SPACE CENTER, Houston -- Astronauts on Sunday exchanged supplies for trash that has accumulated on the International Space Station since shuttles were grounded after the 2003 Columbia tragedy. They also prepared for a second spacewalk set for Monday.

Space shuttle Discovery, which docked at the orbiting outpost Thursday, became the first shuttle to return to orbit last week. Among the mission's goals: Resupply the station and remove the mounds of trash that have accumulated in recent years.

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"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."

The astronauts are unloading 15 tons of cargo. They expect to return to Earth with 13 tons of trash and other items that are no longer needed on the station.

When the station's two-man crew awoke Sunday, they were told that their seven Discovery guests would remain at the station for an additional day.

"Hopefully it is not going to be like the relatives who miss their flight and have to stay another day," Charles Hobaugh, who works in the station's mission control, told crew members by radio.

A ninth day at the station was approved late Saturday, days after NASA decided to ground future shuttle missions because an almost one-pound piece of foam broke free of Discovery's external fuel tank. The piece of foam missed Discovery, but was a haunting reminder of Columbia.

A 1.67 pound chunk of foam shed from Columbia's external tank hit the shuttle's left wing and caused a hole that allowed the searing gases of re-entry to melt the wing from the inside out. The shuttle disintegrated over Texas as it returned to Florida. All seven astronauts died.

NASA spent hundreds of millions of dollars over 2 1/2 years redesigning the external tank, but said last week it did not solve the problem and more work is needed.

Bill Gerstenmaier, the station's program manager, said Discovery will leave the station in much better shape than when it arrived. Water will be left behind, as well as laptops, wash cloths, a printer, dry wipes, floppy disks, food and nitrogen.

"We are going to be in very good shape through the end of the year," Gerstenmaier said.

Shuttles are needed for the supplying and continued construction of the station.

Slideshow

Discovery Images Isolated Images Of Piece Of Foam Falling From Shuttle

When NASA's shuttles were grounded in 2003, the agency began relying on Russian vehicles to deliver supplies to the station. However, the Russian cargo ship Progress cannot haul the weighty items the shuttle can. The next delivery by Progress is scheduled for September.

Gerstenmaier said station managers prepared for a scenario in which there would be no shuttle flights this year. Despite that, he said the grounding of future shuttle missions opens the station to risk.

"We are still susceptible to some large failure of some component that can only be delivered by the shuttle that we don't have a like spare on station," he said.

Among the tasks Discovery's astronauts performed for the station was the reconfiguring of a gyroscope, which failed in March. Four gyroscopes, each weighing 660 pounds, are intended to steer the station, but only two have been working in recent months.

On Monday, during their second spacewalk, Discovery astronauts Stephen Robinson and Soichi Noguchi, planned to replace the other failed gyroscope, which has not worked since 2002.

The pair planned to go over procedures for installing the gyroscope on Sunday with astronaut Andrew Thomas, who was set to direct Monday's replacement.

As they began spacewalk preparations Sunday, Robinson informed Mission Control's Stephen Frick that things might proceed slower than anticipated.

"If you saw what our mid-deck looks like right now, you'd see why," Robinson said as his colleagues transferred items between the shuttle and station. "It's a high traffic zone."

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