The U.S. space agency NASA has ordered astronauts aboard the shuttle Discovery to make unprecedented repairs to its exterior heat shield during a spacewalk Wednesday. Missions officials say there is a possibility that not undertaking the repair could be catastrophic to the shuttle's re-entry next week.

With memories of the 2003 shuttle Columbia disaster still fresh, NASA is taking no chances. Deputy shuttle program manager Wayne Hale says a spacewalking astronaut will either remove or trim two bulging pieces of fabric from the Discovery's underside. Engineers worry that the pieces could threaten the orbiter during the blistering heat of re-entry to Earth.

"We are doing what is necessary to be done to fly this vehicle safely and ensure the crew's safe return to the best of our ability," Mr. Hale says.

The problem involves just two of thousands of heat resistant slices of fabric wedged between ceramic tiles underneath the shuttle, where no spacewalker has ever been. The two pieces of material have slipped and stick out just a couple of centimeters beyond the shuttle's surface. NASA worries that the protrusions can change aerodynamic conditions during Discovery's fiery, high-speed re-entry and add hundreds of degrees of temperature to certain parts of the orbiter.

Wayne Hale

Shuttle engineers debated for three days whether the protrusions might threaten Discovery with destruction similar to that of Columbia, which burned up during re-entry because launch debris had punctured a hole in its wing. Mr. Hale says information about aerodynamics at that high altitude and speed is so scant that, in the end, the technicians could not say whether or not Discovery could tolerate the heat buildup.

"Given the large variation and the large state of the unknown, it was prudent to take action so that we would not have to worry about any of the worst consequences," Mr. Hale says.

U.S. astronaut Steve Robinson, accompanied by Japanese crewman Soichi Noguchi, will make the repair standing on the end a robot arm attached to the International Space Station, where Discovery is docked for a resupply and maintenance mission. Shuttle spacewalk supervisor Cindy Begley says NASA will take standard precautions to prevent the mechanical arm, the astronauts' safety tethers, and their tools from striking the fragile shuttle heat tiles.

"They are going to have to be very careful of the area, not to damage anything while they are there," Ms. Begley says. "We're making sure we're taking as many tools off them as we can and holding their safety tethers back behind them so that (they) can't come and hit the tile."

The unscheduled repair is being added to a spacewalk that was already planned, the third of the Discovery mission.

On Monday, the two astronauts went on spacewalk number two to replace a faulty station gyroscope, a rapidly spinning mechanism that helps the station maintain its stable position. They had repaired another faulty gyro during their first outing Saturday.

The loss of both units had left the station with only two working gyroscopes, the minimum needed to orient the outpost. The loss of a third would have required periodic firings of the station's steering jets to maintain stability, using precious fuel.