chief of NASA's Flight Director Office, told reporters at Johnson Space Center.

The Discovery mission is a final test of NASA's efforts to dramatically improve safety in order to resume regular shuttle launches and complete the ISS.

NASA wants to conduct 16 more flights to complete the ISS by 2010, when the 25-year-old shuttle fleet is scheduled to be retired.

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