Worried about the "uncertainty" of the space shuttle Discovery flying home with two strips of heat shielding protruding from its underside, NASA officials on Monday ordered the first repair of an orbiter in flight.
NASA engineers said Stephen Robinson, 49, an engineer from Sacramento, would be lowered over the side of Discovery to either pull out or snip off two "gap fillers" sticking out between the thermal tiles cloaking the orbiter's belly.
The operation should take a little more than 90 minutes. It is simple enough to be regarded as routine, engineers said, except that it is an unplanned spacewalk and marks the first time in the shuttle's 24-year history that astronauts have been called upon to perform an in-space, onboard repair to their own vehicle.
Wayne Hale, deputy shuttle program manager, told reporters at a Johnson Space Center news conference that handlers decided on the spacewalk because they could not be sure that the protruding fabric strips would not cause dangerous hot spots in the orbiter's heat shielding during reentry.
"There is large uncertainty because nobody has a very good handle on the aerodynamics at those speeds," he said. "Nobody flies Mach 22 at 260,000 feet." A spacewalk, he added, is an "option to set our minds at rest."
Hale said astronauts and engineers on the ground developed the spacewalk plan over the past three days and practiced it "at length." He said the final package of instructions was uplinked to Discovery late Monday night.
Discovery launched last week as the much-heralded first shuttle mission since Columbia disintegrated on reentry 21/2 years ago, but the debut has been marred first by the loss of large pieces of foam insulation from its external fuel tank after liftoff, and now by the protruding gap fillers.
NASA grounded the shuttle fleet after the Columbia disaster and has grounded it again pending a solution to the foam problem. Hale said engineers have also formed a task force to analyze why the gap fillers come out and will change procedures for inserting them in the next two weeks.
The trouble with the hardware has contrasted sharply with the near-flawless performance of the shuttle crew, now in the middle of an eight-day sojourn at the international space station.
Robinson and fellow spacewalker Soichi Noguchi spent Monday removing a space station gyroscope that seized up for unknown reasons three years ago and replacing it with a new gyro brought from Earth.
The station uses four of these "control moment gyroscopes" to fix its position in space and had been surviving with two or three ever since the Columbia disaster. The shuttle is the only spacecraft big enough to carry items as large as the washing-machine-size gyros.
Robinson and Noguchi trained for this exacting task -- shifting 620-pound gyros 65 feet back and forth between the shuttle and the station -- for three years and carried it off with ease. They finished quickly enough to retrieve tools and rearrange equipment in the shuttle's payload bay for the Wednesday spacewalk that will include Robinson's unprecedented journey.
NASA came close to ordering an emergency spacewalk in June 1991 to repair Columbia's 60-foot-long cargo doors, whose broken seal dangled free like loose weatherstripping next to a critical latch. Engineers ultimately ruled out the spacewalk, however, because tests showed that the doors would close tightly even if the seal became caught in the latch.
Technicians use gap fillers, made of a feltlike fabric covered with a heat-resistant ceramic coating, to fill large crevasses in the mosaic-like patchwork of thermal protection tiles on the bottom of the shuttle, or to help stabilize them during the stresses of liftoff and reentry.
The fillers help provide a uniform surface as the shuttle transits the upper atmosphere during reentry, when tile temperatures can rise to 2,300 degrees Fahrenheit.
A flat surface enables the orbiter to enjoy a smooth ride, knifing through a silky "boundary layer" of hot gas that envelops the shuttle as it decelerates. A protruding gap filler, however, can "trip" this boundary layer and create turbulence that results in much higher hot-spot temperatures.
NASA engineers said Discovery's protruding fillers, one near the center line on the forward part of the orbiter and one slightly aft and to the side, are thin, single strips of fabric about six inches wide, sticking up stiffly about an inch above the tile.
Despite a paucity of data, NASA has plenty of experience with gap fillers. Shuttles fly with thousands of them, and engineers over the years have documented scores of protruding fillers after shuttles landed without incident.
Most of these protrusions were shorter than the ones on Discovery, but Hale said there was no way to know whether they had simply burned off during reentry. Until new imaging devices were installed on the shuttle during the post-Columbia redesign, it was impossible to look at a protruding gap filler in space.
Cindy Begley, Mission Control's spacewalk chief, described Robinson's task as "fairly straightforward," unlike Monday's elaborate gyroscope swap. She said engineers picked Robinson because Noguchi already had another assignment early in the Wednesday spacewalk, the time slot for the gap-filler job. "It's a single-person task, and we want to avoid too many tools or other items, including another crew member," she said.
Begley said the team will put Robinson in foot restraints aboard the space station's 55-foot crane. Shuttle pilot James Kelly and mission specialist Wendy Lawrence, the crane operators, will lower him over the side of the orbiter and place him in front of each afflicted gap filler.
The biggest fear about the job is that something will smack into the tiles and, by chipping or dislodging them, cause potentially catastrophic damage to the heat shielding.
Begley said Robinson's tether will be tied behind him, and his tool bag will hold only three items: scissors, forceps and a special cutting tool that looks like a hacksaw blade.
But the first tool will be the sausage-like, white-gloved Mickey Mouse fingers of Robinson's right hand. The hope is that Robinson will easily be able to pull out the offending gap fillers. If that does not work easily, he will go immediately to the hacksaw. The scissors are a last resort.
"I'm always nervous," Begley said about the repair. "It's like everything that happens on the shuttle. The first requirement is to do no harm."
Robinson, left, and Soichi Noguchi teamed up to remove a space station gyroscope that seized up three years ago and replace it with a new one.