HOUSTON--The space shuttle Discovery was given a clean bill of health to return home Monday after mission managers declared a fourth spacewalk would not be needed to repair a damaged thermal blanket on the orbiter's hull.
"We have cleared Discovery to reenter," said Wayne Hale, NASA's deputy shuttle program manager, during a briefing here Thursday at Johnson Space Center (JSC). "We believe the chances of anything untoward happening with this piece of fabric is remote."
The announcement came after a night of wind tunnel tests which proved that sizeable chunks of the small thermal blanket--damaged on launch and puffing out from just below the orbiter's leftmost flight deck window--would likely not rip off and smash into a vulnerable section of Discovery. The blanket clearance was the last piece needed to clear Discovery for landing. Its heat-resistant tiles and reinforced carbon carbon (RCC) panels were cleared during earlier inspections, shuttle officials said.
Hale said that after wind tunnel tests of three concluded this morning, engineers believed there was only a 1.5 percent chance a piece of the blanket could detach from its mooring at time it could cause the worst damage--while the Discovery was locked in supersonic flight between Mach 6 and Mach 1, about 10 minutes before landing. In that worst case scenario, a piece of the blanket about eight-tenths of an inch long could pull free and hit a critical area, such as the orbiter's tail.
"I will not tell you that there is zero risk," Hale said. "Even if this blanket were in pristine shape, there is not zero risk."
However, the risk of landing with the damaged blanket was much less than the risk of once more suiting up Discovery's two spacewalkers, STS-114 mission specialists Stephen Robinson and Soichi Noguchi, and sending them on a repair job.
This could have potentially created more debris that could harm the orbiter or astronauts' spacesuits, Hale said. Similarly, deciding to leave Discovery's crew on orbit to be retrieved by a rescue shuttle--a remote case that would call for the launch of Atlantis orbiter--also carries more risk than landing as there are still concerns about external tank foam shedding during liftoff, Hale said.
The damaged blanket measures about 20 inches (50 centimeters) long and 3.8 inches wide (9.6 centimeter), though only about an eight-inch (20-centimeter) section puffed up during launch.
Flight controllers alerted Discovery's astronaut crew of the good news before mission managers had concluded their daily meeting Thursday, NASA officials said.
"Soichi we have good news," astronaut Julie Payette, serving as Discovery's CAPCOM. "The blanket is safe for return...so no EVA four."
"That's, I would say, good news," Noguchi replied.
Noguchi and Robinson have successfully conducted three spacewalks during their STS-114 mission, the latest including a first-ever repair of the orbiter's tile-lined belly. During that repair, Robinson pulled two space-filling strips of ceramic-lined cloth jutting out from between the heat tiles underneath the orbiter's nose.
Meanwhile, the entire STS-114 crew has completed their primary cargo transfer work, loading tons of fresh supplies into the International Space Station (ISS) and transferring still more tons into a cargo pod for return to Earth, shuttle officials said.
The shuttle is scheduled to land at NASA's Kennedy Space Center on Aug. 8.
"This flight has been conducted flawlessly," Hale said. "It is a thing of beauty."
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