Oct. 17, 2007 — -- The debate: Is it safe to fly the Space Shuttle Discovery when three of the panels on its wings have deteriorated?
NASA safety engineers are split over the decision. In the end they were overruled by senior managers at the Flight Readiness Review, which gave the go-ahead for the shuttle countdown to start this weekend. The panels are made of reinforced carbon, coated with silica, which keep the searing heat of re-entry from burning through the space shuttle's skin. Three of the 44 composite panels that help protect the shuttle show evidence of degradation, which was detected by thermo graphic imaging.
STS 120 Discovery is now scheduled to launch at 11:38 a.m. EDT Oct. 23. The mission is the most complicated space station construction mission to date. It will add a new module to the orbiting outpost, and the crew will relocate a truss, a delicate and painstaking three-day process. Five demanding spacewalks are scheduled including one just to practice damage repair techniques.
Space Shuttle Program Manager Wayne Hale says he is comfortable flying with the panels "as is." He admits the decision to fly came after a very spirited debate that lasted four hours. But in the end, he said, "The preponderance of evidence shows an acceptable risk in flying, not a safe risk, but an acceptable risk. There are things we do not understand about this vehicle after 26 years of flying."
What is the worst-case scenario? According to Hale, "The potential is catastrophic loss of vehicle so we have to pay close attention to it."
Hale doesn't believe that is a serious prospect; he believes the wing panels may simply lose more of their protective coating. But he contends NASA is prepared to deal with it during the mission.
"If we find something during the inspection we have a repair technique," said Hale. "What evidence is there we will be in trouble during the actual re-entry? After working through all that, I think the majority of engineers and I decided that there is a good analysis that we could survive even if the worst thing happens to us during re-entry.
The debate over the safety of flying with a damaged heat shield was triggered by NASA's Engineering and Safety Center, an independent group formed after the Space Shuttle Columbia's accident in 2003. Columbia's breakup was caused by a piece of foam that hit the left wing of the shuttle, creating a hole in the panel that went undetected by the safety team then in place at NASA.
NASA now has the ability to detect damage and goes to extraordinary lengths to find it, using cameras to videotape the launch in detail, and when the shuttle is on orbit, inspecting every inch of the orbiter for damage.
During the last mission, STS 118, video cameras documented nine pieces of foam falling off the tank during launch, with three pieces striking Endeavour. NASA examined 11 suspected damage spots on the shuttle; the one that worried them the most was below the right wing.
A piece of foam the size of a baseball came off a bracket on the shuttle's external feed line 58 seconds into the launch. It bounced into a strut for the tank, ricocheted right into the bottom of Endeavour, carving a 3-inch-by-2-inch hole, all the way through the tile, leaving the felt and aluminum of the shuttle's skin exposed.
Hundreds of engineers across the country spent days analyzing the hole and determined it would be safe for the crew and shuttle to fly home with the damage. NASA doesn't want to take any more chances with damage, so during the next mission two spacewalking astronauts will practice repair techniques just in case a future shuttle crew must fix damage on orbit.
The NASA Engineering and Safety Center has been studying the panel issue since May and still does not understand why the protective coating on some of the wing panels is coming off. It recommended additional testing before Discovery flies and suggested replacing the three suspect reinforced carbon panels, two on the right wing and one on the left wing. That work would have set the launch back by at least 60 days, the time it would take to remove old panels and replace them with new panels.
The seven-person crew onboard Discovery will be commanded by Pam Melroy, the second woman to lead a space shuttle flight.