NASA cleared the space shuttle Discovery on Thursday for its flight home next week after concluding that a frayed thermal blanket below the commander's window is not a serious flight hazard.

Wayne Hale, deputy shuttle program manager, said engineers at NASA's Ames Research Center in California tested similar strips of fabric in a wind tunnel overnight at speeds between Mach 1 and Mach 6 -- six times the speed of sound.

Hale said the engineers found that even if the entire blanket tore away at the worst possible time, it had only a 1.5 percent chance of striking Discovery, and, at worst, it could put a hole in the orbiter's rudder brake but not disable it.

"We have released Discovery to reenter," Hale told reporters in a news conference at Johnson Space Center. "We have decided not to take any action on the thermal blanket because we have determined it is a negligible risk at this time."

Formal clearance for reentry is an important milestone in any shuttle flight -- particularly for Discovery, the first shuttle to fly since Columbia disintegrated over Texas 21/2 years ago, killing all seven astronauts aboard.

NASA had equipped Discovery with a wide array of new sensors, imaging devices and inspection systems, and it had expected to clear the shuttle four days into the 14-day mission. Instead, during launch the external fuel tank shed a large chunk of foam insulation similar to the one that caused the Columbia disaster. NASA has grounded the shuttle fleet until this problem is solved.

In-space inspections before and during the shuttle's nine-day sojourn at the international space station then turned up two protruding "gap fillers" in the underside heat shielding, and the frayed blanket, a rectangular piece of fabric installed just above the D in "Discovery" painted under mission commander Eileen Collins's window.

On Wednesday, spacewalker Stephen Robinson made the first in-flight shuttle repair in history when he removed gap fillers, but Hale told reporters that Discovery might need another spacewalk for the thermal blanket.

On Thursday morning, the Hale-led Mission Management Team conducted its daily review with the blanket as the last impediment to reentry clearance.

In Mission Control, shuttle communicator Julie Payette called Discovery shortly after 1 p.m. Eastern time: "We have good news," she said. "The MMT has reached the conclusion that the blanket underneath the . . . window is safe for return. We have new analysis that showed debris . . . would be no issue.

"So, basically, no EVA 4," she concluded, using NASA shorthand for "extravehicular activity" -- a spacewalk.

"That's, I would say, good news," replied Discovery spacewalker Soichi Noguchi.

The astronauts spent the day filling up the Italian-built Rafaello multipurpose logistics module, a cylindrical storage bin that carried nine tons of spare parts and supplies up to the space station and will return with 10 tons of worn-out gear, broken tools and trash.

On Friday, the astronauts will put Rafaello back into Discovery's payload bay and restow other equipment. The shuttle will undock from the station early Saturday and spend Sunday testing flight control systems before landing at Kennedy Space Center in Cape Canaveral, Fla., before dawn Monday.