Safety repairs ordered on orbiting shuttle

NASA officials have ordered unprecedented repairs to space shuttle Discovery's heat shield to ensure the safety of seven astronauts who will fly the ship back to Earth next week.

The repair job, to be attempted Wednesday, involves the first trip by a spacewalking astronaut to the underside of an orbiting shuttle in the program's 24-year history. The crew member will try to remove two pieces of ceramic fiber material that are protruding from gaps between heat-resistant tiles on Discovery's black belly.

The astronaut chosen for the task is Stephen Robinson, a 49-year-old aerodynamicist born in Sacramento, Calif. Aboard Discovery, he serves as flight engineer.

In a space-to-ground news conference early Tuesday, Robinson acknowledged he's taking a big risk. "The tiles, as we know, are fragile, and an EVA crew member [spacewalker] out there is a large mass," he said.

Robinson will ride the International Space Station's robot arm to reach and smooth the rough spots. He'll be out of his crewmates' line of sight, which is a cause for some nervousness among mission managers. But Hale said engineers have figured out a way to position cameras on the robot arm so they can keep an eye on Robinson during the operation.

The astronaut will try to pull the fillers out with his hands. If that doesn't work, he will use a saw or scissors to cut the material until it is flush with the tiled surface.

Engineers have been developing procedures for the repair since Friday, while mission managers argued with each other about the risks.

Wayne Hale, deputy shuttle program director and chairman of the mission management team, said Monday evening that while the risks of a spacewalk are well understood, the risks of leaving the gap fillers unattended are not. "This is the new NASA," Hale said in a news conference. This is the first shuttle mission in 30 months and was intended to mark NASA's recovery from the February 2003 Columbia accident. But the agency has grounded shuttles again once Discovery returns in order to solve continued problems with dangerous foam insulation shedding from the external fuel tank.

Thanks to the imagery enhancements NASA was required to make in the name of safety before Discovery flew, this is the first time engineers have been able to see and evaluate protruding gap fillers before a shuttle lands. Hale told reporters at Mission Control in Houston that a lack of data from previous flights complicates the question of what might happen if the material is not removed.

One gap filler is located near the orbiter's center line behind the nose landing gear door, farther forward than any observed during postflight runway inspections to date. The other is off center, toward a wing, and farther aft. The protrusions could cause the shield to heat up four times faster and get 25 percent hotter than it is designed to do when Discovery glides through Earth's atmosphere on the way home. Whether the inferno would be too much for the heat shield is what engineers couldn't determine.

"The analysis had so much variability that we just could not preclude bad things," he said, adding that because nothing but the shuttle flies so high or fast - Mach 22 at 216,000 feet -nobody has a very good handle on aerodynamics at those altitudes and speeds.

Hale said all hazards were considered, and that engineers had developed a simple plan for the spacewalk with good safety precautions.

Robinson will do the work early Wednesday, at the start of a previously scheduled outing to do maintenance on the space station.