The six Discovery astronauts and three ISS crew members removed critical cargo from a container delivered by the space shuttle and prepared for the mission's second spacewalk set for Monday.

NASA analysts, meanwhile, spent the night examining images from two remaining areas of potential concern on the spacecraft.

But officials appeared optimistic that Discovery will not need repairs before returning to Earth from just the second shuttle mission since the February 2003 Columbia accident that killed seven astronauts.

The analysts were to decide whether an insulating blanket protruding from Discovery's windows and a gap filler poking from between thermal tiles in the shuttle's underside would pose a problem.

"As I left ... the mission evaluation room, the guys were telling me the windows were not going to be a concern at all," Steve Poulos, the orbiter program manager, told reporters Saturday at the Johnson Space Center in Houston, Texas.

As far as the gap filler, he said: "Likely, we'll be able to show that we are going to clear this one as well, but I don't want to necessarily presuppose the data."

But Poulos said the Discovery astronauts, who arrived at the ISS Thursday, could perform a spacewalk to remove the gap filler if necessary.

Two Discovery astronauts performed a first spacewalk Friday to test a beam that coincidentally could be used to reach the gap filler. The simulation had been planned before last Tuesday's shuttle launch.

Astronauts Piers Sellers and Mike Fossum floated out of the ISS for the excursion, which is part of NASA's intense efforts to dramatically improve shuttle safety.

Sellers first tested on his own the stability of the 15-meter-long (50-foot-long) boom that was added to the equally long robotic arm.

Fossum then joined Sellers at the end of the boom and the pair simulated repairs to see if it can support two people.

NASA officials described the first of three planned spacewalk as a success and said that the elongated robotic arm could be used for repairs if necessary.

"Hopefully we'll never have to use it, but we know we have the capability if we do need to," said shuttle flight director Tony Ceccacci.

The ability to make in-orbit repairs is among several efforts by the US space agency to prevent another tragedy. New repair techniques were tested in the first post-Columbia flight last year.

Poulos said the boom would have to be used if the gap filler in the back of the shuttle's underside needs to be removed.

The astronauts, meanwhile, prepared for the second spacewalk Monday, which will focus on ISS maintenance, and removed cargo from the container, which includes research equipment, clothes and food for the orbiting laboratory.

On a third spacewalk Wednesday, the astronauts will test repairs on pre-damaged samples of reinforced carbon-carbon (RCC), a composite material used on the shuttle's wing leading edges as a heat shield.

Columbia's demise was caused by foam insulation that peeled off its external fuel tank and pierced its heat shield during liftoff, dooming its return to Earth on February 1, 2003.

NASA has since made several fuel tank modifications to limit the size of debris during blastoff.

Officials were pleased with the performance of Discovery's fuel tank during liftoff, saying it shed small pieces of debris as expected but too late into ascent to cause concern.

But if NASA concludes that the shuttle suffered irreparable damage, the astronauts will take refuge in the ISS and wait for a rescue mission.

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