The Discoverycrew is today workeing out plans for an impromptu repair job that will send a spacewalking astronaut underneath the shuttle to work on its fragile belly for the first time in space history.
Working to ensure a safe return to Earth next week, the astronauts pored over instructions sent up from Nasa engineers and began piecing together a makeshift hacksaw that Steve Robinson may use in his task tomorrow.
Nasa wants him to pull out or cut off two thin strips of ceramic cloth, each about an inch (2.5 cm) long, sticking out between the heat-resistant tiles that coat Discovery's smooth underside.
The protruding strips are small but the US space agency fears they could affect aerodynamics and dangerously add to the intense heat on the shuttle as it re-enters Earth's atmosphere on August 8th.
The repair has been tacked on to an already-scheduled spacewalk to install a storage platform outside the International Space Station, which Discovery linked with last week.
The added spacewalk had Nasa engineers on the ground and the astronauts on Discovery scurrying to develop a plan for Robinson, who will be manoeuvred to the shuttle's belly on the space station's 50-foot (15 meters) robot arm.
"Procedures, plans and timelines are all currently being reviewed and updated to accommodate that unprecedented activity by Steve Robinson," said NASA flight commentator Rob Navias. "Some of those procedures are still being refined."
In the shuttle's 24-year history, spacewalking astronauts have never worked on the shuttle's belly before.
Out of direct sight from the shuttle and space station, Robinson will first try to pull out the protruding material, which is a stiff ceramic cloth. If that fails, he will try to cut it with the hacksaw, which Navias described as "basically a handle with a serrated edge."
Robinson must take extra care not to damage the fragile tiles that surround the protruding fillers. They can withstand fierce heat but are easily broken.
"The thinking right now is that the crew member is very unlikely to do any damage to the tile," deputy shuttle program manager Wayne Hale said. "That risk has been mitigated and controlled."
But just in case, he said, a tile repair tool tested by Robinson and spacewalking partner Soichi Noguchi in their first walk on Saturday will be in the shuttle airlock, available for use if needed.
Nasa is undertaking the risky repair because it does not want to lose another shuttle to heat damage, as it did with Columbiaon February 1st, 2003.
Columbia'sdemise was caused not by protruding strips but loose foam from the external fuel tank colliding with a wing at launch and punching a hole in the heat shield. Superheated gases entered the breach during re-entry and the shuttle broke apart over Texas, killing its seven astronauts.
After the accident, Nasa adopted new procedures, spent $1 billion on safety upgrades and built equipment to inspect the shuttle while it is in orbit. Discoveryis making the first shuttle flight since Columbia.
Videos showed loose tank foam at Discovery's launch last week and minor damage to the shuttle but nothing dangerous. The protruding strips are not believed to have been caused by impact damage, but perhaps poor adhesion.
But the recurrence of the foam problem prompted Nasa to order a grounding of its shuttle fleet.