STEVE INSKEEP, host:
NASA has decided to remove two bits of ceramic fabric that are sticking out from under the shuttle Discovery. That requires a space walk tomorrow. Mission managers say the small pieces could disturb the airflow during re-entry and raise temperatures on parts of the heat shield beyond acceptable levels. NPR's David Kestenbaum reports.
DAVID KESTENBAUM reporting:
The two pieces of fabric are called the gap fillers and they fill small open spaces in the heat shield. Both are sticking out about an inch. Engineers have been trying to figure out if that inch matters and they've concluded they can't be sure. Here's the concern. The shuttle begins its descent at 25 times the speed of sound. In the initial stages, the air flows smoothly over the wings, but then it becomes turbulent and creates friction that heats things up. Engineers worry that the small bits of fabric could cause the turbulence to begin earlier than usual, raising the temperature in places. Wayne Hale chairs the mission management team.
Mr. WAYNE HALE (Chairman, Mission Management Team): And at the end of the day, the bottom line is there is large uncertainty because nobody has a very good handle on aerodynamics at those altitudes and at those speeds. Given that large degree of uncertainty, life could be normal during entry or some bad things could happen.
KESTENBAUM: Hale says their best guess is that everything will be OK, but turbulent flow is tough to model mathematically and the shuttle is the only craft that has flown at these speeds and altitudes. Chuck Campbell is an engineer specializing in aerothermodynamics. He and a team of experts have been working on this problem for three days.
Mr. CHUCK CAMPBELL (Engineer): We just don't have much data to work with. If we are a hundred years from now and we had gone through 20 shuttle designs and had the experience of all of that information, we'd be in a different story, but the shuttle is a unique vehicle.
KESTENBAUM: There is one worrisome data point. In November of 1995, the space shuttle Columbia was heading back to Earth when some parts of its heat shield experienced temperatures higher than normal. Instead of 2,300 degrees Fahrenheit, they were 400 or 500 degrees hotter. When Columbia landed, engineers found the reason: a small gap filler sticking out about an inch or so. One of the dislodged pieces in question now is in a more sensitive place further toward the vehicle's nose where the shuttle experiences higher temperatures. At very high temperatures, the heat shields could fail. Cindy Begley, the mission space walk manager, says the plan is to have astronaut Stephen Robinson ride the space station's robotic arm, go under the space shuttle and remove the two gap fillers.
Ms. CINDY BEGLEY (Manager, Mission Space Walk): The first attempt is going to be to pull it out and we don't expect that to take a lot of force. If it seems to be taking a lot of force, then we're going to look at cutting it off.
KESTENBAUM: NASA says the shuttle should be fine without the gap fillers. One provides structural support during launch. The other helps protect the shuttle from heat, but engineers say it's not critical. Cindy Begley says no astronaut has ever taken a space walk below the shuttle's delicate underside.
Ms. BEGLEY: They're going to have to be very careful of the area not to damage anything there while they're there. We're making sure that we're taking as many tools off of them as we can and holding their safety tethers back behind them. You know, that big reel box keeps floating around. We're going to have that tucked behind them so that it can't come and hit the tile.
KESTENBAUM: The decision to remove the gap fillers represents what manager Wayne Hale calls the new NASA, quote, "If we can't prove it's safe, we don't want to go there," end quote. On Sunday as engineers were studying the problem, he emphasized that inspecting the shuttle in this much detail is also new.
Mr. HALE: This flight we have literally put the orbiter thermal protection system under a microscope. I mean, if you look at some of those pictures and you can count the threads in the fabric cloth that was built up, that's the level to which we have looked at the underside of the orbiter, and it's unprecedented. It's an unprecedented period that we looked. We have always just assumed the thermal protection system to be good.
KESTENBAUM: Now he says they know it's good. With the possible exception of the gap fillers, he says, the shuttle seems to be in excellent shape.
David Kestenbaum, NPR News.
INSKEEP: There's more of our shuttle coverage at npr.org. Transcript provided by NPR, Copyright NPR.
