NASA engineers examined new detailed pictures of space shuttle Discovery's heat shield Friday, a day before two astronauts were to embark on the most disorienting task of their 13-day mission: a wobbly spacewalk.

The astronauts beamed down up-close views of six areas of concern on the shuttle's thermal skin, and engineers agreed that three of them posed little risk, said Steve Poulos, manager of the orbiter project office. Two more areas have not been cleared but were unlikely to be an issue, Poulos said.

That left just one concern that cautious NASA managers said they could not discount yet: a piece of fabric filler protruding from the thermal tiles on Discovery's belly.

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Poulos said it was "amazing" how clean the shuttle appeared in all the pictures taken in orbit.

"There is nothing that jumps out in terms of tile damage like we saw on the last flight. Can't explain it necessarily, but it's great to see that level of performance. Overall, it really made it kind of easy on the analysts," Poulos said.

But if that one remaining piece of fabric filler needs to be fixed, it will have to be yanked out by astronauts in emergency repairs during a third spacewalk on July 12.

Because the protruding filler is in a hard-to-reach area, astronauts Piers Sellers and Michael Fossum would have to use a never-before-tried maneuver of working on the end of the shuttle's 100-foot robotic arm and boom extension.

That daring task is just what Sellers and Fossum were coincidentally scheduled to test Saturday morning during a six-and-a-half-hour spacewalk that is described as the zero-gravity equivalent of trying to paint a house while standing on a rickety ladder.

They will stand at the end of the robotic arm and boom to test a technique for repairing the spacecraft's heat shield.

"You're standing at the end of it at night, so you'll feel like you're standing on a diving board or standing at the top of a telephone pole or hanging down from a ceiling," Sellers said in a June interview. "It's disorienting, there's no question."

And if that's not enough, mission control told Sellers and Fossum that they may also have to play "hot potato" with a potentially scorching replacement part for a broken transporter on the international space station. The railcar-like device must be fixed to help with future construction of the orbiting outpost.

The part is stored in a small bag that traps heat. If it gets too hot, mission controllers suggested the astronauts "let it hang from a tether or 'hot potato' it between your hands."

Also Friday, NASA managers announced that Discovery has enough fuel to stay up for a 13th day to squeeze in a third spacewalk for Sellers and Fossum on July 12.

"It's great news because it gives us a whole lot more capability and a little more bang for the buck, if you will, for this shuttle mission," space station astronaut Jeffrey Williams said in an interview with Associated Press Television News.

The shuttle is now scheduled to land at 9:12 a.m. EDT on July 17.

Astronauts spent several hours focusing the cameras on six specific "areas of interest" on the shuttle that had been seen in earlier photographs. Engineers need more information about the heat shield to ensure there is no damage like the kind that doomed Columbia's flight in 2003.

The fabric gap filler is material fitted between tiles to prevent them from rubbing against each other. Two pieces of gap filler had to be removed from Discovery's belly during a spacewalk last year because of concerns they would cause problems during re-entry.

Besides the filler, NASA was looking at a horseshoe-shaped white spot on the shuttle's nose cap that engineers have described to the crew as resembling bird droppings — but not the same as white splotches found earlier on the wing.

The concern was that the spot could be a hole in the thermal-protection system and need repair, but Poulos said the close-up photographs convinced him that it was not a divot.

Two areas of concern that engineers had not discounted, but Poulos said were unlikely to be a problem, were discolorations on Discovery's right wing, including the area where it gets hottest during re-entry. If they need to be fixed, the crew already was slated to test a patch kit, he said.

The crew spent much of Friday hauling thousands of pounds of supplies into the station. Unloading items 220 miles above Earth was a challenge because of the lack of gravity, according to Discovery commander Steve Lindsey.

"You've got to go very, very slow because if you go fast, you kind of run into things and bump into other equipment," Lindsey said in interviews with reporters on the ground. "It's kind of an interesting choreography we have to go through."