Houston ? 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.

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,” 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 this 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.

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.

The shuttle is now scheduled to land at 8:12 a.m. CDT July 17.