HOUSTON - Despite clearing another hurdle in efforts to prove the integrity of the shuttle Discovery's heat shield, mission managers are still studying a puffed-up thermal blanket to ensure it won't rip off during reentry and hit the spacecraft.

NASA's deputy shuttle program manager Wayne Hale said Tuesday that engineers have cleared Discovery's wing leading edges for the return flight to Earth, but are now discussing whether a damaged thermal blanket just below a crew cabin window on the orbiter's nose could potentially inflict damage to the spacecraft if it pulls free.

"The biggest work going on is to determine whether or not it's even possible for that blanket to come off," Hale said during a briefing here at Johnson Space Center.

The results of that analysis should be presented to the flight's mission management team (MMT) in the next 48 hours, he added.

The study comes after engineers had cleared the orbiter's thermal blankets and heat resistant tiles for reentry, but shuttle managers said that evaluation was based on thermal heating concerns which are not an issue for the loose piece of fabric.

Meanwhile, Discovery's seven-astronaut crew is preparing to conduct a 6.5-hour spacewalk early Wednesday to pluck out two pieces of filler material from the orbiter's heat shield and install new hardware to the exterior of the International Space Station (ISS).

A thorough inspection of Discovery's wing leading edges, which are covered heat-resistant reinforced carbon carbon (RCC) panels, yielded a few scuffs marks but nothing shuttle engineers perceived as a flight risk for reentry, mission managers said.

Hale said the 20-inch (50-centimeter) long blanket was apparently damaged during Discovery's July 26 launch by a piece of debris - possibly a paper cover for one of the orbiter's thrusters - which punctured the material and allowed air to puff it up. A nearly 8-inch section (20-centimeter) of the 3.8-inch wide (9.6-centimeter) blanket has puffed up from Discovery's hull, he added.

The stitching seems secure on the blanket and the glue adhering it to Discovery appears in place, but engineers want to track the paths the loose fabric could take if it pulls free during reentry and ensure that the orbiter's tail, rudder speed brake, orbital maneuvering systems (OMS) pods and other areas are not in danger of an impact, shuttle officials said.

"We want to prove to a reasonable engineering level that we don't have any concerns," Hale said.

Launch debris and the shuttle's thermal protection system are serious concerns or NASA and its shuttle engineers. In 2003, the Columbia orbiter was destroyed, its crew lost, during reentry after sustaining critical damage to its wing leading edges by a piece of external tank foam debris during lunch.

After two and a half years of work and redesign to increase shuttle safety, NASA launched Discovery and its STS-114 crew only to see one large piece of foam fall from its external tank from an area previously thought safe by tank engineers. That foam debris did not hit Discovery, but did prompt shuttle officials to ground future flights until they understand and address the problem.

Discovery's STS-114 mission is slated to land at NASA's Kennedy Space Center on Aug. 8.

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