From left, space shuttle program manager Wayne Hale, space shuttle launch director Mike Leinbach, and NASA Engineering and Safety Center director Ralph Roe discuss the shuttle Discovery's flight readiness Tuesday.
NASA late Tuesday cleared the shuttle Discovery for launch next week on a pivotal space station assembly mission even though some safety experts urged a delay so that three slightly degraded heat-shielding panels on the wings could be replaced.
Discovery is scheduled to lift off from NASA's Kennedy Space Center in Florida on Tuesday.
During a two-week mission, the shuttle's seven astronauts plan to deliver and install a gateway component at the space station in preparation for the future installation of European and Japanese science modules.
The decision to proceed with Discovery's launch followed a daylong review at Kennedy in which top space agency officials sifted through information on months of planning for the mission.
"We looked at everything and we're ready to go fly," said NASA's Bill Gerstenmaier, the agency's spaceflight chief, who chaired the review. "We understand the risks in front of us."
A safety concern raised by the NASA Engineering and Safety Center focused on a slight degradation of the coating on three of the 44 carbon composite panels that shield the wings' leading edges from a 3,200 degree Fahrenheit heat buildup that accompanies the shuttle's descent to Earth.
Replacing the panels would have forced NASA to haul Discovery from the launch pad to a protective hangar at Kennedy. The task would have delayed the launch by at least two months.
'We will take action'
The safety center was established after the 2003 shuttle Columbia tragedy as an independent advisory group to those who manage the agency's human and robotic missions. Columbia broke apart over Texas because of what was later determined to be damage to the leading edge of a wing, caused by foam debris during launch.
Last week, Wayne Hale, NASA's shuttle program manager, recommended the agency go ahead with plans for Discovery's launch based on assessments of the panels that showed no worsening in the shield coating after the ship's three most recent missions.
He re-embraced his position after NASA experts debated it for hours.
"At the end of the day, we decided we were in an acceptable risk posture to go fly," Hale told reporters. "We will continue to work hard on it. As we go from flight to flight, if the risk grows to an unacceptable level we will take action."
Cause of damage unknown
The assessments were based on postflight diagnostics using thermography, a technique that examines the panels using heat to reveal internal and external damage. Discovery's damage was confined to narrow regions of the silicon carbide and a glass coating on two panels on the right wing and one on the left.
Since Columbia, NASA has added inspections of the wing panels by the astronauts during the day after they liftoff and the day before their descent to Earth. The shuttle crew now carries some repair tools and materials. If severe damage is detected they could move aboard the space station to await a rescue mission.
Shuttle engineers attributed the erosion of Discovery's protective material to oxidation, a slow degradation in the presence of oxygen, and possibly to the salt in the air on Florida's Atlantic coast.
However, the safety center offered another view - that some other little understood process was responsible for the degradation, making it difficult to reliably predict when a panel might fail, said center director Ralph Roe.
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