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The U.S. space agency NASA is considering whether astronauts aboard the shuttle Discovery need to make an emergency space walk to repair the craft's protective heat shield. Mission controllers worry that filler material sticking out from between some exterior surface tiles could endanger the orbiter by making re-entry hotter than it already is.
The concern is over two pieces of so-called gap filler, thin pieces of ceramic-coated fabric that fill the cracks between the shuttle's ceramic tiles. With the tiles, they shield the shuttle's underside from searing re-entry temperatures.
The gap filler is protruding up to two-and-a-half centimeters in two areas, and NASA engineers say it could upset the smooth flow of air beneath the shuttle as it glides back toward Earth at high speeds next week. The engineers estimate that the turbulence created by these tiny projections could cause a 15 to 25 percent increase in atmospheric temperature already superheated by friction with the vehicle.
Deputy shuttle program manager Wayne Hale says technicians are studying whether this would threaten the orbiter with a calamity like the one that befell the shuttle Columbia, which burned up during re-entry in 2003 because launch debris punctured a hole in its wing.
"They are really operating at the limits of the understanding of aerodynamics, and that is a problem. But the consequence, which is heating, is what we're interested in," Mr. Hale says.
As engineers consider whether the protrusions are dangerous, mission officials are contemplating the possibility of sending astronauts on an unplanned spacewalk to trim them. This would be in addition to the three spacewalks that had been scheduled for the shuttle's current mission to the International Space Station.
Mr. Hale says fixing a shuttle in orbit would be a rare event.
"You know, as opposed to the station where we do maintenance every time, it seems, we visit them, there has been very little, if any, on the shuttle," Mr. Hale says.
The gap filler issue is one of the several questions NASA has had about Discovery's surface, which has experienced problems during this mission. It says several tiles apparently struck by launch debris from the external fuel tank did not sustain serious damage. The space agency also says reinforced carbon panels protecting the front edges of the wings appear safe for re-entry. This is the location where launch debris punctured Columbia's wing. But shuttle engineers continue to study whether a re-entry threat is posed by protective thermal blankets that have come loose near the cockpit window.
The continuation of the launch debris problem that doomed Columbia has forced NASA to suspend future shuttle missions until it can solve it. Because of the new flight moratorium, NASA has extended Discovery's time at the station by one day to give astronauts more time to service the outpost.
That servicing is taking the form of a second spacewalk on Monday. The main task for U.S. astronaut Steve Robinson and Soichi Noguchi of Japan is to install a new stabilizing gyroscope to replace a backup unit that has failed.
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