By Paul Rincon
BBC News science reporter, Houston
An astronaut is to make unprecedented repairs to the space shuttle Discovery, the US space agency Nasa has announced.
Nasa engineers have been working out the best way to remove the material
Stephen Robinson will remove strips that are sticking out between heatshield tiles on Discovery's belly.
Nasa is concerned the dangling material - called gap fillers - could cause part of the shuttle to overheat as it re-enters the atmosphere.
Astronauts have never fixed a shuttle's heatshield on a spacewalk before - or gone under an orbiting shuttle.
The International Space Station's Canadarm2 robotic arm will position Discovery mission specialist Robinson underneath the shuttle so he can pull the gap fillers out.
If this does not work, he will use a saw to hack them away.
This task will be added to the third planned spacewalk of the mission - which may be delayed to give the crew time to prepare.
Wayne Hale, the shuttle's deputy programme manager, said Nasa could not be certain what effect the protruding gap fillers would have as the shuttle heats immensely during re-entry.
On balance, shuttle managers decided that a spacewalk to pull out the strips - though itself risky - would be preferable to letting the shuttle return as it is.
"When we looked at the unknown versus what we do know about [spacewalks], it was a very easy decision," said Mr Hale.
"The bottom line is there is large uncertainty because no one has a very good handle on aerodynamics at those altitudes and those speeds. Given that large degree of uncertainty, life could be normal during entry or some bad things could happen."
A team of managers, engineers and aerodynamicists has been working to address the issue for the past three days.
The shuttle's 15m-long Orbiter Boom Sensor System will be moved alongside Mr Robinson so that astronauts on the station and ground controllers can watch him carrying out the procedure.
"This is the new Nasa. If we cannot prove this is safe, we don't want to go there. It exceeded our threshold and we needed to take action," Mr Hale said.
The underside of the shuttle is exposed to the most intense heating during its re-entry to Earth's atmosphere.
The ceramic material is standing proud of the tiles
Protrusions on this otherwise smooth belly could disturb the air flow during re-entry, causing turbulence that raises temperatures on heatshield tiles downstream.
Chuck Campbell of the shuttle's aerodynamics team said the gap fillers could increase heat loads on shielding tiles by an estimated 10-30%.
"We're talking about several hundred degrees," he explained.
On re-entry the shuttle's underside can be exposed to temperatures of 1,260C (2,300F) or higher.
Damaged tiles are particularly vulnerable to overheating. The phenomenon can also cause problems with the shuttle's flight control.
Two gap fillers are sticking out from between heat shield tiles near Discovery's nose section, one by about 2.8cm (1.1in) and the other by about 2.2cm (0.9in).
Nasa officials say this is not related to damage from foam debris. Instead, the fabric could have been shaken loose by the vibrations during launch.
Shuttle managers have been going back over records of the STS-73 shuttle mission in 1995.
Stephen Robinson will make the walk under the shuttle
On that mission, space shuttle Columbia returned to Earth with a gap filler protruding by 3.5cm (1.4in).
The shuttle experienced early heating on re-entry, but returned to Earth safely.
But eight years later, a suitcase-sized piece of foam fell away from Columbia during its launch, punching a hole through heatshield panels in the left wing.
The damage did not present problems in orbit, but as the vehicle tried to re-enter Earth's atmosphere on 1 February 2003, super-heated gases entered the wing and tore the ship apart.